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July 1973 T I * On Digital Simulation of Multicorrelated Random Processes and Its Applications COPY A. K. Sinha Department of Engineering Science and Mechanics This research was supported by the National Aeronautics and Space Administration, Washington, D. C. Under Grant No. NGL 47-004-067 Virginia Polytechnic Institute and State University https://ntrs.nasa.gov/search.jsp?R=19730021831 2018-07-30T02:44:48+00:00Z

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July 1973

TI

* On Digital Simulation of MulticorrelatedRandom Processes and Its Applications

COPY

A. K. SinhaDepartment of Engineering Science and Mechanics

This research was supported by the National Aeronauticsand Space Administration, Washington, D. C.Under Grant No. NGL 47-004-067

Virginia Polytechnic Instituteand State University

https://ntrs.nasa.gov/search.jsp?R=19730021831 2018-07-30T02:44:48+00:00Z

ON DIGITAL SIMULATION OF MULTICORRELATED RANDOM PROCESSES

AND ITS APPLICATIONS

by

Ajit Kumar Sinha

(ABSTRACT)

Two methods of simulation of multicorrelated random processes

from the given matrix of spectral density function have been pre-

sented. It has been noted that FFT method works as efficiently as

the Trigonometric method and is much faster. It has been found that

there are certain cases in which Trigonometric approach has advan-

tages over the FFT method. Some example problems are solved to show

the usefulness of this approach in solving the problems of linear

and nonlinear random vibrations. It has been observed that this

technique and particularly the FFT method offers a very fast and

convenient alternative for performing random nonlinear response

analysis. Various possible areas in which this approach can be

extended have been also discussed.

TABLE OF CONTENTS

Chapter

ACKNOWLEDGEMENTS iv

LIST OF FIGURES v

LIST OF TABLES . . . . vii

LIST OF SYMBOLS viii

I. INTRODUCTION 1

II. LITERATURE SURVEY 4

1. Review of Existing Simulation Methods 4

2. Review of Stationary Random Response of Multidegreeof Freedom Nonlinear System 10

III. DISCUSSION OF SIMULATION METHODS . 22

1. Trigonometric Model . . . . . 22

2. Fast Fourier Transform Method 33

3. Simulation of Strong Wind Turbulence 38

IV. APPLICATIONS OF SIMULATION TECHNIQUE 57

1. Forced Oscillation of Free Standing Latticed Tower . . 57

2. Nonlinear Random Vibration of String 71

3. Nonlinear Panel Response Due to Turbulent BoundaryLayer 80

V. CONCLUSIONS AND FUTURE EXTENSIONS . "

REFERENCES 102

APPENDICES '. 106

iii

A. Random Variable and Probability 106

B. Fast Fourier Transform 112

C. Matrix Method for Singular Cases 116 f~"i

D. Computer Program 119 /;

VITA 166 /

ACKNOWLEDGEMENTS

The author greatly appreciates the wise counseling, helpful

suggestions, and encouragement of Professor Larry E. Wittig, his

committee chairman. Special thanks must go to Professor M. Hoshiya,

who is now in Japan and under whom the foundation for this work was

laid to start with.

He further thanks each committee member for the aid and

assistance they have provided throughout his tenure at Virginia

Polytechnic Institute and State University.

The author acknowledges the sponsorship of this research by

the National Aeronautics and Space Administration under Grant

No. NGL 47-004-067.

Final thanks must go to Mrs. Peggy Epperly for typing this

dissertation.

iv

LIST OF FIGURES

Figure Page

1. Schematic Sketch of a Continuous Power Spectrum Used toGenerate Time Series 28

2. Schematic Sketch of a Discrete Power Spectrum from aSimulated Time Series 29

3a. Simulated Wind Velocities by Tirgonometric Model . . . . 42

3b. Simulated Wind Velocities by FFT Method 43

4a. Comparison Between the Original Spectrum ofHorizontal Turbulent Wind Velocity at 100 Ft. and theSimulated One by Trigonometric Method 44

4b. Comparison Between the Original Spectrum of HorizontalTurbulent Wind Velocity at 100 Ft. and the SimulatedOne by FFT Method 45

5a. Comparison Between the Original Spectrum of LateralTurbulent Wind Velocity at 100 Ft. and the SimulatedOne by Trigonometric Method 46

5b. Comparison Between the Original Spectrum of LateralTurbulent Wind Velocity at 100 Ft. and theSimulated One by FFT Method 47

6a. Comparison Between the Original Spectrum of VerticalTurbulent Wind Velocity at 100 Ft. and the SimulatedOne by Trigonometric Method 48

6b. Comparison Between the Original Spectrum of VerticalTurbulent Wind Velocity at 100 Ft. and the SimulatedOne by FFT Method 49

7a. Comparison Between the Original Spectrum of HorizontalTurbulent Wind Velocity at 50 Ft. and the SimulatedOne by Trigonometric Method 50

7b. Comparison Between the Original Spectrum of HorizontalTurbulent Wind Velocity at 50 Ft. and the SimulatedOne by FFT Method 51

VI

8a. Comparison Between Original Co-spectrum of HorizontalTurbulent Wind Velocities at 100' and 50' and theSimulated One by Trigonometric Method 52

8b. Comparison Between Original Co-spectrum of HorizontalTurbulent Wind Velocities at 100' and 50' and theSimulated One by FFT Method 53

9a. Comparison Between Original Quadrature Spectrum ofHorizontal Turbulent Wind Velocities at 100' and50' and the Simulated One by Trigonometric Method . . . . 54

9b. Comparison Between the Original Quadrature Spectrumof Horizontal Turbulent Wind Velocities at 100'and 50' and the Simulated One by FFT Method 55

10. Latticed Tower Subjected to Turbulent Wind Load 66

11. Simulated Generalized Forces and Displacement forTower Problem . 70

12. Simulated Generalized Forces for Nonlinear StringProblem 77

13. Simulated Displacement Time Histories for NonlinearString 78

14. Comparison Between Linear and Nonlinear Response ofa String by FFT Method 79

15. Problem Geometry for Nonlinear Plate Vibration Dueto Turbulent Boundary Layer Pressure 81

16. Simulated Generalized Forces and Displacement forthe Plate Vibration 96

17. Comparative Study of Plate Response Due to SubsonicPressure Fluctuations by FFT Method 97

1-A. PDF of Continuous Random Variable 107

LIST OF TABLES

Table Page

1. Concentrated Masses and Projected Areas at PanelPoints for Latticed Tower 67

2. Flexibility Matrix of the Latticed Tower in Y-direction . 68

3. Computed Normalized Mode Shapes with Respect to theBottom Panel for Tower Problem 69

vii

LIST OF SYMBOLS

A Displacement vector of the nth mode

a Velocity of sound for external flow00

a Velocity of sound for cavity flow

a (K), b (K) Time series amplitude coefficients

b.. Modal amplitude of plate vibration' J

C. . (w) Co-spectral density• J

D Plate stiffness

£[•] Expectation operator

F .(t) Dynamic wind load at the location j

F . Static wind load at the location jajF" Random generalized force for plate vibration

F6 f*mn' mn Aerodynamic generalized forces for external and

cavity flow respectively

F(t), T(t) Forcing function vector

f(x,t) Force per unit length of the strings\

f (x , t ) Nondimensional forcing function

f . j ( t ) Simulated time series

G(w) Spectral matrix

G (w) One-sided cross-spectral density

H (w) Element of lower triahgular matrix derived from

6(w)

vi 11

IX

K Stiffness matrix

K Covariance

k,, k2, k. Wave numbers in x-,, x^, x, directions respectively

k, S. Frequency index

M Mass matrix

M. Generalized mass in the tower problem

p(:x,y/t) Turbulent pressure

pe(:x,y,T) Fluid pressure, external flow

P (x,y,t) Fluid pressure due to cavity flow

p"» P^J P° Nondimensional turbulent pressure, external flow

pressure and cavity flow pressure respectively

Q. .(w) Quadrature spectrum

Q.(t ) Generalized force in the tower problem

q n-th generalized coordiante2

q p^U J2, dynamic pressure^

R Cross- correlation functionmnS Cross-spectrum of the fluctuating velocities at

oj okpoints j and k

Cross-spectrum of the generalized force

(w) Cross-spectrum of the generalized coordiante

S . Spectral density of the dynamic displacement

T Initial tension in the spring

t D

U^ Mean external flow velocity

U Convectional speed

•* Iutajt) Nondimensional displacement of the center of the

string

V . Mean wind velocity at the locationajV .(t) Pulsating velocity component

w Angular frequency

W Upper frequency cut-off

w. Lower frequency cut-off

w Nondimensional plate thickness

X_. • Elements of complex vectorPK

x", y, z" . Nondimensional co-ordinate

yk - Dynamic displacement

a (k) Phase angle

a • Modes of natural oscillation

3-|, $2 Damping coefficients

6 Logrithmic damping decrement of oscillation

Aw Frequency increments

c;pk Complex random number

?* Boundary layer thickness

v Poisson's ratio

£ , n Gaussian random variables

p Mass per unit length of the string

p Free stream density! OO

p Cavity flow density

R.M.S. of nondimensional response at the center

of the string

$ Airy stress function for membrane stress

$.(w) Transfer function in the tower problem

$, $c Velocity potential, external and cavity flows

CHAPTER I

INTRODUCTION

In this dissertation, two methods are described to simulate on a

digital computer a set of correlated, stationary and Gaussian time

series with zero mean from the given matrix of power spectral densi-

ties and cross-spectral densities. Some example problems are investi-

gated to show the power of this technique to solve the problems of

linear and in particular nonlinear random vibrations. The development

set forth is for any arbitrary number of correlated series; however,

in practice, the number is limited by the storage capability of the

computer.

The first method is based upon trigonometric series with random

amplitudes and deterministic phase angles. The random amplitudes are

generated by using a standard random number generator subroutine. An

example is presented which corresponds to three components of wind

velocities at two different spatial locations for a total of six

correlated time series. Selected spectral densities computed from the

simulated time series are compared to the original spectral densities

from which the time series were generated.

In the second method, the whole process is carried out using the

Fast Fourrier Transform approach in place of trigonometric series. It

is found that this method gives more accurate results and works about

twenty times faster for a set of six correlated time series.

1

To show one of the many areas of application of the present

method of simulation, namely the class of random structural vibration

analysis, the following problems have been investigated: (1) The

linear vibration characteristics of a long tower under the action of

correlated wind loads have been presented. Taking it to be a fourteen

degrees of freedom system, the time history of the displacement of the

top of the tower has been plotted considering up to three modes of

vibration. The usefulness of this method in the case of linear

analysis can be significant and very important, e.g., looking for the

occurrence of maximum structural response rather than the r.m.s. value

of response. This knowledge will be very useful for the reliability

study of structures under random loads; (2) One of the most interesting

and significant applications of the proposed method is the simulation

of random generalized forces. The necessity of simulating random

generalized forces arises when the dynamic response analysis is per-

formed in time domain either for the purpose of obtaining information

beyond the second order statistics (such as first passage time distri-

bution) or when the structure is nonlinear and therefore an approximate

random response is sought by simulating the excitation.

In order to assert the validity of the preceding discussion, the

problem of the nonlinear vibration ofa string has been solved in the

time domain by simulating the random generalized forces.

Finally to show the application of this method to a more complex

problem, the random nonlinear vibration of a flexible plate immersed

in a fluid flow on one side and backed by a fluid filled cavity of

finite dimensions on the other side is considered. The nonlinear

plate stiffness induced in the plate by out-of-plane bending and the

mutual interaction between the external and the internal fluid flow

is included.

The FFT simulation technique is utilized for the response

analysis of the plate undergoing large deformation. The same problem

has been done by Shinozuka [1] where he has taken a multidimensional

trigonometric series model for the generalized forces. The analysis

is performed in the time domain rather than in the frequency or wave

number domain as is usually done in linear response analysis. The

-numerical example has been presented for subsonic flow over the plate.

CHAPTER II

LITERATURE SURVEY

Part A: Review of Existing Simulation Methods

Numerous papers dealing with simulation of random process have

been published in recent years. Although many authors dealt with the

simulation of single random processes utilizing trigonometric series

[2], filtered white noise [2], filtered shot noise [3] and correlated

random pulse trains [4] etc, only Hoshiya and Tieleman [5], Shinozuka [6]

and Borgman [7] studied the simulation of multicorrelated processes.

I. In the simulation of ocean surface elevation, Borgman used wave

superposition by choosing the frequency in such a way that the ampli-

tude of each wave function was an equal portion of the cumulative

spectrum. Borgman also presented a method for simulating several

simultaneous time series by passing a white noise vector through

filters. He proposed the following model [7].

m °°y*(t) = A

m = 1, 2, ... M (2-1)

where,

ym(t) = m th time series

x.(t) = independent random inputs

= kernals

Let Smr(f) represent the cross spectral density between ym(t) and

yr(t). Kernals k (T) and its Fourier Transform T^A(f) are deter-

mined from the relation

rS t f\ _ y TT- / f\v t f\ c f f\

•mT"\^/ ~ " ^m-i V^/*^-»--i \^/^-,^-S \^/ •uij. ^J t I X i

j=i

r = 1, 2, ..., m and (2-2)

m = 1, 2 M

where the bar denotes the complex conjugate.

This system of equation can be solved sequentially (taking KJJ to

have zero gain for j = 1, 2, 3 ...) as

K2l(f) "/SJlTfT (2.3)

K22(f) = ES22(f) - |K21(f)|2]%tc.

This, in turn, determines the digital filter co-efficients, anmj,

needed to approximate the kernel associated with the system function

kmj(f). Then the final simulation equation reduces to

N Nym(kAt) = ^ ^ a^x., , k-n,

m = 1, 2, ... M (2-4)

in which x^, n for n = 1, 2, 3, ... is the j th generated sequence ofJ

independent, zero mean, unit variance, normal random variables.

II. Later Hoshiya and Tieleman [5] considered the following trigo-

nometric model for simulating two correlated random processes:

x(t) = E (a,-cos w.t + b.-sin w,-t)= J J J J

ny(t) = E (CCOS (Wt + a) + d-sin (Wt + a) (2-5)

where a,-, b-, c- and d- are random variables and Oj is deterministic.

The two stationary normal processes are correlated if there exists a

non-zero correlation between a,- and c,- and/or b,- and dn-. The co-J J J J

variance CXyj between the two random variables a^ and Cj is defined

as

cxyj = Etajcj] = EtbJdjl

and the standard deviation at frequency w- , axj and Oyj are given as

axj = (w-jMw (2-6)

ayj = ^(WjjAw (2-7)

where Sx(w,-) and Sy(wj) are the discrete form of the power spectra for

processes x and y respectively.

Expressions for the co-spectrum and the quadrature spectrum are given as

2CCy(Wj)Aw = axjayjpxy.jcos aj (2-8)

2QCy(Wj)Aw = -axjayjPxyjSin a-j (2-9)

whence,

and

The random variables a. and b^ are generated independently from aJ «

normal distribution with zero mean and a standard deviation

and Oyj = 2Sy(wj)Aw respectively. Then Cj and d:

are generated as follows. Since a< and c^ are both normally dis-J J

tributed, the joint probability density function of a^ and c* isF a-21 a;

.2

axj°yj

and.the probability density function of aj is

P(aj) =

The conditional probability density function of Cj for given 3j is

(2-iic)

Consequently, the conditional probability density function of Cj for

given aj is also Gaussian with mean Pxyj j and a standard deviation

i- Similarly, the conditional distribution of d- for a givenjand standard deviationb- is Gaussian with mean

xjThus random variable Cj is generated from Gaussian distribution with

mean Pxyj f9-; and a standard deviation of ayj 1-Pxyj' Similarly, thexj

random variable d- is generated from a Gaussian distribution with meanJ

i/ ?"and a standard deviation of a.,,- l - p -J Jj

8

III. In his paper [8], Shinozuka proposed a different

trigonometric model for the simulation of multivariate random pro-

cesses. He used a series of cosine functions with weighted ampli-

tudes, almost evenly spaced random frequencies and random phase

angles with uniform distribution. He considered a set of n sta-

tionary random processes fo.j(t) (i = 1, 2, ... n) with a specified

cross-spectral density matrix S°(w) = [S°f.f.(w)], where S°-p.f.(w)I J I J

are mean square spectral densities if 1= j and cross spectral

densities of fOj(t) and foj(t) if i 4= j.

He proposed the following model

x c o s [w j k t+ qjk + e^^)] (2-12)

where,

w^ (k = 1, 2, ... N) are random variables identically and independent-

ly distributed with the density function

gj(w) = iH-jjMlVj2 (2-13)oo

with a,2 = J |H in.(w)|2dw (2-14)J _oo JJ

and Q.. (k = 1, 2, ... N) are also identically and independently dis-

tributed with the uniform density l/(27r) between 0 and 27r. HJ.J(W)

are obtained from

Si.j(w) = Z. H1k(w)Hjk(w) (2-15)K~ I

and,

H^(w)(2-16)

(2-17)

The series simulated by the above technique have the asymptotic

ergodicity.

IV. Comparison of Various Approaches.

It appears that method of simulation proposed by Shinozuka [8] is

the most efficient so far. Thismodel [8] seems to be more general than

that due to Hoshiya and is more efficient than that due to Borgman [7].

Since the measured cross spectral densities are usually given

numerically in terms of the real and the imaginary parts or the

modulii and the angles, the computation work associated with finding

|H-jj(w)|, from which "YJJ(W) = |H1-j(w)|/Hj1-(w) and the angles 6-jj(w) .,

can be directly evaluated in the case of Ref. [8]. Therefore, method

of simulation presented here appears more practical than that proposed

in Ref. [7], which requires (a) the inverse Fourier Transformation of

N(N+l)/2 functions of w, H-J-J(W) and (b) the same number of integra-" <

tion in the time domain. Also, form of simulated functions con-

sisting of sum of cosine functions can be proved extremely advan-

tageous, when used as input, in evaluating the corresponding output

of a linear system.

10

PART B.

The main usefulness of the simulation method is in the area of

(a) a numerical analysis of dynamic response of non-linear structures,

(b) time domain analysis of linear structures under random excitations

performed for the purpose of obtaining the kind of information, such as

first excursion probability and exact time history of a sample

function, that is not obtainable from the standard frequency domain

analysis, and (c) numerical solution of stress wave propagation

through a random medium and eigen value problems of structures with

randomly non-homogeneous material properties. The usefulness of this

method has been demonstrated in the later chapter of this dissertation.

In this connection, it looks appropriate to make a relative study of

the prevalent methods of handling the non-linear random vibration

problems.

Review of Stationary Random Response of

Multidegree of Freedom Nonlinear Systems

Fokker-Planck Approach.

One exact method of studying the stationary random response of

a nonlinear system is the Fokker-Planck approach. If the excitation

is a Gaussian white noise, then the transitional probability density

of the response process is governed by the Fokker-Planck equation.

The equation governing the first probability density function for the

stationary response has been solved under the following rather

restrictive conditions:

11

(1) the damping force is proportional to the velocity

(2) the excitation is a Gaussian white noise

(3) the correlation function matrix of the excitation is

proportional to the damping matrix of the system. Under the above

conditions, the equation of motion may be written as follows:

(2-18)MX + Cx + = f(t)3x

= 0

with

= 2YC6(T) (2-19a)

where y is a constant, u(x) is the potential energy of the system and

/ 3u \

(2-19b)3.xl

3'u3x«

\Suppose that there exists an orthogonal matrix A which can simul-

taneously diagonalize the mass matrix M and the damping matrix C:

(2-20)

where V and A are two diagonal matrices. Then, upon using the trans-

formation x = Az and noting that

" ' 3u(I)jj=l J'm J>< 3zk

m = 1 , . . . n

3X3u(

3X 3z

_ 3u(z

3z(2-21)

12

Equation (2-18) becomes

Vz + AZ + j = ATf(t) = b(t) (2-22a)

and the correlation function matrix of b~(t) is given by

R^T) = 2YAS(T) . (2-22b)

The stationary Fokker Planck equation associated with (2-22a) is given

by

(2-23,

where A- and v- denote the j th diagonal element of A and V, p is thei

abbreviation for the first probability density of the Markovian/z\

vectorf^J. The solution to (2-23) may be written as follows:

p(z,z) = 3 expj-^E VjZ.,.2 + u(I)]J. (2-24)

This solution was first obtained by Ariarathan [9] for a two-degree-

of freedom system, and it was extended to the above form by Caughey

[10]. The constant 3 in (2-24) is a normalizing factor such that

r f°°••• j p(F iz)dz1...dzndz1...dzn = 1- (2-25)

In the original co-ordinates, equation (2-24) becomes

p(x,x) = B exp« - 7|9x'Mx + ufx) >. (2-26)I l V

It will be noted that the terms in the square brackets are

respectively the kinetic energy and the potential energy of the system.

13

Equation (2-26) may also be written as

p(x,x) = B exp {--gi xMTx> exp {- lu(x)> (2-27)

Hence x" and x~ are linearly independent.

Normal Mode Approach

Consider an n-degree-of-freedom system governed by the equation

of motion

MX + C^x" + K(°)X + y?(x,x) = f(t), (2-28)

u = a small parameter

The matrices c'°).. and K*0' are respectively the damping matrix and

the stiffness matrix of the system due to the linear part of the•

damping forces and the spring forces, and yc (x,x~) represents the non-

linear forces of the system. Here, T(t) is a stationary Gaussian random

vector. Without loss of generality, one can assume that the mean vector

of T my = 0.

In using this approach, the following two conditions must be

satisfied: (1) the linear system obtained by neglecting the nonlinear

term g"(x~,x~) in (2-28) must possess normal modes; (2) the correlation,

function matrix R^T) must be diagonalized by the same matrix which

diagonalizes the matrices M, C*°' and IO°^.

The second condition is quite restrictive and is seldom realized in

real systems.

Assume that the above restriction can be met, then there exists

a matrix A such that

14

ATMA = I

= wk(o)5ki

* KJ (2-29}( *

ATRf(t)A = D(T), dkj = dk(T)6kj.

By using the transformation x~ = Az~, Eq. (2-28) reduces to

z + A(o)t + n(o)I + yATg(z,I) = ATf(r) = F(t) (2-30a)

Where the correlation function matrix of b is

MT) = D(T) . (2-305)

In component form Eq. (2-30a) becomes

.. . - j » z ) = bj(t) (2-31a)K~" I

and Eq. (2-30b) becomes

E[bk(t)b.j(t+T)] = dk(T)6k.j, j, k = 1, ... n. (2-31b)

The differential equation in (2-31 a) may be written as

' j = 1 , ... n

where the deficiency term e..- is given by

._ _+ y E akjgk(z,z), j = 1, ... n (2-33)

K*~ I

2If the quantities Xj and Wj are chosen in such a way that some

measure of the deficiency term is minimized, then it seems reasonable

that the statistics of the response of the nonlinear system can be

approximated by those of the linear system described by

Zj + XjZj + Wj2Zj = bj(t), j = 1, ... n (2-34)

15

At this stage, the differential equations are uncoupled and the

excitation b(t) is an uncorrelated vector process. Hence each un-

coupled differential equation can be solved separately.p

In order to determine Xj and Wj , Caughey [11] chose them so as

to minimize the mean square value of the deficiency term e. This can

be achieved by requiring that

] = 0 \1\j = 1, ... n. (2-35)

f?1?] = 0'

Substituting (2-33) in (2-35) and interchanging the order of dif-

ferentiation and expectation, we obtain

~" I

= (Wj(o))2 + u Z akjt[z.jE[z.jgI<(z",z)] (2-36)l\~" I

W;

Equations (2-34) and (2-36) can be used to find various mean square

values of the response process.

In certain cases, the contribution from the first mode may be

dominant. In these cases, we may let x. = a^-iz-i in the aboveJ w

derivation. Then

n

3=1 (2-37)n n . 9

XT = X^ + y Z a. j lE[z1g..(z1,z1)]/Etz1^].J "~ *

16

This is rather a rough approximation, but it is very simple and in some

cases, it does give reasonably approximate solutions.

Perturbation Approach

Consider the same problem as defined in the previous section, whose

equations of motion are

MX + G(°)X" + K ^ ° % + yg"(x",x) = ?(t) (2-38)

Assume that y is so small that the solution of Eq. (2-38) can be

approximately represented by [12]

x" = Xg + yx"-j (2-39)2

Substi tut ing (2-39) into (2-38), neglecting terms involv ing y,n 0 1y3, ... and equating corresponding coefficients of y and y yields

the following sets of linear differential equations:

Mx0 + C(°)x0 + K

(°)x0 = f(t) (2-40a)

MXT + C(°)x-, + K(°)X-, = -g"(x0(t),x0(t)) (2-40b)

Correct to the same order of accuracy, the instantaneous correlation

matrix for displacement becomes

E[xxT] = E[x0x0T] + y{E[x071

T] + E^Xg1]}- (2-41)

Note that

E[XOX-|T]= (EtXoX-,1])1. (2-42)

The matrix E[XQXQ] can be found from (2-40a) by the various approaches

as in linear analysis, and EJ'XQX' ] may be evaluated as follows.

Since (2-40a) and (2-40b) are linear, their stationary solutions are

XQ = J G(t-T)f ( i )dT (2-43a)

17

dT , (2-43b)

where G(t) is the common impulse response function matrix of (2-40a)

and (2-40b) and g(x) is the abbreviation of (XQ(T) ,XO(T)). Thus,

E[XOX.,T] = - f f G(t-T1)E[f(T1)g

T(T2)]

x GT(t-T2)dT1dT2 . (2-44)

The matrix E[?(T| )gT(T2)] can be evaluated with the help of proper-

ties of Gaussian processes. Therefore, we can find E ^ g x ] and

hence E[xxT].

Usual ly , the evaluation of the integrals in (2-44) is not easy, so

Tung [13] has developed a different approach to generate E[xxT]

from (2-40a) and (2-40b). He applies Foss's method [14] to uncouple

(2-40a) and (2-40b) into first order differential equations and then

solves the resulting equations to f ind the various instantaneous

correlation matrices.

This approach wi l l fail if the damping matrix f/°^ is a nul l

matrix. In this case, Equation (2-40a) does not have a stationary

solution since all its correlation functions w i l l f ina l ly go to

inf ini ty . Another l imitation of this approach is that not onlymust the

nonlinearity of the system has to be small , but also the excitation

has to be suff icient ly low.

Generalized Equivalent Linearization

The normal mode approach is qui te powerful if it applies. How-

ever, due to the conditions imposed on the excitation, its appl icat ion

18

is rather limited.

In his thesis Yang [15] introduced a more general approach.

Except that the excitation must be stationary, the only additional

restriction to this approach is thatthe excitation be Gaussian. In

this approach, we define an auxiliary set of linear differential

equations for the original nonlinear system. The solution of the

original nonlinear system is approximated by the solution of an

auxiliary set and the unknown co-efficients are chosen in such a way

that some measure of the difference between the two sets of equations

is a minimum.

Consider the following linear equation

MX + Cx~ + Kx = ?(t) (2-45)

and the nonlinear system given by

fft + g(x,x~) = f(t). (2-46)

The difference between (2-46) and (2-45)

e~= g(x~,x) - ex" - Kx" (2-47)

The necessary conditions to minimize E.[e~e] are given by

SE[?Te] = 2E[eT 3ef 1 = 2E[e,x.] = 0

ik_ _ ^ (2"48)

3E_[eJ_e]_= 2E[eT e ] = 2E[e.x,] = 0-3k-,. I 3k i k J J

J ix J IV

Upon using Eq. (2-47), they become in the matrix form

E[exT] = Etg^x.xlx"1] - E[xxT] - KE[xxT] = 0

_T _^-,-T .*-T —TE[ex ' ] = E[g(x,xlx'] - CE[xx ' ] - KE[xx ' ] = 0

It has been shown that the conditions in Eq. (2-49) do define a

minimum for E[e~Te"][15].

19

In order to solve Eq. (2-49) for K and C, it is first necessary to_ JL __ .T __ : __ J.T __ T _ I.T

express E[g(x,x)x ' ] and E[g(x,x)x ] in terms of E [xx ' ] , E [xx ' ] and

E[xx~T]. Let ykr denote the displacement of k th mass relative to

the r th mass and let the approximate force acting on the k th mass

by the nonlinear element connecting the k th mass and the r th mass

be denoted by Skr(ykr»ykr) . Then• •' •

E[gk(x",x),x.j] = z E[Skr(ykr,ykr)x.j]

r4k (2-50)

E[g k(x>x)x j ] = z

where the sum is taken over all nonlinear elements connected to the

k th mass. Since x~ is a Gaussian vector, it follows that the• •

quantities ykr, y. , x-> and Xj will be Gaussian distributed. Hence

E[Skr(ykr,y|<r)xj] = E[Skr(ykr,ykr)ykr]

E[Skr(ykr,ykr)x.j] = E[Skr(ykr,ykr)ykrl

x E[ykrx..]/E[y r]

Define

Ykr = E[Skr(ykr,ykr)ykr]/E[yk2]xkr = E[Skr(ykr,ykr)ykr]/E[yk2]

Then Eq. (2-51) reduces to

E[Skr(ykr.Ykr)Kj] = r "-"".J- (2.53)E[Skr(ykr,ykr)x..] = E[(Ykrykr *

20

Hence, there exists a linear system with spring constant X^ and

damping co-efficient y^ defined by Eq. (2-52) such that if the

nonlinear system is replaced by this linear system, the expectation_ ^ . .— _ • • -p

values E[g(x,x")x ' ] and E[g(x,JOx ] will not be changed.

Substituting Eq. (2-53) in Eq. (2-50) gives

E[gk-(x,x)x.,] = E[l(Ykrykr

_ _ . . (2-54)E[gk(x,x)x.,] = E[Z(Y|<rykr + Xkrykr)xj]

Let the stiffness matrix and the damping matrix of the linear system

defined by Eq. (2-52) be denoted by K^e) and C^. Then Eq. (2-54)

can also be written as

(2-55)

E[gk(x,x)x.,] = E[j i(CJe)xs + KJe)xs)Xj]

since the right-hand side of (2-54) and (2-55) are just two different

representations of the total force acting on the k th mass. In

matrix form Eq. (2-55) becomes

E[g"(x,x)xT] = c(e)E[xx"T] + K

E[g"(x,x)x"T] = c ~ "

Substituting Eq. (2-56) in Eq. (2-49) and solving for K and C whichT. .

minimizes E[e e], yields

(K - K^bEIxx"1] + (C - C ( e))E[xx~T ] = 0 /

(K - K ( e))E[xxT] + (C - C ( e ) )E[xxT ] = 0

21

which can also be written as

E[xxT] E[xxT]

E[xxT] E[xxT]

( K -

(C -= 0. (2-58)

If the square matrix is non-singular, the only solution to Eq. (2-58)

is

K =(2-59)

If the square matrix is singular, (2-59) is not the only solution.

But in this case any solution to Eq. (2-58) will lead to the same

minimum for E[ee']. So we still can use Eq. (2-59).

Thus, it has been shown that the linear system formed by

replacing each nonlinear element by a linear spring and a linear

damper defined by Eq. (2-52) will minimize E[ee~T] provided that the

excitation is Gaussian.

It should be pointed out that the smallness of the nonlinear

term in the original equation was not a presupposition in the develop-

ment of the above method. In general, the accuracy of this method

depends on the smallness of the nonlinearity. When the nonlinear

terms are not small, this method is often an iterative procedure.

This concludes the relevant literature survey. In the light

of the review study in this chapter, we will be better able to

appreciate, in the next chapters, the power and usefulness of the

simulation approach in the analysis of random linear and nonlinear

problems.

CHAPTER III

DISCUSSION OF SIMULATION METHODS

In this chapter we will present two methods to simulate a set

of Gaussian processes (f-|(t), f2(t), ... fm(t), ... f (t)} when the

power spectral density function of each process and cross spectral

density functions between any two processes are known. These

spectral densities are usually arranged in the form of a matrix

commonly known as spectral matrix. This matrix is given by

G12(w) ... G1M(w)

G(w) =G21(w) G22(w) ... G2M(w)

GM](w) GM2^W) •••

(3-1)

where Gmn(w) is the one-sided cross spectral density between the pro-

cess fm(t) and fn(t). When m + n, Gfnn(w) is complex with real part

Cm^w) called the co-spectrum and imaginary part - Qmnlw) called the

quad^spectrum. When m = n, the quad-spectrum is zero and then

Gf^w) stands for power spectrum for that particular process.

I. Trigonometric Model

Here we will show that a set of random processes can be simu-

lated by a trigonometric series of the form

N= z {ai l(k)cos [wkt + on(k)]

K~ I

+ bn(k)s1n [wkt + an(k)]>(3-2a)

22

23

Nf2(t) = Z {a21(k)cos [wkt + o21(k)] + b21(k)sin [wkt + a21(k)]

K I

+ {a22(k)cos [wkt + a22(k)] + b22(k)sin [wj^t + a22(k)]>

(3-2b)

fm(t)=k^

{aml(k)cos [V + <W<>] + bml<k>s1n

am2(k)cos [wkt + (^(k)] + ^(kjsln [wkt

More generally, the above set of processes can be represented as

m Nfm(t) = Z Z {amp(k)cos .[wkt t -anptk)]

p~" I K~~ I

+ bmp(k)sin [wkt + o p(k)]. (3-3)

The coefficients amp(k) and bmp(k) are Gaussian random variables with

zero mean that have the following additional properties: (i) amp(k)

and bnq(2,) are always independent and for the case when m = n, p = q

and k = 1 they are identically distributed, (ii) amp(k) and anqU)

(or t>mp(k) and bnq(S,)) are statistically correlated if p = q and

k = a, otherwise they are independent.

The phase angles (k) are deterministic. Since the co-

efficients amp(k) and bnq(k) are Gaussian distributed it follows from

24

the Central Limit Theorem that the time series fm(t) will also be

Gaussian distributed.

The form of the simulated time series given by Eq. (3-3) lends

itself to a very simple physical interpretation. For the first time

series (m = 1) the index of summation p takes the value of unity.

For the second time series (m = 2) the index p takes the values

unity and two. The value of p = 2 adds to the second series two

terms that are independent of the first series. Physically it adds

some energy to the second process independent of the first process.

Likewise for each new process two additional terms are tacked on which

allow that process to have some energy that is independent of all the

previous processes.

In order to see how the co-efficients amp(k) and bn (£), and

the phase angles amp(k) should be determined it is necessary to con-

sider the cross correlation between the time series fm(t) and fn(t).

Here for convenience we will assume m £ n. The cross correlation

between these time series is given by

R^T) = E[fm(t)fn(t +T)]

m n N N= E[ Z Z Z Z

p=l q=l k=l £=1

{amp(k)anqU)cos [wkt + ^(kjjcos [w£(t + T) + an

+ amp(k)bnqU)cos [wkt + ^(kjlsin [wA(t + T) + o

+ bmp(k)bnq(A)sin [wkt + amp(k)]sin [w£(t + T) + anq(£)]>]

(3-4)

25

The expectation operator can be brought inside the summations,

and since the expectation of a sum is the sum of the expectations, we

get

m n N NR^T) = Z E E E

p=l q=1 k=l A=l

{E[amp(k)anq(£)]cos [wkt + o^pfkj jcos [wk(t + T) + anqU)]

+ E[amp(k)bnqU)]cos [wkt + amp(k)]sin [wA(t + T) + anqU)l

+ E[bmp(k)anqU)]sin [wkt + amp(k)]cos [w£(t + T) + anqU)]

+ EtbmpCkJbnqf^Jls in [wkt + ^(kJlsin [w£(t.+ T) + anq(£)]}

(3-5)

Because amp(k) and bnq()l) are always independent the middle terms in

the above expression are both zero. Furthermore, since amp(k) and

anq(^) (°r bmp(k) and bnq^^ are independent unless p = q and k = £,

Eq. (3-5) reduces to

m N

{E[amp(k)anp(k)]cos [wkt + cxmp(k)]cos [wk(t + r)

+ E[brnp(k)bnp(k)]sin [wkt + (%p(k)]sin [wk(t + T) + anp(k)]}

(3-6)LetKmnp(k^ = Etamp(k)anp(l<)]. That isK f f lnp(k) is the covariance

between amp(k) and anp(k). Because b^pfk) has the same distribution

as amp(k), we also have K^p (k) = E[bmp(k)bnp(k)] . Using these

relationships and a trigonometric identity Eq. (3-6) becomes

m N) = E E K^pdOcos [wkr - ofTlp(k) + anp(k)] (3-7)

p=l k=l

26

The right hand side of Eq. (3-7) is independent of the time t; there-

fore, the assumed trigonometric form for the time series yields

stationary random processes.

Noting the fact that the spectral density is the Fourier Trans-

form of the correlation function, the one-sided cross-spectral

density for processes fm(t) and fn(t) is given by

2 j" l\nn(T)iJWTdT, w> 0(3-8)

0, w < 0

The next step is to substitute Eq. (3-7) into Eq. (3-8) and perform

the integration. However, it should be recalled that KmnpCO in Eq.

(3-7) is the expected value of amp(k) times anp(k) (or bmp(k) times

bnp(k))- Therefore, by substituting Eq. (3-7) into Eq. (3-8) what

we obtain is an expected value for the cross-spectral density. Thus

we have

m N2 Z Z

r ,-JWTx I cos [wkx - omp(k) + anp(k)]e di}

w > 0

(3-9)0, w < 0

Carrying out the integration gives

m N2ir Z Z Kj^p^Mw - wk)

E [ G m n ( w ) ] = 1 P=1 k=1

x {cos [Ofpptk) - anp(k)] - j sin[ofop(k) - anp(k)]} , w _> 0

Q, w < 0 (3-10)

where 6(w - w^) is the Dirac delta function.

27

We thus see that for the assumed trigonometric time series given by

Eq. (3-3) Gftn is composed of both real and imaginary terms. When

m = n, Gjnn corresponds to a power spectral density and the imaginary

portion is zero. A schematic plot of Eq. (3-10) is shown in Figure

1. The frequencies w^ are chosen such that

wk = w£ + (k - ^.)Aw, k = 1, 2, ... N

Aw = Wu " w* (3-11)N

where wu and w,, are respectively the upper and lower cut off fre-

quencies of the spectra that is used to generate the time series.

Suppose an actual power spectrum G(w) for which we want to obtain a

simulated time series is shown in Figure 2. We can slice this

spectrum up into N intervals of width Aw as shown. The amount of

energy contained in the slice centered at w = W|< is Gfw^Aw. We want

the expected value of the spectrum corresponding to our simulated

time series to have the same amount of energy at w = w^. This means

that the expected amplitude of the delta function at w = w|< should be

G(w)Aw. Similar criteria must be applied for the cross-spectra of the

simulated time series.

We assume that at any frequency w = W|< the cross-spectral

matrix can be factored into an upper and lower triangular matrix,

where the upper triangular matrix is the complex transpose of the

lower triangular matrix, as indicated below:

28

'mmt

t t 1 I J1

W, W2 Wk WN

w, wu

w

Fig. 1 . Schematic sketch of a discrete power spectrumfrom a simulated time series.

29

'mm

" W, WoW,

W

AW

Fig. 2. Schematic sketch of a continuous power spectrumused to generate a time series

30

Gn(wk) G12(wk)

G21(wk) G22(wk)

. G l M (w k f j

. G2M(wk)

GMl(wk) GM2(wk) GMM(wk)

Hn(wk) 0 ... 0

H21(wk) H22(wk) ... 0

Hm(wk) HM2(wk)

Hn(wk) H21(wk) ... HM1(wk)

0 PT22(wk) ... HM2(wk)

0 0 HM M(w k)

( - 1 2 )

The bar is used to denote the complex conjugate.

If Eq. (3-12) holds then

Ww) = Z VW)VW) (3-13)

for n <_ m. Eq. (3-13) can be rewritten with the limit of summation

taken as m instead of n since Hmn(w) = 0 for m > n. Thus Eq. (3-13)

becomes

m _W"> - z Hmp(w)Hnp(w) (

P=l

Using the above relation,solutions are obtained as

where [fo(wk) is the m th principal minor of G(wk) with D0 being

defined as unity, and

31

H (w, } = H (wi ) 6(1, 2, , p - 1 , .m\"nipple' Hpp^wk; vl 2, p - 1. V,

D k ( w ) ( 3 - 1 6 )

.p = 1, 2 M, m = p + 1 M

where,

"11 6-10 G-i , I Gln

G/l, 2, ..., p-1, m\M, 2, ..., p-1, p;

321

Gp-l,l, GP-1,2 ' ' ' ' Gp-l,p-l GP-1,P

Gml > GmZ ' • • • • • ' jp-l ^p

is the determinant of a s'ubmatrix obtained by deleting all elements

except (1, 2, ... p-1, m) th rows and (1, 2, ... p-1, p) th columns of

G(wk).

It is noted that the above solutions are valid only when the matrix

G(w) is Hermitian and positive definite, as can be seen from Eq.

(3-15).

Because the cross-spectral density matrix G(w) is known to be only

non-negative definite, special consideration is needed in those cases

where G(w) has a zero principal minor. This is discussed in Appendix

C.

For the band of frequency [WL - -^ Aw, wu + rAw] we want the ex-l\ ^ "• C.

pected value of the energy in the spectrum of the simulated time

series to be equal to the energy Gmn(w)Aw of the original spectrum.

Thus equating the energy in the band centered at w = wk as given by

Eqs. (3-10) and (3-14) we get

2TrKnlnp(k){cos [Ompfk) - o^k)] - j sin [^(k) - o^k)]}

= Hmp(wk)Hnp(wk)Aw (3-17)

32

If we write

(3-18a)

(3-18b)

= RE[Hmp(wk)] + 3 IM[Hmp(wk)] (3-18c)

then by substituting Eq. (3-18a) into Eq. (3-17) we can see that

= lHmp(wk)ltcos sin

From Eqs. (3-18b) and (3-18c) we find

-1 IM[Hmp(wk)](3-20)RE[Hmp(wk)]

Thus it follows, as stated earlier, that the phase angles are deter-

ministic.

We recall that

(3-21a)

(3-21b)

That is we want amp(k) and bmp(k) to be independent random variables

with their covariances given by the right hand side of Eq. (3-19).

This can be done in the following manner, if we let

E[amp(k)a (k)]or

E[bmp(k)bnp(k)]

and

(3-22a)

(3-22b)

where m = 1, 2 M; p = m, m + 1, ... M and f and n are in-

dependent Gaussian random variables generated on a digital computer.

Because the £'s and n's are Gaussian distributed, the a's and b's

will also be Gaussian distributed.

33

Substituting Eq. (3-22a) in Eq. (3-21a) or Eq. (3-22b) in Eq. (3-21b)

and taking the expectation of both sides we see that in order to

make the left-hand-side equal to right-hand-side, the mean of £ or

n should be zero and their variances should be unity.

From Eq. (3-21 a) we have

and

(3-23b)

Thus the terms amp(k) (or bmp(k)) satisfy the properties stated earlier

and they also satisfy Eq. (3-17).

This concludes our explanation of how to simulate several multi-

correlated time series.

After discussing the Fast Fourier Transform in the next section, we

will give an example problem to check and compare the two methods.

II. Fast Fourier Transform (FFT) Method

Assume that a set of discrete time series can be represented by the

following expression:

fpn ' JT SXPkEXP(J T1] (

where ,

p = number of time seriesn = time indexk = frequency index

34

The terms Xpk are elements of a complex vector generated in the

following way

{Xpk> = f?h1*HkHCpk> (3-25)

where,

h = At, discrete time increment

[Hk] is a lower triangular matrix obtained from the given

spectrum matrix as discussed in the preceding section, that is, it

satisfies the relation

[Hk][Hk]T = [6k]

and, ?pk = spk.+ j npk

(3-26)

(3-27)

where £ni. and nnk are independent Gaussian random numbers such that-pk

E[Cpk] = E[npk] = 0

E[^J = E[n2k] = 0.5

Writing Eq. (3-25) in expanded form

(3-28a)

(3-28b)

lk2k

Mk

N_ik2h

H]lk 0 . . . . 0

H21k H22k ' ' ' ' °

HMlk HM2k • • H MMk

C2k

CMk

(3-29a)

That is, we can write

pk = [aDiH (3-29b)

(3-29c)

where the bar denotes complex conjugate.

35

Then,

V i - " cr<Xr*,] " 2h E[

N M M

Moving the expectation operator inside, we can write

<3-30b)

Making use of properties of complex random number as given in terms

of £'s and n's by Eqs. (3-28a) and (3-28b), it can be easily seen

that

where 6qs and S^ are Kronecker delta functions;substituting (3-30c)

into (3-30b), we get

M M My T H H (S & f^-^la^

q=l s=l

M(3-31b)

Taking the inverse Fourier Transform of Eq. (3-24), we get

X,nk = SPk n=0 r"

Therefore,

xp(N-k) = ^

N-l(3-32b)

n=0 r"

36

Since f is real, comparing Eqs. (3-32a) and (3-32b), we see

that

Xp(N-k) = xpk (3'33)

Thus, it is only necessary to generate Rvalues of X for each time

series because the other half of the values of X are just the complex

conjugate of the first half of the values of X.

Now, it is necessary to show that time series represented by

Eq. (3-24) do indeed have the proper power and cross-spectral

densities.

From Eq. (3-24)

Since f is real, taking the complex conjugate on both sides of

the above equation, we can write

Thus, the cross correlation between time series p and time series r

is given by

Rpr(n,m) = Etfpnf^] (3-34a)

where, n and m stand for the time indices of process p and process r

respectively.

Substituting for fpn and f^ from Eq. (3-24) and Eq. (3-24a)

respective7y in Eq. (3-34a), we get

R p r (n ,m) = E[Jj-2 I I XpkX r J lEXP[j MjpM-jj (3.34b)

37

Moving the expectation sign inside

1 N-l N-l

k=0 &=0

x EXP[j N " — ] • (3-34c)

From Eq. (3-31b)one obtains

- _ N M

q=l

Substituting this in Eq. (3-34c)

-, N-l N-l MRpr(n,m) = " - - - . . - - ^

x Exp[j i]. (3_34d)

Unless k = H, Eq. (3-34d) is zero.

Hence,

Rpr(n,m) = H"''" -) • (3-34e)

That is, the correlation is a function of the time lag which

proves that the process is stationary.

Finally we can write Eq. (3-34e) as

The spectral density function is the inverse Fourier Transform of

Eq. (3-35). That is

6prk = 2h "Vllprdn-iOEXPM Mtol] (3-36a)K~U

- S Hpqkff k (3-36b)q=l

38

which is equal to the elements of the spectral matrix between the

process p and process q.

Thus, we see that the assumed form of the time series given by

Eq. (3-24) has the same spectral density as the target spectral

density function.

Simulation of Strong Wind Turbulence

In order to check the two simulation techniques described above

namely the trigonometric model and Fast Fourier Transform model, we

generated six correlated time series representing the fluctuating

part of wind velocities. These six series represent the three com-

ponents of the wind at two different spatial locations. Each simu-

lation technique was checked by comparing the spectra of the simulated

data to the spectra used to generate the data. The input spectra

used for these time series are based on wind velocity measurements

made by several investigators [16, 17, 18]. These spectra will not

be discussed here in detail. Under strong wind conditions turbulence

is due to mechanical mixing caused by fractional effects. In this

case, the turbulence can be considered a stationary random process with

zero mean and a Gaussian distribution.

The following expressions for the spectra were used for this

simulation:

u* ((1.0+1.5 (ig.

nC22(n) _ __2

u*2 ((1.0 + 1.5 (7.368f1)°'781)2.134

39

nC33(n) 3.36f]

u*? " 1 + 10f,5/3 ^

nC4 4(n)_ 272.2f2 _ _ _ _ _ ( j_4o)

U*2

nC55(n) _ 46.46f2

u*2 ((1.0 + 1.5 (11.06f2)°-781)2-1

, 3.36f2

u*2 1 + 10f25/3

nCl3(n) _ 12.(3-43)

u*2 (1.0 + 7.5f!)8/3

nC46(n) _ 12.5f2

u*2 (1.0 + 7.5f2)8/3

C]4(n) = (C11(n)C44(n))}5(exp(-19Af))}scos (2irAf) (3-45)

Ql4(n) = (C11(n)C44(n))Js(exp(-19Af))35sin-(2TrAf) (3-46)

C25(n) = (C22(n)C55(n))}2(exp(-13Af)^cos (4irAf) (3-47)

Q25(n) = (C22(n)C55(n))^(exp(-13Af))}5sin (47tAf) (3-48)

C36(n) = (C33(n)C66(n))J2(exp(-13Af)^cos (4irAf) (3-49)

^36^) = (C33(n)C66(n))Js(exp(-13Af))3Ss1n (4irAf) (3-50)

C34(n) = (C13(n)C46(n))^(exp(-16Af))^cos (3irAf) (3-51)

C16 = C34 (3-52)

Q34(n) = (C13(n)C46(n)')Js(exp(-16Af))J5s1n (3rrAf) (3-53)

Q16(n) = Q34(n) (3-54)

where C^ and Q^j represent the co-spectrum and quadrature-spectrum

parts of the spectral density function respectively.

40

Note that C^ = Cj^

and Qfj --Q-n

The elements of the components of the spectral matrix, which have not .

been listed above, have been assumed to be zero. The following actual

values were used for simulation

z1 = 100 ft, z2 = 50 ft

v-| = 60 ft/sec., V2 = 50.974 ft/sec.

f-| = nz-]/v-|, f2 = nz2/v2

u*2= (5.212)2 = 27.1649 ft2/sec2

f = nAz/7 = n (100 - 50)/(60.0 + 50.974)/2

where 7= average velocity

n = frequency in cycles per sec.

Suffix 1 or 2 refers to the station at 100 ft or 50 ft respectively.

Results

Fig. 3-a shows a set of six simulated correlated time series

based on the trigonometric model. Time series one, two and three repre-

sent the wind velocities in the mean wind, cross wind and vertical

direction respectively at a height of 100 ft. Time series four,

five and six are similar simulations for a spatial position 50 ft.

below the first point. Series one and four are positively correlated,

while series three and six are negatively correlated to one and four.

Series two and five, the series representing the cross wind, are

positively correlated to each other but uncorrelated to the other

four series. For these series, velocities were calculated every second

41

for 2048 seconds. The number of frequencies used, equal to N in

the analysis, was 600.

Fig. 3-b shows a similar set of simulated time series based on

the Fast Fourier Transform method. It is interesting to note the

similarity of the outcome. The number of frequencies used, equal

to N in the analysis, has to be the same as the number of time points

that is 2048.

A comparison between the spectra used to generate the simulated

data and the spectra of the simulated data is presented in Figures

4-a through 9-a for the trigonometric model and in Figures 4-b

through 9-b for the FFT model. Figures 8-a and 9-a are respectively

the co- and quad-spectra between time series one and time series four

based on trigonometric model. Figures 8-b and 9-b are the FFT counter

part. In each case the solid line represents the original spectrum

and the dashed line represents the spectrum of the simulated time

series. The spectra were calculated from the simulated data using

the "Cooley-Tukey" method of calculating the correlation function

first, and then the "Tukey window" was used for smoothing purposes.

As can be seen the comparison between the actual spectra and the

simulated spectra is quite good in all cases. We did not consider it

necessary to show all 36 possible power and cross-spectra. The

agreement looks the worst for the quad-spectrum between series one

and four (Figures 9-a and 9-b). The vertical scale for this compari-

son, however, is very expanded and about all we can conclude is that

both the original and simulated spectra are very close to zero. Also,

42

00

•^ o

uT op

I:\£* o?

00

o

*' i \ «"•' *' JM/f'tv'I'11 • i1 ME»y < 11 '•'*$t< I ''t'1! iM ,u .v **?"\ if

' ' / .' I

FIG 3A. SIAJLATED WIND VELOOTES BYTRIGONOMETRIC MODEL

THE TOTAL RECORD LENGTH IS 2048 SECONDS

43

ft 00

o

00

%H> wA|

FIG 3B. SIMULATED WIND VELOCITIES BYFFT METHOD

THE TOTAL RECORD LENGTH IS 2048 SECONDS

44

.00 L 08I

2 51 3 It

Fig. 4a. Comparison between the original G-|](w) and the spectrumof the first simulated time series based ontrigonometric model. (Vert. Scale: 100(M2/sec)/in;Horz. Scale: 0.53(rad/sec)/in)

Hci _

45

Oao'

. S3 2W

Fig 4b. Comparison betwejen the original G-j- |(w) and thespectrum of the first simulated time series basedon FFT model. (Vert. Scale: 100(M2/sec )/1n;Horz. Scale: 0.63(rad/sec)/in)

46

8

8R

.00 1 88 Z 51

Fig. 5a. Comparison between the original G22(w) and the spectrumof the second simulated time series based ontrigonometric model. (Vert. Scale: (50 M2)/sec)/in;Horz. Scale: 0.63(rad/sec)/in)

47

clQ--t"

ub

a/•-i

c. 00 1 25

Wi 83 51 3 L4

Fig. 5b. Comparison between the original GWw) and thespectrum of the second simulated time series basedon FFT model. (Vert. Scale: (50M2/sec)/in; Horz.Scale: 0.63(rad/sec)/in)

3F

8

48

8d.

enLJ

8in'

oao.00 L 25w Z SI 3 LI

Fig. 6a. Comparison between the original G^w) and thespectrum of the third simulated time series basedon trigonometric model. (Vert. Scale: (5 M^/sec)/in;Horz. Scale: 0.63(rad/sec)/in)

49

mu

63 I 25W

i 8S 2 51

Fig. 6b. Comparison between the original 633 ) and thespectrum of the third simulated time series basedon FFT model. (Vert. Scale: 5(M2/sec)/in'; Horz.Scale: 0.63 (rad/sec.)/in)

50

.00

Fig. 7a. Comparison between the original 644(1/0 and thespectrum of the fourth simulated time series basedon trigonometric model. (Vert. Scale: (150 M2/sec)/in; Horz. Scale: 0.63(rad/sec)/in)

C! .

b

51

U

u-i _J

00 L 25W

1 8S 2 51

Fig. 7b. Comparison between the original 644^) and the

spectrum of the fourth simulated time series basedon FFT model. (Vert. Scale: 150(M2/sec)/in; Horz,Scale: 0.63(rad/sec)/in)

52

<n

R-i

WZ 51

Fig. 8a. Comparison between the original C-| (w) and the co-spectrum between simulated time series one and fourbased on trigonometric model. (Vert. Scale:81.25(M2/sec)/in; Horz. Scale: 0.63. (rad/sec)/in)

53

. GO . 63 1 25W

2 51

Fig. 8b. Comparison between the original G-| (w) and the co-spectrum between simulated time series one and fourbased on FFT model. (Vert. Scale: 81.25(M2/sec)/in;Horz. Scale: 0.63(rad/sec)/in)

54

.DO

-" 1.63

1i 25

W

11 SB

\2 51

13

Fig. 9a. Comparison between the original Qi4(w) and the quad-spectrum between simulated time series one and fourbased on trigonometric model. (Vert. Scale:4.0(M2/sec.)/in; Horz. Scale: 0.63(rad/sec)/in)

55

-ro

L 25W

i 88

Fig. 9b. Comparison between the original Qi4(w) and the quad-spectrum between simulated time series one and fourbased on FFT model. (Vert. Scale: 4.0(M2/sec)/in;Horz. Scale: 0.63 rad/sec)/in)

56

it is interesting to note that, in general, the comparison between

the simulated spectra and the actual spectra looks better in the

case of FFT approach.

Comment

Considering the above results we feel that both the simulation

methods presented work very well. The only drawback we see is the

amount of computer storage that is necessary. It is noted that FFT

method works about twenty times faster in terms of computer time

compared to trigonometric model. But FFT approach has a drawback in

that the number of time points is equal to the frequency points and

we can't restart the simulation at any given time t. It has to

start with t =0. Also, the time interval is related to frequency

intervals. The trigonometric approach does not suffer from these

disadvantages. Here, we can start the simulation beyond any given

time and also the time interval is not dependent on the frequency

discretization. Because of the presence of sine and cosine functions

in the trigonometric model, it can be very useful in the response

analysis of linear system. Thus, we see that whereas the Fast

Fourier Simulation method has a tremendous advantage in terms of

computer time, trigonometric model is more useful in certain cases.

CHAPTER IV

APPLICATIONS OF THE SIMULATION TECHNIQUE

Example 1. Forced oscillations of a free standing latticed tower.

To show the application of the simulation method for a linear-

problem the dynamic response of a three-legged, latticed steel

tower, shown in Figure 10, due to wind load in the y direction was

investigated. The tower was idealized as a space truss, and the

structural members were assumed to resist axial loads only.

For the dynamic analysis, the tower was modeled mathematically

as a discrete system of fourteen masses lumped at the panel points.

Horizontal loads were assumed to be applied at the panel points, and

secondary stresses were assumed to be small.

The assumptions made in deriving this analytical model are

summarized below:

1. The tower is a linear elastic space truss;

2. Motion in two orthogonal horizontal directions are uncoupled;

3. Vertical motions are negligible;

4. Loads are applied only at panel points;

5. Secondary stresses are negligible;

6. Masses are concentrated at centroids of planes at panel

points;

7. The base of the tower is rigid; and

57

58

8. The deformations are small enough to be geometrically linear.

Under these assumptions, the column elements of the flexibility matrix

were computed by successively applying a unit load horizontally at

each panel point in y direction. The flexibility coefficients are

shown in Table 2. The mass matrix is diagonal.[37]

After the flexibility and mass matrices for the tower had been

computed, the natural frequencies and mode shapes were obtained in

the usual manner as described in the following section.

The equation of motion for an undamped, multi-degree of freedom,

discrete mass systems are given by

My + Ky = F(t) (4-1)

where M = mass matrix,

K = stiffness matrix of the system,

F(t) = forcing function vector of the system,

y = displacement vector,

and,

y = acceleration vector of the system.

When the system is vibrating harmonically at a natural frequency,

F(t) is zero and the displacement vector may be written as

y = A~nsin(wnt) (4-2)

where An = displacement vector of the n-th mode

wn = n-th natural frequency.

Eq. (4-T) then becomes

-wn2MA~nsin (wnt) + KA~nsin (wnt) =5" (4-3)

59

oDividing Eq. (4-3) by wn sin (wnt) and rearranging terms, then

n

Premultiplying both sides of Eq. (4-4) by the inverse of K

K"lMAn =-L2K"1KAn (4-5)V

and replacing K" by S, the flexibility matrix of the system, and

K K = I ~ identity matrix, then

SMAn = -VAn (4-6)wn

Denoting D = SM as the dynamical matrix and An = — JF» tne standardwnalgebraic ei gen value problem is obtained as

D A n = X n A n (4-7)

The computed first three eigen values and their corresponding eigen

vectors are shown in Table 3. Three natural periods were 0.496 sec.,

0.158 sec., and 0.080 sec. in the y direction.

After having determined the modal shape and natural frequencies,

we will proceed to do the dynamical analysis as follows: Let the

masses Mj, concentrated at points 1, 2, 3, ... j, k, ... r of the

system be subject to disturbing forces.

F-j(t) = Faj + Foj(t) (4-8)p

where Faj = °-5pCXjS.jVa.j "" tne static wind loading acting at the

point under consideration, vaj is the average value of the longitudinal

component of the velocity at this point,

F0j(t) = Faj( — °J - + _2ill - ) is the disturbing force relevant

60

for the pulsating part of the velocity v0j(t),

p is the air density,

CV1- is the drag coefficient of the construction at the level j,AJ

and Sj is the area of the projection of the structure at the level j

on the plane perpendicular to the direction of the wind.

Now, let

ryk = £ qi(t)a i(xk) (4-9)

i=l J

where yk = dynamic displacement

Oy-jCxk) = modes of natural oscillation

q.j(t) = generalized coordinates.

Then the equation of motion for the i th mode is

q,(t) + (u + Tv)Wi2qi(t) =i (4-10)

Mi4 r 2

where u = _ - r° •4 + r0

2 '

rThe generalized force Q-j(t) =. Z Fj(t)« -j(xj) , and the generalized

J ~~ *

r 2mass Mjg = I Mjayj(xj) where

J ~" I

w^ = i th natural circular frequency of the system

6 = logarithmic damping decrement of oscillation

Neglecting the square term in the expression for the disturbing

force, which is very small compared to the first term, we can write

F0j(t) - Faj{!

61

Now,

" a

Then

Qi( t ) = mean generalized force = ZFajOy.j(xj) (4-13)

BQ-J(T) = correlative function of the generalized

force = E E Q j d O Q j U + T)]

i aj

{Fakay1(xk)(l +2V°k(t+T))}] (4-14a)vaj

Multiplying these two expressions and moving the expectation sign

insideN N

j=l k=l

N N

j=l k=l

x E[V0j(t)vok(t + T)] (/

vajvak

= Cross correlative function for the generalized force =

,(t + T)]

j—i K—i

62

N •

j=l k=l

!! " . , * , ,. vn1(t)vnlf(t + T)

Taking the Fourier Transform of the correlative and cross correlative

functions we get

= spectrum of the i th generalized force

N N= .?, . /

J ~ I K I

f (1 + 4 E[VQj(t-)Vok(t * T)] e-i

v v

e-iWTdT

a j a k

Assuming the mean wind to be invarient with time

N N

x (6(w) + _ 4_Sv o 1 v . ) (4-16b)vajvak

where SVQJ-VQ^ is the cross-spectrum of the fluctuating velocities at

points j and k. Note that Spi(w) will be complex because Sv0jVok might

have real and imaginary parts.

However, the imaginary part can be neglected if this turns out

to be very small compared to the real part.

Similarly,

SQ I-Q£(W) = cross -spectrum of the generalized force

N .NSQiQ£(w) = Z I FajFak«yi(

xj)ayA(xk)

J— I K I

Svojvok) (4-17)

63

For considering the dynamic effect, we don't consider the static de

flection and in such cases the spectral density for the fluctuating

part only will be

N NSQi(w) = ^ ^aj^yi^jS^k)

and,

N N

?QiVw) ' .^ k^

Now, to find; the expression for the transfer function of the system,

let

and, q . ( t ) = ^

where $.j(w) is the transfer function.

Substituting these expressions in Eq. (4-10) we get

-W2$.(w)eJ'wt + (u + iv)wi2$,(w)eJ Mig

or' ^ Mig[-W2 + (u + i

-w^ + (u - iv)wji ~ -2M ig(-w + (u + i v J w J C - w + (u - iv^-

-w2 + (u - iv)wi2<3> i (w) = - — r-x - ;; — 5 — o~T — 57 (4-20)

' ^ + -4

64

Since u2 + v2 = 1

Then,

Sq-j(w) = spectrum of the generalized coordinate

Sq1(w) = SQ.J (w) | $.j (w) |2

SQI(w)Sql(W)=MigV-2uw

2Wi

2 + Wi

2)

and using the relation

we get the cross-spectrum of the generalized coordinate as

s . (w) = SQiQ£(w)[w4 - u(Wl2 + w£

2)w2 + iv(Wi2 -

yk(t) = Z qi(t)ayi(xk)i=l

Similarly for any point H

r= I.

s=l

Then correlation function between y^t) and

r r

Then the spectral density of the dynamic displacement

•f

i2W2 + W-j4)(w4 - 2uW£2W2 + W/)

(4-23)

Now, for the motion of any point k

r

• (4"24)

65

r r= £ Z ayi(xk)ays(x£)Sq1qs (4-25a)

r r _= Z Z ayi(xk)ayS(xa)$i(w)$(w)SQiQjl (4-25b)

r r

N N 4Sv0jVok

(x^(* ^ws2)w2 + ivCwj2 - ws

2) + w^W

i2w* + Wi4)(w4 - 2uws2w2 + ws

4)

when k = Z, -it gives the power spectrum of the displacement.

Thus, having obtained the expressions for cross and power spectrum,

we can simulate the displacement at any point. Thus the spectral

density function of the displacement being known, FFT simulation

method was used to simulate the displacement of the top of the tower

shown in Fig. 10 whose important parameters are listed in Tables 1-3.

The value of the co-efficient of drag was taken to be 0.6 and one per-

cent of the critical damping was used.

Fig. 11 shows the simulated samples of the displacement and the

generalized forces. The first three samples are the time histories of

the generalized force for the 1st, 2nd and 3rd mode of motion respec-

tively and the fourth sample shows the time history of the normalized

displacement of the top of the tower due to all the three modes

combined together.

66

PANELPOINT

SECTION A-A

Fig. 10. Latticed steel tower subjected to turbulent windload.

67

TABLE 1

CONCENTRATED MASSES AND PROJECTED AREAS

PanelPoint

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Elev.(ft)

150

147

143

137

130

123

115

107

97

87

75

61

45

25

ProjectedArea (sq- ft.)

2.87

5.22;;

8.06

8.75

10.32

11.97

14.12

16.76

19.30

23.32

28.08

32.35

43.45

60.37

D.L. (Ib.) atPanel Point

1810

510

750

810

920

1790

1190

1360

2940 •

1900

3580

2940

5670

5240

NOTE: 1. Values for projected area are for one face only.

2. Values for D.L.(Dead IrQad) are for entire tower.

68

•z.o1— 1H-| |

j

Of

I—IQ>_•z.t—t

(/} "

CM

1— QL

UU

LU .>£

_J

I-H

OQ o .C

<C

>-i O

i—

U- C

1 1 .-_

u~ *f

1 1 1 % *

o>-h-_JCQ

1— 1XLU

1rrJLJ_

«*

r—co•—CMr—_r—OCD00r~-

voin«a-CO

CM|—

|— _

(Oc:o0)«— (O

<O i-

o -o

•r—i. 4->J,^ —

to-dro

V)

r*~rCMCD

CM

-in

«a-vo

coCM

CM

0 0

w—

CD

Is**

CO O CO

CM en

inCO

CM

CM

o o o

CM i—

vo in

O

^J- VO VO

I-- **• O

VO

CO

CO

CO

CM

o o o o

co o en o CM

i— CD

en CD

vo•—

co m

i— iv.«d-

co co

co CM

CD O O O CD

r- CM

co Tj-

in

r-senvoi—omINCOr—CDcoinoCM0inoCMCMooCOCMoCM00COCMOVO

pH

CO

r—OCO

f"-v

J-•—oovoor-.^ j.>—o.voCOr—OCO

CDi—0,_ooCMOr-

0CMO

0of~*

I—0—V0

CMi—OVOvocoo,_f**,.>I—oCMVOinoCMCMvoor-vovor—CDoo

voCOf*Sfc

0ocoj-00oCDoCO0oV0

r-~0oCMenooCOf_

I—oCOCOCM

ovoCM

r—oCOoCO0CD

voCMino•oCM0vooo00f**»VOooenCOr-oo0oCOoo,-J.inCO0CDfoCDo0COvnenoocoCOCDoovoooCDo

f«-cooo.3-o•51-0' •oCOin^oo_0ino•0CDCOinoo00fin00CMf—vooCDin^voOoJ.Jvooo$vo0oCO0f-.oor—

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CDCMOCOooCOCMCOOoCDj-coooop*s.COooCOCOcooovooo0CMCM

^ooCMCO*$•

Ooo*S"ooCM

CM'OoCOCM—OCDCOf!•«

OOVOin_o•CDfN»,

vo1—Oor*N»r*sfci»0ovoCOr—oocj~CDOO

CMOCM0OCDOCMO0VO

CMO

OOCMCM0OCOCMCMOOCO

00•si-CDOOenooo_inoCDCDcoinoooj-mo0•ovoinoooN.mooo00inoooCDinooooooCDoVOoo0,VOoooCMvooooCMVOooCD*

69

TABLE 3

COMPUTED NORMALIZED MODE SHAPES(Normalized with respect to bottom panel)

Y DIRECTION

PanelPoint

1

2

3

4

5

6

7

8

9

10

11

12

13

14

FirstMode

39.93

38.31

36.15

33.00

29.50

26.22

22.71

19.47

15.82

12.56

9.18

5.96

3.19

1.00

SecondMode

-5.60

-4.73

-3.59

-2.00

-0.38

0.93

2.10

2.95

3.61

3.83

3.72

3.13

2.20

1.00

ThirdMode

1.78

1.23

0.52

-0.41

-1.14

-1.56

-1.63

-1.44

-0.90

-0.21

0.63

1.29

1.56

1.00

70

v?3

f*\ ^u

] «:

*V/wvv 'M^ ' " w v^/

FIG. 11. GENERALIZED FORCES AND DISPLACEMENTFOR LATTICED TOWER:(A) GENERALIZED FORCE POM FIRST MODE

(B) GENERALIZED FORCE FOR SECOND MODE

(a GENERALIZED FORCE FOR THIRD MODE

(D> NORMALIZED DYNAMIC D8PLACEMENT OF THE TOP OF THE TOWER

71

Example 2. Nonlinear Random Vibration of String

One of the most significant applications of the simulation tech-

nique is for simulating random generalized forces. The necessity of

simulating random generalized forces arises when the dynamic response

analysis is performed in time domain for the purpose of obtaining

information beyond second-order statistics or when the structure is

nonlinear and therefore an approximate random response .is sought by

simulating the excitation. In order to assert the validity of the

preceding discussion, the problem of nonlinear vibration of a string

will be considered in this example followed by the problem of non-

linear vibration of a plate in the next example.

The governing differential equation for nonlinear vibration of

a string is

9u + clu = fT + AE3t2 3t l ° 2L

where p is mass per unit length, C is the linear viscous damping, T0

is the initial tension, f(x,t) is force per unit length of string, A

is the cross-sectional area of string, and the boundary conditions are

u(o,t) = u(L,t) =0 . (4-28)

Assuming an approximate solution of the form,

Mu(x,t) = Z bn(t)sin f^ (4-29)

n=l L

one obtains

72

M ..

" - . (or,

M .. M

(4-3,)

Multiplying both sides of Eq. (4-3l) by sin ""* ,nH • *obtains '"""" — andmegrating, one

f(x,t)sinL

Dividing by p throughout, we get

(4-32b)

where 8 = ^

or,

-1:./ r(x.t).i, spdx (4.32c)

73

Assuming that the fundamental frequency of the corresponding linear

spring is 0.1 Hz or I°_ = 4 sec , we can write Eq. (4-32c) asPL2

b'n + 26bn + [1 + jf- S (]Sl)2bk2](2n7T)2bn

= |k f f(x,t)sin ^x (4-33)To J0

L'0

The right-hand side of Eq. (4-33) is the random generalized forces to

be simulated.

( _,. i . rmrxi .ix-,

uc i i iiiu i a ecu .

Let fm(t) = f f(t,Xl)sinJo

fn(t + T) = I f(t + T,x?)sin

Then,

r n vu T -i) - TVL T T,*.2j'o

= correlation function of the generalized force

in(T) = E[fm(t)fn(t + T)]

'E[f(t lXl)f(t + T,x2)]

. imrxi .x sin ——Lsin

in "'sin __±dx1dx2 (4-34)'o Jo ' ~ L L

where R(T,X-| ,x2) is the correlation function of the actual force.

Taking the Fourier Transform of Eq. (4-34) we get

.i ,1. . l i e / \ • roTXi . nirx2. , /. _r\

mn'w' = 5(w,x-i ,x2)sin —j-—sin —•—dx-idx2 (4-35;o o

where Snin(w) and S(w,x-j ,x2) are the spectral density functions of the

74

generalized force and actual force respectively.

For the particular case we take (Ref. [8]),

S(w,Xl ,x2) = S0(w)e-alwl lxrxzl (4-36)

where

(4-37)

a and a are constants and Of gives the standard deviation of the

actual force.

Substituting Eq. (4-36) in Eq. (4-35)

c- i \ rL fLc / \ -a!w l lxi-X2l . irmxi . mrx?. .S,nn(w) = S0(w)e *'sin - " s i n — dx2} j L Lo o

= S ( w ) I m n (4-38)

where Imn

or,

r . imrx, r -a|w|xifxl a|w|x? . mr= sin -T-1 e e Sin

Jo L L ;o

alwlxirL -a|w|x? . mrx2 i' ' ' e ' ' sin ±dx2 dx-|+ e

'x

w)2 +

fL1 + si

J /\. ,,.x dx, + sin-H-« '"'"Mndx

75

(aw)2 + (p

(aw)2 + (HU

which can be further simplified as

(4-39)

So,

/nnr\ /f iTTx+ L L

m 2 n - (-Dm+n - e-l-l^c-i(aw)2 + (p2

(4-40)

A numerical example was carried out for the case where 3 = 0.1 x

2ir sec"1 , a = 4ir sec'1 , aL = 0.7 sec, 1°. = 0.05, L = 25 in. , T0 =AE

76

100 lb., p = |JT Ib/in, E = 3xl07 psi.

From the derived expression for the spectral density function, the

generalized forces for 1st, 2nd and 3rd modes were simulated by the FFT

method. They are shown in Fig. 12. Equations of motion (4-33) for

n = 1, 2, and 3 were reduced to six first order differential

equations which were solved numerically by using the Modified Pre-

dictor-Corrector method. Fig. 13 shows the simulated samples of the

displacement at the midspan. The first three time series are the

generalized displacement histories for the 1st, 2nd, and 3rd modes

of motion respectively. The fourth one gives the actual displacement

due to all three modes of motion at the center of the string. The

root mean square aQ of nondimensional response

at the midspan is plotted as a function of the root mean square

of nondimensional forcing function

f(x,t) =_

in Fig. 14. The total number of 2000 time points with time increment

of 0.025 sec. were used for response calculation. For the sake of

comparison, the response of the linear string was also plotted on the

Fig. 14.

Conclusion

By comparing the results with the one obtained by Shinozuka [8]

for the same problem, it is found that they are very close. This

demonstrates the usefullness of the FFT simulation method. It is very

77

!«*-"

£j

o

h i *• ? - i f . ';IL y fcifl A«-:^ .ufe j ^1^ B ^^ .'ii ^ >• -W V H^s M i i; - « ii'psj'' • ilM i ^u^!j

U)

•a

a.

m

(C)

lUji iiI'IY"' vyi

WlJiitv J^/lfflJV'M »• V wy / '/ 'An

FIG. 12. GENERALIZED FORCES FOR NONLINEAR

STRINQ:(A) JWNDIMENSJONAL GENERALIZED FORCE FOR 1ST MODE

(B) NONOIMEHStONAL GENERALIZED FORCE FOR 2m> MODE

(C) NONDIMENSIONAL GENERALIZED FORCE FOR 3m MODE

78

in

o Av . - ' l / l / l / 1

(B)

FIG. 13. DISPLACEMENT TIME HISTORIES FOR

NONLINEAR STRING:(A) NONDtMENSIONAL GENERALIZED DISPLACEMENT FOR Irr MODE

(B) NONDIMEN3WNAL QCNERAUZED DISPLACEMENT FOR 2ND MODE

< (C) NONDIMEN8IONAL OENKRALI2ED DISPLACEMENT FOR Sm MODE

(0) NONDIMEN8IONAL DISPLACEMENT TIME HISTORY AT THE CENTER

OF THE STRINO

79

8(O

±0

0-J

z

zI

I I

I

O

0)

*

•—

o

CD•

O(0O

iqdrod

CVJd

OJ

00OCDOCSJ•

o

od

oc>

OXLUUJ

5oi

o_c•M<u

-QCD

03QJ

l/icoO

-(O

)t-0)c-Eo-ocfOs-<o0)t:

o -g

O)

_S

§

DC .J2(Oo

.o

lN3W

30V

1dS

ia

HV

NO

ISN

3lNia-N

ON

dO

SIN

d

80

interesting to note that total computer time for 2048 simulated

points was less than 2 minutes which shows the advantage of time-

saving due to FFT method. To simulate the same number of points

using trigonometric model takes about 30 minutes of computer time.

Example 3. Nonlinear Panel Response Due to Turbulent- Boundary Layer.

The random vibration of a flexible plate immersed in a fluid

flow on one side and backed by a fluid filled cavity on the other

side has been considered in this analysis (Fig. 15). The nonlinear

plate stiffness, induced in the plate by out-of-plane bending and

the mutual interaction between the external and internal fluid flow,

is included. The same problem has been considered by Shinozuka [1]

where he used multidimensional simulation method to find the deflec-

tion at the center of the plate. In the present work, the

FFT simulation approach has been used to do the response analysis at

the center of plate. The results are compared with the ones obtained

in Ref. [1].

The deflection w of a simply supported plate having geometric

nonlinearity is described, in dimensionless form, by two partial

differential equations

V4W = $yy- + *t#fyy -2*jy - W~

- 3 w t - w ^ - + p ' + p e + pc (4-41)f\

V4! = 12(l-v2){w5<y - w yy} (4-42)

with-boundary conditions

81

PLATE

Fig. 15. Problem'geometry for nonlinear plate vibration due toturbulent boundary layer pressure.

82

w(0,y/f) = w(l,y,t) =0

w(x,0,t) = w(xM,D =0

ltt) = 0

yyt) = 0

where x~ = x/a, a = plate length

y = y/b, b = plate width

w = w/h, h = plate thickness

$ = nondimensional airy stress function for membrane stresses

v = Poisson's ratiot 1 /D

t = nondimensional time = -H/— whereabr PD = Eh3/12(l-v2), plate stiffness

E = modulus of elasticity

3 = damping co-efficient

a2b2

p = . p p(x,y,tf) where p(x,y,t:) is turbulent pressure

2h2Pe = ^p pe(x,y,:t) where pe(x~,y,tf) is fluid pressure due to

external flowty f\

pc = ^TQ-p^tx.y.T) where pc(5T,yYt) is fluid pressure due to

cavity flow

In solving Eqs. (4-41) and (4-42), the plate deflection w is expanded

in terms of the normal modes of the corresponding linear plate:

w(x",y,T) = E E bmn(lf)sin nmxsin mry (4-44)m n

where bmn(tf) is the modal amplitude.

83

After the stress function $ is evaluated [19], the assumed modal

solution is satisfied in the Galerkin sense by computing its integral

which is average weighted in turn by each term of Eq. (4-44). The

result is a system of simultaneous nonlinear integral-differential

equations for b—.(t) which involve the following generalized forces

_ .?u2 rl rl•Fmn(t) = TIT" p(x,y,tf)sin mrrxsin nhU J0 J0

a rl rlh ., 9 P VA jy , t j s in nrnrxsi

p II J J

. ! . ! _ _1 1 n fv v T^"in imr¥-in i i |j \ A ,jr » uy b i n iiniAoi

hp°°a- ^o ^o

irydxdy

n mryd^dy-

n mryd^d-

(4-45a)

(4-45b)

(4-45c)

where pbo = free stream density

DO, = mean external flow velocity

ac = velocity of sound, cavity flow

To promote computation ease, a two mode approximation corresponding .

to x coordinate and one mode approximation corresponding to y co-

ordinate to Eq. (4-44) has been assumed. Then the solution for $ [19]

satisfying Eq. (4-42) is given by

+ [(a2 + v)bn2 + (a2 + 4v)b21

2]a2x2}

En2a2 bn2rr~7i cos 2irx - biiboicos irx

bllb21 b212 -STTX + - c o s 4-rrx]

(4'46)

84

where a = jr

Substituting Eq. (4-46) and Eq. (4-44) into Eq. (4-41) we get the

following set of equations

b^ + 3^ + (C1Q + C-,^-,2 + C12b22)b11

+\CFC + AFi6) (4-47a)

= 4(F2 + ACF2C + AF2

e) (4-47b)

where b-j = b-j ^, bg = b2-|, Fj = FJ-J , F2 = F2-j, etc.

C = (a + l)2ir4IU Q

r - ( a. 4\2 4U20 ~ (a + ~J *

cll = |^4[(l-v2)(a2 + 72) + 2(2v + a2 +*r Ct (X

2(5v + a2 +

c21 = c12

+ 2(8v + a2 + 4)]a

3-j and $2 are the damping coefficients for the first and second mode

respectively. The generalized aerodynamic force of the external flow

is given by

Fm6 = * Q™, (4-48)r

where Q^ is given in references [20] and [21]. For high mach

85

numbers, Eq. (2-48) can be approximated to

[Embr(t] + Pmlb r(t)] (4-49)r

where

- 0 for r = m

for r = m

= 0 for r=f m, M = mach no.

Again based on Ref. [21], the generalized cavity force is given by

Thus having obtained the approximate expressions for the generalized

forces due to external flow and cavity flow, we proceed to find the

generalized forces due to turbulent pressure fluctuations as follows

by usingthe FFT simulation technique: the semi-empirical formula of

cross-spectral density for subsonic boundary layer turbulence is

given by [22]

.-6S(w,k-| ,k2) = 0.715 x 10

0.8exp(-0.47w^-)Uoo

(4-51)

for 0 £ w < °°

where, w = freq. in radians, k-j and k2 are wave numbers in the x and y

86

directions respectively. Uc = 0 . 6 5 U», q = dynamic pressure =2

pU» /2, c = boundary layer thickness.

Let £-| = x2 - x-j

Then the spectral density function

rJ

0.1w 0.715w j- -

IT Uoo

where C0 = 3.7exp(-2jg-)+ 0.8exp(-0.47S - 3.4exp(-87;§)Uoo (fo ^Joo

(4.52)

Now, the generalized force due to turbulent pressure fluctuation is

given as

F (t) = «2b2 f1 f1 ,_ -h

r rJ J PtxT^.Dsln'o 'o

sin m2Try,dx-j ,dy -j (4-53a)

f1 f1P(x2,y2,t + T)sin

J n J n

sin (4-53b)

87

Then

n * n « n * n

x sin m-jTTXiSin moTryiSin

or,

x sin m-|Trx, sin

(4-54)

where R,,, m n n (T) is the cross correlation function of the generalized

force and Rp()T-j ,x~2,y^ »y2»T) is the cross correlation function of the

turbulent pressure fluctuations.

Taking the Fourier Transform of both sides of Eq. (4-54) we get

x sn m-j-n-

(4-55)

where Sm m n n (w) and S(w",Ci.Co) are the spectral density functionsllinllloMi >*O • ^

of the generalized forces and turbulent pressure fluctuations

respectively and w is the nondimensional frequency.

Nondimensionalizing Eq. (4-52) with respect to w, £-j and

stituting in Eq. (4-55) for S(w,Ti »O we get

1 1 i i — n Iwa .— — ,0.715wbx r1 f1 f1 [Vlxz-x!.|^e-|y2-yi|—or-

•'o •'o •'o •'o

., ,awx s ' -sin rn-iTTX^sin m2"F]Sin njirx^

x sin n2Try2d)ridX2dy^d72 (4-56)

In order to solve the integral in Eq. (4-56) we proceed as follows:

let

IVl = Jo

sin m-iTTX-jsin n-|irX2dxidx2 (4-57a)

rl ,_ _ .0.715waI 6 - | y 2 - y i D C

'osin n^TTyiSin n2Try2dyi dy2 (4-57b)

then

,1

2 -c- •'o

ri= J

-0_.lwa

0.1 wa- i -°.-.'lw.?x- iw§=7e -DT*! 1

e ^^2e-JD?<2

xl

x sin n-|TrX2dX2]dXi (4-58)

Evaluating the integrals inside the square bracket separately:

89

— O.lwa- ." -

e e sin n-iTrx2dx2

•r' nO.lwa-

e c cos l osin

O.lwa-2

e sin. fAl uc

•f

0.1wa-

e cos

0.1wa-.pi ~°c~^2iJ e

Jo

O.lwa_

- cos

cos (n^ + g3-)^

wa

T2

wa

A

O.lwa

"~B

1 waf (^TT - Uc)

B

COS

90

Q.lwa 1,Uc B

O'.lwa-

A

+ (lyr* )s1n (njir + j

0.1 wa J-i'~OT~A}

where, A = (Q-lwa.)? + (n + wa}2^*C ' ^C

Integral Table [23] was used to evaluate the above integrals

Now, we want to evaluate

-0.1 wa_ wa—~ ~r

_ e e sinxl

-O.lwa-?

wa- _.• i; •" cos

-0.1 wa-Uc

X2" -|-e "{cos (n--rr - " cos ("^ + -

C . D

-e

-O.lwa . , wa»— / wax / wav--y—sin (n-j-rr - u^Jx-j - (n-jir - ^-)cos (n-jTr - u^;x1

B

91

-O.lw—

-0.1wa wa»— wa.-

i -.

A

where,

-O.lwa

C = e uc {

-O.lwa

D - e U c ' <

-O.lwa

(n,, - f ) - (n^ - g|)cos (»,, -

E = e c

uc « "t

-O.lwaF = e

Uc {'V^cos (niir + ^v*

Multiplying Eq. (4-59a) by

adding them, we get

-Q.lwa- 0-1wa- wa-

[e-uT^lfXleUc V^2s1n

and Eq. (4-59b) by

O.lwasin

(4-59b)

and

92

.lwa .,

-O.lwa- O.lwa-

-0.1wa-

De

,1r0.1wa.

.1 rQ.1waUc °s

-O.lwa- 0.1 war,1 ,-O.lwa ~TF~~X1 . ./"OrFe

-O.lwa- O.lwc^.f0.1waQ ~0c xl pp Uc 1-,

'2Bl~Dc~^

Substituting Eq. (4-59c) in Eq. (4-58) and carrying out the inte-

gration, we get

T jO-lwa O.lwa,.;2BUC

J6m1n1

(4-59c)

wa wa -O.lwa

0.1 warC . D , Uc ,C2 , D2-,{4A + 4B}e {AT + BT}

-0.1 waO.lwaJ 1 , ~DE~TE1 . Fl,

- 4A}e {AT + BT>O.lwa

Uc •C ^4A " 4BnAT

O.lwa r

93

O.lwa

where

Al =

Cl

Dl

El

FT

Gl

C2

D2

2BUc(nvfTr + ^ir]

«% __ j. Wet M _ wa -

4A 4B J Bi

O.lwa

J % + W}e U° {£T + BT}

-O.lwa• O.lwa/1 1 Io Ur rCl . DliJ "TJ^W ' 4A} {AT + BT}

O.lwa. rF E i Mr fC2 + D2^J 14A " 4B"/e C 1AT FT

wa_,2Uc'

- ) - (11,11 - w)cos (m,ir -

gi)sin

- (ny

(4-60)

. gi) . („,,„ . «|)COS („,,„ +

G2 = - gf) + (V - j"),

94

Now,

,- - ,0.715wb•ju:I_ _ = e - ' -<• sinm2n2 J0 J0

Carrying out the integration we get

Q.715WD

uc

Uc

-0.715wb

+ (-I)"2*1)}] (4-61)

Then

'(I * -' (4-62)

where Im n and Im n are given by Eq. (4-60) and Eq. (4-61) respec-

ti vel y .

Thus, the spectral density function of the generalized force due to

turbulent pressure having been obtained, the generalized forces were

simulated by using the FFT simulation technique.

As pointed out earlier, to work out a numerical example a two

mode approximation in the x direction and one mode approximation in

the y direction was taken. In order to compare the results with ones

given in ref. [l],the following numerical values taken in [1] were used:

a = 10 in., b = 20 in.

d = 5 in. , v = 0.3

p = O.lh lb/in2, E = 107 psi

95

Uc = 0.65 UU, ap = 0.0056 q

pbo = pc = 0.00089 slugs/ft3

aoo = ac = 995 ft/sec

U» = 800 ft/sec,'5 = 0.157 in.

The thickness of the plate was varied in a way to adjust the non-

dimensional pressure, p", for the convenience of illustration. The

damping coefficients 3] and 32 were taken as 1 percent of the

critical damping of the 1st and 2nd modes of linear plate, respec-

tively.

In order to solve the equations (4-47a) and (4-47b), 4096

discrete values of both F](tf) and F ftf) were simulated from the

derived expression for spectral density function using the FFT simu-

lation technique. Eqs. (4-47a) and (4-47b) were reduced into four

first order differential equations, and a modified predictor corrector

method was used to solve for b-| (t) and 2 ) with zero initial con-

ditions. Fig. 16 shows the displacement time history at the center

of the plate and the generalized forces for the first and second mode.

The nondimensional R.M.S. response at the center of the plate versus

the nondimensional R.M.S. pressure is plotted in Fig. 17 with and

without the cavity effect. For the purpose of comparison, response

corresponding to linear plate is also plotted in Fig. 17.

Conclusion

It is seen that the results obtained here compare very closely

with the one given in Ref. [1] for the sub-sonic case. Results obtained

in [1] are based on the multidimensional simulation analysis where

96

oo£

52

Si

8M

H

£LUOLJ-

_U-

§ S

S

97

oOX

oI§sIE<UJ

10oxOroOx00

10gxCD

roOxrogXCM

s_01<o

UJ

trIDCOCOUJ

trQ.

COzUl

o.QotoO)

a> •

«/> "O

c o

o -c

Q +

->in

OJ

OJ LJ_

03CLJD

4-

EO

O

OCOon

T3

(O3

3

in o3

O) r—

«0

S-

J-

3<o

inQ

. «/)E

0)

o s-

cj

n.

10in

oqd

lN3IM

30V

ndS

ia 1V

NO

ISN

3WIQ

-NO

N

JO S

Wd

98

Shinozuka has taken only 100 frequency points which looks like a very

small number considering the fact that turbulent pressure fluction

takes place over a wide frequency range. From this point of view,

our analysis seems more accurate because we have divided the

spectrum into 4096 points. The total computer time to simulate 4096

values of both F-j (t) and F2(t) and to obtain 4096 points for both

b-|(tf) and b£(t) was only 3 minutes 30 seconds. It is needless to

point out that the time required to simulate 4096 points for the

generalized forces by using the trigonometric model of simulation

will be much much more. This once again brings out the immense use-

fulness of FFT simulation approach wherever it is applicable.

CHAPTER V

CONCLUSIONS AND FUTURE EXTENSIONS

Two methods of simulation of a multivariate and multicorrelated

random process have been presented and these methods have been

applied to analyze some example problems of linear and nonlinear

random vibration.

Comparing the two methods of simulation, we conclude that both

the methods work very well. The only drawback is the amount of

computer storage, which can become prohibitive if there are too many

correlated processes to be simulated or if a very large number of

discretized points are necessary for analysis. It is noted that FFT

method works much faster in terms of computer time compared to the

.trigonometric model. This speed ratio is directly proportional to

the number of series and discretized points to be simulated. For

example, the time required to simulate the six components of wind

turbulence, with each pf the time series having 2048 discretized

points, was twenty times less in the case of FFT method compared to

the trigonometric approach. But FFT approach has a drawback in that

the number of time points is equal to the number of frequency points

and we cannot restart the simulation at any given time t. It has to

start at t = 0. Also, the time interval is related to the frequency

interval. The trigonometric approach does not suffer from these

99

100

disadvantages. Here, we can start the simulation beyond any given

time and also the time interval is not dependent on the frequency

discretization. Thus, we see that whereas the FFT method has a

tremendous advantage in terms of computer time, the trigonometric

method can be more useful in certain cases.

As to the usefulness and importance of the simulation technique,

we conclude that it offers probably the best means of performing the

nonlinear response analysis of random vibration problems when the

spectral function for the forcing function is specified. In light

of the review study in Chapter II, we feel that the simulation approach

and particularly the FFT method gives a much faster solution with

lesser number of constraints. It is noted that FFT method works as

well as the ones in Ref. [1, 6, 8] for analyzing the nonlinear re-

sponse and this approach is much faster than that of Shinozuka

[1,6,8].

The methods of simulation presented here can be extended to in-

clude multidimensional homogeneous and nonhomogenous processes. For

example, writing

m N-| N2 Nn

p=l k^l k2=l kn=l

X COS [W-i. t + Wo,, XT + +

x sin [W|. t + w2. x-| + ... + wn. xn + (k-j ,k2...kn)]

[5-1]

101

one can simulate a multivariate multidimensional process using trigo-

nometric method. Shinuzuka [6, 8] has already extended his trigono-

metric model to include the multidimensionality and the above proposed

extension will not be much of a problem. But it will be really

exciting to extend the FFT approach to a multivariate-rnulticiimensional

case. It does not seem very difficult particularly when the sub-

routine HARM can take care of up to 3-dimensional inverse Fourier

Transform. We feel that it should work without trouble, but it does

need a little closer look. It might be also interesting to extend the

FFT approach to include the nonhomogeneous process by utilizing the

concept of evolutionary power spectrum. This will be useful in simu-

lating trasient and siesmic response and will also lead to considerable

time saving over Shinozuka's method [8].

There are lots of other areas in which the simulation approach

can be very useful and about which we have not discussed in this

dissertation. For example, in the area of reliability analysis and

in the response analysis of structures when the spatial variation of

material properties are specified. These are some of the many areas,

the simulation technique and particularly the FFT approach can be

extended. These extensions look very practical.

REFERENCES

[1] Vaicatis, R., Jan, C. M. and Shinozuka, M., "Nonlinear PanelResponse from a Turbulent Boundary Layer," AIM Journal,Vol. 10, No. 7, July, 1972.

[£l Goto, H., Doki, K., and Akiyoshi, T., "Artificial EarthquakeWaves by Computer," Proceeding of Japan EarthquakeEngineering Symposium, 1966.

[3] Amin, M. and Ang, A. H. S., "Nonstationary Stochastic Model ofEarthquake Motions," Journal of Engineering MechanicsDivision, ASCE. Vol. 94.

[41 Liu, S. C., "Dynamics of Correlated Random Pulse Trains,"Journal of Engineering Mechanics Div., ASCE, Vol. 96,Aug., 1970.

[5] Hoshiya, M. and Tieleman.H. W., "Two Stochastic Models forSimulation of Correlated Random Processes," V.P.I. Tech.Report, VPI=E-71-9, June, 1971.

[6] Shinozuka, M. and Jan, C. M., "Simulation of MultidimensionalRandom Processes II," Tech. Report No. 12, ColumbiaUniv., N.Y., April, 1971.

[7] Borgman, L. E., "Ocean Wave Simulation for Engineering Design,"Journal of Waterways and Harbors Div., ASCE, Vol. No.WW4, November, 1969.

[8] Shinozuka, M., "Simulation of Multivariate and MultidimensionalRandom Processes," Journal of Acoustical Society ofAmerica, Vol. 49, No. 1, January, 1971.

[9] Ariarathan, S. T., "Random Vibration of Nonlinear Suspensions,"Journal of Mechanical Enqr. Science, Vol. 2, pp. 195-201(1960).

[10] Caughey, T. K., "Derivation and Application of the Fokker-Planck Equation to Discrete Nonlinear Systems Subjectedto White Random Excitation," Journal of AcousticalSociety of America. Vol. 35, No. 11, 1963.

102

103

[11] Caughey, T. K., "Equivalent Linearization Technique," Journalof Acoustical Society of America, pp. 1706-1711 (1963)

[121 Crandall, S. H., "Perturbation Technique for Random Vibrationof Nonlinear Systems," Journal of Acoustical Society ofAmerica, Vol. 35, No. 11, Nov., 1963.

[13] Tung, C. C., "The Effects of Runway Roughness on the DynamicResponse of Airplanes," Journal of Sound and Vibration,Vol. 5, pp. 164-172 (1967T

[14] Foss, K. A., "Coordinates Which Uncouple the Equations ofMotion of Damped Linear Systems," Journal of AppliedMechanics, Vol. 25, pp. 361-364 (19587!

[15] Yang, I., "Stationary Random Response of Multidegree-of-FreedomSystems," Ph.D. Thesis, California Institute ofTechnology, submitted March 10, 1970.

[16] Blackadar,A. K., Dutton, J. A., Panofsky, H. A. and Chaplin, A.,"Investigation of Turbulent Wind Field Below 150 MAltitude at the Eastern Test Range," NASA CR-1410, August,1969.

[17] Panofsky, H. A., "Properties of Wind and Temperature at RoundHill, South Dartmouth, Massachusetts," Research andDevelopment Tech. Report ECOM-0035-F, Penn. State Univ.,Univ. Park, Penn., Aug., 1967.

[18] Fichtl, G. H. and Mcvehil, G. E., "Longitudinal and LateralSpectra of Turbulence in the Atmospheric Boundary Layer,"AGARD Conference Proceedings No. 48, NATO, Munich, Sept.,1969.

[19] Bolotin, V. V., Nonconservative Problems of the Theory ofElastic Stability, Pergamon Press, 1963, pp. 280-289.

[20] Dowell, E. H., "Generalized Aerodynamic Forces on a FlexiblePlate Undergoing Transient Motion," Q. Appl. Math., Vol.24, 1967.

[21] Dowell, E. H., "Transmission of Noise from Turbulent BoundaryLayer Through a Flexible Plate into a Closed Cavity,"Journal of Acoustical Society of America, Vol. 46, No.1, 1969.

[22] Bull, M. K., "Wall-Pressure Fluctuation Associated with Sub-sonic Turbulent Boundary Layer Flow," J. Fluid Mech.,Vol. 28, Part 4, 1967.

104

[23] Gradshteyn, I. S. and Ryzhik, I. M., Table of Integrals Seriesand Products, Academic Press, 1965.

[24] Bendat, J. S. and Piersol, A. G., Random Data: Analysis andMeasurement Procedures, Wiley-Interscience, New York,1971.

[25] Lin, Y. K., Probabilistic Theory of Structural Dynamics,McGraw-Hill Book Company, New York, 1967.

[26] Meirovitch, L., Analytical Methods in Vibration, The MacmillanCompany, London, 1967.

[27] Dutton, J. A., Panofsky, H. A., Deaven, D. C., Kerman, B. R.and Mirabella, V., "Statistical Properties of Turbulenceat the Kennedy Space Center for Aerospace Vehicle Design,"NASA CR-1889, August 1971.

[28] McVehil, G. E. and Camnitz, H. G., "Ground Wind Characteristicsat Kennedy Space Center," NASA CR-1418, Sept. 1969.

[29] Maestrello, L., "Measurement and Analysis of the ResponseField of Turbulent Boundary Layer Excited Panels," J. ofSound and Vibration. 2, 1965, pp. 270-292.

[30] Shinozuka, M. and Wen, Y. K., "Monte Carlo Solution of Non-linear Vibrations," AIAA Paper No. 71-213, AIAA 9thAerospace Science Meeting, New York, 1971.

[31] Fung, Y. C. and Barton, M. V., "Shock Response of a NonlinearSystem," J. of Applied Mech., Sept., 1962.

[321 Gardener, N. J., "Response of Cooling Tower to Turbulent Wind,"J. of Strc. Div. of ASCE. Oct., 1969.

[33] Vickery, B. J., "Wind Action on Simple Yielding Structures,"J. of Engr. Mech. Div. of ASCE, April 1970.

[34] Beer, F. P., "On the Response of Linear System to Time-Dependent,Multidimensional Loadings," Journal of Applied Mechanics,March 1961.

[35] Shinozuka, M. and Astill, C. J., "Random Eigenvalue Problemsin Structural Analysis," AIAA Journal. Vol. 10, No. 4,1972.

[36] Hasselman, T. K. and Hart, G. C., "Modal Analysis of RandomStructural Systems," J. of Engr. Mech. Div., ASCE, June1972.

105

[37] Proceedings of Seminar on Wind Loads and Structures, sponsoredby NSF, Japan Soc. for Promotion of Science and Univ. ofHawaii, Oct., 1970.

APPENDIX A

1. Random Variable and Probability Distribution Function

Let X be any random variable which varies with statistical regu-

larity. Since X is a random variable, any function of X is also a

random variable.

The first-order probability distribution function of X can be

described by a graph as in Fig. A-l which shows the probability den-

sity function p(X). The hatched area gives the probability that X

lies between a and b. ItMs apparent that the probability is zero

that X actually assumes any given value x. Also we havei

p(X)dX =1- (A-l)i . . .

The cumulative distribution function (CDF) defines the probability of

occurrence of a value of X less than or equal to a given value x, i.e.,

P(x) = j p(X)dx. (A-2)-00

According to fundamental theorem of integral calculus,

• P(x) (A-3)

wherever this derivative exists.

Properties of CDF

(1) CDF is a non-decreasing function of x

(2) P(x) varies between 0 and 1 as x varies from -» to °°. Thus

P(-») = 0 and PH = 1.

Several random variables X,, Xp, . . .X are said to be jointly dis-

tributed if they are defined as functions on the same sample description

106

107

POO

0 X

Fig. 1-A. PDF of Continuous Random Variable X.

108

space. We shall refer to p(X,, X 2 , . . .X ) as the joint probability

density of these random variables. Thus, the probability that a, <

X, < b,, a0 < X0 < bo s . ..a., < X^ < b_ is given by1 12 2 2 n n n 3 J

f1 [ 2...[ n p(X1 ,X2 , . . .Xn)dX1dX2 . . .dXn . (A-4)J a_ J a _ J ;>ll '"2 ~an

Also,

j ] • • • ] p(X1,X2,...Xn)dX1dX2...dXn = 1 . (A-5)—oo — oo »oo

The joint probability distribution of two random variables X, and X2

is given by

p(XrX2) (A-6)

and

p(a-, < X, < b, and a2 < X2 < b,,) •

= I ] I 2 p(X1,X2)dX1dX2 (A-7)

Supposing that X is now a random function of spacial coordinates

x^ or time t, the first and joint probability distribution functions

defined above are valid for any given values of x_ or t. That is, the

probability that X (x or t) lies between a and b is given byrb

p(X(x, or t))dX(x or t) (A-8)^a

and

P(x) = J p(X(x or t))dX(x or t) • (A-9)

—CO

2. Mean. Mean Square, Variance and Standard Deviation

The ensemble average

109

= Jyx = E[X] = / X p(X)dX (A-10)

—CO

defines the mean of X or the expected value of X. The operator E is

used to denote this kind of average. The mean square value of X is

given by

E [X2] = I X2 p(X)dX- (A-ll)

An important statistical parameter is the variance of X which is the

ensemble average of the square of the deviation from the mean. Thus

the variance of X is given by

a2 = E [(X-yx)2] = f(X-yx)

2 p(X)dX

An alternate expression is

a2 = E[X2] - yx2 • (A-13)

When the mean is zero, the variance is identical with mean square.

The square root of Eq. ( 13 ), that is a, is called the standard

deviation. The co-efficient of variation is defined by

V- 'g - (A-14)yx

which tells the degree of variation.

3. Random Process

Let us consider random functions X(t) and Y( t ) in which t

represents any variable, say, special co-ordinates or time.

no

For a random process, we may express the correlation between

X(t-j) and X(t2) by means of the autocorrelation function

Rx (trt2) = E [X(t}) X(t2)]

oo • oo

= J f X(t!) X(t2) pttUT), X(t2))dX(t1) dX(t2) (A-15)

-00 -CO

The prefix auto refers to the fact that X(t-|) and X(t2) represents a

product of values on the same sample at two different coordinates.

If X(t^) and X(t2) are statistically independent, we have

p(X(t1), X(t2)) = p(X(t!)) p(X(t2)) and hence

RxU^ t) = E [X( t ! ) ] E [Xit2)] (A-16)

Cross correlation function is defined for two samples X( t - [ ) and

Y(t2) by

t2) = E [X( tT ) Y(t2)]

OO OO

= f f x(tl) Y(t2) P(X(t i) , Y(t2 ) ) dX(t ! ) .dY(t2 )

—OO —CO (A-17)

Correlation coefficient is given by

E [(X(t,) - E [X(t1)])_(Y(t2) - E [Y(t2)])]PYY = ! /. v /.i— (A-18)MXY ax(t-|) ay(t2)

It can be shown that

-1 < PXY £ 1 (A-19)

and the intermediate values of PXY measure the degree of linear

statistical dependence between X(t]) and

in

4. Stationary Random Process

A random process is said to be stationary if its probability

distributions are invariant under a shift of the co-ordinates. In

other words, the family of probability densities applicable at t-j also

applies at ^- In particular, the first order probability density

function p(X) becomes universal distribution independent of time.2

This implies that all the averages baseed on p(X) , say, E [X ] and 0 ,

are constants independent of time.

For a stationary process, the autocorrelation or cross cor-

relation functions become functions of the difference between t, and

t2 and independent of t, or t2.

Therefore, for a stationary process, Equation (A-15) becomes

RX(T) = E [X(t!) X(t!+T)]s T = t| - t2 (A-20)

and Equation (A-17) is

RXY(T) = E [X(t-,) Y(t! + T ) ] , T = t} - t2- (A-21)

APPENDIX B

Fast Fourier Transform

^ Restricting the limits to a finite time interval for the record

x(t), say in the range (o, T), the finite range Fourier Transform is

defined as

T

X(f, T) = J x(t)e'J'2Trftdt (B-l)

oAssume now that time x(t) is sampled at N equally spaced points a

distance h apart, then for arbitrary f, the discrete version of

Equation (B-l) becomes

N-lX(f , T) = h I xn Exp[-j 2ir fnh] (B-2)

n=0

Selecting the discrete frequency values as

fk = kf = T= Nh k = 0, 1, 2, ... N-l (B-3)

At these frequencies, the transformed values give the Fourier com-

ponents defined by

Y _ X(fk, T) N-- j xn exP I"

k = 0, 1, 2 ..., N-l (B-4)

where h has been included with X(f|<, T) to have a scale factor of

unity before the summation. Note that results are unique only out to

112

113

k = 2 since the Nyquist cutoff frequency occurs at this point. Fast

Fourier transform (FFT) methods are designed to compute these

quantities, X^, and can also be used to compute the co-efficients of

the regular Fourierseries Aq and Bq in the expression

N/2xn = x(nh) = Ao + 2 Aq cos(^-pL)

q=l

N T2 ~

+ S Df, f 4 M / tTTQn \ / n r \•* bq sirU .." ) • (B-5)q=l N

To simplify the notation further, let

w(u) = exp [-j p] . (B-6)

Observe that w(N) = 1 and for all u and v,

w(u + v) = w(u) w(v) • (B-7)

Also, let

X(k) = Xk and x(n) = xn (B-8)

Then Equation (B-4) becomesN-l

X(k) = £ x(n) w(kn), k = 0, 1, 2, ... N-l « (B-9)n=0

Equations (B-4) and (B-9) should be studied so as to be easily

recognized as the Fourier transform of x(n) when x(n) is expressed by

a series of N terms. Such equations require a total of approximately" O

fr complex multiply-add operations to compute all of the X(k) terms

involved.

Fast Fourier Transform procedures are now based upon decomposing

N into its composite (nonunity) factors, and carrying out Fourier

transform over the smaller number of terms in each of the composite

114

factors. In particular, if N is the product of p factors such that

N = irr.,- = r^2 ... rp (B-10)

i = 1

where the r's are all integers greater than unity, then it can be

proved [24] that X(k) in Equation (B-9) can be found by computing

in an iterative fashion the sum of p terms,

N 9— Fourier transform requiring 4r-j real operations each,

N 9— Fourier transform requiring 4r? real operations each,r2j (B-ll)

N 9— Fourier transform requiring 4rD real operations each.rP H

Hence the total number of real operations becomes

P4(N + Nr2 + Nr3 + + Nrp) = 4 Z r2 (B-12)

The resulting speed ratio of these FFT procedures to the standard

method is then2

speed ratio = — p = -^— (B-13)4N Z ri 4 E ^

speed ratio for powers of two:

If N = 2P, then z r,- = 2p = 2 logo Ni=l

In this case, the speed ratio by equation (B-13) appears to be

N Nspeed ratio =

115

However, a doubling of the speed can be achieved in practice by

noting that the values for w(kn), when N is a power of 2, all turn

out to be +1 or -1, so as to yield a higher speed ratio of the order

of

speed ratio = Jj- (B-14)

For example, if N = 2'^ = 8192, Equation (B-14) gives a speed ratio

of i^ 158.

APPENDIX C

Expanding Eq. (3-14), with H(W|<) being a lower triangular matrix, one

obtains

+ Hm"n + •••- Hmn"nn« for n < ra,

Gmm=

From Eqs. (3-22a), (3-22b), and (3-3) and (C-l) one can see that

m th component process is simulated from Hmp(w|<) (p = 1, . . . , m) and

the coherence between the m th component and n th component with

n < m is generated by (w ) and Hnp(w|<) (p = 1, 2 ..... n). When

a vanishing minor is due to a zero mean square density of, say, the

m th component process at a frequency w^, then the elements of cross-

spectral density associated with this component are also zero at this

particular frequency. It is easy to show that a sufficient condition

for this case is

Hmn(w|<) = 0, for n = 1, 2, ... m

Hnm(w|<) = 0, for n = m + 1, .... M

The remaining elements of Hmn(w|<) are then determined from the sub-

matrix obtained by deleting the m th row and m th column from the

matrix G(wk).

A coherence of unity between two component processes (the m th

and the (m + 1) th) at a frequency w^ will also give rise to vanishing

116

117

principal minors: This implies a complete linear dependence at this

frequency between the two processes and hence the existence of the

transfer function 3(wk) such that

G^, m+l(wk) = |3(w)|2Gmm(wk)

(C-2)GJJ, , m+1 (v,'k) = 3 (w} Gflim (wk)

Therefore, at this wk, it can be shown that G(wk) will take the

following form:

Gin G-i j G-i ,j+l G,..ii ij i ip/i

Gill GMj G>> 1+1 GMMMl Mj M, MM

G-i T G-i . 3G_- _• G-i MII Ij ij in •

G^i G.:,- 3G-;-; Gn*M_ _ .o _ (C-3)

1

GM1 GM.i 3GM.i GMM

In this case, H(wk) can be solved for a particular frequency in the

following fashion.

First, reduce the size of both matrices G(wk) and H(wk) by

deleting the (j + 1) th row and (j + 1) the column, then solve for

the reduced H(w. ) from the reduced GCw^), by the method mentioned in

118

Chapter I I I . Final ly , the deleted elements of H ( w k ) , as can be seen

by direct substitution in Eq. (C-l ) , are given by

Hj+ 1 ,p = FH.jp) for p = 1, 2, ... j

H j+1 ,j+l = 0 , (C-4)

H,,,j+l = 0, for p = j+2, M

If two component processes, again, say, the j th and (j + 1) th,

are completely correlated, then $(w) as defined for the previous case

will reduce to a real constant $0 independent of the frequency. This

will lead to Dpfw^) = 0, p = j + 1, ... M, for all frequencies. For

this case, it is necessary to simulate only m - 1 component processes

with the (j + 1) th removed from the system, since it can be obtained

by

fj+1(t) = 00fj(t)

With the proper combination of the above procedures, it is possible

to deal with the following situations:

(i) more than one component of GCw^) vanishes;

(ii) more than two components are completely coherent or

correlated;

(iii) the combination thereof.

APPENDIX D

COMPUTER PROGRAM

This section of the report deals with the listing of some of

the important input parameters in the computer programs included.

On the left-hand column is the computer language (FORTRAN) symbol and

on the right-hand side, just opposite, is the equivalent variable

used in the derivation of the equations in Chapter III and Chapter

IV.

Listing for Program for Simulation by Trigonometric Method:

K KA, frequency index

ornp(k) ALPHA(I,KK,KA), deterministic phase angles

Wu WU, upper cut-off frequency

M£ WL, lower cut-off frequency

Gmn S(I,J), element of spectral density function

Hjn- HHH(I,J), elements of lower triangular matrix

amp» kmp AA(I,KK,KA), BB(I,KK,KA), random Gaussian variables

Aw DW, discretized frequency interval

m KK,- number of time series

£p, TV VI , V2, random number generated by Gauss subroutine

At DT, time interval

fm(t) F(I,JJJ), representation of time series

119

120

Listing for Program for Simulation by FFT Method:

K KA, frequency index

G(I,J) S(I,J), input spectral density function

Hp k HHH(I.J), elements of lower triangular matrix

np|<, L^ Z1(K), Z2(K), independent Gaussian random numbers

Cp XI(K), complex random number

h DT, time interval

Xpk DDX(I,KA), elements of complex random vector

fpn F(I,J), time series representation

Listing for Program for Nonlinear Response

Analysis of String

Of SIGF, standard deviation of the forcing function

L TL, length of the string

a ALPHA, a constant

T0 TO, initial tension in the string

p ROE, density per unit length

w-] , w2, w3 Wl, W2, W3, natural frequency of vibration for

1st, 2nd and 3rd modes respectively

A AR, area of cross section

S0(w) SO(KA), spectral density function of actual force

Smn(w) S(I,J), spectral density of the generalized force

fm(t) F(I,J), generalized force representation

bn(t) AN(t), derivative of modal displacement

bn(t) BN(t), modal displacement

121

bn(t) ANPR(t), second derivative of the modal dis-

placement

Listing for Program for Nonlinear Panel

Response Problem:

3-i BET1, percentage of critical damping for 1st mode

of vibration

32 BET2, percentage of critical damping for 2nd mode

of vibration

cfp SIGP, r.m.s. pressure of boundary layer pressure

fluctuations

Uoo UINF, mean external flow velocity

a SA, plate length

b SB, plate width

v GNU, Poisson's ratio

Uc DC, convectional speed

M CM, mack number

p ROE, plate density per unit area

d SI, cavi ty depth

u SV, sound velocity

C ' XSI, boundary layer thickness

F ALP

D RIG, plate stiffness

Ac DEMC, constant

X DEM, constant

122

q SQ, dynamic pressure

C0 CONST, constant

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VITA

AjH Kumar Sinha was born OR in .

In 1960 he graduated from local government high school and entered

the St. Xavier's College, Ranchi, India, from where he obtained his

Intermediate Science degree. In 1962, he was admitted to Bihar

Institute of Technology, Sindri, India, where he received his B.S.

degree in Mechanical Engineering in 1966. After having worked as a

lecturer at the same institute for two and one-half years, Mr. Sinha

joined the University of Mississippi Graduate School in September of

1969 from where he transferred to Virginia Polytechnic Institute in

September 1970 to work towards the completion of Ph.D. requirements

in the Engineering Mechanics Department. Upon completion, Mr. Sinha

plans to work for the United Engineers and Constructors, a subsidiary

of Raytheon Corp., in Philadelphia.

166