1 strain rate, rotation rate and isotropy in the previous lecture the strain rate tensor ij and the...
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1
STRAIN RATE, ROTATION RATE AND ISOTROPY
In the previous lecture the strain rate tensor ij and the rotation rate tensor rij were defined as
i
j
j
iij
i
j
j
iij x
u
x
ur,
x
u
x
u
2
1
The shear stress tensor ij was represented as
vijijij p
where ijv denotes the viscous stress tensor, and ij
v was related only to the strain rate tensor ij, in the most general linear form
klijklvij A
This was done because we want to relate ijv to how a body deforms,
not how it rotates. But to do this we must establish that ij does indeed characterize deformation, and rij does indeed characterize rotation.
2
STRAIN RATE, ROTATION RATE AND ISOTROPY
A deformable body may be deformed in two ways: extension shearing. Extensional deformation is illustrated below:
Shear deformation is illustrated below:
3
We first consider extensional deformation. A moving deformable body has length x1 in the x1 direction. The velocity u1 is assumed to be changing in the x1 direction, so that the values on the left-hand side and right-hand side of the body are, respectively is
STRAIN RATE, ROTATION RATE AND ISOTROPY
In time t the left boundary moves a distance u1t, and the right boundary moves a distance [u1 + (u1/x1)x1]t
11
111 x
x
uuandu
1x
1x
1u 11
11 x
x
uu
tu1
txx
uu 1
1
11
1x
4
The initial length of the body is x1. The length of the body after time t is given as
STRAIN RATE, ROTATION RATE AND ISOTROPY
The extensional strain rate is the rate of length increase of the body per unit initial length per unit time =
tutxx
uux 11
1
111
tu1
txx
uux 1
1
111
[(new length) – (old length)]/(old length)/(t) =
1
1
1
1111
111
x
u
tx
xtutxxu
ux
5
Note that u1/x1 > 0 for an elongating body and u1
/x1 < 0 for a shortening body. The corresponding extensional strain rates in the x2 and x3 directions are, respectively,
STRAIN RATE, ROTATION RATE AND ISOTROPY
3
3
2
2
x
uand
x
u
These extensional strain rates relate to the diagonal components of the strain rate tensor 11, 22 and 33 as follows:
3
3
3
3
3
333
2
2
2
2
2
222
1
1
1
1
1
111
x
u
x
u
x
u
2
1
x
u
x
u
x
u
2
1
x
u
x
u
x
u
2
1
6
Now we consider shear deformation. Consider the points A, B and C below. The velocities in the x1 direction at points A and C are, respectively,
STRAIN RATE, ROTATION RATE AND ISOTROPY
and the velocities in the x2 direction at points A and B are, respectively,
A B
C
1x
2x
22
111 x
x
uuandu
11
222 x
x
uuandu
2x
1x
2u 11
22 x
x
uu
22
11 x
x
uu
1u
7
1x
2x
STRAIN RATE, ROTATION RATE AND ISOTROPY
The body undergoes shear deformation over time t. That is:• Point A moves a distance u1t in the x1 direction and u2t in the x2 direction;• Point B moves a distance [u2 + (u2/x1)x1]t in the x2 direction; and• Point C moves a distance [u1 + (u1/x2)x2]t in the x1 direction.
tu1
txx
uu 2
2
11
tu2 txx
uu 1
1
22
2x
1x
8
1x
2x
STRAIN RATE, ROTATION RATE AND ISOTROPY
Recall that for small angle , sin . The angles and created in time t are defined below. These can be approximated as:
tu1
txx
uu 2
2
11
tu2 txx
uu 1
1
22
2x
1x
tx
u
x
tutxxu
u
2
1
2
122
11
tx
u
x
tutxxu
u
1
2
x
211
22
9
1x
2x
STRAIN RATE, ROTATION RATE AND ISOTROPY
The strain rate due to shearing can be defined as the angle increase rate ( + )/t, or thus
tu1
txx
uu 2
2
11
tu2 txx
uu 1
1
22
2x
1x
1
2
2
11
2
2
1
x
u
x
u
t
txu
xu
t
)(
10
The strain rate due to shearing due to shearing d( + )/dt in the x1-x2 plane is related to the component 12 of the strain rate tensor as
STRAIN RATE, ROTATION RATE AND ISOTROPY
t
)(
2
1
x
u
x
u
2
1
1
2
2
112
The corresponding components of the strain rate tensor due to shearing in the x2-x3 and x1-x3 planes are correspondingly
1
3
3
113
2
3
3
223
x
u
x
u
2
1
x
u
x
u
2
1
11
STRAIN RATE, ROTATION RATE AND ISOTROPY
It is thus seen that the strain rate tensor ij does indeed characterize the rate at which a body is deformed by elongation or shearing. We now must establish that the tensor
i
j
j
iij x
u
x
ur
does indeed characterize rotation.
We approach this indirectly, by first defining circulation. Circulation is an integral measure of the tendency of a fluid to rotate. Let C denote some fixed closed circuit within a fluid (across which fluid can flow freely), and let denote an elemental arc length tangential to the circuit that is positive in the counterclockwise direction.
sd
sd
C
The circulation is defined as:
C
sdu
12
STRAIN RATE, ROTATION RATE AND ISOTROPY
To illustrate the idea of circulation, we consider two simple examples.
The first of these is constant, rectilinear flow in the x direction with velocity U, so that (u, v, w) = (U, 0, 0). The circuit has length L in the x direction and length H in the y direction. The circulation around the circuit is:
HU
1
2
3
4
L
00)UL(0UL
sdusdusdusdu
sdu
4321
C
sd
sd
sd
sd
Thus the is no circulation around a circuit in rectilinear flow.
x
y
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STRAIN RATE, ROTATION RATE AND ISOTROPY
Now we consider the case of plane Couette flow with
Note that u = 0 where y = 0 and u = U where y = H. Now
HU
1
2
3
4
L
UL0)UL(00
sdusdusdusdu
sdu
4321
C
sd
sd
sd
sd
Thus there is circulation, and it is negative (i.e. directed in the clockwise direction).
0,0,
H
yU)w,v,u(
x
y
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STRAIN RATE, ROTATION RATE AND ISOTROPY
The concept of circulation is closely related to the concept of vorticity. Consider a loop around a region with area dA, such that the circulation around the circuit is d. The vorticity is defined as
dA
d
The vorticity of a fluid at a point is equal to twice the angular velocity of the fluid particles at that point. This can be seen by considering a fluid particle rotating with angular speed , at the center of a circle with radius dr. The arc length of the periphery of the circle is 2dr, the area of the circle is (dr)2, and the peripheral velocity is dr. Thus
dr
2)dr(
dr2dr
dA
d
)dr(dA
dr2drd
2
2
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STRAIN RATE, ROTATION RATE AND ISOTROPY
The vorticity can be related to the velocity field as follows. Consider the elemental rectangular circuit below. The components of velocity at the center of the rectangle are (u, v). The component of the velocity normal to the circuit on segments 1, 2, 3 and 4 are
1.
2.
3.
4. y
1
2
3
4
x
sd
sd
sd
sd
(u,v)
x
y
y2
1
y
uu
y2
1
y
uu
x2
1
x
vv
x2
1
x
vv
Thus
y)x2
1
x
vv(x)y
2
1
y
uu(
y)x2
1
x
vv(x)y
2
1
y
uu(d
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STRAIN RATE, ROTATION RATE AND ISOTROPY
Reducing,
y
1
2
3
4
x
sd
sd
sd
sd
(u,v)
x
y
And thus since = d/dA,
yxdA
yxy
u
x
vd
y
u
x
v2
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STRAIN RATE, ROTATION RATE AND ISOTROPY
Now we return to constant rectilinear flow and plane Couette flow.
For rectilinear flow (u, v, w) = (U, 0, 0) and
x
y
U
0y
u
x
v2
Therefore the illustrated red paddle will not rotate as it moves with the flow.
For plane Couette flow (u, v, w) = (Uy/H, 0, 0) and
H
U
y
u
x
v2
The illustrate red paddle will thus rotate in the clockwise direction as it moves with the flow, with angular speed = U/(2H).
x
y
U
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STRAIN RATE, ROTATION RATE AND ISOTROPY
So far we have considered only 2D flows in the (x, y) plane, in which case angular velocity and vorticity are directed along the z axis. The appropriate 3D extension is
j
kijkii x
u2orux2
kx
u
x
uj
x
u
x
ui
x
u
x
u
uuuxxx
kji
ux
2
1
1
2
1
3
3
1
3
2
2
3
321
321
Or expanding out
Thus for example
y
u
x
v
x
u
x
u
2
1
1
23
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STRAIN RATE, ROTATION RATE AND ISOTROPY
The rate of rotation tensor rij is directly related to the vorticity vector i and thus to the angular velocity i. That is,
kijk
12
13
23
3
2
2
3
3
1
1
3
2
3
3
2
2
1
1
2
1
3
3
1
1
2
2
1
i
j
j
iij
0
0
0
0x
u
x
u
x
u
x
u
x
u
x
u0
x
u
x
u
x
u
x
u
x
u
x
u0
x
u
x
ur
(Note: do not confuse the Levi-Civita third-order tensor ijk with the strain rate tensor ij.)
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STRAIN RATE, ROTATION RATE AND ISOTROPY
Our goal is to relate the viscous stress tensor ijv to a measure of the rate of
deformation not the rate of rotation, of a fluid. Thus we relate ijv to ij
rather than ui/xj.
The most general linear relation between ijv and ij is
klijklvij A
The relation can be simplified by assuming that it is a) isotropic, so that the form of the relation is invariant to coordinate
rotation, and has the same physics in any direction, and b) symmetric, so that ij
v = jiv.
We do not go through the complete details of isotropy here. We showed in Lecture 2, however, that pressure p is isotropic. More specifically, where ij
p denotes the part of the stress tensor associated with pressure,
ijpij p
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STRAIN RATE, ROTATION RATE AND ISOTROPY
The most general second-order isotropic tensor Aij takes the form
ijij CA
where again C1, C2 and C3 are arbitrary scalars. Thus the relation
reduces to
where C is an arbitrary scalar. (In the case of ijp, C = -p.) It turns out that
the most general fourth-order isotropic tensor is (Aris, 1962)
jkil3jlik2klij1ijkl CCCA
klijklvij A
ji3ij2klklij1vij CCC
But for an incompressible fluid
0x
u
k
kkkklkl
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STRAIN RATE, ROTATION RATE AND ISOTROPY
Thus
But
i
j
j
iij x
u
x
u
2
1
ji3ij2vij CC
or
But ij is a symmetric tensor, i.e. ij = ji. Further defining C2+ C3 = 2, where = the dynamic viscosity of the fluid, the relation reduces to
ijijvijijijij
vij 2pp,2
i
j
j
iijij x
u
x
up
The above relation defines the constitutive relation for a viscous Newtonian fluid. Note that the form guarantees symmetry in ij.