speculation about near-wall turbulence scales nina yurchenko, institute of hydromechanics national...

Speculation about near-wall turbulence scales

Nina Yurchenko,Institute of Hydromechanics

National Academy of Sciences of Ukraine, Kiev

[email protected]

mailto:[email protected]

About Near-Wall Turbulence Scales

2

STRATEGY

• To study practical issues of similarity between transitional and turbulent structure in near-wall flows

• To generate/maintain streamwise vortices with given scales in a turbulent boundary layer

• To optimize integral flow characteristics through modification of turbulence properties


3

L

S

y

z

U

z

z

z

U1

y3

y2

y1

U2

U3

Normal and spanwise velocity profiles and streamwise vortices in a boundary-layer

TOP: Inflectional normal profiles of averaged velocity measured for different spanwise coordinates

BOTTOM:Wavy spanwise profiles of averaged velocity at different distances from a surface

MIDDLE:Hypothetical vortical structure corresponding to the measured velocity fields


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Evolution of a streamwise vortical structure in boundary layers:

a) Development or generation of streamwise vortices followed by formation of normal shear layers between two counter-rotating vortices,

b) Deformation of a vortex shape due to an amplified instability mode of the shear layer

c) Aggravation of the vortex deformation – restriction of the amplitude growth

d) Breakdown of the normally stretched vortices; formation of a new compact structures under centrifugal forces or under control conditions shown as .

a b c d

Energy replenishment


5

Goertler stability diagram describing behavior of streamwise vortices in a BL

z

G

1.0

1

=1890 650 210 100 30 100.0

10.0

0.01 0.1 10.0 1.0 0.1

experiments by Aihara et al (1985)

Experiments by other authors

experiments by Yurchenko (1983) 1G G

0

G=23/2 U0-1R-1/2, z=2/z;

1- neutral curves (numerical)by Floryan & Saric (1986);

1n and 2n– 1st and 2nd modes found numerically

as a guidance to choose a vortical structure scale optimal for a given flow control problem


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Knowledge of physical mechanisms of vortical evolution of a near-wall flow is prerequisite to development of efficient

approaches to flow control

Convex surfacez

Concave surface

U(y) velocity profiles at z=0, z/4, z/2

Z, spanwise X, streamwise

Y, normal

U0

Counter rotating streamwise vortices

Flush-mounted heated elements

U(z) velocity profile


7

• 200 by 200 mm size • 12% relative thickness• R = 800 mm or 200 mm• direct / inverse position in the flow• 6 sections of heated elements

Variable control parameters: scale of generated vortices,

z = 2.5 mm or 5.0 mm; ΔT(z), or electric power consumed for

heating; a number and combinations of

independently heated sections

Test models

y

z

x S 1 S 2

S 3

z

S 4 S 5 S 6

FL O W

R – basic radius of convex and concave parts


8

CONTROL PARAMETERS:

Flush-mounted streamwise elements are organized into independent electrically heated sections on both sides of the model imposing various space scales of disturbances. Typical regular spanwise temperature difference ΔT(z)=35

z1 z2

BASIC FLOW PARAMETERS in aerodynamic experiments:

U=10 - 20 m/s, R=200 и 800 mm.

(1)

Y

X

Mz Flow

(1)

(1)(2)

Z

X

Test section

Model – backward position

Model – forward position


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Reference, ΔT=0

λz1=λ2G=84

λz2= λ1G=236

Тz

Heated strips 0.0025 0.0050 0.0075 0.0100 0.0125 0.0150 0.0175

0.002

0.004

0.006

0.008

0.0025 0.0075 0.0125 0.0175

0.002

0.004

0.006

0.008

0.0025 0.0050 0.0075 0.0100 0.0125 0.0150 0.0175

0.002

0.004

0.006

0.008

Lam

inar

cas

e

Tur

bule

nt c

ase Reference, ΔT=0

λz=0.0025 m

λz=0.0050 m

Streamwise vortices of

different scales generated in

boundary layers

LEFT: Transitional boundary layer: G=8; Тz=300

RIGHT:Turbulent boundary layer:Re=5105 ; Тz=350, x=0.19


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Wind tunnel

• Closed-return type

• Elliptical test section 75 x 42 x 90 sm.

• Up to 30 m/s free-stream velocity

• External 3-component strain gage balance with strip support

• Precision 20 mN

• Resolution 2 mN


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Test models

• Two multi-layer composite shells with internal wiring to provide low thermal conductivity of the material and thus on a model surface

• Glued together with a model holder

• Mounted between test-section sidewalls to form a 2D flow


12

Time series during 350 s for a selected angle-of-attackand a heating sequence, off-on–off:

50 s – testing of a cold model 170 s – heating ON 130 s – heating OFF,

model cooling stage

Measurements

-0.02

-0.015

-0.01

-0.005

0

0.005

0 50 100 150 200 250 300 350

dCy @+16.5deg.

dCy @+10.5deg.

-0.01

-0.005

0

0.005

0 50 100 150 200 250 300 350

dCx @+16.5deg.

dCx @+10.5deg.

-0.05

0

0.05

0.1

0 50 100 150 200 250 300 350

dL/D @+16.5deg.

dL/D @+10.5deg.

Increments of Lift coefficient Cy, Drag coefficient Cx and Lift-to-Drag ratio vs time


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Results

• R800 model in a direct position, sections #2, 3, 5 and 6 are ON

• Angles-of-attack: 9, 10 and 23 deg.

• Free-stream velocity 15 m/sec.

• ΔTz = 40


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RESEARCH CONTINUITY:flows controlled with spanwise-regular plasma discharges

generated near the wall

y z

x

z

Basicflow

MW generator

E 0

MWradiation

U(z)

U(y)

Plug-in assembly of plasma actuators


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INTERDISCIPLINERY RESEARCH: Moscow Radio-Technical Institute;

Institute of Hydromechanics NASU, Kiev National Aviation University of Ukraine, Kiev

• Greater practical applicability of the method: possibilities to control flows around moving or rotating parts (e.g. in turbine cascades) or in inaccessible places or in a hostile environment;

• Design and operation flexibility and efficiency;

• Localized / intermittent plasma generation – energy saving technology;

• Broader range of control parameters including nonstationary effects due to application of MW field in a pulse mode of a chosen configuration.


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Temperature variation in boundary layers

downstream of plasma sources

0

100

200

300

400

500

600

700

800

900

1000

0 0.05 0.1 0.15 0.2 0.25 0.3

T

x

laminar

turbulent

The spanwise array of high-temperature (1000C) sources is placed at 1mm over the wall


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0 .000 0 .002 0 .004 0 .006 0 .008 0 .010 0 .012 0 .014 0 .016 0 .018 0 .0200 .000

0 .002

0 .004

0 .006

0 .008

0 .010

0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.0200.000

0.002

0.004

0.006

0.008

0.010

-700-600-500-400-300-200-100-50-2502550100200300400500600700

0 .000 0 .002 0 .004 0 .006 0 .008 0 .010 0 .012 0 .014 0 .016 0 .018 0 .0200 .000

0 .002

0 .004

0 .006

0 .008

0 .010

x = 0.10 m

x = 0.19 m

x = 0.05 m

Calculated streamwise vorticity fields in spanwise cross-sections

downstream of localized thermal sources

x = 0.05 m, 0.01 m, 0.19 m; z = 5 mm (left column), z = 10 mm (right column)


18

Sketch of the wind-tunnel facility designed for aerodynamic

tests under conditions of MW radiation and plasma generation

Eiffel chamberand magnetron

system

Diffuser

Nozzle

Test section

Absorber of MW radiation

FLOW


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BL control using a spanwise linear array of localized plasma discharges

z, scale of generated vortices

FLOW

x

z y FLOW

x

z y FLOW

x

z y

x

z Insert with plasma actuators

R

MW-initiation of localized plasma discharges over a test model

Sketch of the plug-in assembly of plasma actuators mounted in the model wall


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CONCLUSIONS:

• Inherent to flow streamwise vortices can be energized to result in efficient control of boundary-layers.

• Laminar-turbulent transition was delayed from ~ 27% of a cord to ~ 40% in a controlled case (ΔT = 40С) under imposed z-regular disturbances of an appropriate mode.

• Certain combinations of thermal-control parameters improve the aerodynamic performance of the model.

• Further optimization of flow control is under way based on MW-controlled plasma arrays over a surface.


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Acknowledgments

This material is based upon work supported by the European

Office of Aerospace Research and Development, AFOSR, AFRL under the Partner Project P-053, 2001-03, of STCU (Science and Technology Center in Ukraine) and the CRDF GAP grant # UKE2-1508-KV-05, 2006-09.

The author acknowledges with thankfulness contributions of

Drs. Pavlo Vynogradskyy (measurements) and Natasha Rozumnyuk

(computation).


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speculation about near-wall turbulence scales nina yurchenko, institute of hydromechanics national...

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