1. Speculation about near-wall turbulence scales Nina Yurchenko, Institute of Hydromechanics National Academy of Sciences of Ukraine, Kiev nina.yurchenko@silvercom.net

2. 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. About Near-Wall Turbulence Scales 3 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

4. About Near-Wall Turbulence Scales 4 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. About Near-Wall Turbulence Scales 5 Goertler stability diagram describing behavior of streamwise vortices in a BL G=  2 3/2 U 0 -1 R -1/2,  z =2  / z ; 1- neutral curves (numerical) by Floryan & Saric (1986);  1n and  2n – 1 st and 2 nd modes found numerically as a guidance to choose a vortical structure scale optimal for a given flow control problem

6. About Near-Wall Turbulence Scales 6 Knowledge of physical mechanisms of vortical evolution of a near-wall flow is prerequisite to development of efficient approaches to flow control Convex surface z Concave surface U(y) velocity profiles at z=0, z /4, z /2 Z, spanwise X, streamwise Y, normal U0U0 Counter rotating streamwise vortices Flush-mounted heated elements U(z) velocity profile

7. About Near-Wall Turbulence Scales 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 R – basic radius of convex and concave parts

8. About Near-Wall Turbulence Scales 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 MzMz Flow (1)  (2) Z X Test section Model – backward position Model – forward position

9. About Near-Wall Turbulence Scales 9 Reference, ΔT=0 λ z1 =λ 2G =84 λ z2 = λ 1G =236 ТzТz Heated strips Laminar case Turbulent case Reference, ΔT=0 λ z =0.0025 m λ z =0.0050 m Streamwise vortices of different scales generated in boundary layers LEFT: Transitional boundary layer: G=8;  Т z =300  RIGHT: Turbulent boundary layer: Re=5  10 5 ;  Т z =350 , x=0.19

10. About Near-Wall Turbulence Scales 10 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

11. About Near-Wall Turbulence Scales 11 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. About Near-Wall Turbulence Scales 12 Time series during 350 s for a selected angle-of-attack and 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 Increments of Lift coefficient C y, Drag coefficient C x and Lift-to-Drag ratio vs time

13. About Near-Wall Turbulence Scales 13 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. ΔT z = 40 

14. About Near-Wall Turbulence Scales 14 RESEARCH CONTINUITY: flows controlled with spanwise-regular plasma discharges generated near the wall y z x z Basic flow MW generator E 0 MW radiation U(z) U(y) Plug-in assembly of plasma actuators

15. About Near-Wall Turbulence Scales 15 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.

16. About Near-Wall Turbulence Scales 16 Temperature variation in boundary layers downstream of plasma sources 0 100 200 300 400 500 600 700 800 900 1000 00.050.10.150.20.250.3 T x laminar turbulent The spanwise array of high-temperature (1000  C) sources is placed at 1mm over the wall

17. About Near-Wall Turbulence Scales 17 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. About Near-Wall Turbulence Scales 18 Sketch of the wind-tunnel facility designed for aerodynamic tests under conditions of MW radiation and plasma generation Eiffel chamber and magnetron system Diffuser Nozzle Test section Absorber of MW radiation FLOW

19. About Near-Wall Turbulence Scales 19 BL control using a spanwise linear array of localized plasma discharges MW-initiation of localized plasma discharges over a test model Sketch of the plug-in assembly of plasma actuators mounted in the model wall

20. About Near-Wall Turbulence Scales 20 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 = 40  С) 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.

21. About Near-Wall Turbulence Scales 21 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).

22. About Near-Wall Turbulence Scales 22