Speculation about near-wall turbulence scales

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

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 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 About Near-Wall Turbulence Scales

4. Energy replenishment a b c d • Evolution of a streamwise vortical structure • in boundary layers: • 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 . About Near-Wall Turbulence Scales

5. Goertler stability diagram describing behavior of streamwise vortices in a BL as a guidance to choose a vortical structure scale optimal for a given flow control problem 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 About Near-Wall Turbulence Scales

6. U(z) velocity profile U(y) velocity profiles at z=0, lz/4, lz/2 Counter rotating streamwise vortices Flush-mounted heated elements Y, normal Concave surface U0 Z, spanwise X, streamwise Convex surface lz Knowledge of physical mechanisms of vortical evolution of a near-wall flow is prerequisite to development of efficient approaches to flow control About Near-Wall Turbulence Scales

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

8. Test section (1) (1) Y Mz Model – forwardposition a Flow X (1) (2) Model – backwardposition z1 z2 Z X BASIC FLOW PARAMETERSin aerodynamic experiments: U=10 - 20 m/s, R=200 и 800 mm. CONTROL PARAMETERS: Flush-mounted streamwise elements are organized into independent electrically heated sections on both sides of the model imposing various space scales ofdisturbances. Typical regular spanwise temperature difference ΔT(z)=35 About Near-Wall Turbulence Scales

9. Reference, ΔT=0 λz1=λ2G=84 Heated strips λz2= λ1G=236 Тz Reference, ΔT=0 Streamwise vortices of different scales generated in boundary layers LEFT: Transitional boundary layer: G=8; Тz=300 RIGHT: Turbulent boundary layer: Re=5105 ; Тz=350, x=0.19 Laminar case Turbulent case λz=0.0025 m λz=0.0050 m About Near-Wall Turbulence Scales

10. Wind tunnel • Closed-return type • Elliptical test section75 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 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 About Near-Wall Turbulence Scales

12. Measurements Increments of Lift coefficient Cy, Drag coefficient Cx and Lift-to-Drag ratio vs time 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 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. • ΔTz = 40 About Near-Wall Turbulence Scales

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

16. 1000 T laminar 900 800 turbulent 700 600 500 400 300 200 100 0 x 0 0.05 0.1 0.15 0.2 0.25 0.3 Temperature variation in boundary layersdownstream of plasma sources The spanwise array of high-temperature (1000C) sources is placed at 1mm over the wall 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) About Near-Wall Turbulence Scales

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

22. About Near-Wall Turbulence Scales