Speculation about near-wall turbulence scales Nina Yurchenko, Institute of Hydromechanics National Academy of Sciences of Ukraine, Kiev
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
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 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
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
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
About Near-Wall Turbulence Scales 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
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 = m/s, R=200 и 800 mm. (1) Y X MzMz Flow (1) (2) Z X Test section Model – backward position Model – forward position
About Near-Wall Turbulence Scales 9 Reference, ΔT=0 λ z1 =λ 2G =84 λ z2 = λ 1G =236 ТzТz Heated strips Laminar case Turbulent case Reference, ΔT=0 λ z = m λ z = 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
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
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 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
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
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
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 Temperature variation in boundary layers downstream of plasma sources T x laminar turbulent 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 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
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, , of STCU (Science and Technology Center in Ukraine) and the CRDF GAP grant # UKE KV-05, The author acknowledges with thankfulness contributions of Drs. Pavlo Vynogradskyy (measurements) and Natasha Rozumnyuk (computation).
About Near-Wall Turbulence Scales 22