Experimental study of flow separation

Transcription

1 Experimental study of flow separation Fawzi FADLA TEMPO Laboratory (DF2T Team) - Valenciennes

2 Outline Introduction & Background Motivations & Methodology 2D-Bump separation (adverse pressure gradient) Velocity fields -Reynolds number dependency -Unsteady vortex motions (for low Reynolds number case) Wall shear-stress -Electrochemical methods -Wall shear-stress statistics (Comparison with DNS data from LML) Conclusions GDR Configuration (sharp edge) at TEMPO Laboratory 1/18

3 Introduction & Background Flow separation from a surface Sharp edge Smooth body (subjected to an adverse pressure gradient) Key Parameters : Reynolds number Re t = u t.(h/2)/n or u t.d/n Aspect ratio Channel flow l/h Boundary layer flow l/d 2/18

4 Motivations Reynolds number dependency on the separation area (aspect ratio) Unsteady phenomena (flapping, Kelvin Helmholtz instability,...) Experimental Validation (ER2 Team, LML «DNS simulation») Control of separation Methodolology Wind tunnel Spatial caracterisation (PIV) Channel Flow Wall-shear stress statistics (Electrochemical method ) 3/18

5 2D-Bump separation - Reynolds number dependency Wind tunnel configuration for spatial analysis Test section aera # 2 m x 2 m Bump position (7m to the nozzle) Velocity range U 0 # 0.4 m/s 1.0 m/s 4/18

6 2D-Bump separation - Reynolds number dependency Setup - PIV 2D 2C 2 caméras CCD 2048x2048 pixel 2 85mm optical lens Windows : 415mm x 215mm Spatial resolution: 1,35mm Acquisition : 2000 fields, F acq : 7.5Hz U 0 5/18

7 2D-Bump separation - Reynolds number dependency Existence of a critical Reynolds number for l/d= Re t = Critical Reynolds number evolution for different aspect ratio (l/d)? Separation behavior for Wall bounded flow & Confined flow (same aspect ratio)? 6/18

8 2D-Bump separation - Unsteady vortex motions (for low Reynolds number case) Re t = 240 l/d = 0.67 Flapping, Shedding & Kelvin Helmholtz instability 7/18

9 2D-Bump separation Future work Different flow configuration (Channel & boundary layer ) Different aspect ratio (for both configurations) Spectral analysis (HWA) «currently underway» 8/18

10 2D-Bump separation - Electrochemical methods Basis/ Principles - Permit to measure the instantaneous wall shear stress - Electrolytic solution - Wall shear stress is a function of current (I). t = I 3 Literature: James E. Mitchell and Thomas J. Hanratty - A study of turbulence at a wall using an electrochemical wall shear-stress meter. JFM, vol.26(1), /18

11 2D-Bump separation - Wall shear-stress statistics Channel flow configuration for wall shear-stress measurements Different configurations: h=10mm without bump (Re t #74-400) h=20mm with bump (Re t # ) 10/18

12 2D-Bump separation - Wall shear-stress statistics Experimental validation without bump (h=10mm) Keirsbulck L., Labraga L. and Gad-el-Hak M. (2012) Statistical Properties of Wall-Shear-Stress Fluctuations in Turbulent Channel Flows, International Journal of Heat and Fluid Flow, in press. [DOI: /j.ijheatfluidflow ] (in press). 11/18

13 2D-Bump separation - Wall shear-stress statistics Experimental validation without bump (h=10mm) - Good agreement with DNS Data 12/18

14 2D-Bump separation - Wall shear-stress statistics Numerical result with bump -Marquillie et al. 13/18

15 2D-Bump separation - Wall shear-stress statistics Comparison with DNS data from Marquillie et al. (h=20mm) 14/18

16 Conclusions Experimental wall-statistics available for Channel flow (h=10mm, Re t =74-400) Usefull database for preliminary investigation of flow control on a 2D bump Next step Complete this work testing different configurations & aspect ratios Flow control 15/18

17 Information about GDR Configuration at TEMPO Laboratory Same as Orléans GDR model Velocity range: 0.4m/s to 60m/s LDV measurements PIV (spatial resolution: 4.8mm) 16/18

18 GDR Configuration at TEMPO Laboratory Thacker Present Position X = -240 mm (0 mm : edge) d 99 (mm) t w (kg/m.s 2 ) u t (m/s) /18

19 GDR Configuration V réf = 10 m.s -1 Re h = 0, V réf = 20 m.s -1 Re h = 1, V réf = 30 m.s -1 Re h = 1, /18