Forcing the shear layer of a backward-facing step flow

Transcription

1 Forcing the shear layer of a backward-facing step flow using DBD plasma actuator J.-L. Aider* T. Duriez* J. E. Wesfreid* G. Artana *Laboratoire PMMH UMR7636 CNRS ESPCI - France Laboratorio de FluidoDynamica (LFD) Universidad de Buenos Aires - Argentina 1

2 Outline 1. Context 2. Experimental set-up 3. DBD actuation 4. Characteristics of the unperturbed mean flow 5. Modification of the mean-flow characteristics 6. Analysis using phase averaging and Time-resolved visualizations 7. Spatial growth of the perturbations 8. Estimation of the streamwise wavelengths through phase averaging. 9. Conclusion 2

3 Backward-facing step flow Lr Uo δ xr h Lr 3

4 Backward-facing step flow A very simple geometry but a complex flow 4

5 Backward-facing step flow The transitions of the shear layer is one of the key phenomena Onset of the Kelvin-Helmholtz instability Reh = 250 Reh = 450 5

6 Backward-facing step flow The transitions of the shear layer is one of the key phenomena Onset of the 3D destabilization of KH vortices λ 5h 6

7 Objective of the experimental study One can use many different strategies to control the flow separation: Upstream of the separation At the separation Downstream the separation Constant or time dependant Control using pulsed excitation just upstream the step edge 7

8 Experimental setup Characteristics of the wind tunnel and BFS geometry Lz=100 H1=100 Uo y H=20 δ z O H0=120 x 800 Square cross-section Uo = 3 to 15 m.s-1 i.e. ReH = UoH/ν = to Expansion ratio E =

9 Actuation Actuation using a ionic wind induced by a DBD actuator Lz=100 Uo Plasma dischar ge H1=100 y H=20 δ z O 30 H0=120 x 800 Constant HV = 6kV Induced velocity uj 4 m.s-1 High frequency AC duty cycle 27% 9

10 PIV and visualization set-up 18mJ double cavity YaG LASER Seeding with oil microdrop lets Uo Plasma dischar y ge H1=100 z O 30 PIV window H=20 δ x 800 For each Reynolds numbers and excitation frequencies: 4Hz double frame CCD Camera 700 pairs of images Synchronisation with the excitation frequency for phase reconstruction

11 PIV and visualization set-up 2W Laser Seeding with oil microdrop lets Uo Plasma dischar y ge H1=100 z O 30 PIV window H=20 δ x 800 For a single Reynolds number and different excitation frequencies and duty cycles: High-speed Phantom camera 500Hz sampling rate images are recorded

12 Characteristics of the BFS flow Evolution of the Ux velocity field as a function of the Reynolds number

13 Characteristics of the BFS flow Visualization of the vortex shedding in the shear layer for Re H = 8000

14 Characteristics of the BFS flow Evolution of the recirculation length Lr as a function of the Reynolds number

15 Visualisations of forced shear layer.

16 Modification of the mean-flow characteristics Evolution of the recirculation length Lr with the forcing frequency for ReH = 3050 f/f0 =1

17 Phase-Averaging

18 Spatio temporal diagrams

19 Streamwise wavelength of the perturbation

20 Streamwise wavelength of the perturbation

21 Streamwise wavelength of the perturbation

22 Streamwise wavelength of the perturbation

23 Role of the phase velocity Retour aux visus

24 Spatial transient growth of the perturbation

25 Conclusions 1. Large gains on xr over a large range of frequencies 2. Optimal frequencies around f0 with critical behaviour 3. Two different behaviours with respect to the frequency ratio. For f<f0 control of the shear layer based on vortex pairing where phase velocity seems to be the key element. Importance of the nature of the perturbation. For f>f0 frequency locking on the forcing frequency 4. 5.