Shock induced boundary layer separation and associated unsteadiness

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1 Shock induced boundary layer separation and associated unsteadiness Pierre Dupont thanks to J.P. Dussauge, J.F. Debiève, L. Larchevêque, S. Piponniau

2 Summary Low frequency unsteadiness in turbulent Shock induced separated flows: origins? Expe/numerical set up and results: LF and MF time scales Similarity with incompressible separated flows: Mach number and density influences Some passive controls attempt Conclusions

3 Flow unsteadiness visualisation: experiments vs LES EXPE Tomographic video of the interaction. Not time resolved LES Schlieren time resolved

SWBLI unsteadiness: origin? Plotkin, 1975 expe Touber et al.

filter Upstream Turbulence Upstream influence (megastructures >10δ 0

, JFM 2009 Shock Wave Decelerated region self sustained unsteadiness

4 SWBLI unsteadiness: origin? Plotkin, 1975 expe Touber et al., JFM 2011, LES System shock +interaction considered as a low pass filter Upstream Turbulence Upstream influence (megastructures >10δ 0 ) Ganapathisubramani et al, JFM 2007 Humble et al., JFM 2009 Shock Wave Decelerated region self sustained unsteadiness in the decelerated region; shock waves are passive IUSTI expe, Robinet theo, DNS, P. Martin JFM 2012 DNS

1% u et 1% p ) Ø Continuous (5 hours) Ø adiabatic or heated wall LES: L.

5 IUSTI: High Speed Flows «facilities» Expe: Ø M<3, Tt=293K,. 1atm<Pt<1atm Ø 17cmx12cm test section Ø Hypoturbulent (0.1% u et 1% p ) Ø Continuous (5 hours) Ø adiabatic or heated wall LES: L. Larchevêque Ø ONERA FLU3M LES solver Ø Turbulent inflow boundary condition (Synthetic Eddy Method (Jarrin et al.))

6 SWBLI: low frequency unsteadiness U1 U 7<Θ<10 SL 0.03 arbitrary scale X0 f.l SL = U1 SL 0.03 SL 0.5

7 Medium frequency unsteadiness in the interaction (LES) Intercorrelation between the shock position and the pressure fluctuations for medium frequencies Agostini et al. (AIAA J. 2012) Intercorrelations isovalues Mach waves orientation Correlations aligned with the Mach waves generated by the convective stuctures

8 SL 0.5 Mixing layer downstream separation shock New eduction scheme from LES data (see Larchevêque this morning) coherent convective structures in the mixing layer (Q criteria) Z/L X* Z/ trajectories of the coherent convective structures along the interaction: shed downstream Z/L X* Z/ 0

9 Activité post-doctorat PIV + Time Resolved estimators -> TR velocity fields Collaboration avec l Institut PPRIME (Poitiers) Linear Stochastic Estimation Estimator(s) Time Resolved correlated with v LSE LSE LSE No TR Piv Objective : To estimate the velocity fields from a conditional information in one or more points

10 Linear Stochastic Estimation: spatio-temporal analysis Collaboration CNES/IUSTI/Institut Pprime S. Piponniau, E. Collin, P. Dupont, and J.F. Debiève.. Int. J. of Heat and Fluid Flow, Time resolved velocity fields

11 Common scheme for separated flows (incompressible-compressible) Spark Schlieren + PIV (vortex detector) ü a Mixing Layer is developping downstream of the separation point (MF convective unsteadiness) ü large scales shed into the downstream flow ü LF fluctuations of the ML (flapping) ü LF and MF correlated shock motions

12 Compressible vs incompressible separations M=2.3 M=0 Kiya and Sasaki, JFM 1983 S Lc 0.03 S L 0.7 shedding S L 0.5 S L 0.12 flapping

13 Low Frequencies: subsonic separated flow vs shock induced separation S L S L =f L/U Mach 5 M 3 % par 4! Low frequency unsteadiness: reflection, compression corner, blunt fins, over expanded nozzles M=0 Origin of this large Mach dependency? hypothesis: compresible mixing layer properties

14 Model for LF unsteadiness in separated flows Mass conservation in the bubble. Initially: M b ρ 0 L 1 h Mass lost per unit time by entrainment on the low velocity side of the mixing layer : δ 1 Dividing stream line Ṁ ej y0(x=l1 /2) δ 2(x=L1 /2) ρudy δ 2 Characteristic time to drain mass: T = M b Ṁ ej = 1 f L 1 /2 è spreading rate of the ML! compressibility effects

15 S L Compressible mixing layer downstream separation shock LF MF Flapping Shedding M=0, Separated: S L,BF =0.12 and S L,MF =0.7 M=2.3, SWBLI: S L,BF =0.03 and S L,MF =0.5 Mixing layer with compressibility effects on: S. Piponniau et al., JFM, 2009 Spreading rate: S L,BF Φ(M c ) g(r,s) Dupont et al., JFM, 2006 Convection velocity S L,MF U c S tr U 1 δ normalized spreading rate weak function of velocity and density ratios

16 Low Frequencies: subsonic separated flow vs shock induced separation S L =f L/U % par 4! 7 6 ±20% 6 S L 0.06 S L * M Shock reflection, compression corner, blunt fins, overexpanded nozzles M=0 3 Mach M 0 5 S L Φ(M c )g(r,s) = L/h 3 Aspect ratio of the bubble

17 Oblique Shock Reflection over a Heated Wall: compresssibility vs density effects PIV data: rms normal velocity θ=8 cell size: 0.8mmx0.4mm 2 T w /T r =1 T w /T r =2 Increase of interaction length ( 50%) when heated for constant deviations

18 Oblique Shock Reflection over a Heated Wall: time scales no wall sensors external scanning: characteristics (LF) and Mach waves (MF) Agostini et al. AIAA 2012 Shock (LF+MF + ) ML (MF + ) Shedding (MF - +BF) Heated wall

L =0.5 0.4 S L =0.03 0.35 0.3 0.25 ne(n) 0.2 0.15 0.1 f (Hz) 0.

19 Oblique Shock Reflection over a Heated Wall: time scales Hot wire data (external scanning): ü LF shock unsteadiness ü MF unsteadiness (ML) ü shedding ü downstream S L = S L = ne(n) f (Hz) 0.05 TURBINTERMED - ERCOFTAC Spring Festival avril Toulon X(mm) S L

SL Compressible mixing layer downstream separation shock M c = U a 1 + a 2 S L = S L φ(m c ) g(r, s) s=density ratio T w 1/2 0.03 T w -1 Φ(M c ) Linear stability analysis g(r,s) 0.025 0.02 0.

20 SL Compressible mixing layer downstream separation shock M c = U a 1 + a 2 S L = S L φ(m c ) g(r, s) s=density ratio T w 1/ T w -1 Φ(M c ) Linear stability analysis g(r,s) g(r,s) -15% s=1 s=0.56 (ad.) s=0.28 (heated) 1 M c Two effects of inverse influence! r r s=1 s=0.56(adiab.) s=0.28 (T w /T r =2)

21 Oblique Shock Reflection over a Heated Wall: time scales S L = S L φ(m f(hz) c ) g(r, s) Aspect ratio L/H as in adiabatic cases (6-7)

22 Effect of passive control: AJVG Attempt to control the interaction by upstream air injection: Generate streamwise vortices

23 Effect of passive control: AJVG (stereoscopic PIV data) Formation of counter rotating streamwise vortices demonstrated: Also known from subsonic literature for jets in crossflow Fric & Roshko (1994) Induction of a high and low speed modulation in the upstream boundary layer M c and s

24 Characterisation of the separation with AJVG Reduction in separation length for maximal downstream position of shock Reattachment point moved upstream with respect to reference (no AJVGs) Reduction in separation bubble height with jets: no more reverse flow at h=3mm Longitudinal mean velocity profile h=1mm reference no jets jet 0.4 bar, min separation jet 0.4 bar, max separation Longitudinal mean velocity profile h=3mm reference no jets jet 0.4 bar, min separation jet 0.4 bar, max separation 300 L: ref 350 Umean [m/s] h=1mm L: max L: min Umean [m/s] h=3mm X [mm] X [mm]

25 Frequency measurements: spectrum f.e(f) Increase in shock frequency (Lî ) S L unchanged No jets 200 Hz P 0jets =0.4 bar 300 Hz

26 Conclusions Ehrenstein JFM 2008 Ø Turbulent shock induced separation present large similarities with incompressible separated flows: in both cases, low and medium frequencies unsteadiness have to be related to the dynamics of the separated region. Same aspect ratios observed (6-7). Ø Similar convective coherent structures (S L =0.5 vs 0.7) Ø Mixing layer compressibility and density effects seem abble to predict the large difference between incompressible (S L 0.12) and compressible (S L 0.03) low frequency unsteadiness (ML flapping?) Ø Passive control change mainly upstream mean conditions (effects on L through δ 0 * and C f0 ) with limited effects on unsteadiness Ø Ø Similar mechanisms as in incompressible separation possible? (see Ehrenstein: several modes interacting) Incompressible results could suggest effective active control strategy. Mach number influence? Density influence?

27 Thank you for your attention!

Boundary Layer Interactions

Unsteadiness Unsteadiness of of Shock Shock Wave Wave / / Turbulent Turbulent Boundary Layer Interactions Noel Clemens Department of Aerospace Engineering and Engineering Mechanics The University of Texas