1. Dynamique du recollement turbulent pendant la crise de traînée du cylindre circulaire Olivier Cadot Sanjay Mittal Aditya Desai Charad Saxena Brajesh Chandra National Wind Tunnel Facility IIT Kanpur, India

2. Oil print on the wall (Schewe 1986) Drag crisis (Roshko 1961) Bubbles transitions (Bearman 1969)

3. 10 m/s < U 0 < 75 m/s Turbulence intensity < 0.3% Pressure coefficient measurements Cp(t) = 2(p(t)-p 0 )/  U 0 2 Pressure scanner 32HD @ Measurement Specialties 32 ports, 500Hz per port NWTF

4. U0U0   =12° 0 30 ports for C p ( ,t)

5. Time response fc~150 Hz -2 Autopower spectrum C p (84°,t) subcritical régime (short time dynamics) bistable régime (long time dynamics) U Cp (84°,t) f (Hz) U=35 m/s U=59 m/s pressure scanner, sampling rate : 500Hz

6. Sectional drag and lift vs. Reynolds number Analysis of C p ( ,t ) vs. Re first transition one side reattachment both sides reattachment

8. Periodic dynamics tU/D~5

9. Symmetric disturbances having long time dynamics tU/D~5000 #1 #2 #1 #2

10. #1 #2 Symmetric perturbation #1 #2 Discrete transition leading to a bi- stable behavior. Larger adverse pressure gradient after the transition. Consistent with the symmetric reattachment of Higuchi et al. (1989).

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13. Conditionnal averaging over quadrants :

16. 50.5% 49% 0.5% 37% 1% 62%

17. Statistics of changes of states kdt =320 d/U Consistent with binomial law : Slope -2

18. Conclusion Reattachment is discrete and random : the mean pressure distribution is given by the time proportion of well defined states. Symetric and antisymmetric couplings are observable. AS coupling S coupling Re Due to reattachment somewhere else on the cylinder ? Dynamics along the span ?