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LOW SPEED STREAKS INSTABILITY OF TURBULENT BOUNDARY LAYER FLOWS WITH ADVERSE PRESSURE GRADIENT

Jean-Philippe Laval
CNRS, UMR8107, F-59650 Villeneuve d'Ascq, France, Univ Lille Nord de France, F-59000 Lille, France

Matthieu Marquillie
Université de Nice-Sophia Antipolis Laboratoire J.-A. Dieudonné Pare Valrose; F-06108 Nice Cedex 2; CNRS, UMR 8107, F-59650 Villeneuve d Ascq; Univ Lille Nord de France, F-59000 Lille, France

Uwe Ehrenstein
Université de Nice-Sophia Antipolis Laboratoire J.-A. Dieudonné Pare Valrose; F-06108 Nice Cedex 2; Aix Marseille Univ, IRPHE UMR 6594, CNRS, F-13384 Marseille 13, France

Sinopsis

A direct numerical simulation (DNS) of a turbulent channel flow with a lower curved wall is performed at Reynolds number Reτ ~ 600 at inlet. Low-speed streak structures are extracted from the turbulent flow field and base flows formed with conditional streaks averages, superimposing the mean streamwise velocity profile, are used for linear stability analyzes. The instability in the presence of a strong pressure gradient at the lower wall is shown to be of varicose type, whereas at the upper wall (with weak pressure gradient) varicose and sinuous unstable modes can coexist. The onset of the dominant instability mechanism of varicose type is shown to coincide with the strong production peaks of turbulent kinetic energy near the maximum of pressure gradient on both the curved and the flat walls. The size and shape of the counter-rotating streamwise vortices associated with the instability modes are shown to be reminiscent of the coherent vortices emerging from the streak skeletons in the direct numerical simulation.