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ISSN オンライン: 2642-0554

SMALL SCALE PRODUCTION IN THE COHERENT STRUCTURES OF A SHEAR FLOW OVER AN OPEN CAVITY

Laurent Jacquin
Department of Fundamental and Experimental Aerodynamics ONERA, 8, rue des Vertugadins, 92190 Meudon- France

Nicolas Forestier
Department of Fundamental and Experimental Aerodynamics ONERA, 8, rue des Vertugadins, 92190 Meudon- France

Philippe Geffroy
Department of Fundamental and Experimental Aerodynamics ONERA, 8, rue des Vertugadins, 92190 Meudon- France

要約

The present study aims at showing an example of shear flow in which small scale perturbations are directly produced inside strongly organized vortex or coherent structures. The flow under consideration is a mixing layer which develops over a cavity in a high speed flow where it is forced by a low frequency acoustic resonance. The mixing layer is observed to organize itself into highly coherent 2D vortices which have been scrutinized by means of a conditional LDV technique as in Lyn & Rodi (1995). When considering the triple decomposition of the flow velocity which discriminates the "coherent" motion (phase average) from the random motion, it is observed that the latter concentrates into the vortex cores. A detailed analysis of the flow indicates that a linear elliptic instability mechanism could be responsible of this small scale production. This departs from the classical picture where small scale turbulence is preferentially produced by continual stretching of three-dimensional vorticity in the saddle regions between vortical structures. This difference could be attributed to the highly coherent organization of the vorticity which is found in a cavity flow and which is rarely encountered in other forced mixing layers.