Jérémy Basley An Experimental Investigation on Waves and Coherent Structures in a Three-Dimensional Open Cavity Flow

A space-time study of a three-dimensional nonlinearly saturated open cavity flow is undertaken using time-resolved space-extended experimental data, acquired in both cross-stream and spanwise planes, in incompressible air and water flows. Through multiple modal decompositions in time and space, the waves and coherent structures composing the dynamics in the permanent regime are identified and characterised with respect to the instabilities arising in the flow. Effects of nonlinearities are thoroughly investigated in the impinging shear layer, regarding the self-sustained oscillations and their interactions with the inner-flow. In particular, the analysis conducted throughout the parameter space enlightens a global connection between the selection of locked-on modes and the amplitude modulation at the impingement and the mode switching phenomenon. Furthermore, observations of low frequencies interacting drastically with the shear layer flapping motion underline the existence of intrinsic coherent three-dimensional dynamics inside the cavity in spite of the shear layer disturbances. Linear stability analyses have demonstrated the onset of centrifugal instabilities along the main recirculation. In the present investigation, we focus on the dynamics after saturation occurred. It reveals numerous space-time coherent structures, whose properties are quantified and classified with respect to the underlying instabilities. We observe travelling or standing spanwise waves, as well as steady structures. Finally, some patterns exhibited by the saturated structures suggest that the nonlinear mechanisms governing the mutations of the flow after the linear regime could gain more insight in the frame of amplitude equations.