Control of the antisymmetric mode (m = 1) for high Reynolds axisymmetric turbulent separating/reattaching flows

Abstract

The separated flow over a three-dimensional axisymmetric step controlled by means of continuous jets is investigated numerically at a high subsonic regime using Zonal Detached Eddy Simulation (ZDES). The main objective is here to analyze the influence of two controlled devices acting in different regions of the separated flow, i.e., in the shear layer or in the recirculation area. Contrary to most flow control strategies that aim at reducing the drag, the final purpose of this study consists in controlling the antisymmetric azimuthal mode (m=1) responsible for side loads occurring on a massively separated afterbody. Thus, the design of controlled cases has been motivated both by a literature review which is detailed but especially by the previous identification of a potential area of receptiveness linked to an absolutely unstable area. The related achievement expected lies in increasing the three-dimensionality of the flow but decreasing its large scale coherence. For both controlled configurations, instantaneous, statistical, and fluctuating properties are scrutinized. In particular, the analysis exhibits that the backflow has to be considered stronger in time compared to its mean and broadly distributed in space which impacts the amount of energy needed to counteract this backflow and thus to control the flow. Then, a two-point spectral analysis is performed on the entire wall of interest of the afterbody providing a spatial representation of the fluctuating energy for a selected frequency. This last investigation permits to understand the influence of the control on the mechanisms related to the dimensionless frequency StD=0.2 and its associated antisymmetric mode. Finally, this analysis permits to predict the effect of the control on the change in the nature of the instability contained in the potentially receptive zone transitioning from fully absolute to fully convective.