Experimental Investigation of the Bi-stable Behavior in the Wake of a Notchback MIRA Model 2019-01-0663

This paper reports an experimental investigation of the wake flow behind a 1/12 scale notchback MIRA model at Re=UL/ν=6.9×10^5(where U is free-stream velocity, L the length of the model and ν viscosity). Focus was placed on the flow asymmetry over the rear window and trunk of a notchback MIRA model. One hundred and three measurement points were used to map the pressure field, while the wake topology was investigated by means of 2D Particle Image Velocimetry. The analysis of the instantaneous pressure signals over the notch configuration clearly shows that the wake presents a bi-stable behavior in the horizontal direction, characterized by the switches between two preferred positions leading to a statically symmetric wake, but this phenomenon is not found in the vertical direction. The instantaneous wake dynamic highlights three forms that correspond to two bi-stable states and switch state respectively, confirming the analysis result of the instantaneous pressure signals. The sequence of these asymmetric states is random. In order to enable a deep insight of the bi-stable states, phase-averaged on the pressure data and the unsteady wake flow has been done to extract the two distinct states and switch state. The statistical analysis on the switch probability between the two states provide a comprehensive description of the mechanisms of the switch between the bi-stable states. Besides, this phenomenon is of a long timescale with the order of 600L/U, which is hundred times the common vortex shedding. High-frequency measurements were also carried out with a synchronized two-component hot-wire to study the vortex shedding behavior.