Hysteresis in slanted-base-cylinder afterbody flows

Abstract

Afterbody flows such as those behind cargo aircraft type configurations display rich fluid dynamic behavior, including flow separation at the base of body, complex three-dimensional vortical structures and wake turbulence. A representative configuration for study is a cylinder with axis parallel to the freestream and an upswept base. At low upsweep angles, measured relative to the axis, the flow is characterized by a pair of streamwise vortices. As the angle is increased, the flow transitions to a wake closure. Experimental evidence shows that near the critical angle, (ϕ≃45°), either of the two patterns may be obtained, depending on the history of the flow i.e., the flow exhibits hysteresis. The features associated with this phenomenon are examined numerically using a validated Large-Eddy Simulation approach. A systematic campaign varying both Reynolds number and upsweep angle confirms the presence of hysteresis at ReD=25,000 and ϕ=45°. In the vortex regime, the three-dimensional separated flow from the sides rolls up into a horseshoe-type vortex with legs representing a streamwise oriented pair displaying coherent but counter-rotating vorticity field, while a diffused wake-like regime is obtained in the other. Sectional projected streamlines highlight the key kinematic differences between the two regimes. The change in base pressure due to very different separation patterns is analyzed to quantitatively examine the large drag disparity observed in experiments. Finally, instantaneous flowfields are used to obtain the shedding characteristics as well as the nature of wake turbulence in both regimes. The present study has relevance to flows in aerodynamic vehicles as well as in automobiles, and may aid in possible control strategies for drag reduction.