Unsteady flow structures around a high-drag Ahmed body

Abstract

This work aims to gain a relatively thorough understanding of unsteady predominant coherent structures around an Ahmed body with a slant angle of $25^{\circ }$, corresponding to the high-drag regime. Extensive hot-wire, flow visualization and particle image velocimetry measurements were conducted in a wind tunnel at $\mathit{Re}=(0.45{-}2.4)\times 10^{5}$ around the Ahmed body. A number of distinct Strouhal numbers (St) have been found, two over the rear window, three behind the vertical base and two above the roof. The origin of every St has been identified. The two detected above the roof are ascribed to the hairpin vortices that emanate from the recirculation bubble formed near the leading edge and to the oscillation of the core of longitudinal vortices that originate from bubble pulsation, respectively. The two captured over the window originate from the hairpin vortices and the shear layers over the roof and side surface, respectively. One measured in the wake results from the structures emanating alternately from the upper and lower recirculation bubbles. The remaining two detected behind the lower edge of the base are connected to the cylindrical struts, respectively, which simulate wheels. These unsteady structures and corresponding St reconcile the widely scattered St data in the literature. The dependence on Re of these Strouhal numbers is also addressed, along with the effect of the turbulent intensity of oncoming flow on the flow structures. A conceptual model is proposed for the first time, which embraces both steady and unsteady coherent structures around the body.