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Flow structure around a low-drag Ahmed body

Published online by Cambridge University Press:  26 February 2021

K. Liu Open the ORCID record for K. Liu [Opens in a new window] ,
B.F. Zhang Open the ORCID record for B.F. Zhang [Opens in a new window] ,
Y.C. Zhang  and
Y. Zhou
K. Liu
Affiliation:
Center for Turbulence Control, Harbin Institute of Technology, Shenzhen518055, PR China
B.F. Zhang*
Affiliation:
Center for Turbulence Control, Harbin Institute of Technology, Shenzhen518055, PR China
Y.C. Zhang
Affiliation:
State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun130122, PR China
Y. Zhou*
Affiliation:
Center for Turbulence Control, Harbin Institute of Technology, Shenzhen518055, PR China
*
† Email addresses for correspondence: zhangbfu@hit.edu.cn, yuzhou@hit.edu.cn
† Email addresses for correspondence: zhangbfu@hit.edu.cn, yuzhou@hit.edu.cn
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Abstract

The wake of an Ahmed body may be divided into high- and low-drag regimes where the rear slant angle (φ) is in the ranges of 12.5°–30° and larger than 30°, respectively. This work aims to gain a relatively thorough understanding of unsteady predominant coherent structures around an Ahmed body of φ = 35° in the low-drag regime. Extensive hot-wire, wall pressure, flow visualization and particle image velocimetry measurements have been conducted at Reynolds number $Re \in [0.3,2.7] \times {10^5}$, based on the square root of the model's frontal area. A total of five distinct Strouhal numbers have been identified in the wake. One of them, Stw ≈ 0.30, is captured behind the vertical base, which is associated with the structures that emanate from the upper recirculation bubble and pinch off from the lower bubble, respectively. It is found that Stw scales with a characteristic length αS, which reflects physically the bubble size, and the Strouhal number $St_w^ + $ based on αS is a constant 0.20, irrespective of the value of φ. A corner vortex rolling upstream is observed near the lower end of the slanted surface, whose formation mechanism and dynamical role are discussed. The Reynolds-number effect on the flow is also documented. Based on the present and previously reported data, a conceptual flow structure model is proposed for a low-drag Ahmed body, including both steady and unsteady coherent structures around the body.

JFM classification

Wakes/Jets: Separated flows Vortex Flows: Vortex interactions Wakes/Jets: Wakes
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 913 , 25 April 2021 , A21
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Copyright
© The Author(s), 2021. Published by Cambridge University Press

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