Strouhal Numbers of Unsteady Flow Structures Around a Simplified Car Model

Abstract

This work aims to gain relatively thorough understanding of unsteady flow structures around an Ahmed body and associated predominated Strouhal numbers \( {\text{St}}_{\sqrt A } \) based on the square root of the frontal area A. Extensive hotwire and flow visualization measurements were conducted in a wind tunnel at \( \text{Re}_{\sqrt A } = 6.59 \times 10^{4} \) over the roof, the rear window, and behind the vertical base of this body with a slant angle of 25°. Four distinct Strouhal numbers have been identified. \( {\text{St}}_{\sqrt A } = 0.196 \) and 0.140 are detected above the roof. \( {\text{St}}_{\sqrt A } = 0.196 \) is captured again over the rear window, along with another unsteady structure of \( {\text{St}}_{\sqrt A } = 0.265 \). Behind the vertical base, the wake is dominated by the quasi-periodic structures of \( {\text{St}}_{\sqrt A } = 0.442 \). A conceptual flow structure model is proposed based on the present data as well as those in the literature.