Strouhal Numbers of Unsteady Flow Structures Around a Simplified Car Model
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
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Abstract This work aims to gain relatively thorough understanding of unsteady flow structures around an Ahmed body and associated predominated Strouhal numbers StpffiffiAffi based on the square root of the frontal area A. Extensive hotwire and flow visualization measurements were conducted in a wind tunnel at RepffiffiAffi ¼ 6:59 104 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. StpAffiffiffi ¼ 0:196 and 0.140 are detected above the roof. StpAffiffiffi ¼ 0:196 is captured again over the rear window, along with another unsteady structure of StpAffiffiffi ¼ 0:265. Behind the vertical base, the wake is dominated by the quasiperiodic structures of StpffiffiAffi ¼ 0:442. A conceptual flow structure model is proposed based on the present data as well as those in the literature.
1 Introduction There has been recently a renewed interest in finding new technologies to reduce aerodynamic drag and hence fuel consumption in automotive industry because of the lasting high fuel costs in the past few years as well as the issue of global warming. Active control has been identified to be the most likely technique that may achieve substantial drag reduction further. The Ahmed body is perhaps the B. F. Zhang Y. Zhou (&) Department of Mechanical Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong e-mail: [email protected] S. To State Key Laboratory of Ultra-precision Machining Technology, Department of Industrial and System Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong Y. Zhou Shenzhen Postgraduate School, Harbin Institute of Technology, Shenzhen, China
Y. Zhou et al. (eds.), Fluid-Structure-Sound Interactions and Control, Lecture Notes in Mechanical Engineering, DOI: 10.1007/978-3-642-40371-2_26, Springer-Verlag Berlin Heidelberg 2014
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most widely used simplified vehicle model in the active drag reduction research (Vino et al. 2005; Thacker et al. 2010). One piece of the crucial information required for the active control of drag is the Strouhal number StpffiffiAffi of unsteady flow structure around the Ahmed model. Previously reported StpffiffiAffi is rather scattered, 0.18 * 0.50 over the rear window of u = 258 and 0.36–0.53 behind the base. This work aims to clarify this issue and conduct a relatively thorough investigation on StpffiffiAffi in flow around the Ahmed model based on extensive hotwire and flow visualization measurements.
2 Experimental Details Experiments were conducted in a closed-circuit wind tunnel, with a 2.4 m-long square test section (0.6 9 0.6 m). The 1/3 scaled Ahmed body was placed on a raised floor, which is used to control the boundary layer thickness. Figure 1 shows the dimensions of the model and the definition of the right-handed Cartesian coordinate system (x, y, z), with the origin O at the midpoint of the lower edge of the model vertical base. Measurements were conducted at a free-stream velocity (U?) of 8.33 m/s, corresponding to RepffiffiAffi ¼ 6:59 104 . A single ho
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