The flow structure around an Ahmed vehicle model and its active control

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The flow structure around an Ahmed vehicle model and its active control

 

Author: Zhang, Bingfu
Title: The flow structure around an Ahmed vehicle model and its active control
Degree: Ph.D.
Year: 2014
Subject: Automobiles -- Aerodynamics
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Mechanical Engineering
Pages: xx, 143 leaves : illustrations ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2763006
URI: http://theses.lib.polyu.edu.hk/handle/200/7780
Abstract: The lasting high fuel cost has recently inspired resurgence in drag reduction research for vehicles. This work aims to gain a relatively thorough understanding of unsteady predominant coherent structures around an Ahmed vehicle model with a rear slant angle of 25° corresponding to the high-drag regime and, on the basis of this understanding, to explore an effective active technique to reduce its aerodynamic drag. Extensive hotwire, particle image velocimetry (PIV) and flow visualization measurements were conducted around this body in a wind tunnel at Re = 4.5 × 10⁴ -2.4 × 10⁵ based on the square root of the frontal area A of the 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 each St has been identified. Two detected above the roof are ascribed to the hairpin vortices emanated from the separation bubble formed near the leading edge and the oscillation of the core of longitudinal vortices originated from the bubble pulsation, respectively. Two captured over the window originate from the hairpin vortices and the shear layer over the side surface, respectively. One measured in the wake results from the alternate emanation of structures from the upper and lower recirculation flow regions, respectively. Two detected behind the lower edge of the base are due to the lower and gap vortices, respectively. These unsteady structures and corresponding St provide an explanation on the rather scattered St data in the literature. The dependence on Re of these Strouhal numbers is also addressed. A conceptual model for the flow structure around the body is proposed.
The quasi-periodical organized structures emanated from the two recirculation flow regions behind the base have been found to be closely associated with the aerodynamic drag of the model. The ensemble-averaged velocities, streamlines, vorticity contours, and second moments of fluctuating velocities were then obtained and discussed to characterize this quasi-periodical structure. A physical process is proposed for the generation of the quasi-periodic structure, which is fully consistent with experimental observations. On the basis of the improved understanding of flow physics, an active control technique is developed using a combination of steady blowing over the rear window and behind the base. All active control experiments were conducted at Harbin Institute of Technology. The aerodynamic drag of the Ahmed body was measured using a force balancer at Re = 1.3 × 10⁵ - 2.0 × 10⁵. Steady blowing S1 was applied along the upper edge of the rear window, which has been demonstrated to be effective in weakening the separation bubble on the rear window. This actuation led to a maximum drag reduction by 11% at Re = 1.3 × 10⁵ and 12% at Re = 1.7 × 10⁵ and 2.0 × 10⁵. Steady blowing S2 was also deployed along two side edges of the rear window to break the well known longitudinal C-pillar vortices, and the drag can be reduced by around 6% at all the Reynolds numbers. The effectiveness of combining the two steady actuations S1 and S2 in reducing the drag was evaluated at Re = 1.7 × 10⁵, achieving a maximum drag reduction of 16%, higher than any previous reports based on steady blowing. The influence of steady blowing S₃ and S₄, applied along the upper and lower edges of the base, respectively, on the drag of the Ahmed body has been investigated. Results show that the drag can be either substantially reduced or increased, depending on the angle of the steady blowing relative to the free-stream velocity. The combination of S₃ and S₄ at their optimal blowing angles achieved an impressive drag reduction of 17% at Re = 1.7 × 10⁵. The drag reduction reached 25% with S₁, S₂ and S₃ all switched on and 28% with S₁, S₂ and S₄ all switched on, which is significantly higher than those reported in the literature, and is in fact very close to the target (30%) set by automotive industries (Bruneau et al. 2011). Interestingly, the drag reduction achieved was only 25% with all the four actuations S₁, S₂, S₃ and S₄ on. Finally, the net aerodynamic power savings produced by these active control approaches are estimated to evaluate the efficiency of the control.

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