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Aspect ratio and the dynamic wake of the Ahmed body

Citation

Venning, J and McQueen, T and Lo Janoco, D and Burton, D and Thompson, M and Sheridan, J, Aspect ratio and the dynamic wake of the Ahmed body, Experimental Thermal and Fluid Science: International Journal of Experimental Heat Transfer, Thermodynamics, and Fluid Mechanics pp. 1-10. ISSN 0894-1777 (In Press) [Refereed Article]

Copyright Statement

© 2021 Published by Elsevier Inc.

Abstract

The dynamic structures in the wake of the Ahmed body with a 25° slant angle were investigated. It is well established that the wake is dominated by two, counter rotating, streamwise vortex structures (C-pillar vortices). The influence these vortices have on the flow over the slant back was altered by modifying the width of the body between 60% to 120% of the standard width. The wake was measured using time-resolved particle-image velocimetry in several streamwise and spanwise planes for a height-based Reynolds number of 2.6 x 104. The wake measurements were decomposed with a spectral proper orthogonal decomposition technique, revealing two oscillatory modes. Firstly, the C-pillar vortical structures alternately contract and expand with a Strouhal number of 0.24, mildly dependent on the width. Secondly, the boundary layers on the sides of the body separate at the end of the body, forming shear layers with a dominant frequency of St = 2.30. Reducing the width of the body to 60% rotates the symmetry axis of the shedding mode. Furthermore, changing the spacing between the C-pillar vortices, by changing the width of the body, alters the nature of the spanwise vortical structures that form behind the base of the model. Intermediate width bodies produce two stable spanwise vortices. For wide bodies, the flow remains separated over the entire slant, and the top spanwise vortex does not form. Rather, the flow over the slant resembles a free shear layer with a Kelvin–Helmholtz-like instability leading to periodic shedding of small-scale vortex structures. Conversely, for narrow bodies, this phenomenon is observed at the bottom of the body and the lower spanwise vortex no longer forms while the flow over the slant remains attached.

Item Details

Item Type:Refereed Article
Keywords:Ahmed body, Trailing vortices, aspect ratio, time-resolved PIV, Spectral POD
Research Division:Engineering
Research Group:Fluid mechanics and thermal engineering
Research Field:Aerodynamics (excl. hypersonic aerodynamics)
Objective Division:Expanding Knowledge
Objective Group:Expanding knowledge
Objective Field:Expanding knowledge in the physical sciences
UTAS Author:Venning, J (Dr James Venning)
ID Code:146694
Year Published:In Press
Deposited By:NC Maritime Engineering and Hydrodynamics
Deposited On:2021-09-22
Last Modified:2021-10-13
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