The wakes of Ahmed bodies with varying aspect ratio
Venning, J and Burton, D and Lo Jacono, D and Thompson, M and Sheridan, J, The wakes of Ahmed bodies with varying aspect ratio, Book of Abstracts of the 9th European Fluid Mechanics Conference, 9-13 September 2012, Rome, Italy, pp. 98. (2012) [Conference Extract]
An Ahmed body is a generalized three-dimensional model that generates a similar
flow structure to that of road vehicles. One of the key features of this flow is the
generation of C-Pillar vortices off the rear slant edges. These vortices form a pair of
large, counter-rotating, unsteady structures of a conical shape which are generated as
the flow moves across the rear slant. It has been demonstrated that these vortices
play a significant role on the drag force acting on the body. The effect on the drag
by separating these vortices from each other (by increasing the width of the body) is
therefore of interest.
The findings are presented of an investigation into the wakes of 25.7% scale Ahmed
bodies with slant angles of 25◦ and with varying widths: 0.6, 1, and 1.25 times the
standard Ahmed dimensions (denoted Narrow, Standard and Wide respectively).
The models were tested in a water channel with flow speeds up to 0.5 m/s. Particle
image velocity (PIV) was used in the spanwise direction (ie. yz-plane) which allowed
the capture of the C-Pillar vortices. Figure 1 shows the velocity field and vorticity
in this plane. With increased body width, there is a decrease in the strength of
the C-Pillar vortex; additionally, a large vortex of opposite sign is evident below the
The effect of the aspect ratio on the drag force will also be discussed. In addition
to the mean velocity field, time-resolved PIV was gathered, allowing the dynamics
of the vortices to be analysed. The analysis of the frequency spectra throughout the
wake will be presented and discussed.
Ahmed body, vehicle aerodynamics, vortex flow, PIV, dynamics