BME Department of Fluid Mechanics, Conference on Modelling Fluid Flow (CMFF’12)

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The unsymmetric flow around the Ahmed body
Walter Meile, Thomas Wanker, Günter Brenn

Last modified: 2012.05.22.

Abstract


The Ahmed body has become a standard body of reference for aerodynamic flows around passenger vehicles. The literature shows a variety of data and simulations of the flow, in particular for symmetric conditions, i.e., with zero yawing angle. For this situation, measurement data have widely served as benchmarks for the validation of simulation tools.

The present paper reports about results from an extensive experimental study of the flow around the Ahmed body for various yawing angles. The Ahmed body was equipped with rear geometries at two different slant angles. The experiments were carried out in the 2m model wind tunnel of the institute at Reynolds numbers between 0.69×106 and 2.79×106. Further to force measurements, the flow was visualized and measured by means of particle-image velocimetry (PIV).

The experiments revealed an instability of the flow at a yawing angle around 12°, which leads to temporal fluctuations of the aerodynamic forces acting on the body. The flow turns out to be bi-stable. The temporal characteristics of the change between the two states of flow reveal no periodicity, but rather chaotic behaviour. The simultaneous measurement of forces and flow field allow the physical reasons for the fluctuating forces to be identified.

The paper will present all details of the experiments and the results, which allow the dynamic behaviour of the Ahmed body under asymmetric flow conditions to be predicted with high accuracy.


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