Ahmed body

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Latest revision as of 05:46, 6 October 2015

Brief Description

Fig. 1: Ahmed model. Dimensions are in mm (Fig. from [4])

The Ahmed body was described originally by S. R. Ahmed in 1984 [1]. Three main features were seen in the wake:

1. The A recirculation region that is formed as the flow separates at the top of the vertical back surface of the model

2. The B recirculation region that is formed due to the separation at the base of the model.

3. The c-pillar vortices that form as the vorticity in the side boundary layers roll up over the slant edges.

The wake was shown to be highly dependent on slant angle. For slant angles less than 12°, the flow remains attached over the the slant. The flow is essentially two-dimensional and has low drag. Between 12° and 30° the flow becomes much more three-dimensional as the c-pillar vortices form. These reach maximum strength at 30°. The drag increases significantly as the low pressure cores act on the rear surfaces. Past 30° the flow separates fully off the slant. This results in a sudden decrease in drag and weaker c-pillar vortices.

Effect of Aspect Ratio: Venning et al [7] showed that the aspect ratio of the rear slant had a significant effect on the wake. The wider bodies ceased to reattach at slant angles of 25°, suggesting that the critical angle lowers as aspect ratio increases. They also provide vortex core location from experimental data that can be used for validation [8].

Description of the test case: The Ahmed body (Fig. 1) was first defined and its characteristics described in the experimental work of Ahmed [1]. Two configurations with slant angles of 25°and 35°are considered as a test case. For this configurations detailed LDA Measurements have been performed by Becker, Lienhart and Stoots [2,3] in the LSTM low-speed wind-tunnel with a cross-section of 1.87x1.4 m2 (width x height) with a bulk velocity of 40 m/s. The test-section of the wind-tunnel was 3/4 open (only ground plate present). The distance between the body and the plate representing the ground is 50 mm. In the experiment perform by the Ahmed [1], flow velocity was taken 60 m/s, Reynolds number was 4.29 million based on model length.

References:

[1] S.R. Ahmed, G. Ramm, Some Salient Features of the Time-Averaged Ground Vehicle Wake, SAE-Paper 840300, 1984

[2] H. Lienhart, S. Becker, Flow and Turbulence Structure in the Wake of a Simplified Car Model, SAE 2003 World Congress, SAE Paper 2003-01-0656, Detroit, Michigan, USA, 2003

[3] H. Lienhart, C. Stoots, S. Becker, Flow and Turbulence Structures in the Wake of a Simplified Car Model (Ahmed Model), DGLR Fach Symp. der AG STAB, Stuttgart University, 15-17 Nov., 2000

[4] C. Hinterberger, M. García-Villalba, W. Rodi, Large Eddy Simulation of flow around the Ahmed body. In "Lecture Notes in Applied and Computational Mechanics / The Aerodynamics of Heavy Vehicles: Trucks, Buses, and Trains", R. McCallen, F. Browand, J. Ross (Eds.), Springer Verlag, ISBN: 3-540-22088-7, 2004

[5] S. Krajnovic, L. Davidson, Large eddy simulation of the flow around a simplified car model, SAE 2004 World Congress, SAE Paper 2004-01-0227, Detroit, Michigan, USA, 2004

[6] M. Minguez, R. Pasquetti, E. Serre, High-order large-eddy simulation of flow over the “Ahmed body” car model, Phys. Fluids, 20, 9, 2008

[7] Venning, J., Lo Jacono, D., Burton, D., Thompson, M., and Sheridan J., The effect of aspect ratio on the wake of the Ahmed body. Experiments in Fluids 56, 2015.

[8] Venning, J., Vortex locations for the longitudinal structures in the wake of the Ahmed body. DOI: 10.13140/RG.2.1.1301.4882