Flow Around a Simplified Car Body (Ahmed Body)

Experiments by H. Lienhart, S. Becker, C. Stoots

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Flow Configuration

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The experiments were performed in the LSTM low-speed wind-tunnel, which has a cross section of 1.87 × 1.4 m2. Figure 1 below illustrates the geometry of the Ahmed body.

Figure 1: Ahmed body geometry

The diameter of the stilts on which the body was mounted was 30mm. These extended into the floor of the tunnel, leaving a distance of 50mm between the ground and the Ahmed body.

Two slant angles for the rear part of the body were studied: 25o and 35o. In both cases, the length of the slanting section was the same (222mm). These angles were chosen to be either side of the critical angle of 30o at which separated flow occurs within the wake of the slant.

The geometry of the front section of the body is available in Ahmed-front-geo.dat.

Flow Parameters

Kinematic viscosity of air: 15 × 10-6 m2/s
Bulk velocity: Ub = 40 m/s
Height of body: h = 288 mm
Reynolds number based on height of body: Re = 768,000

Measurement Technique

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The experiments were performed in the LSTM low speed wind tunnel. The studies were conducted in a 3/4 open test section (ie. floor, but no sides or ceiling) with a blockage ratio of 4%. The wind tunnel can generate flow velocities from 3 to 55 m/s with average turbulence intensities of less than 0.25%, and all measurements for the Ahmed body were taken at bulk air velocities of 40 m/s. A computer-based feedback system was utilised to ensure constancy of the test section bulk velocity and air temperature.

Hot-wire measurements of the velocity profiles 400 mm upstream of the AHmed body were obtained to serve as inlet conditions for numerical simulations. These were performed with a two-component hot-wire system, which was rotated to obtain the third component.

Flow visualization using oil streaks was performed for both model shapes. These showed complex three-dimensional flow patterns and confirmed earlier findings that a small change in the slant angle around the critical 30o causes a dramatic change in the flow pattern.

A two-component LDA system was used to make measurements of all three velocity components in the symmetry plane from upstream of the body to some distance downstream behind the closure of the wake. LDA measurements were also made in several transverse planes in the wake.

Available Experimental Data

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Previous and Reference Numerical Solutions

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The case has been studied in both the 9th and 10th ERCOFTAC (SIG-15)/IAHR workshops on Refined Turbulence Modelling.

Related Publications

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The measurements were performed withinthe European project MOVA (Models for Vehicle Aerodynamics, 1998-2001).

  1. Ahmed, S.R., Ramm, G. (1984) Some salient features of the time-averaged ground vehicle wake, SAE Technical Paper 840300.
  2. Lienhart, H., Stoots, C., Becker, S. (2000) Flow and turbulence structures in the wake of a simplified car model (Ahmed model), DGLR Fach Symp. der AG STAB, Stuttgart University.
  3. Lienhart, H., Becker, S. (2003) Flow and Turbulence Structure in the Wake of a Simplified Car Model, SAE Paper 2003-01-0656, Detroit.