cases:case082

# Flow Around a Simplified Car Body (Ahmed Body)

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

Fig. 1: Ahmed body geometry

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

Two slant angles for the rear part of the body are reported here: 25o and 35o. In both cases, the length of the slanting section was the same (222 mm). 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 curved section of the body is available in ahmed-front-geo.dat.

#### Flow Parameters

• Kinematic viscosity of air: $15 \times 10^{-6}$ m2/s
• Bulk velocity: $U_b = 40$ m/s
• Height of body: $h = 288$ mm
• Reynolds number based on height of body: $Re = 768,000$

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.

Measurements available include mean velocity and Reynolds stress profiles on a number of planes, and pressure measurements on the rear part of the body:

• LDA mean velocity and Reynolds stress profiles on a number of planes parallel and normal to the streamwise direction
• Hot wire boundary layer measurements of mean velocity and Reynolds stresses at a number of locations close to the body surface
• Surface pressure coefficients on the rear part of the body

Sample plots of selected quantities are available.

The data can be downloaded as a single compressed tar archive or as individual files from the tables below.

Note that in describing the coordinates at which the following data was taken, $x=0$ is the rear end of the body, $y=0$ is the symmetry plane and $z=0$ is the ground plane.

Geometry

The geometry of the front curved part of the body is given in ahmed-front-geo.dat.

Inlet Profiles

Inlet profiles of mean velocities, Reynolds stresses and triple moments are provided 400 mm upstream of the front end of the body at three $y$ locations:

 $y=0$ mm inlet-y000.dat $y=200$ mm inlet-y200.dat $y=400$ mm inlet-y400.dat

LDA Measurements

LDA measurements of mean velocities, Reynolds stresses and triple moments are available on a number of planes:

25o Slant Angle Case
$x$-$z$ Planes $y$-$z$ Planes
$y=0$ (whole plane) Ahmed-25-y000-whole.dat $x=-178$ mm Ahmed-25-xm178-yz.dat
$y=0$ (over slant) Ahmed-25-yp000-xz.dat $x=-138$ mm Ahmed-25-xm138-yz.dat
$y=100$ mm (over slant) Ahmed-25-yp100-xz.dat $x=-88$ mm Ahmed-25-xm088-yz.dat
$y=180$ mm (over slant) Ahmed-25-yp180-xz.dat $x=-38$ mm Ahmed-25-xm038-yz.dat
$y=195$ mm (over slant) Ahmed-25-yp195-xz.dat $x=0$ Ahmed-25-xp000-yz.dat
$y=-195$ mm (over slant) Ahmed-25-ym195-xz.dat $x=80$ mm Ahmed-25-xp080-yz.dat
$x=200$ mm Ahmed-25-xp200-yz.dat
$x=500$ mm Ahmed-25-xp500-yz.dat
Boundary layer profiles on symmetry plane (u,w components only): Ahmed-25-yp000-bl.dat
35o Slant Angle Case
$x$-$z$ Planes $y$-$z$ Planes
$y=0$ (whole plane) Ahmed-35-y000-whole.dat $x=-88$ mm Ahmed-35-xm088-yz.dat
$y=0$ (over slant) Ahmed-35-yp000-xz.dat $x=0$ Ahmed-35-xp000-yz.dat
$y=100$ mm (over slant) Ahmed-35-yp100-xz.dat $x=80$ mm Ahmed-35-xp080-yz.dat
$y=180$ mm (over slant) Ahmed-35-yp180-xz.dat $x=200$ mm Ahmed-35-xp200-yz.dat
$x=500$ mm Ahmed-35-xp500-yz.dat
Boundary layer profiles on symmetry plane (u,w components only): Ahmed-35-yp000-bl.dat

The above data can also be downloaded as Excel spreadsheets: ahmed_25_lda.xls and ahmed_35_lda.xls.

Hot Wire Boundary Layer Measurements

Hot wire measurements of the mean and rms velocity across the boundary layers on the central part of the body are available at the locations indicated in figure 2. They were performed in the 25o case, but are probably not very sensitive to the slant angle of the rear part.

Fig. 2: Hot wire measurement locations

Pressure Measurements

Pressure measurements over the rear part of the body:

25o Slant Angle Case 35o Slant Angle Case
Ahmed-25-press.dat Ahmed-35-press.dat

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

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

1. Ahmed, S.R., Ramm, G., Faltin, 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.

Indexed data:

case082 (dbcase, flow_around_body, semi_confined_flow)
case082
titleFlow around a simplified car body (Ahmed body)
authorBecker, Lienhart, Stoots
year2000
typeEXP
flow_tag3d, separated, bluff_body