Aerodynamics: Flow around the Ahmed Body

This validation case belongs to fluid mechanics, representing the aerodynamics of the Ahmed body study. The aim of this test case is to validate the following parameters:

Drag coefficient computation
Velocity profiles

The simulation results of SimScale were compared to the experimental data presented in [Ahmed]$^1$.

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Geometry

The geometry is created based on the simplified aerodynamic body used by Ahmed et al$^1$. See Figure 1 for dimensions and Figure 2 for the geometry. The slant angle ($\phi$) is set to 25°. The body is placed in a wind tunnel 6 $m$ x 5 $m$ x 13.5 $m$ in order to limit the aerodynamic blockage effect.

ahmed body geometry — Figure 2: Three-dimensional view of the geometry used in the study

Analysis Type and Mesh

Tool Type: OpenFOAM®

Analysis Type: Turbulent Incompressible fluid flow

Mesh and Element Types:

Mesh	Mesh Type	Number of Cells
Mesh 1	Standard	3,746,887
Mesh 2	Standard	6,915,859
Mesh 3	Standard	10119321

Table 1: Mesh details

The Standard Mesher algorithm with tetrahedral and hexahedral cells was used to generate the mesh, with refinements near the walls and in the wake region (see Figure 3).

ahmed body simulation mesh — Figure 3: Mesh #3 of the three meshes investigated in the Ahmed body validation case

A typical property of the generated mesh is the $y^+$ (“y-plus“) value, which is defined as the non-dimensionalized distance to the wall, learn more. A $y^+$ value of 1 would correspond to the upper limit of the laminar sub-layer.

Wall treatment

Full Resolution in the near-wall region: The first cell lies at most at the boundary of the laminar sub-layer and no further. Here, $y^+$ value is 1 or below.
Use of wall-functions to resolve the near-wall region: There is no need to place cells very close to the laminar sub-layer, and typically $30 \le y^+ \le 300$.

An average $y^+$ value of 1 was used for the inflation layer around the body, and 150 for the floor. The $k-\omega$ SST turbulence model was chosen, with full resolution for near-wall treatment of the flow around the body and with wall function for the floor.

Simulation Setup

Material

Fluid

Air with a kinematic viscosity of 1.5 x 10^-5 $kg/ms$ is assigned as the domain fluid.

Boundary Conditions

The boundary conditions for the simulation are shown in Figure 4 below:

Boundary Condition	Face	Value
Velocity inlet $[m/s]$ Turb. kinetic energy $[m^2/s^2]$ Specific dissipation rate $[1/s]$	Inlet	60 0.135 180.1
Pressure outlet $[Pa]$	Outlet	0 (Fixed gauge pressure)
Slip wall	Side and top faces	–
No slip wall – Wall function	Bottom face (Ground)	–
No slip wall – Full resolution	Car body	–

Table 2: Boundary Conditions for the Ahmed Body simulation

The free stream velocity of the simulation is 60 $m/s$, so that the Reynolds number based on the length of the body $L$ is 4.29e6. Those are the same values presented in the original experiment of Ahmed and Ramm$^1$.

Reference Solution

The experimental solution is presented in Figure 4 in the reference paper$^1$ giving the value for the drag force coefficient for the slant angle $\phi$ = 25°:

$$ C_{d} = 0.2875 $$

Result Comparison

Drag Coefficient

The drag force is defined as

$$ F_{d}={\frac {1}{2}}\rho \,U^{2}\,C_{d}\,A_x $$

where $A_x$ (0.115 $m^2$) is the projected area of the Ahmed body in the streamwise direction and $F_{d}$ the drag force. The drag force and drag coefficient were determined by the integration of surface pressure and shear stress over the entire Ahmed body (except for the 4 stilts acting as support).

Mesh #2 was selected due to its accuracy and favorable results in relation to the simulation time. The resulting drag coefficient of the Ahmed body, closest to the reference solution as yielded by Mesh #2, was computed to be 0.2915, which is within a 1.217 % error margin of the measured value.

Table 2 shows the result of the mesh independence study:

Mesh	DRAG FORCE $[N]$	DRAG COEFFICIENT	REFERENCE	ERROR [%]
Mesh #1	73.776	0.2985	0.2875	3.65
Mesh #2	72.043	0.2915	0.2875	1.217
Mesh #3	70.186	0.2835	0.2875	-1.39

Table 2: Results comparison and computed errors

The difference in the error percentage magnitude between Mesh #2 and Mesh #3 is 0.173, indicating that the drag coefficient values are acceptable.

Wake Flow Patterns

The velocity streamline contour of the mean flow obtained with the simulation is reported in Figure 5, together with experimental results of reference.

ahmed body wake velocity vectors — Figure 5: Velocity vectors and contours plotted with SimScale’s online post-processor

ahmed body wake experimental results — Figure 6: Experimental results for comparison showing a schematic of the streamlines over the Ahmed body

References

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

Note

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Last updated: August 29th, 2025

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