Documentation

Aerodynamics: Flow around the Ahmed Body

Overview

The aim of this test case is to validate the following parameters of steady-state incompressible turbulent flow around the Ahmed Body with a pre-referenced case [1]:

  • Drag Coefficient
  • Velocity Profiles

The geometry is uploaded on to the SimScale platform and meshed using the snappyHexMesh tool. A fine resolution is provided behind the Ahmed Body to detect wakes.

See the validation project

Geometry

The geometry is created based on the simplified aerodynamic body used by Ahmed et al [1]. See Fig.1.a for dimensions and Fig.1.b for the geometry. The slant angle (ψ

ψ

) is set to 25 degrees. The body is placed in a wind tunnel (6m×5m×13.5m

6m×5m×13.5m

) in order to limit the aerodynamic blockage effect.

Fig.1.a. Dimensions of the Ahmed Body
Fig.1.b. Geometry used in the study

Analysis type and Domain

The snappyHexMesh tool was used to generate the mesh, with refinement near the walls and in the wake region (see Fig.2.).

Fig.2. Mesh used for the SimScale case

A typical property of the generated mesh is the y+

y+

 (“y-plus”) value, which is defined as the non-dimensionalized distance to the wall; it is given by y+=uy/ν

y+=uy/ν

. A y+

y+

 value of 1 would correspond to the upper limit of the laminar sub-layer.

  • 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+
    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 30y+300
    30y+300

     

    .

An average y+

y+

 value of 1 was used for the inflation layer. The kω

kω

 SST turbulence model was chosen, with full resolution for near-wall treatment of the flow.

Tool Type: OPENFOAM®

Analysis Type: simpleFoam

Mesh and Element types:

Table 1: Mesh Metrics
Mesh type Number of volumes Type
snappyHexMesh 38 million

38 million

 

3D Hex

Simulation Setup

Fluid:

Air with kinematic viscosity of 1.5×105kg/ms

1.5×105kg/ms

 is assigned as the domain fluid. The boundary conditions for the simulation are shown in Table 3.

Boundary Conditions:

Table 2: Boundary Conditions for Ahmed Body simulation
Parameter Inlet Top Face Bottom Face Lateral Faces Outlet Body
Velocity 63.7 m/s

63.7 m/s

 

Symmetry 63.7m/s

63.7m/s

(Moving Wall)

Symmetry Zero Gradient Full Resolution
k

k

 

21.9

21.9

 

Symmetry Wall Function Symmetry Zero Gradient Full Resolution
Omega 29215

29215

 

Symmetry Wall Function Symmetry Zero Gradient Full Resolution
Pressure Zero Gradient Symmetry Wall Function Symmetry 0 Pa Full Resolution

The free stream velocity of the simulation is U=63.7 m/s

U=63.7 m/s

, so that the Reynolds number based on the height of the body H

H

 is Re=1.2×106

Re=1.2×106

. It is of the same order of magnitude used in the original experiment of Ahmed and Ramm [1].

Results

Drag Coefficient

The drag coefficient is defined as

CD=Fd/0.5×rho×U2×Ax

CD=Fd/0.5×rho×U2×Ax

 

where Ax(0.112 m2)

Ax(0.112 m2)

 is the projected area of the Ahmed body in streamwise direction and FD the drag force. The time-averaged drag force was determined by integration of surface pressure and shear stress over the entire Ahmed body. The resulting drag coefficient of the Ahmed body was computed to be 0.306

0.306

 which is within a 2.86%

2.86%

 error margin of the measured value of 0.298

0.298

 [1].

Wake Flow Patterns

The velocity streamline contour of mean flow obtained with the simulation is reported in Fig. 4 together with experimental results of reference [2].

References

[1] (1234) S.R. Ahmed, G. Ramm, Some Salient Features of the Time-Averaged Ground Vehicle Wake, SAE-Paper 840300, 1984
[2] `Serre, Matthieu Minguez, Richard Pasquetti, Emmanuel Guilmineau, Gan Bo Deng, Michael Kornhaas, Michael Schäfer, Jochen Fröhlich, Christof Hinterberger, Wolfgang Rodi, On simulating the turbulent flow around the Ahmed body: A French–German collaborative evaluation of LES and DES.`_

Disclaimer

This offering is not approved or endorsed by OpenCFD Limited, producer and distributor of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks. OPENFOAM® is a registered trade mark of OpenCFD Limited, producer and distributor of the OpenFOAM software.

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