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 ( $6 m \times 5 m \times 13.5 m$

6 m \times 5 m \times 13.5 m

) in order to limit the aerodynamic blockage effect.

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.).

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 $y^{+} = u^{*} y / ν$

y^{+} = u^{*} y / 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, $y^{+}$
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 $30 ⩽ y^{+} ⩽ 300$

An average $y^{+}$

y^{+}

value of 1 was used for the inflation layer. The $k - ω$

k - ω

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	$38 m i l l i o n$ $38 m i l l i o n$	3D Hex

Simulation Setup

Fluid:

Air with kinematic viscosity of $1.5 \times 10^{- 5} k g / m s$

1.5 \times 10^{- 5} k g / m s

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	$63.7 m / s$ $63.7 m / s$ (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 $R e = 1.2 \times 10^{6}$

R e = 1.2 \times 10^{6}

. 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

$C D = F d / 0.5 \times r h o \times U 2 \times A x$

C D = F d / 0.5 \times r h o \times U 2 \times A x

where $A x (0.112 m^{2})$

A x (0.112 m^{2})

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]	(1, 2, 3, 4) 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.

Last updated: November 25th, 2019

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