Required field
Required field
Required field

# 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 2. See Fig.1.a for dimensions and Fig.1.b for the geometry. The slant angle ($$\psi$$) is set to 25 degrees. The body is placed in a wind tunnel ($$6 m \times 5 m \times 13.5 m$$) 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-plus“) value, which is defined as the non-dimensionalized distance to the wall; it is given by $$y^+ = u^*y/\nu$$. A $$y^+$$ 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, $$y^+$$ 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. The $$k-\omega$$ 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:

## Simulation Setup

Fluid

Air with kinematic viscosity of $$1.5 \times 10^{-5} kg/ms$$ is assigned as the domain fluid. The boundary conditions for the simulation are shown in Table 3.

Boundary Conditions

The free stream velocity of the simulation is $$63.7\ m/s$$, so that the Reynolds number based on the height of the body $$H$$ is $$1.2 \times 10^{6}$$. It is of the same order of magnitude used in the original experiment of Ahmed and Ramm 3.

## Results

Drag Coefficient

The drag coefficient is defined as

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

where $$A_x$$ ($$0.112m^2$$) 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 $$0.306$$ which is within a $$2.86%$$ error margin of the measured value of $$0.298$$ 4.

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 5.