Ahmed Body

Introduction

S.R. Ahmed used a simplified automobile model to experimentally observe the change in flow structure as a function of rearward inclination angle. Since then, this problem has been used to validate automotive external aerodynamic analyses. This example uses steady-state incompressible flow conditions for a velocity of 40 m/s at a rearward tilt angle of 25°.

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

In the paper Drag coefficient(Cd) is 0.285. Simulation result is 0.287, 0.7% difference.

Simulation conditions are as follows.

solver : buoyantSimpleNFoam
turbulence model : $Realizable$ $k-epsilon$ model
density : 1.2 $kg/m^3$
viscosity : 1.8e-5 $kg/ms$
flow condition : 40 $m/s$ at inlet

Start BaramFlow and load mesh

Run the program and select [New Case] from the launcher. In the launcher, select [Pressure-based] for [Solver Type] and [None] for [Multiphase Model].

Use the given polyMesh folder. In the top tab, click [File]-[Load Mesh]-[OpenFOAM] in that order and select the polyMesh folder.

General

For this example, we’ll use default conditions.

Models

For this example, we’ll use $Realizable$ $k-epsilon$ model for turbulence.

Materials

Material properties of air is as follows

air
- Density : 1.2 𝑘𝑔/㎥
- Viscosity : 1.8e-5 𝑘𝑔/𝑚s

Boundary Conditions

Set the boundary type and values as shown below.

minx : velocity Inlet
- Velocity Specfication Method : Magnitude, Normal to Boundary
- Profile Type : Constant
- Velocity Magnitude : 40 (m/s)
- Turbulent Intensity : 1 (%)
- Turbulent Viscosity Ratio : 10

maxx : Pressure Outlet
- Pressure : 0 (Pa)

miny : Wall (Velocity Condition : Translation Moving Wall)
- Velocity : (40, 0, 0) (m/s)

bottom, leg, nose1, nose2, nose3, nose4, nose5, rear, side, slant, top : Wall
- Velocity Condition : No Slip

minz, maxz, maxy : symmetry

Reference Values

Set the Reference Value for the aerodynamic coefficient calculation as follows.

Area : 0.056(kg/m², (50% of the cross-sectional area perpendicular to the flow direction)
Density : 1.2 (kg/m³)
Length : 1 (m)
Pressure : 0 (Pa)
Velocity : 40 (m/s)

Numerical Conditions

In this example, we’ll change the settings as shown below.

Pressure-Velocity Coupling Scheme : SIMPLE
Discretization Scheme
- Pressure : Momentum Weighted Reconstruct
- Momentum : Second Order Upwind
- Turbulence : Second Order Upwind
Under-Relaxation Factors
- Pressure : 0.3
- Momentum : 0.7
- Turbulence : 0.7
Convergence Criteria
- Pressure : 0.001
- Momentum : 0.001
- Turbulence : 0.001

Monitor

Monitor the force coefficients of car.

Select [Monitors]-[Add]-[Forces] and set values as shown below.

Initialization

Set values as follows

Velocity
- X-Velocity : 40 (m/s)
- Y-Velocity : 0 (m/s)
- Z-Velocity : 0 (m/s)
Pressure
- 0 (Pa)
Turbulence
- Scale of Velocity : 40 (m/s)
- Turbulent Intensity : 1 (%)
- Turbulent Viscosity Ratio : 10

Enter the value and click the Initialize button at the bottom. Then click the [File]-[Save] menu to save the case file.

Run

In Run Conditions, set the following settings and proceed with the calculation.

Number of Iterations : 2000
Save Interval : 300
Data Write Format : Binary
Selct [Parallel]-[Environment] in menu. Set Number of Cores as you want and select [Local Machine] for [Parallel Type].

When the calculation is started, you can see the graphs of Residuals and Force monitor as shown below.

Post-processing

Scalar distribution at boundary

BARAM uses ParaView for post-processing. To start post-processing, click the ParaView button in [External Tools]. In this example, we will draw the pressure distribution and streamlines.

Change the Case Type to Decomposed Case.