한국어

Car External Flow

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.

In [Mesh Regions] select boundaries
- Bottom, internalMesh, leg, miny, nose1, nose2, nose3, nose5, rear, side, slant, top

Change solid color to p_rgh.

Streamline

Draw the streamline of the flow around the vehicle.

Utilize the [Extract block] function to extract the geometry of the vehicle and floor as shown below.

Change p to [Solid Color].

In the Pipeline Browser on the left, click baram.foam once to activate it.

Click [Stream Tracer] icon.

Change settings as follows

Seed Type : Point Cloud
Center : (0.7, 0.1, 0.1)
Radius : 0.2
Number of Points : 100
Coloring : Vorticity

You can see streamline as shown below.

DriveAer

Download mesh

Download simulation

Introduction

DrivAer is a full-scale vehicle model for exterior design and aerodynamic testing used in the field of automotive engineering to simulate and evaluate the exterior shape and aerodynamic characteristics of vehicles. The model was introduced to bridge the gap between simplified models and highly complex production vehicles, and CAD files and experimental results for many types of geometries are publicly available.

This example uses a fastback model with side mirrors and wheels.

https://www.epc.ed.tum.de/en/aer/research-groups/automotive/drivaer/

Use steady-state incompressible flow conditions with moving ground and rotating wheel.

The experimental results in the paper show that drag coefficient(Cd) is 0.247 for ASME and 0.243 for SAE, and the simulation results show that Cd = 0.243, which is consistent with the SAE experimental results.

The simulation conditions are as follows

solver : buoyantSimpleNFoam
turbulence model : \(Realizable\) \(k-\epsilon\) model
density : 1.205 \(kg/m^3\)
viscosity : 1.82e-5 \(kg/ms\)
flow condition : 30 \(m/s\) at inlet