AhmedBody
Introduction
S.R. Ahmed used a simplified automobile model to experimentally observe how the flow structure changes with the angle of inclination of the rear. Since then, this problem has been used to validate automotive external aerodynamic analyses. In this example, a mesh for external flow analysis is created for a geometry with a 25° rear tilt angle.
ref : S.R. Ahmed, G. Ramm, Some Salient Features of the Time-Averaged Ground Vehicle Wake, SAE-Paper 840300, 1984
Geometry
Import geometry
Use given ahmed.stl file as geometry.
Click the [Import] button at the bottom to select the ahmed.stl file.
When the geometry file is read, a window appears as shown below.
Create farfield boundary for the external flow analysis of a car.
Click the [Add] button and select [Hex6] and set values as follows
- Name : Hex6_1
- Type : None
- MIn. : (-4.5 -0.05 0)
- Max. : (10 5 3)
Since it is a left-right symmetric shape, we give it a minimum z coordinate of 0 to make the mesh only half in the +z direction.
In Geometry, select a face and right-click and select Edit/View. In the window that opens, change the name as follows
- Hex6_1_xMin → inlet
- Hex6_1_xMax → outlet
- Hex6_1_yMin → wall
- Hex6_1_yMax → sky
- Hex6_1_zMin → left_side
- Hex6_1_zMax → right_side
Create hexahedra for refinement
To specify a volume to refine mesh, click the [Add] button and select [Hex] and set values as follows
- Name : Refinement1
- Type : None
- MIn. : (-0.6 -0.05 0)
- Max. : (1 0.4 0.27)
A Refinement1_surface is created in [Geometry]. Select this and right-click Edit. Change the [Type] to [None].
Once again, click the [Add] button and select [Hex] and set values as follows
- Name : Refinement2
- Type : None
- MIn. : (-0.9 -0.05 0)
- Max. : (1.7 0.6 0.5)
A Refinement2_surface is created in [Geometry]. Select this and right-click Edit. Change the [Type] to [None].
The final geometry looks like this
Click the [Next] button to move on to the next step.
Region
Click the (+) icon at the top to create a region. Move the mouse to the intersection of the lines that appear in light green color in the graphics window and position it inside the computation domain. Click the [Add] button to complete the setup.
Click the [Next] button to move on to the next step.
Base Grid
Select [Use Hex6] option and set the number of grids to 100, 35, and 20. Click the [Generate] button to generate the base grid.
Click the [Next] button to move on to the next step.
Castellation
Use default conditions for [Configuration] and [Advanced].
Define mesh refinement for Refinement1.
Click the (+) icon under [Volume Refinement] and set the following settings for Refinement1
- Group Name : Refinement1
- Volume Refinement Level : 5
- Volume : refinement1
Once again, click the (+) icon and set the following settings for Refinement2
- Group Name : Refinement2
- Volume Refinement Level : 3
- Volume : refinement2
To enable parallelization, click [Parallel]-[Environment] in the menu and enter the desired value for [Number of Cores].
Click the [Refine] button and the castellation step starts.
Click the [Next] button to move on to the next step.
Snap
Use the default settings and click the [Snap] button to start snap.
Click the [Next] button to move on to the next step.
Boundary Layer
Click the (+) icon under [Configuration] to add a [Setting], and set it as follows
- Number of Layers : 5
- Thickness Model Specification : First and Expansion
- Size Specification : relative
- First Layer Thickness : 0.1
- Expansion Ratio : 1.2
- Min. Total Thickness : 0.3
- Boundary : nose1, nose2, nose3, nose5, rear, side, slant, top, leg, bottom
Click [Apply] button to start boundary layer step.
The final mesh looks like this
Click the [Next] button to move on to the next step.
Export
Click [Export as BaramFlow project] to export the mesh to the desired location.