Week 4 Challenge : CFD Meshing for BMW car

Aim:

For the given BMW car model, check the geometrical errors on half portion. Create and assign PID’s, surface mesh the half model and do symmetry to the other side.

Target length for the different parts of a model are as follows

• Body: 3mm
• Lights: 4mm
• Grills: 1mm
• Rims and Tyres: 2mm
• Optical Rear View Mirror: 2mm
• Window: 2mm
• Silencer: 3mm
• Logo: 1mm

Procedure:

• Open ANSA, change to CFD mode then click on the “?” symbol another window is opened in which scroll down you will find “-performance_mode 3” copy and paste it near the “?” symbol click ok.
• Drag and drop the Turbocharger model in the ANSA window.
• Check for the units of the model and also check the resolution, as the curves are not clear. Hence change 1mm length for perimeter.

• Click on Tools >> Checks >> Geometry >> Execute. Now we could see the error which we need to clean up in topology in order to resolve the error.
• It is necessary to resolve both the geometric and intersection errors.
• Since the model is symmetric in nature so delete one half of the model, do not delete the logo.

• In order to simulate the external flow over car we need to avoid multiple volume, so we need to create a single car volume model.

Grills:

• Leave only the desired grill to work and hide all other parts.
• Using various ANSA operation remove all single cons, Triple cons and Intersection.
• Try not to create small faces, as it may lead to Skewed elements while enabling the quality criteria.

Top Grill:

Bottom Grill:

Bottom Left Grill:

Logo:

Head Light:

Rear View Mirror:

Side Symbol:

Window:

Floor:

Wheels:

PID Assigning:

• Check the geometry check again for the model, all the intersection errors are resolved only Single cons at the symmetry plane is visible which will be eliminated when we mirror the other half.

• Let’s start creating the PID’s for the model. Open the PID table New >> Shell Property >> Provide name >> Ok, similarly provide all the names.
• Right click the PID name >> Apply >> Select that part >> Middle click the mouse. Similarly follow for all components.
• In order to delete unwanted PID Click Compress >> Ok.

• In this model we will switch ON the quality criteria to check the Skewness, maximum & Minimum length.
• Open Quality Criteria >> un check the unwanted criteria in the shell & solid. Further Click Open Edit Criteria Visibility >> un check the unwanted criteria’s >> Ok.

• Let’s start component wise surface mesh. We will start with the minimum element size and then proceed to maximum.

1mm Mesh: Grills & Logo

• From the PID table hide all the other component.
• Mesh >> Perimeter >> Spacing >> Auto CFD >> Perimeter >> Select the desired component >> Provide the Max. & Min. range for the element length >> Middle click mouse.
• Macros >> Select the desired component >> Provide the Max. & Min. range for the element length >> Middle click mouse >> OK. This Condition is provided to ANSA so that during mesh generation it is limited to the provided range.
• Mesh >> Length >> Perimeter >> 1mm >> Ok >> Macros >> 1mm >> Ok.
• Mesh Generation >> Choose Tria element.
• Mesh Generation >> CFD >> Visible. Mesh is created.

• Resolve the off element by Right on it solve, Reconstruct or Reshape >> Ok

• Now, Freeze the entire component, without doing so mesh generation will not happen at any of the component due to varying mesh size at the intersection. Hence, we will freeze the mesh component at the end of mesh generation.

2mm Mesh: Rims, Tyres, Rear View Mirror, Window

• From the PID table hide all the other component.
• Mesh >> Perimeter >> Spacing >> Auto CFD >> Perimeter >> Select the desired component >> Provide the Max. & Min. range for the element length >> Middle click mouse.
• Macros >> Select the desired component >> Provide the Max. & Min. range for the element length >> Middle click mouse >> OK. This Condition is provided to ANSA so that during mesh generation it is limited to the provided range.
• Mesh >> Length >> Perimeter >> 2mm >> Ok >> Macros >> 2mm >> Ok.
• Mesh Generation >> Choose Tria element.
• Mesh Generation >> CFD >> Visible. Mesh is created.

• Now, Freeze the entire component, without doing so mesh generation will not happen at any of the component due to varying mesh size at the intersection. Hence, we will freeze the mesh component at the end of mesh generation.

3mm Mesh: Body & Silencer

• From the PID table hide all the other component.
• Mesh >> Perimeter >> Spacing >> Auto CFD >> Perimeter >> Select the desired component >> Provide the Max. & Min. range for the element length >> Middle click mouse.
• Macros >> Select the desired component >> Provide the Max. & Min. range for the element length >> Middle click mouse >> OK. This Condition is provided to ANSA so that during mesh generation it is limited to the provided range.
• Mesh >> Length >> Perimeter >> 3mm >> Ok >> Macros >> 3mm >> Ok.
• Mesh Generation >> Choose Tria element.
• Mesh Generation >> CFD >> Visible. Mesh is created.

• Now, Freeze the entire component, without doing so mesh generation will not happen at any of the component due to varying mesh size at the intersection. Hence, we will freeze the mesh component at the end of mesh generation.

4mm Mesh: Lights

• From the PID table hide all the other component.
• Mesh >> Perimeter >> Spacing >> Auto CFD >> Perimeter >> Select the desired component >> Provide the Max. & Min. range for the element length >> Middle click mouse.
• Macros >> Select the desired component >> Provide the Max. & Min. range for the element length >> Middle click mouse >> OK. This Condition is provided to ANSA so that during mesh generation it is limited to the provided range.
• Mesh >> Length >> Perimeter >> 4mm >> Ok >> Macros >> 4mm >> Ok.
• Mesh Generation >> Choose Tria element.
• Mesh Generation >> CFD >> Visible. Mesh is created.

• Now, Freeze the entire component, without doing so mesh generation will not happen at any of the component due to varying mesh size at the intersection. Hence, we will freeze the mesh component at the end of mesh generation.

Symmetry Model:

• Once the surface mesh over the model is defined, Using Transform >> Copy >> Entities >> Select the whole model leaving the logo as it is not symmetry >> Middle Click Mouse >> Symmetry >> Three-point symmetry >> Select any three points where you need the symmetry plane >> Middle click mouse.

• Now, the whole model is created this method of deleting one half and resolving errors in other half, meshing the model and creating the other half using symmetry command is very much efficient & less time consuming.

Final Mesh Model:

Wind Tunnel Creation:

• Now, un freeze all the component one’s individual mesh of the component is completed.
• We will start Creating Wind Tunnel around the car model.

• Measure the car model length (x), Inlet of the wind tunnel will at 4x from the front of car, Outlet will be 6x from the back of the car, Top of tunnel will be 3x from top of the car, both sides of the tunnel will be 1x from side of the car. Bottom of the tunnel has no empirical relation just create in such a way that it does not intersect the car model.
• Back to TOPO mode take a hot point at the bottom of the car and create a reference point below it, with this point as ref. start creating the front and back point, offset the points to sides.

• Create curve joining the end points and then using face command create the bottom plane.

• Using Offset command, offset the bottom plane to the top plane using the top reference point and retain the original plane.

• Now using the face command >> Cons >> create the side faces. Assign PID for Top, Bottom & Side faces.

• Using PID hide all other components leaving only top plane and using Spacing provide the max. & min. Length for perimeter & Macros. Using Mesh >> CFD Visible command surface mesh the top face.
• Mesh the top surface by coarse mesh and bottom surface by fine mesh, so that growing mesh is obtained at the side faces.

Bottom Face Mesh:

Top Face Mesh:

• Spacing >> Auto Manual >> provide the D-start value as bottom plane mesh value & D-stop as Top plane mesh value >> Select the all side Edges >> Middle Click Mouse.

Volume Mesh:

• Volume Mesh >> Define >> Delete the Un-Wanted volume, Keeping only the Tunnel volume. Since the analysis is to study the external flow over the car model.

• Type of volume mesh needs to be defined so select the Unstructured Mesh >> Tetra CFD for all the Volume.
• Right click in the mesh status >> Mesh >> Ok for all the Volume.
• In order to check the volume mesh, we use cut plane option. Cut Plane >> New >> Default XY (desired plane to view cut section) >> select the node as reference for the plane position >> Ok.

Result:

Sr. No.	Element Size	No. of Element	Component
1	1mm	84303	Grills & Logo
2	2mm	143219	Rims, Tyres, Rear View Mirror, Window
3	3mm	158259	Body & Silencer
4	4mm	1976	Lights
5	-	772432	Whole car model

• of elements in Surface mesh is 781626 for whole geometric model.
• of elements in Volume mesh is 7808949 for whole geometric model.

Conclusion:

• Thus, given Geometrical model is resolved from all the errors and Volume mesh is created for the provided Mesh element size.

Resolved ANSA Model File Link:

https://drive.google.com/file/d/1_sq39M9savgtNSntSke-eFOstRCyUafG/view?usp=share_link