MODELING AND VOLUMETRIC MESHING OF WIND TUNNEL TO STUDY AERODYNAMICS ON BMW M6

Modelling AND VOLUMETRIC MESHING OF WIND TUNNEL TO STUDY AERODYNAMICS ON BMW M6

GOAL: To create a Wind Tunnel around BMW M6 car and create a volumetric mesh to perform an external CFD flow simulation

PROCEDURE: The BMW M6 model when imported into the ANSA workspace consists of various topological errors, these errors should be resolved to create a surface mesh, many errors occur in surface meshing which should also be resolved to create a volumetric mesh

STEP1: Importing and setting up the model

STEP2: Identifying and Rectifying the Topological errors

STEP3: Setting up PIDs and defining the element length

STEP4: Setting the mesh parameters and creating a surface mesh

STEP5: Resolving the errors in the surface mesh

STEP6: Creating the mirror model

STEP7: Modeling the wind tunnel

STEP8: Creating a volumetric mesh

STEP1: Importing and setting up the model

• Browse for the model and open
• Enable shadow mode, Set draw mode in PID mode, enable cross hatches
• Enable draw CONS in PID, MID ...mode, like in ent mode in presentation parameters
• Toggle the detail on demand level to the far left

Model after set up

STEP2: Identifying and Rectifying the Topological errors

• When Macros is disabled the design line becomes visible, delete them using the universal delete option

Design lines BMW M6

• As the model is symmetrical half of the model can be deleted, and after creating a surface mesh can be mirrored back, this reduces the time drastically
• don't delete the tires and rims as they might not be symmetrical
• Hide the wheels before deleting or they might also get deleted
• Select the universal delete option and set the selector in entity mode and drag over half of the model and delete it
• Remove the letters BMW on the logo as the are cause difficulties for the volumetric mesh, even though their effect on aerodynamic drag is negligible
• Set the element length as 1 unit to make the model much more accurate

• Find the single cons and start to rectify them, in many cases the surfaces might have to be deleted and new surfaces should be created to rectify errors

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• also, make sure that there are no thickness volumes left in the model as they might cause complications in choosing the final volume that needs to be meshed
• Only the outer surfaces are needed to create a shell of the model, so remove all the internal components as well

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• look for overlapping surfaces and delete them as they design errors and create errors in surface mesh rectify the various errors in grills so that the intersections are corrected and it's sealed properly so it can become the part of the outer shell, in order to achieve this many hot points might need to be created from mesh elements

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• There are many topological errors which require deleting and creating new surfaces

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• perform geometry check to find any other errors, neglect the single cons which are formed due to deletion of another half the model

after rectifying the topological errors set up the PIDs and define the element lengths for different PIDs

STEP3: Setting up PIDs and defining the element length

Setting PIDs is important to differentiate between components and ease up the working on the components individually

set the element lengths accordingly as a fine mesh for every element would be very time consuming and will be computationally expensive

STEP4: Setting the mesh parameters and creating a surface mesh

• After defining the element lengths define the mesh parameters

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• now create a free mesh for all the components
• Go to checks manager and perform a surface mesh check

STEP5: Resolving the errors in the surface mesh

• The errors which are related to minimum size can be overcome by decreasing the minimum element length as there are many intricate surfaces which are important to airflow and are smaller than the given minimum element length
• The errors related to sharp edges can be resolved by decreasing the check angel as surfaces in the grills form very sharp angles
• The errors related to proximities can are mostly formed by very close surfaces lying next to each other, these errors can also overcome by reducing the maximum angle
• The errors related to skewness need to be corrected by a combination of RECONSTRUCT, MOVE SURFACE, SPLIT, SWAP, CUT

BODY AFTER GETTING MESHED

STEP6: Creating the mirror model

• Now as all the errors have been resolved the other half of the model can be created by using COPY, SYMMETRICAL options and selection of three points which can form a plane
• makes sure the rims and tires are in hiding mode as they are not symmetrical, they cannot be mirrored
• now do the same process for the front wheels as well as one of the wheels is completely distorted and cannot be restored in any way
• but now the thread patterns on the left wheels are different from those of right side wheels, so copy the thread from the right rear tire and paste it onto the front
• the copy operations work with the help of two guide points which represent starting and ending elements

COMPLETE MODEL

STEP7: Modeling the wind tunnel

• As the meshing has been done on we need to create a wind tunnel around the BMW model with the following dimensions
• fornt= 4xlength of the car
• rear =6xlength of the car
• top=4xlength of the car
• bottom=100mm
• we need to create faces with these dimensions with the help of point
• These points are created such that each of them is perpendicular to each other
• After creating these planes perform the intersection operation to trim the extra bits of the surfaces

WIND TUNNEL

• now that the wind tunnel has been modelled, it has to be surface meshed first such that the mesh size gradually grows from top to bottom
• this can be done by using the spacing option under perimeters in the mesh utility menu
• chose geometry biasing option and set the min value as 20 and max as 100
• after creating the mesh clean up the model by performing the compress option

STEP8: Creating a volumetric mesh

• Now that the entire surface meshing is done volumetric meshing can be done
• under the V-MESh utilities, menu go to volumes and select define and click on ok
• Select the required volume that has to be meshed volumetrically meshed and delete the remaining volumes in the dialogue box and close the window
• select the tetra CFD under unstructured mesh menu and drag over the model which has to be meshed and confirm it

after getting meshed the model looks like this

VOLUMETRIC MESHED WIND TUNNEL

CHANGE OF MESH LENGTH AS IT MOVES FAR AWAY FROM THE MODEL