Week 4 Challenge : CFD Meshing for BMW car

Aim:

• To clean the geometry.
• Mesh the components part by part.
• Eliminate off elements present in the mesh.

Take away:

At the end of this article one will be able to,

• Use different tools to clean the geometry.
• Understand the quality criteria assigned to each and every PID.
• How to tackle the off elements if present in the mesh.

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NOTE: THIS PROJECT CONTAINS LOT OF ERRORS. REPETATION OF THE SAME TOOL WILL NOT BE DEMONSTRATED.

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PRE CLEAN UP GEOMETRY:

• There are lot of red and blue cons.
• Red cons are edges with one surface attached and blue cons are edges where three surfaces meet.
• So the goal here is to remove red cons and avoid the formation of blue cons.
• This will be done part by part for easy access.

Essential Tools:

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--> Resolution:

• As observed the geometry has a very poor resolution.
• This can affect the physics while running the simulation.
• This can be done by navigating to LENGTH option in MESH DECK (MESH DECK >> PERIMETERS >> LENGTH).
• Assign a value of 1mm.

• Resolution is much better than before.
• Now we can resolve the errors part by part.

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>> Exploring the UNIVERSAL DELETE option:

• Since the model is symmetrical, we will be deleting one half of the car geometry, clean the remaining half and the apply symmetry optoin after completeing the clean up.
• UNIVERSAL DELETE option is present in the top ribbon.

• Now we will workout on the remaining half of the geometry.

>> Exploring the PASTE option:

• This is used to paste one con onto other con.
• Select PASTE option. (TOPO DECK >> CONS >> PASTE).

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Bottom grill clean up:

• There are lot of unnecessary surfaces.
• Objective here is to keep a common volume inside the car so we will be deleting/modifying few surfaces.

• The above selected surfaces are not required. So we can go ahead and delete the surface.
• We have to create a comman volume.

>> Exploring the PROJECT option:

• This will project the cons on the surface.
• Select PROJECT option. (TOPO DECK >> CONS >> PROJECT).

>> Exploring the NEW option:

• This tool is used to create new surface/face.
• Select NEW option. (TOPO DECK >> FACES >> NEW).

>> Exploring the EXTEND option:

• This tool will extend the cons on a particular surface of interest.
• It only works for red cons.
• Select EXTEND option. (TOPO DECK >> FACES >> EXTEND).

• The final objective is shown below:

• The red cons will go away once we do symmetry.

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Side grill clean up:

• Objective is to create a common volume.
• Eliminate the red cons and avoid blue cons.

• We will be removing the triple cons first.

• It is quite evident that there is no triple cons present there after using the UNIVERSAL DELETE option.

• The ribs in the above figure will form its own local volume.
• Inorder to avoid local volume formation, we must open the ribs from the rear end.
• Delete the surfaces as shown in the figure below:

• Now its time to make some modification to the existing surface.
• This is done using the CUT option. (TOPO DECK >> FACES >> CUT).

• Similarly, we will be modifying the upper surface.
• We will be creating lines to delete unwanted surfaces.
• This can be done using CUT option to create the lines. (TOPO DECK >> FACES >> CUT).
• And lateron surfaces can be deleted using UNIVERSAL DELETE option.

>> Exploring the FILL HOLE option:

• This tool is used to fill an empty surface.
• Select FILL HOLE option. (TOPO >> FACES >> FILL HOLE ).

>> Exploring the PARAMETRICAL option:

• This tool allows us to create a hot point at some distance from any local reference location.
• Select PARAMETRICAL option. (TOPO DECK >> HOT POINTS >> PARAMETRICAL).
• The final output should be as shown in the figure below:

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Logo clean up:

• Major clean up is required at the bottom of the logo.
• We must clean the the unwanted surfaces using UNIVERSAL DELETE option.
• Final output must be like the figure shown below:

• We can go ahead and delete the internal surfaces newa the windshield and rear of the car.

• Resolved output can be seen below:

• There are no single and triple cons present near the windsheild, rear and the windows.

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Upper grill clean up:

• Obejctive here is to keep common volume inside the car.
• Make sure that we do not form local volume.

>> Exploring the PROJECT option:

• This tool allows us to project cons on seleted surface/face.
• Select PROJECT option. (TOPO DECK >> CONS >> PROJECT).
• We will be projecting cons on to surface as shown in the figure below:

>> Exploring the PROJECT CUT option:

• This tool allows you to create a cut perpendicular to the choosen surface and hot point of a con.
• Select PROJECT CUT option. (TOPO DECK >> FACES >> PROJ.CUT).

• Next step is to delete the top surfaces using UNIVERSAL DELETE option as shown below:

• We will be cleaning up the bottom surface of the grill to patch the region and not form a localized volume.

• Now we can go ahead and create new surfaces as shown below:

• Lastly we have to close the region.

>> Exploring the MULTI PROJECT option:

• This tool helps us to create multiple hot points with respect to local reference hot points.
• Select MULTI PROJECT option. (TOPO DECK >> HOT POINTS >> MULTI PROJECT).
• This solves the final piece off this part as shown below:

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Doorside feature cleanup:

• As visible in the above picture, there are lot of single cons and triple cons with intersecting surfaces.
• We need to clean up those areas with appropriate tools.

>> Exploring the INTERSECT option:

• This tool is used when we see there are three surface meeting and not forming a triple con.
• Select INTERSECT option. (TOPO DECK >> FACES >> INTERSECT).

• There is face which needs to be trimmed, but the cons are incomplete.

• With the help of PROJ. CUT option we can delete that surface.

• Next is to extend few surfaces to eliminate the chances of formation of localized volume.

• Still some modification is required. It is done using EXTEND, UNIVERSAL DELETE, CUT etc and many other options.
• Finally, the end result should be some what like as shown in the figure below:

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Mirror clean up:

• Objective here is to remove the triple cons.

• As you can see there are no triple cons present.

• Since we do not want localized volume to be formed, we must delete this region.

• The clean up mirror is done.

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Tire clean up:

• We will be removing triple cons and overlapping surfaces if present.
• This can be done by hiding few surfaces and deleting unecessary surfaces using UNIVERSAL DELETE option.

• Similarly procedure is for the second tire.

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Rear car clean up:

• Triple cons are visible. Simply delete with UNIVERSAL DELETE option.
• The rectified figure can be found below:

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Floor clean up:

• There are lot of surfaces penetrating into different surfaces.
• The objective here is to removme those surfaces.
• This can be done using UNIVERSAL DELETE option.
• If any surfaces meet and do not form any cons, then it is best prcatice to perform INTERSECT option. (TOPO DECK >> FACES >>INTERSECT).

>> Exploring CONS2CURVE option:

• This tools helps in creating curves out of existing cons.
• Objective here is to leave footprints for later use.
• Select CONS2CURVE option. (TOPO DECK >> CURVES >> CONS2CURVE).
• It will change the con colour to purple.

• There are lot of uneven surfaces which needs to be deleted.

• Since there are lot of surfaces intersecting and not forming an cons, we must use INTERSECT option.
• Select INTERSECT option. (TOPO DECK >> FACES >> INTERSECT).

• Comparing befor and after of the floor, the after is much cleaner and has les overlapped surfaces as seen in the below figure:

• This concludes the topology clean up of each and every part.

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Assigning PIDs:

• The next phase of the pre-processing is to assign PIDs to respective component.
• This allows you to work with the model fluently without any chances of facing any error or creating confusion over similar looking parts.
• Figure below shows the default PIDs assigned to the component:

• To create new PID, right click and you will see NEW option. Selecting NEW, will allow you to set and apply PIDs.
• Some created PIDS are shown below:

• PIDs highlighted in blue are newly created PIDs.
• Inorder to APPLY PIDs, we will isolate the component and then assign the PIDs.
• For example, in order to assign the Logo PID, we will isolate the logo and then assign it by right clicking on logo and select APPLY.

• After assigning the bulb will turn on.
• Similar is the procedure for assigning other PIDs.

• As seen in above figure, all the newly created PIDs have glowing bulb, which means all are assigned properly.
• Now next step is to delete the default created PIDs by ANSA.
• This can be done by selecting COMPRESS option present in ribbon at top left corner.

• This is the end of assigning PIDs.

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Meshing:

>> Exploring the QUALITY CRITERIA option:

• This tool allows us to set set the quality of mesh before hand.
• Later it meshes the surface/volume maintaining the bounds of the specified quality criteria.
• Some of the quality checks can be found in the below figure:

• Lets say the element skewness is less than 0.5 (see above figure), then it will show that particular element in green colour.
• Next step is to assign mesh elements part by part but one at a time.
• Below is the list of target lengths:

1. Body - 3 mm
2. Lights - 4 mm
3. Grills - 1 mm
4. Rims and Tyres - 2 mm
5. Optical Rear view mirror - 2 mm
6. Window - 2 mm
7. Silencer - 3 mm
8. Logo - 1 mm

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Meshing the logo:

• Before initiating the mesh, MESH PARAMETER and QUALITY CRITERIA must be assigned.
• Target Length = 1 mm
• Maximum Length = 2 mm
• Minimum Length = 0.4 mm
• Select LENGTH option. Assign same mesh length value for both perimeter and macros (MESH DECK >> PERIMETERS >> LENGTH).

• Select SPOT MESH option. (MESH DECK >> MESH >> MESH GENERATION).

• In order to check quality criteria matches or not, we must enable HIDDEN mode.
• At the left hand side one can see the criteria.

• Off elements can be removed using RECONSTRUCT option.
• Select RECONSTRUCT option. (MESH DECK >> SHELL MESH >>RECONSTRUCT).

• As seen, there are no off elements present after using RECONSTRUCT option.

Meshing the Grills:

• Before initiating the mesh, MESH PARAMETER and QUALITY CRITERIA must be assigned.
• Target Length = 1 mm
• Maximum Length = 2 mm
• Minimum Length = 0.3 mm
• Select LENGTH option. Assign same mesh length value for both perimeter and macros (MESH DECK >> PERIMETERS >> LENGTH).

• Clear the off elements using RECONSTRUCT option.

Meshing the Window:

• Before initiating the mesh, MESH PARAMETER and QUALITY CRITERIA must be assigned.
• Target Length = 2 mm
• Maximum Length = 3 mm
• Minimum Length = 1 mm
• Select LENGTH option. Assign same mesh length value for both perimeter and macros (MESH DECK >> PERIMETERS >> LENGTH).

• Mesh can be seen below:

• No presence of off elements. As it can be seen in the below figure:

Meshing the Lights:

• Before initiating the mesh, MESH PARAMETER and QUALITY CRITERIA must be assigned.
• Target Length = 4 mm
• Maximum Length = 5 mm
• Minimum Length = 3 mm
• Select LENGTH option. Assign same mesh length value for both perimeter and macros (MESH DECK >> PERIMETERS >> LENGTH).

• Mesh can be seen below:

• Number of off elements are 11.
• This can be resolved by using SPLIT option. (MESH DECK >> ELEMENTS >> SPLIT).
• By clicking the desired edge it will split that edge.
• If this does not work, use SMOOTH option. (MESH DECK >> SHELL MESH >> SMOOTH).
• Even this does not work then use MOVE option. (MESH DECK >> GRIDS >> MOVE).

• No off elements are present.

Meshing the Rims and Tyres:

• Before initiating the mesh, MESH PARAMETER and QUALITY CRITERIA must be assigned.
• Target Length = 2 mm
• Maximum Length = 3 mm
• Minimum Length = 1 mm
• Select LENGTH option. Assign same mesh length value for both perimeter and macros (MESH DECK >> PERIMETERS >> LENGTH).

• Mesh can be seen below:

• There are lot of off elements near the nut bolt area.
• Can be resolved by performing topo clean up.

• No off elements are present.

Meshing the Silencer:

• Before initiating the mesh, MESH PARAMETER and QUALITY CRITERIA must be assigned.
• Target Length = 3 mm
• Maximum Length = 5 mm
• Minimum Length = 1 mm
• Select LENGTH option. Assign same mesh length value for both perimeter and macros (MESH DECK >> PERIMETERS >> LENGTH).

• Mesh can be seen below:

• It does not have any off elements.

Meshing the Rear View Mirror:

• Before initiating the mesh, MESH PARAMETER and QUALITY CRITERIA must be assigned.
• Target Length = 2 mm
• Maximum Length = 3 mm
• Minimum Length = 1 mm
• Select LENGTH option. Assign same mesh length value for both perimeter and macros (MESH DECK >> PERIMETERS >> LENGTH).

• Mesh can be seen below:

• There are no off elements.

Meshing the Body:

• Before initiating the mesh, MESH PARAMETER and QUALITY CRITERIA must be assigned.
• Target Length = 3 mm
• Maximum Length = 5 mm
• Minimum Length = 1 mm
• Select LENGTH option. Assign same mesh length value for both perimeter and macros (MESH DECK >> PERIMETERS >> LENGTH).

• While using perimeters option, deselect the edges in conjucntion with the body PID.
• It will assure the mesh of the other elements do not vanish.
• To remove the off elements, one must assign the global element criteria.
• Considering the smalllest feature in the car and possible areas of skewness formation.

• Finally, there are no off elements present and we can head to create wind tunnel.
• Surface meshing of PIDs end here.

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Symmetry of the model:

• Select proper view for proper selection of the model.
• Select TRANSFORM option present at the right hand side of the viewport.
• Select COPY option. It will ask you to select the entities to copy.
• Select the entire car except the logo.

• Select any three point along the line representing as the symmetry plane.
• Take a peek at the figure below, representing the entire car.

• Next step is to create Wind Tunnel.

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Wind Tunnel:

• There are few standards to be followed to make a wind tunnel.

>> Exploring the RELATIVE option:

• This tool helps in creating a point relative to an already selected point in either x,y or z direction.
• Select RELATIVE option. (TOPO DECK >> POINTS >> RELATIVE).

• A point is created.
• With the similar approach we can create more points using RELATIVE option.
• Front Point = 5854 mm.
• Rear Point = 8410 mm.
• Top Point 3900 mm from top point.
• Side Point = 1300 for both sides.

• These points can be used to create curves.
• Select CREATE option. (TOPO DECK >> CURVES >> CREATE).

• Using this one can create surface/face.

>> Exploring the OFFSET option:

• This tool helps in creating a surface/face offset from selected entity.
• Select OFFSET option. (TOPO DECK >> FACES >> OFFSET).
• Offset value = 4275 mm.
• Finally, the wind tunnel looks like this:

Meshing the Wind Tunnel:

• As we are closer to the car, we expect to have a finner mesh and as we move away from the car, we expect to have a coarser mesh.
• Turn off the maximum and minimum length quality critera
• Top surface = 100 mm.
• Bottom surface = 20 mm.
• For side, inlet and outlet we have to give variable mesh length.
• This can be done by using SPACING option.
• Select SPACING option. (MESH DECK >> PERIMETERS >> SPACING).
• It will ask few inputs. Please refer below figure:

• Select the vertical edges of the box as shown in the figure below:

• Finally the mesh in the front and side looks like the figure below (same is for other wind tunnel surfaces):

• Finally there are no off elements and all the surfaces are meshed.

NOTE : In this task volumetric mesh is not performed as it will take lot of time to mesh. Hence the discussion is also not done.

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Conclusion:

• Learnt how to create a half model.
• Topology clean up part by part with the help of different tools.
• Assigning appropriate PIDs.
• Meshing PIDs one at a time.
• Deligating with off elements and it rectification using different tool approach.
• Creating symmety of the half model.
• Creating wind tunnel and assigning relevent mesh length and variable mesh length.

LINK TO THE MODEL.