ANSA- TurboCharger Preparation and Meshing

TURBOCHARGER MESHING

Objective:

To clean the turbocharger model to be free from topological errors, assign functional PIDs,

Methodology:

Stage 1: (Geometry Clean-up)

Looking for single CONs is general way of spotting topological errors.

1. Look for interfering surfaces, and try to create a closed model.
   
   -Here, some surfaces seem to have dual colouring, which can be indicative of two surfaces at the same location.
   
   
   The pink surface might be left over from the CAD process, and needs to be removed as it is not necessary. Hide green surface using PID selection, then delete the pink surface using: Universal delete option, as shown below.
   
   
   
2. Fill surfaces to complete incomplete geometry.
   
   -From the geometry it can be seen that two regions needs to be filled with surfaces,
   
   
   REGION :1
   Since it a form of hole, first we can check with CONS> Fill Hole option. This option selects a set of CONs in a loop.
   
   
   REGION :2
   Fill Hole option does not seem to work for region 2, due to the curvature of the surface. This region requires the appropriate CON to be split into multiple entities to improve the surface obtained.
   
   
   The above action can be performed using: Hot Points> Parametric- The con is selected by clicking on the CON near the hot point from where the parametric scaling has to start. A value between 0 to 1 has to be entered to insert a hot point at that parametric position on the CON.
   
   
   The CON is split into many entities until an approximate required accurate surface can be obtained.
   
   
3. Solve for intersecting surfaces.
   
   -The surface below seems to have extruded beyond the bounding surface. This requires the face to be trimmed and the CON merged with CON of the bounding surface.
   
   Using CONS> Project- Select CONS to be projected, Select the face on which it is to be projected.
   
   The excess extruded portion of the face can now be deleted as it has been split at the projected CON. But now there are two single CONs left over at the same location, which needs to be merged to form one double CON. Using CONS> Paste- Select the two CONs. The two CONs have now been merged
   
   
   -Two surfaces are seen to be intersecting, so it necessary to solve for them.
   
   Using Faces> Intersect- Select the two sets of intersecting Faces. This splits the intersecting surfaces at the intersection. The necessary faces can be retained, and the others can be deleted following Intersection.
   

The resulting model has been solved for topological errors.

Stage 2: (Assigning PIDs)

The turbocharger model can be assigned different properties based on function of each part. It allows for separation of different components into different observable groups. The properties can be managed in the Properties Manager.

To assign PID, use Faces> Set PID- Select faces, Select property to assign to faces or "New" can be used to create new PID.

Stage 3: (Defeaturing/ Model simplifying)

For the portion shown below it is necessary to merge the ends (shown as white lines) of connected components to simplify geometry.

• At each of the joints (same diameter connections), use Faces> Topo- Select the junction CONs. This fuses the CONs of the adjacent components, creating a higher order CON.
• Deleting the end faces at the connecting junction creates a double CON, making the components into one region.
• This is repeated for each connecting junction, until a single region is obtained for the portion shown above.

For the interfering portion, project CON (shown below in white) onto face, and delete the interfering portion of pink shaft.
Use CONS> Project- Select CONs to project, Select face onto which CONs are to be projected.

Gaps in the impeller are connected to form a complete circle, to then be connected to the shaft.
Use Faces> Cut- Choose the two hot points to connect with CON.

The CON of the shaft (marked below) is projected onto the face, and the end face of the shaft is deleted along with the new face formed from projecting the CON.

At the end of this stage, volumes have been created to suit the final stage of volumetric meshing.

Stage 4: (Surface Meshing)

Different PIDs are meshed to different sizes based on necessity in regard to fluid flow. The following mesh sizes will be used:

• Blade Stage 1- 1mm
• Blade Stage 2- 1mm
• Compressor Casing- 5mm
• Impeller- 2mm
• Inlet casing cover- 5mm
• Shaft rotor - 1mm
• Turbo casing - 5mm

1. Each component above is made to "Show only" from the properties window. It is better to proceed from components of coarse mesh length to those of fine mesh length, as this keeps us on the conservative side.
2. The mesh length is assigned to the CONs on that part using: Perimeter> Length in the mesh module.
   
3. After all the CONs have been assigned length, the entire geometry can be meshed together using Mesh Generation> Free Mesh> Visible.
   

This produces a complete surface mesh. At regions where there is a transition of Mesh length, the mesh size varies gradually.

Stage 5: (Remaining geometry clean-up)

After surfaces have been meshed, they have to be checked for consistency. The following portions can be seen to present an anomaly, which have two surfaces at the same location. And the two overlapping surfaces have different mesh sizes which exaggerates the problem.

• For these two surfaces, the shaft surface is retained and the other deleted. This produces single CONs.
• The single CONs are pasted onto the shaft surface using CONS> Project- with some additional settings as shown below
  
• This results in the creation of triple CONs.
  
• All the newly created CONs on the shaft are given new mesh length of 1mm (same as that of other CONs of the shaft). The model is then be remeshed.

Stage 6: (Mesh Refinement)

The mesh is refined using a simple rule: an edge is given the smaller of the element size of the parts that share the edge. The entire geometry is meshed using this rule.

Poor mesh quality can be observed in some regions where two edges of very different element sizes meet. This can be resolve by making the variation gradual between the element sizes.

In the figure below, the CON of the casing- 5mm, meets with the CON on the shaft- 1mm.

A patch of is created around this region using Macros> Cut- Select the two points to be joined, from the Mesh module.

The element size is changed to 3mm using Perimeter> Length. The final meshed patch looks as shown:

Stage 7:(Volumetric Meshing)

To prepare the model for volumetric meshing, closed volumes are to be created within the turbocharger. Surfaces are created at the inlet and outlet of the compressor casing and turbine casing. To create surface use Faces> New- then choose appropriate surface type and then the surface.

After all the faces are closed, they are meshed. The final part looks as shown:

To create volumes, in the Volume Mesh module, Volumes> Define- Whole db; This created volumes in as many closed volumes are available in the surface model.

All the created volumes may not be required, so each of them are checked for their necessity. To view the list of all the created volumes, Volumes> List.

Only volumes that are necessary for flow analysis are retained and all the others are deleted. Only two volumes are retained as shown below:

Now the volume is meshed using, Unstructured Mesh> Tetra CFD- Select the volumes. The final meshed volumes are shown below:

Conclusion:

Volume mesh can be done only after the surface mesh has been completely done on the bounding surfaces. Only volumes relevant to the flow are considered for volume meshing as shown above. In CFD, Tetra mesh is used to reduce the effort in creating hex elements.