Week 3 Challenge : CFD meshing on Turbocharger

INTRODUCTION :

Turbocharger : A turbocharger, colloquially known as a turbo, is a turbine driven forced induction device that increases an internal combustion engine's efficiency and power output by forcing extra compressed air into the combustion chamber with the help of exhaust from an engine.

ANSA :

• ANSA is a Computer-aided engineering tool for Finite Element Analysis and CFD Analysis widely used in the automotive industry. It is developed by BETA CAE Systems. The software is distributed world wide by a number of BETA CAE Systems subsidiaries and business agents.
• ANSA maintains the association between CAD geometry and the FE mesh. This means that the FE meshes are better representations of their geometric parents. Also it is easy to maintain and update any changes in the geometry by simply reworking the updated area instead of recreating the FE from scratch.
• It carries several proprietary algorithms for meshing suitable for both CFD and structural models. ANSA was initially standing for 'Automatic net generation for structural analysis', but the software has gone beyond that very quickly. ANSA has broadly six menus which are used to do various activities those are: TOPO MESH, VMESH ,DECK-SOLVER, MORPH ,HBLOCK.

Different type of modules in ANSA :

• TOPO - Set of tools for Geometry clean-up and for setting up of PIDs.
• MESH - Tools to generate Surface Mesh or 2D Mesh.
• VOLUME MESH - To generate solid or 3D Mesh.
• SOLVER DECKS - Tools based on different solvers such as ABAQUS, RADIOSS ,ANSYS etc.
• MORPH - Advanced tool to capture geometry more accurately while meshing.

Solution procedure :

• For solving we follow the below procedure ,

1. Import the CAD model.
2. Topo/Geometry cleanup.
3. Geometry check-up.
4. Setting-up of relevant PIDs.
5. Defining the element size or length.
6. Selection of Meshing type.
7. Generating surface meshing the CAD model .
8. Generating volumetric mesh.

1) Importing of CAD model :

• To import the CAD model we follow below steps ,

Go to file -> open -> browse the location of the CAD model -> select it click on open.

• After opening the model at first you will see the errors like glitching of the model,intersections , single cons etc. Let us clear the topo errors one by one.

2) Cleaning topological errors :

a) clearing glitching :

• In the above figure you will see a glitching in the model this happens when there is a surface beneath it.It is a design error.you can see it the below image .when you remove the below extra surface you will see that glitching is vanished.

• In the below figure you will see that there is not glitch which is seen when you remove the pink surface in the above image .

• In the above figure after cleaning glitching you will see many errors like open surfaces ,intersection errror etc.,.Lets clear all the topo errors one by one.

b) Deleting extended surface at compressor casing outlet :

• You will see an extra extended surface is present at the compressor casing outlet ,you can see inabove figure .using project tool project the lower con to the upper surface to divide the etra surface .Then paste the cons using topo or paste .Then you will see a triple con is formed at the intersection in below figure .Then extra surface should be deleted.

c) Creating a new surface at the compressor casing inlet :

• At the compressor casing inlet you will see a open surface bhaving single cons .By using fitted options under New (under faces ) we create a new surface .We can also patch bycreating hot points and creating small small surfaces.

d) Intersecting the surfaces :

• Then the next single con error is seen where there is no intersection when intersected.Then we have to use intersect . By using intersect tool intersect themand delete the unwanted extra surfaces.
• In the below figure you can see no intersection between the surface before intersecting.

• In the below figure you can see the final required surfaces after intersecting and deleting the extra surfaces.

e) Creating new surface on turbo casing :

• The next topo error is open surface for turbo casing.In the below image you can see the open surface for turbo casing .

• If we create a new surface by using coons , you will see a irregular surface is formed.Then we have to insert some hot point on the edges.For creating of hot points we use parametrical under hot points and give length as 0.5. Then it create hot point at the center.
• We create many such hot points on both sides and create new surfaces using it , so that we get a smooth surface covering the casing .
• If you create a new surface by using less hot points we may not get smooth surface as required. So we create more hot points on each side and then create new surface individually between two hot points .Then you see exact well defined required surface is formed.
• In the below figure you can see that exact required surface is formed.

f) Combing shaft rotor and blades as single volume :

• In the below image you can see a lot of triple cons , this is due to the reason that there are surfaces in between each and every blade.We need to delete all those extra surfaces and make these triple cons into double cons . So that we have whole blades and shaft as one volume .

• Here every blade is a seperate volume .Here we have to combine all of them in to one volume of different PID.We delete the extra surfaces between rotor and impeller and other blades to convert them as a single volume .
• You can also observe that there is glitching in the above figure this is due to the reason that there is a surface beneath it.Then project the con of blade on rotor and delete the inner surface and all extra surfaces between them .
• Finally you can see in the below image that all the triple cons are removed and rotor and blades are converted to single volume .

3) Checking for geometrical errors :

• Then let us check for any geometrical errors still exist. To do geometrical checks go to checks -> geometry -> then execute .
• Then you see that there are two geometrical errors , which are triple cons belongs to turbo casing ,which are necessary as all the three surfaces forming the triple cons are important ,so we keep them .

• In the below image you can see the wireframe mode of our model ,where you observe there is ony one triple con which is important ,and other are double cons .So we can conclude that we have cleared all topological errors and we can further move to assigning the relevant PID's .

• But before assigning the PID's ,we have to close the geometry wherever it is open i.e., at inlets ,outlets etc., so that we can do volumetric mesh .If it not closed ANSA will show an error ,so it is necessary to close the geometry

4) Assigning the Relevant PID's :

• In this step we assign the PID for different parts and give a name and colour to differentiate between them.This is also done to specify names to different boundary conditions for various parts.We need to activate cross hatches before setting-up PIDs.
• If you open properties manager at the top , you will have some default PID's which are assigned , so we will just change name for those PID's. For remaining new surfaces created to close the geometry we have to create new PID. Inorder to create new PID , right click on properties manager -> click new -> then select pshell -> then again select pshell -> then give name in the popup box -> then ok .You can change the colour by double clicking on the colour box -> then ok .
• In the similar way create PID for inlets, outlets etc., .
• Here the shaft rotor has 2 different PID's then we assign both of them to a single PID.
• Then next thing is assign these PID's to the components of the geometry .
• To apply , we select /click on the PID to apply in the PID manager -> then right click on it -> then click on apply -> then in the geometry select the cross hatch of the surface you want to assign -> then middle click . In the similar way we have to apply PID's we have created . To give the PID we select with the different type of selection available to select the exact surface needed.
• Then after assigning the PID , delete the empty orginal PID's which has no components assigned by using compress .
• In the below image you can see that whole geometry is closed and also the list of PID's we have assigned.

5) Defining the element size/length :

• Then the next step to be done is to surface mesh the model by giving different mesh lengths for different PID.Here we give blades and rotor a target length of 1 mm,casings a length of 5 mm and impeller 2mm.You can observe here that blades are given lower length because it is a very vital component in the CFD analysis and it should be as refined as possible.

6) Selection of meshing type & generating surface meshing of the CAD model :

• Here we will select the Meshing algorithm.We follow below steps for that ,

Go to Mesh Generation -> spot Mesh -> tria -> drag and select the required model -> click on the scroll button/ middle mouse button on mouse to mesh (or) we can directly select visible under drop down menu of spot mesh instead of drag and select the entire model after seperating them by using NOT .

• After meshing you can see problems . As we mesh the shaft rotor first and then we mesh the impeller the mesh of shaft rotor is gone after mesh of impeller is done this is happened because at the interface of these two it is registered that we gave a length of 1 mm for the shaft and 2 mm for the impeller it over wrote mesh of 1 mm then at the interface the ANSA got confused either to give 1mm or 2 mm so it basically deleted the mesh.then size of element at the interface changes to 2mm.Then mesh the model again .Same problem is seen at the intersection of turbocasing and rotor and compressor casing and rotor.To solve this problem ,instead of changing length again we directly mesh the geometry .
• Then after meshing you see a disparity here on compressor casing due to the element length 1 at the interface and 5 on the compressor casing as 5 suddenly changing to 1 the elements are not good quality.Then to get rid of this we create a patch so that we get a balanced gradient by using cut under macros in mesh deck.
• The model after generation of mesh is shown in below image.

7) Generating volumetric mesh :

• Then only thing left to do is volumetric mesh.Before doing volumetric mesh we have to define it.After defining it generates 8 volumes automaticallyYou can see it in below figure .

• Delete the unnecessary volumes generated .We need the volumes which occupy the interior of compressor and turbocasing.Here volume occupied by the compressor outercasing,blade shaft and tubro casing is deleted because the assembly has a volume by here in CFD analysis we will be looking for flow happening over the shaft or where fluid flow, but not inside or thickness of the geometry.Since inside is hallow and no flow happen and we are considering the flow over the blade and other which are not essential are deleted.
• After deleting we have 4 volumes where flow is happening then we have to do volumetric mesh for them .
• For volumetric mesh to be done under unstructured mesh use tetra CFD .By removing some parts of the elements we can confirm visually volumetric mesh is created by seeing multiple elements inside and slots for blade properties.
• In the below figure you can see the volumetric mesh has been done to the four volumes where flow is happening.

• .You can see the cut sectional view in the below figure .

• you can see the final model after creating the volumes in the below image.

LEARNING OUTCOME : In this process we have learned how to rectify geometry errors and mesh the given CAD model and perform volumetric mesh .We have learnt to crate a well defined surface we need more hot points as we have seen this while creating a face over turbocharger.Then we learnt Setting-up of relevant PIDs to different parts and then defining diffeent element size or length then selection of meshing type then how to generate surface mesh.For volumetric mesh to be done the model should be surface mesh before.The volumetric mesh element size depends on the element size of surface mesh.Then we finally learnt how to generate volume mesh. We also observed that ,as we deleted the volume formed by the rotor you can observe hallow space at the place where rotor is present after volumetric mesh.

CONCLUSION :After the volumetric mesh has been done you can see there are 919472 tria elements and 9942473 tetra elements.You can observe it in below image.