VOLUMETRIC MESHING OF THE 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.

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   ANSA maintains the association between CAD geometry and the FE mesh. ANSA stands for \'Automatic net generation for structural analysis\'.

 ANSA phases : Import cad geometry -> geometry clean up -> skin or mid surface generation -> shell or solid meshing -> element quality check -> mesh improvement -> apply load                                   -> export.

OBJECTIVE     : 

                         To mesh the give cylinder model by using element size as 1 units by using ANSA.  

 Meshing       :  A mesh is a representation of a larger geometric domain by smaller discrete cells.By meshing you break the domain into pieces and each piece represent an element.   

  Meshing process : define element size -> choose size of element -> selecting type of mesh to done -> quality criteria check -> remesh for quality improvement if needed. 

 PROCEDURE    :    

1)ImportingTthe CAD Model

2) Cleaning Topological Errors 

3)Assigning The Relevant PID

4)Surface meshing The CAD model

5)Volumetric Mesh

1)Importing The CAD Model :   

                             First of all import the CAD model in the ANSA.By seeing the model you can see many geometrical errors .If no cons are seen then go to quality criteria  -> presentation parameters -> visualization.

2) Cleaning Topological Errors :

                               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.

a) clearing glitching :

                                        FIGURE 1

In the  above figure (fig 1) 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 (fig 2).when you remove the below extra surface you will see that glitching is vanished. 

                                               FIGURE 2

In the below figure(fig 3) you will see that there is not glitch which is seen when you remove the pink surface in the above image(fig 2).

                                            FIGURE 3

In the above figure(fig 3).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 ( fig3).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 (fig 4) .Then extra surface should be deleted.

                                            FIGURE 4

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 coons create a new surface (fig 5).

                                                 FIGURE 5

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.

                                           FIGURE 6

In the above figure(fig 6) you can see no intersection between the surface before intersecting.

                                         FIGURE 7

Figure 7 is 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.then create a new surface by using coons. At first when you create the surface directly by using coons you will see a irregular surface is formed.Then we have to insert hot point on the edges.you can see creating of hot points in the below image (fig 8).use parameterical under hot points and give length as 0.5. Then it create hot point at the center. 

                                             FIGURE 8

Even after creating a hot point between edge and creating a surface you will see some irregular surface is formed (fig 9)by making intersection ,which is incorrect.

                                            FIGURE 9

This happened as two single hot point in between them is not sufficient to create a new surface . So create another two hot points on each side and then create new surface individually bebetween two hot points .then you see exact well defined required surface is formed.

                                         FIGURE 10

In the figure 10 you can see that exact required surface is formed.

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 if there are any errors it shows  in a window .If no errors are there you can proceed further.Here in fig11 you see that there are no geometrical errors.

                                         FIGURE 11

3)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.To assign PID first select  set PID under faces -> then select any face  -> click middle mouse we get properties. To create a new PID for it then select new  create a new one and assign a name and double click on it assign.In this case the PID\'s are already created,here lets change the name of already assigned PID\'s.Here the shaft rotor has 2 different PID\'s then we assign both of them a single PID.you can observe it in figure 12 that old PID of other half shaft rotor bulb is white  in colour which means no surface is assigned to it.

                                          FIGURE 12

Combing shaft rotor and blades as single volume :

      Here evrey blade rotor is a seperate volume .Here we have to combine all of them in to one volume of different PID.Use topo to join 2  surfaces .For the impeller you see a discontinuity in triple con due to blade surface.Use cut to form a seperate surface for blade and rotor.Then again do topo to get a whole triple con uniformly.Then delete the extra surfaces between rotor and impeller to convert them as a single volume.

                                       FIGURE 13

In the figure 13 you see that by using cut we made impeller rotor part as one surface.

                                          FIGURE 14     

 In fig 14 you see that inside new 2 surfaces are formed after topo. Then we delete both the surfaces to make rotor and impeller as one volume.

                                                 FIGURE 15

In figure 15 youu can see that triple con is converted to double con after deleting the surfaces.In the same way we combine the remaing blades and rotor into a single volume.

                                           FIGURE 16

                                            FIGURE 17

In the figure 16 you can observe that when selected one blade only one is selected.In the fig 17 you can observe that when one blade is selected all are selected.It means they are converted to single volume.You can also observe that there is glitching in figure 17 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 will see that rotor and blades are converted to single volume.

  Then compress file which will will remove permanently deleted and and unused entities, even the unused PID\'s are also deleted.and you also see PID\'s with full light.

4)Surface meshing The CAD model :

   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 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.lets solve one by one.

                                                  FIGURE 18

After meshing you can see 3 problems in figure 18

Problem 1 : 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 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 computer 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 .

Problem 2 : Element length of shaft is 1 and element length of compressor casing is 5mm You can see mismatch at their interface also if you hide the casing top part qnd see you see a overlapping of mesh of both at the interface .To get rid of this problem you have to delete the shaft surface or compressor casing extra part at the interface.After deleting then we have to fix the rotor shaft with the compressor casing .You can observe it in figure 19

                                        FIGURE 19

Problem 3 : Same problem is seen at the intersection of turbocasing and rotor.Then we have to fix it  in the same way by deleting extra and projecting and paste the surfaces.

Creating A Patch : When after deleting the extra surafce in Topo mode when you go to mesh mode you see that the mesh of compressor casing andturbo casing  has been gone this is due to as we connected the two surface of different mesh length.Then remesh them by giving the lengths at the interface as 1mm as it will be finer mesh.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 or an area in between by giving a mesh length in between 1 to 5 here we have give the length as 3 mm for the patch or created area. as we get a balanced gradient.

For the creating of patch use  cut uder macros and cut the area required on the compressor casing.then give the laength as 3 mm by selecting length under perimeters by selecting the area.you can see the patch created in the figure 20.In the same way we have to create patch for the inner surface.

                                      FIGURE 20

Closing the other open surfaces and giving them PID\'s :

To define the volume we need closed surfaces .We close the open faces by creating a face over it It is done bu using coons by selecting edges.Then after creating faces we have to assign a pid for them and mesh them by using free under mesh generation.You can see them in the figure 21.

                                            FIGURE 21

5)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 the figure 22.

                                                     FIGURE 22

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 flow which is happening over the shaft not inside 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 .you can see the required volumes in figure 23.

                                                FIGURE 23

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.

                                          FIGURE 24

In figure 24 you can see the volumetric mesh has been done to the four volumes where flow is happening.You can see the cut section in the figure 25.

                                           FIGURE 25

you can see the final model after creating the volumes in the Figure 26.

                                            FIGURE 26

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. We have also learnt that volumetric mesh is like clay box and propeller when you keeo propeller in clay box and removed then you see impression of propeller in clay in the same way volumetric mesh also created and also learnt 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.As we have 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 926312 tria elements and 9902216 tetra elements.You can observe it in figure 27.

                              FIGURE 27