Week 3 Challenge : CFD meshing on Turbocharger

1. Aim

The aim of this challenge was to perform surface and volume meshing on a Turbocharger model based on pre-determined entities, such as given target length values of its different components using element type as Tria in case of the former i.e., surface meshing, and Tetra in case of the latter, in ANSA-CFD.

2. Procedure

The approach adopted towards performing surface and volume meshing for the given Turbocharger model in ANSA-CFD involved the following steps:

1. Import the given model from the concerned directory containing the ".ansa" file extension using the "Open" option under the "File" menu.
2. After the model is loaded and appears in the "workspace", the "TOPO" module responsible for geometry creation, manipulation and clean up needs to be enabled.
3. For the given model, it was realised that the geometry had a number of overlapping and missing surfaces of which the former required thorough inspection and deletion of unnecessary surfaces while the latter was also identified as "Single Cons" errors when the "Geometry Checks" was performed, which were required to be reconstructed as part of the Topology cleanup, in this case using the "New" button under the list of "Faces" button functionalities. Before performing any surface creation or deletion operations, it was decided to make use of the "Topo" button under the list of "Faces" button functionalities to automatically create the necessary topological conditions between adjacent CONS on the entire model. The "New" button utilised for creating new surfaces consisted of different options or algorithms to essentially define the missing face/surface such as "COONS" which makes use of an interpolation principle based on selected boundary curves, "EXISTING SURF" for drawing a new face on an existing surface, "PLANAR" for a planar face (as the name suggests) and "FITTED" for a non-planar face with regards to the selected boundaries. All of the above alternatives, except for "EXISTING SURF", were made use of depending on the characteristics of the missing faces along with the "LOOP" option enabled in certain cases especially when "COONS" option was preferred as it was able to predict the list of edges, perimeters or curves forming a close loop thereby saving effort and time. There were a few surfaces where the "New" button functionalities could not be implemented directly as the detected surfaces using the previously mentioned algorithms did not close the required surfaces satisfactorily and in those cases, "Hot Points" were inserted on the "CONS" using the "Project" or "Multi-Project" button under the list of "Hot Points" button functionalities or the "Parametrical" button within the same section of buttons to define a new Node or Hot Point parametrically such that the input parameter is a value varying between 0 and 1 representing the percentage of the length of the 3D-curve or CONS. Similar button functionality of "Project" under the list of "CONS" button functionalities was also made use of to both split a surface overlapping into another surface that could later be deleted using the "Delete" button of the Geometry Group, and to define a list of surfaces belonging to a particular part. Another button by the name "Intersect" under the list of "Faces" button functionalities was executed to define the intersection between two groups of faces that penetrated each other, in this case at an angle, and consequently to specify a closed volume between them, the unnecessary surfaces were deleted using the "Delete" button of the Geometry Group.
4. The above step 3. ensured that all the overlapping and missing surfaces, including "Single Cons" errors, were well taken care of but there were also a number of "Triple Cons" errors present in the model which was revealed when the "Geometry Checks" was performed again, some of which contained surfaces of different parts that were not necessary and these were accordingly removed using the "Delete" button of the Geometry Group.
5. At this stage and for the given model, no further geometry associated operations were required, as was also confirmed by executing another "Geometry Checks" except for a few "Triple Cons" which were required surfaces to describe features of essential parts for the given Turbocharger model, and surfaces comprising particular bodies were assigned within the PID manager such as "Blade stage-1", "Blade stage-2", "Impeller", "Shaft rotor", "Turbo_casing", "Turbo_casing_inlet", "Turbo_casing_outlet", "Compressor_casing", "Compressor_casing_inlet", "Compressor_casing_outlet" and "Inlet_casing_cover", which also would act as boundary conditions during the pre-processing stage for a CFD simulation, before proceeding to the next step of surface meshing. It should be noted that due to the complex features of the given model and four out of eleven components forming the internal parts of the Turbocharger model, visibility tools such as "Or", "And", "Not", "All" and "Lock" were made use of to enable ease in selection of the concerned faces as part of their respective bodies.
6. Meshing operations were defined within the "MESH" module which for ANSA-CFD contains buttons for both surface and volume meshing within the same window.
7. The first functionality selected was "Init" under the list of "Perimeters" buttons to essentially reset the "NUMBER" and "LENGTH" aspects of selected perimeters which is also recommended as a best practice when a new mesh needs to be created from an existing or incorrect mesh in order to avoid dealing with issues such as conflicts.
8. The meshing parameters were defined by accessing the "Auto CFD" option within the "Spacing" functionality under the list of "Perimeters" button functionalities which is to define the distribution of nodes based on element length value. For the selected feature, two options are available, "Perimeters" to set the element length value on CONS (Curves On Surfaces) and "Macros" to perform the same operation on "FACE,SHELL". In the given case, using either of the options, values of "Minimum Target Length" and "Maximum Target Length" under the "Spacing Parameters" tab of "CFD SPACING PARAMETERS" window were set to 3mm and 7mm for a target length of 5mm, 1mm and 5mm for a target length of 3mm and 0.5mm and 2mm for a target length of 1mm respectively for the above mentioned PIDs as seen in the table below:

   Sr. No.	PID	Target Length [mm]
   1	Blade stage-1	1
   2	Blade stage-2	1
   3	Impeller	2
   4	Shaft rotor	1
   5	Turbo_casing	5
   6	Turbo_casing_inlet	5
   7	Turbo_casing_outlet	5
   8	Compressor_casing	5
   9	Compressor_casing_inlet	5
   10	Compressor_casing_outlet	5
   11	Inlet casing cover	5

   Table: PIDs and their surface mesh target length values
9. The above step also reflects in the values of the same attributes when "Mesh Parameters" option is selected with the only change being the element type which was selected as "Tria", and step "8." needed to be repeated within the "Quality Criteria" button functionality by enabling "min length" and "max length" quantities and reusing the previously mentioned values.
10. Post-definition of the necessary meshing parameters, the "Select" sub-option within the "CFD" functionality under the list of "Mesh Generation" button functionalities was selected along with the "PID" surface option for the appropriate PID in order to generate the required surface meshes on the different components of the Turbocharger.
11. Similar to the surface meshing, volume meshing was also created for select defined volumes which included the internal voids of the Compressor Casing, Turbo Casing and Inlet casing cover along with the volumes removed as a result of the internal components of the shaft rotor, Blade stage - 1, Blade stage - 2 and Impeller from the previously mentioned components. The required volumes were demarcated using the "Define" functionality under the list of "Volumes" button functionalities in the "Mesh" Modules Buttons followed by selecting the "Remesh" function with "Tetra CFD" option to generate tetrahedral volume cells.

3. Results

Fig. 1 represents the Turbocharger geometry (after Topology cleanup and PID assignment), Fig. 2 to Fig. 8 indicates the surface meshes along different views and internal components, including the transition between them, for the different target lengths as was specified in Table 1, and Fig. 9 to Fig. 11 highlight the volume meshes and its sectional views for the Compressor Casing and Turbo Casing with the internal components of the shaft rotor, Blade stage - 1, Blade stage - 2 and Impeller extracted from the respective bodies. It can be observed that the element type across the model is "Tria" for the surface meshing and on using the "Measure" tool with option set to "Nodes" it was found out that the average distance between two nodal points over the entire surface was representing their respective target length values approximately. Similarly, it can also be noticed that the element type for the volume meshes are "Tetrahedral".

Fig. 1: Turbocharger Geometry (Compressor casing and turbo casing bodies are transparent to reveal internal components)

Fig. 2: Surface Mesh of Turbocharger for target length values as specified in Table 1

Fig. 3: Surface Mesh of external components of Turbocharger for target length values as specified in Table 1 along the side view

Fig. 4: Surface Mesh of external components of Turbocharger for target length values as specified in Table 1 along the top view

Fig. 5: Surface Mesh of external components of Turbocharger for target length values as specified in Table 1 along the bottom view

Fig. 6: Surface Mesh of internal components (Blade stage - 1 and shaft rotor) of Turbocharger for target length values as specified in Table 1 along the front view

Fig. 7: Surface Mesh of internal components (Blade stage - 2 and shaft rotor) of Turbocharger for target length values as specified in Table 1 along the front view

Fig. 8: Surface Mesh of internal components (Impeller and shaft rotor) of Turbocharger for target length values as specified in Table 1 along the front view

Fig. 9: Volume Mesh of Turbocharger for select volumes (internal voids)

Fig. 10: Volume Mesh highlighting cell size transitions between the Internal volume of the compressor casing, and shaft rotor, blade stage - 1 and blade stage - 2 components

Fig. 11: Volume Mesh highlighting cell size transitions between the Internal volume of the Turbo casing, and shaft rotor and Impeller components

4. Learning Outcomes

The learning outcomes from this challenge can be outlined as follows:

1. Identification of overlapping and missing features, and errors in geometry using "Geometry Checks" and deletion and/or construction or re-construction of the same using different modules buttons under "Topo" section as part of the Topology cleanup operation.
2. Assignment of PIDs through appropriate collection of surfaces belonging to the concerned body offering better control on meshing requirements as well as providing the benefit of re-using them at the time of defining boundary conditions.
3. Use of "Init" functionality as a best practice method before defining meshing parameters.
4. Setting of different parameters under various button functionalities such as "Auto CFD", "Mesh Parameters" and "Quality Criteria" to set minimum and maximum target length values of elements and the element type.
5. Creation of surface mesh using the "Select" and "Visible" sub-options within the "CFD" functionality.
6. Creation of volume mesh using the "Remesh" button functionality and "Tetra CFD" option within the "VOLUME" window.

5. Conclusion

The surface and volume meshing of the given Turbocharger model and its components using different button functionalities was well understood using ANSA-CFD. It was realised that a surface and volume mesh with the former described on the values of minimum and maximum target length for element type as Tria and Tetrahedral in case of the latter can be created and that the same model can be further refined and/or updated depending on the prevailing boundary and operating conditions of the concerned fluid flow application.