Engine-Transmission-Gear box Surface wrap meshing and OpenFOAM thermal simulation
JORGE LOZANO
Advanced CFD Meshing using ANSA
Challenge - 5
Engine-Transmission-Gear box Surface wrap meshing
Objective
In this challenge a surface meshing is performed, adjusting each individual surface wrap for the three separate parts with wrap size for the engine as 8 mm and for the other two parts as 5 mm, upon the creation of the surface wrap, the original CAD parts must be deleted and the file must be compressed.
The file parts must be combined and import into one common file, assigning separate parts entities in the manager and delete unnecessary PIDs and parts, concerning a single volume for a further thermofluidic analysis.
Theory
ANSA is an advanced multidisciplinary CAE pre-processing tool that provides all the necessary functionality for full-model build up, from CAD data to ready-to-run solver input file, in a single integrated environment.
ANSA is the users’ preference due to its wide range of features and tools that meet their needs. The list of productive and versatile features is long and the alternative tasks and processes to be completed using them are countless.
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
The advantage of Wrapping is that it saves considerable amount of modeling time in the creation of watertight surface mesh, compared with traditional techniques that employ a series of steps. The traditional steps include:
- isolation of relevant geometry from detailed design data;
- treatment of design and translation related issues such as cracks, holes, gaps etc.
- and topological connection between intersecting parts that are often tedious and time consuming.
Constant Length
Constant Length Wrap is a very fast algorithm that can create a watertight triangular surface mesh of uniform size from an assembly of unconnected and intersecting components. It can be used for various analyses like CFD, NVH and more.
Skewness
The skewness of a grid is an apt indicator of the mesh quality and suitability. Large skewness compromises the accuracy of the interpolated regions. There are three methods of determining the skewness of a grid.
Another common measure of quality is based on equiangular skew.
where:
Skewness' of 0 is the best possible one and a skewness of one is almost never preferred. For Hex and quad cells, skewness should not exceed 0.85 to obtain an accurate solution.
For triangular cells, skewness should not exceed 0.85 and for quadrilateral cells, skewness should not exceed 0.9.
Operations:
1.Activate the function and in the window that opens specify
a- Outer or Inner wrap: select if you want the exterior or the interior watertight surface mesh
b- Target Element Length: specify the size of the shell mesh. The smaller the size the more detail will be captured but the more possible leaks may take place if the original model has any
c- Smooth Factor: The resulting surface mesh may deviate slightly from the original model as it is smoothed in space. The magnitude of the smoothing varies from 0 (none) to 1 (max)
d- Mode: Select which mode should be used for the nodes and edges of the wrap mesh
Fig.1 Creates a watertight surface mesh around an assembly which may have FEM and/or macros.
e- Wrap PID: Specify if the wrap mesh should copy the PIDs of the original model or if it should be placed in a new PID
f- Excluded volumes seed points: Specify points in space where you do NOT want the wrap octree to leak into. In this was if there is a leak ANSA will stop and create curves paths through it so that the user can patch them
g- Work on: Select if you want to wrap the visible or the selected entities
2.Press Next
3.The created octree is displayed. If several volumes are detected the user can select which individual ones should be wrapped
4.Press Next to see the final wrap mesh.
Procedure
The following CAD models are represented to be the engine, gear box and transmission assemblies in Fig.2:
Fig.2 CAD surface models
In Fig.3, Fig.4 and Fig.5 are shown the same CAD models separately for which in the objective is to hide the important PID parts in the 3-D assembly and eliminate the extra parts that helped the CAD engineer design several assembly parts that must be eliminated in the final CAD but are recognized in the export file to ANSA, seeing as well the single and triple coons (contours) that wont be needed for this analysis or mesh application.
Fig.3 Engine model errors and clean model respectively.
Fig.4 Transmission clean model.
Fig.5 Gear box model errors and clean model respectively.
One of the commands to consider is the release coons to separate all surface faces and consequently fill all holes necessary such as for holes, bolt connections and modify (cover or open) any areas where considered by the engineer according to the simulation to perform.
Once having all coons released and all important holes filled with an extra surface, for each separate surface will be selected in the mesh right-top window MESH, to respectively select in the Element section: constant Wrap opening the left window shown, adding the respective Target element length that for the engine is 8 and for the other parts will be 5 and by selecting next a preview tridimensional block mesh is shown to obtain the engineer approval and finish to perform the selected surface mesh.
As follow in Fig. 6, Fig. 7 and Fig. 8 are shown each one of the wrap surface meshing having the respectively for the gear box having 39386 total triangular surface mesh, the transmission 38185 triangular elements and the engine 91587 triangular elements all of them maintaining the surface PID for later simulations.
Fig.6 Gear box model wrapping surface meshed.
Fig.7 Transmission model wrapping surface meshed.
Fig.8 Engine model wrapping surface meshed.
Once the meshing is confirmed to be a good for simulation mesh it’s the next step to delete the original CAD models and only maintain the surface elements with delete command, and once doing that merge all three parts into one only file as in Fig.9, showing the correct alignment of the parts
Fig.9 Assembly model complete wrapping surface meshed.
In Fig.10 it’s showing an interface between the engine part and the already cutted out gear box, for whith the connection between the last one and the transmission will be appearing the same, it’s necesarry to connect by elements both sections not having and intersection or single coons, by using the DELETE element command , and applying the NEW element or FILL command we are able to delete unnecesary elements (interfering), and fill a surface full of elements to cover the cutted patch, and create new mannual elements from nodes or faces.
Fig.10 Engine and Gear box models interference wrapping surface meshed.
In Fig.11 the image represents the assembly completed and by hiding some PID’s its able to show the surface without volume with the respective PIDs, having the posibilit to performe and external or internal volume mesh depending on the case analysis to perform.
Fig.11 Transverse opened models wrapping surface meshed.
The requirements are done but by having applied a tetrahedral volume mesh obtaining 1245228 elements in the body shown in Fig. 12 and Fig. 13 with transverse cuts, it’s considered to have a fine mesh according to the size of the element and the whole volume.
Fig.12 Volume and surface meshed geometry with a sliced opened section.
Fig.13 Transverse cut geometry with tridimensional elements.
Finally, in Fig.14 the model browser window is open, where the surfaces and volume PID’s are shown, apperaing to have three surface parts having each of them sub- PID’s surfaces and a whole volume PID, this being possible by performing a good application of 2-D and 3-D mesh.
Fig.14 Mesh with its respective PIDs.
Another method to realize the meshing for the simulation is by cleaning the geometries one by one, and consequently by merging all three parts into one assembly, releasing all coons and filling all holes necessary, then by applying a constant mesh or variable mesh to the whole assembly which in Fig. 16 is seen a final result of a complete surface mesh, having on PID for the part and the volume and selecting the 3D mesh to complete the same exercise.
Fig.15 Merged parts into one assembly.
Fig.16 Constant element length merged surface mesh.
Having in Fig. 17 are the complete meshed assembly parts and in its right a sliced section to show the tridimensional elements created having a constant length element of 5mm in all three section
Increasing the element number and having only one general part section instead of three as mentioned in the requirement objectives, but having an even faster pre-processing time of the process, although it would affect in the processing time by having the same equations or engineering theory in each element but for more of them compared to the original (first) meshed assembly.
Fig.17 Optional second process type meshes model.
For both meshes specifically the first one which will be used to perform a transient thermal analysis by using OpenFoam and applying the laplacianFoam solver havin the Laplace equations to obtain the temperature in each node or element through time, the skewness, warping, angle and negative vol. parameters are checked to maintain a good and considerable optimal mesh quality too obtain good and expected results without having any kind of deviation in the results, representation shown in Fig. 18.
Fig.18 Quality view for skewness, warping, angle and negative vol. checkup.
Application
Internal heat transfers-transient diffusion analysis
By using a processor (OpenFOAM), and a post processor (paraView) it’s able to see the application and one simple possibility for exporting this surface and volume mesh as a complete material to see the thermal diffusion and obtain results from which the engineer might be able to apply theory and experience to performed a better engineering design.
The solver applied is the Laplacian solver:
Fig.19 Section parts for simulation.
By applying the following fvScheme file:
Fig.20 fvScheme simulation file, used in OpenFOAM.
The simulation was ran in OpenFOAM as a parallel Scotch simulation along 4 processors, in which the sections are shown in Fig. 21.
Fig.21 Model sections for parallel analysis
The following images show the thermal diffusion in 10 minutes from being at 273 K to having a top surface shells at 473 K, and the middle section at 323 K as well as the front surface, showing a 3D video representing the variation over time of temperature for the assembly.
In Fig. 22, Fig. 23 and Fig.24 are showing three images from which he simulation is varying its temperature according to the diffusion gradient for the whole volume, considering that it has only one material, not being a real or applicable analysis but for learning and application view, but viewing the possibilities for ANSA to export the preprocessing files in this case to OpenFOAM but having the possibility of having a really good meshing algorithm and still being able to apply and export the initial conditions and PID necessary to just run the case in a processor software. Video 1 shown the Temperature vs. Time color gradient over the geometry analyzed.
Color gradient was selected according to the colors being able to see by a thermographic camera which in real life it’s possible to obtain these values in real time and could be compared to these computational results.
Fig.22 Initial temperature conditions- assembly image.
Fig.23 Image while performing the analysis- temperature conditions.
Fig.24 Final temperature conditions- assembly image.
Video 1. Temperature vs Time in the Automotive Assembly.
Conclusion
By applying the wrap surface mesh technique it’s shown to have a faster time processing and then merging the parts to make a whole assembly and being able to perform an internal volume to export the mesh once applied all PID necessary.
Quality surface and volume mesh are validated according to Ansys- Fluent parameters with skewness, warping, angle and negative volume.
The surface mesh was performed successfully performing afterwards a simulation in OpenFOAM, by having three different parts with an 8mm engine, and 5mm transmission and gear box surface element size mesh, maintaining a complete volume by merging all three surfaces.
References
https://www.openfoam.com/documentation/guides/latest/doc/guide-schemes-laplacian.html