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First, the region that will be simulated must be determined. This is known as the Background Mesh Box (Also commonly called the bounding box or domain). Typical guidelines are to use 2-8 times the car’s dimensions as the dimensions of the domain. Less space is needed on the sides, moderate space is needed in front of the car, and the majority of space should be behind the car. So for MSXIV (Approximately 5 m long, 2 m wide, 1 m tall), the domain should extend about 15 m (= 5 m * 3) in front of the car, 35 m (= 5 m * 7) behind the car, and 5 m above and on either side. Now, it is important to note that there is a plane of symmetry running down the center of the car. This means that we only need to simulate half of the car. The resulting drag (and other forces) can be doubled when post-processing, since each half of the car should have identical drags. This can drastically reduce the number of elements in your mesh, without sacrificing resolution, and is something that was not taken advantage of for the design of MSXIV. The end result is a domain that is still 50 m long and , 5 m tall, but only and 5 m wide, offset to one side of the car.
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Using a mesh that is 20x20x200 is not fine enough to represent the flow patterns around a vehicle . This is why refinements existso refinements will be added around the car. Locate the Refinements section in the navigation panel, and begin by adding a bounding box refinement. Change the Face to Ymin, Min thickness to 0 m, and Final thickness to 12e-3 m. Save the refinement by clicking the blue check mark in the top right of that panel. Add a region refinement. Change the Refinement mode to Distance and use values similar to those from Table 2.1. These values can vary depending on how many elements you want in your mesh.
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Distance [m] | Level [-] |
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0.75 | 4 |
1.255 | 3 |
2.5 | 2 |
Assign this refinement to the car geometry by clicking the car in the geometry viewer (you may need to click inside the box that says Pick Volumes first so that the program knows you’re ready to select the geometry, ensure the box is blue). Add 2 more geometry refinements, this time leave the modes as Inside. Set the level of the first one to 2, and assign it to the larger Cartesian box geometry primitive that was created earlier by clicking the sliding switch to the left of the label at the bottom of the panel. Then set the level of the second one to 3 and assign it to the smaller Cartesian box geometry primitive that was created earlier. The next refinement is called a Surface refinement. I started using it because I find it helps with the step afterwards (forming the boundary layers). Set the Min level and Max level to 5 and assign it to the car entity. We don’t need cell zones for this type of simulation.
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To select all faces on the vehicle quickly simply hold b, click, and drag a box that encompasses the geometry. |
Those are all the refinements that I generally use, however the problem is that if this mesh is generated now the boundary layers will be missing. Maybe not all of them, but the vast majority in my experience. Proper boundary layer modelling is critical for predicting fluid behaviour around the vehicle so this must be corrected. My understanding of why this happens is that all of the layers are actually formed as requested during mesh generation but many of these layer elements do not meet quality standards and are removed. The solution is to either create higher quality layer elements or to relax the quality parameters, allowing lower quality elements to be used. Both of these require the advanced settings section I mentioned earlier (Return to Mesh in the navigation panel and scroll down to find the advanced settings).
Scroll down until the Snap controls, change the settings indicated in Table 2.2, then scroll to the Layer adding controls. Change the following settings , and make the changes indicated in Table 2.3:
Table 2.2: Snap controls
Setting | Default Value | New Value |
|---|---|---|
Mesh to geometry conformation | 5 | 3 |
Tolerance | 2 | 1 |
Solver iterations | 150 | 300 |
Table 2.23: Advanced layer addition settings.
Setting | Default Value | New Value | |||
|---|---|---|---|---|---|
Slip feature angle | 75 | 60 | |||
Relax iterations | 8 | 20 | |||
Surface normals max smoothing iterations | 2 | 20 | |||
Internal mesh max smoothing iterations | 5 | 1050 | |||
Layer addition thickness max smoothing iterations | 10 | 50 | 100 | Max layer iterations with strict quality controls | 20 |
Layer addition max iterations | 50 | 200 |
These changes are mostly to allow more iterations to generate higher quality layer elements, with the exception of the last onethe layer elements so that they are of higher quality. A few of these settings also speed up the mesh generation process. Then scroll down to Mesh quality controls and make the changes indicated in Table 2.34:
Table 2.34: Mesh quality settings.
Setting | Default Value | New Value | |||
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Max non orthogonality | 70 | 7565 | |||
Min volume | 1e-131e | -18 | Min determinant | 0.001 | 1e-61e+30 |
Relaxed max non orthogonality | 75 | 80 |
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70 |
You can now hit the Generate button in the bottom right of the settings panel.
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I am quite certain these These settings are likely not optimal. From the limited testing that I did, these gave the best mesh results. Future projects should allow some time for finding more optimal mesh settings, even before the mesh independence study, although there are still bad spots on the mesh. Some of the iterations I’ve added might also be unnecessary, and the mesh generation might be able to be sped up by reducing some of the maximum iterations for certain processes. |
Reviewing the Mesh
After the mesh has been generated, navigate to mesh and click the yellow button that appears to select the mesh as your domain. The icon besides Mesh in the navigation panel should change from yellow to green. Before proceeding, we should check to make sure it looks right and that no mistakes have been made. SimScale has several tools for this, some of which have been recently added. The first and most basic is the mesh element count, found in the general mesh settings under Mesh selection. If your mesh is way smaller or way larger than expected, something may be wrong and any issues should be addressed before moving onto the simulation. A more detailed inspection can be done by looking at the mesh in the 3-D display. The symmetry plane provides a convenient cross-section to look at the boundary layers running along the middle of the car, and the mesh clip tool can be used to create any other cross-sections. Finally, you can navigate to the Mesh quality section to conduct more quantifiable analysis of your mesh quality, although it does take a while to load.
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On meshes with around 8M elements or more, mesh clips will start failing. |
You may also have noticed a section called Meshing log, this can be used to see what the mesh generation algorithm is doing while the mesh is being generated. I would consider this to be an advanced tool, but it can be useful for troubleshooting problems. For example, this is the main tool I used to identify which of the advanced mesh settings needed to be changed to retain more boundary layersThis can be difficult to read at first, but it does provide a lot of information about the mesh.
If the mesh is an appropriate size, with an appropriate number of boundary layer cells, and no obvious defects you can continue to the actual simulation.