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Once the size of the domain is determined, the resolution on the surface of the car must be decided upon. This is partially determined by how the boundary layer will be modelled. There are 2 common strategies for this: using full resolution or using wall functions. Whichever strategy is chosen will determine the length height of the boundary layer elements in the direction perpendicular to the surface, and the . The lateral dimensions of the elements should be similar. This determines the size of elements on the car’s surfaceclose to the height.

Then refinements should be used to create appropriately sized elements where neededbetween the car’s surface and the domain walls. Essentially, the area around the car should have the finest elements, then the edges of the domain should have the coarsest elements, and there should be a gradient of element sizes between them. There should also be moderately fine elements in the wake of the vehicle.

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That should bring up global mesh settings, meaning that these settings apply everywhere in the domain (Other settings may only apply to specific regions). Change the algorithm from Standard to Hex-dominant parametric. This gives more control over the mesh and primarily uses hex-based elements rather than tet-based elements. Now expand the Geometry primitives section in the navigation panel and select the Background Mesh Box. A panel should open with the minimum x, y, and z coordinates, along with the maximum x, y, and z coordinates. I used minimum values of 0, 0, and -35, and maximum values of 5, 5, and 15. Next, select the Material Point section. This point determines which volume is divided into elements, so make sure the point is located within the background mesh box, but outside of the car geometry. While we’re here, add a 2 Cartesian box geometry primitiveprimitives. Select a region trailing from the car , for example with one box inside the other one. For example use 0, 0, -20 and to 2, 2, -4 for one, and 0, 0, -16 to 1.5, 1.5, -4 for the other. This is to enclose the wake behind the car, with 2 different mesh densities. Return to the general mesh settings. Beneath the algorithm setting, there should be a section for Bounding box resolution. I’ve been using values of 1620, 1620, and 160200.

You’ll notice that there are a bunch of settings beneath these if you scroll down. I consider these to be advanced settings, and the defaults usually work for most cases. Not our case though, as there are some settings that need to be changed. For now, scroll down to the Layer adding controls and turn off Layer size and I’ll return to this section later for the rest of the changes. The reason I make this change now is that it changes the meaning of some other basic settings that we are going to set next and then I prefer to tweak the advanced settings at the end, since they might take a few tries to get right.

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General Refinements

Using a mesh that is 16x16x160 20x20x200 is not fine enough to represent the flow patterns around a vehicle. This is why refinements exist. Locate the Refinements section in the navigation panel, and begin by adding a bounding box refinement. Change the Face to Ymin, Min thickness to 1e-6 0 m, and Final thickness to 15e12e-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 the values similar to those from Table 2.1. These values can vary depending on how many elements you want in your mesh.

Table 2.1: Refinement levels used around MSXIV.

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 another 2 more geometry refinementrefinements, this time leave the mode modes as Inside. Set the level of the first one to 32, and assign it to the larger Cartesian box geometry primitive we was created earlier by clicking the sliding switch to the left of the label at the bottom of the panel. This next refinement depends on the kind of mesh you’re making. If you want to make a full resolution mesh, I would add it. If you’re using wall functions, you probably don’t need it. I will use it here though because it seems to help 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). It’s called a Surface refinement. 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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The last refinement we need is a boundary layer refinement. This allows the simulation to properly model the effects of the boundary layer on the airflow around the vehicle. Add the Inflate boundary layer refinement and set the layers, expansion ratio, min thickness, and inner layer thickness to the desired values. For a full resolution, I’ve been using 20 , based on whether you want to use wall functions or full resolution. I would start with wall functions, as they are computationally cheaper and I’ve had better convergence with them. I have been using 10 layers, at an expansion ratio of 1.252, a minimum thickness of 0 m, and an inner layer thickness of 26.96e6e-3 m. Select all the faces of the vehicle.

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 boundary layer separation, which can lead to massive increases in drag. This means we need to find out why the boundary layers are not being formed and then correct this if we want to accurately predict the drag of our vehicle. Why this happens is likely quite complicated and influenced by many factors. My understanding is that all of the layers are 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 more control over the mesh generation algorithm, which requires 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 Layer adding controls. Change the following settings indicated in Table 2.2:

Table 2.2: Advanced layer addition settings.

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These settings are to allow more iterations to develop high quality layer elements, with the exception of the last one. Then scroll down to Mesh quality controls and make the changes indicated in Table 2.3:

Table 2.3: Mesh quality settings.

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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 added after MSXIV’s aerobody design was locked. 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. The next quick check is to scroll down to the bottom of the Mesh quality controls and expand the new section called Event log. This section has element counts for each type of element. I believe the Number of prisms refers to the number of layer cells and can be used in the same way as the total element count. Fewer prisms than usual can indicate that too many layers were removed, which should be inspected and addressed before continuing. I was able to get around 16-18k 22k prisms for full resolution meshes, maybe closer to 8k for wall function meshes. 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 detailed and quantifiable analysis of your mesh quality.

Expand
title Mesh Results
Image RemovedImage Added Figure 2.1: Symmetry plane of the mesh generated by the discussed settings. Image RemovedImage Added Figure 2.2: Closer view of the boundary layer elements around the front of the vehicle. Image RemovedImage Added Figure 2.3: Closer view of the boundary layer elements around the pontoon, created with a mesh clip.

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