A common technique in distributing ducted flow involves thin guiding vanes, or baffles. One of the biggest hurdles to modeling baffles is how thin they are relative to the rest of the model. If you were to model their true thickness, you typically have a choice between poor quality, skewed elements and an excessively high mesh count. Instead, thin baffles are often approximated as infinitely thin. There are several ways to create these zero thickness baffles, and the method you choose will affect your meshing strategy as well as your boundary condition setup. Today we will focus on meshing challenges, and next week in Part 2 we will cover setup in CFX and Fluent.
Default Baffle Meshing; Non-Manifold Faces
The traditional method for creating baffles involves grouping surface bodies and solid bodies into a Part in DesignModeler (or into a Component using the Merge Topology option in SpaceClaim). This results in a single solid body in the Mesher with internal faces where the surface bodies intersected it.
These internal faces (with the same body on both sides of the face) are referred to as Non-Manifold Faces, and had to be created using the method above because most geometry editors don’t actually allow non-manifold faces. In the Mesher, non-manifold faces limit the types of mesh methods that you can use: except in rare cases, you must use tetrahedral elements (which will typically drive up your mesh count and negatively affect the convergence rate)
.Inflation on these non-manifold faces is also problematic. For a finite thickness wall, inflation would simply wrap around the thin face from one side to the other; but for an infinitely thin baffle there’s no way for it to wrap around without producing a degenerate (i.e. collapsed) element. Instead, the Mesher will stair-step the inflation down to nonexistence by the time it reaches the end of the baffle.
Not only does this create poor quality pyramid elements where the stair step occurs, it tends to result in high y+ and bad element transition ratios, which can be just as problematic. It is generally recommended to fix this behavior using one of the options below.
Option 1: Adjacent Nonconformal Interfaces
The problems listed above stem from the fact that the baffles are composed of non-manifold faces. The fix is then to convert them exterior faces instead!
Rather than having surface bodies that end in the middle of a solid body, we could slice the model in such a way to make each passage between the baffles its own body, and then combine these bodies into a Part. We can then give a Named Selection to these faces to be treated as walls.
The bodies adjacent to these passages will need to be nonconformally meshed though (i.e. be a separate Part so they do not share nodes). If you were to leave everything as a single Part (or Merged Topology Component), The inflation layers would branch out and produce many of the same problems we saw with the tetrahedral inflation layers.
Option 2: Extending Baffle Faces
A drawback of Option 1 is that it typically results in a lack of mesh refinement immediately downstream of the baffles, which is an area where you often need more resolution.
Instead of placing nonconformal interfaces directly adjacent to the baffles, you could offset these and extend the baffle faces up to these offsets (alternatively you could not use any nonconformal interfaces and just extend the baffles to the ends of the model). The trick would be to only select some of the faces between these bodies to be baffles. Any face between bodies but within a multibody part that is not explicitly given a Named Selection will be treated as interior and not block the flow.
Option 3: Nonconformal Interfaces as Baffles
The above options use multibody parts to define the baffle locations; if you are using a swept mesh this forces all the passages to use the same number of divisions in the sweep direction. If each passage were drastically different in size, you might want to use different mesh controls for each passage, while still maintaining hexahedral elements and baffles in between. This can be done with a noncoformal mesh between the passages.
Like before, each passage between the baffles is split up into its own body, but also remains as its own Part. Then, we only need to specify where the interfaces go. Faces where the baffle doesn’t exist will need to be set as interfaces, but baffle locations will only require interfaces if we need to pass data across them (like heat flow, species diffusion through a membrane, etc.).
In our next blog, we’ll discuss how to handle the setup of infinitely thin baffles in CFX and Fluent.