CFD over an (imported geometry) structure touching the flow volume bottom plane


I'm trying to simulate the wind flow over a membrane structure according to your "Double-Sided Membrane" and "Flow Over a Double-Sided Membrane" tutorials and I've run into a problem with correctly defining geometry in case the structure is standing on the ground.

My structure is a saddlespan with 2 side walls which are anchored directly to the ground. The geometry is imported from an .obj file. I'm doing everything according to the tutorials, but on the mesh generation stage there is always an error like this: "Warning: Flow volume [www.72755.5] Tetulator: Failed: Front advance failed: 3028 faces remain". When looking on the Area color map I can see that the mesh on the structure and on the ground plane is not actually continuous at the edges which touch the ground (screenshot: ), so I believe it most probably is the problem. Although it seems logical to me that when I subtract my structure faces from the flow volume all the intersection edges should also be generated. I even tried to move the flow volume a bit higher to make sure that my structure actually intersects the ground plane, but the same error occurs. But when I move the volume lower so that the structure is floating in the air — the mesh is generated just fine. So I'm a bit of at a loss here, how should this intersection be properly done?

Here are some links to the simulation files:
The file with all the geometry right before subtracting imported mesh from the volume:

The file ready to run the mesh generation (where the error occurs):

Topology only Boolean operations are restricted

Topology only Boolean operations are restricted to only include the geometry explicitly selected for the operation, so in your case it will not imprint edges on a coincident face.

Anyway, it appears that your structure is closed, i.e., there is no path for air to enter it. In this case you need to have a base face that closes your structure, then:

  1. Subtract (Topology Only) the structure's base face from the outer boundary base face
  2. Stitch (Topology Only) the remaining faces including the top portion of your structure to form the final flow volume

The double-sided face conversion is not applicable to your geometry if it is in fact closed.

Thanks! I've tried that and


I've tried that and now the mesh on the ground and on the structure seems to be stitched together, but I'm still getting the same type of error: "Warning: Flow volume [www.86678.5] Tetulator: Failed: Front advance failed: 48 faces remain". I've checked everything I could think of and the mesh seems to look nice and smooth, I can't see any sliver elements or major size inconsistencies. Vol ratio doesn't even show any "blue" elements, the worst values are about 0.3.
Here are the screenshot of the E ratio:

and the troubled model file:

Also I have a further question about modeling: what should I do when my structure is not closed and partially touches the ground? I.e. this saddlespan can be made with one, two or no side walls, so all these variants have to be analyzed.

Unconnected edges present

It appears there are unconnected edges in your volume. To correct, use the Edge->Connect tool on all edges and setup the Substance again, this produced a volume mesh without errors for me.

For your open membrane cases you need to:

  1. Subtract (Topology Only) all the edges (attached to the membranes that touch the ground) from the boundary base face in one operation
  2. Stitch (Topology Only) all the faces except for the membranes to reform the flow volume
  3. Subtract (Topology Only) the membrane face from the flow volume in one operation
  4. Assign all the membranes as 2-sided in one operation

Thank you! The unconnected

Thank you!

The unconnected edges happened to be the problem in each case.