SimScale CAE Forum

Meshing problem

I am relatively new to CFD-Simulation and I ran into a problem while working on a project. I am trying to model a centrifugal pump with a pipe attached.

I am using OnShape to create the pump with the pipe for analysis and when I import the CAD objects, the geometry is reported as water tight. However, after meshing, there is a warning (in yellow/orange), that the “tessellated surface is not closed”. So I am wondering why that would happen if the drawing is water tight. Also, even though the material point is inside the object, the outside was meshed so I am assume there is a hole somewhere.

Does the mesh just need to be refined more?

Is it possible for the meshing process to leave behind holes even though the CAD is water tight?


Hi @mas985,

your mesh is not set up in the way it has to be for this type of analysis you are trying to do. At the moment it looks like you are trying to perform an external flow analysis which is probably not your intention. I would suggest you have a look at the properties of this project here: Centrifugal Pump With MRF

Let me know how things go!



Thanks. I started with that project and made some modifications to the CAD. However, I noticed that project also has the same errors so I was assuming that it was not correct either.

But can anyone answer the questions I posed above?

Hi @mas985

Yes it needs to be refined way more than it is at the moment.

Could you elaborate what you mean by that applied to your model?



After importing the CAD from OnShape, there is a message that states the “Geometry is water tight”.

However, after meshing, there is a note/warning/error that the “The tessellated surface is not closed”.

What does that mean?
Is that why the mesh was outside instead of inside?
What caused the mesh to be outside instead of inside when the material point was placed inside?

I am trying to understand in detail what went wrong and why.


Hi @mas985 and sorry for the delayed answer.

I had a look at your project already. Please let me confer with my colleague about your project first. I will come back to you as soon as I have some information about the CAD handling system @AnnaFless mentioned in yesterday’s answer and if this is causing some issues in your case.



I simplified the analysis to a single pipe with a 90 ELL and use the automatic meshing for CFD and I think I ran into the new CAD handling issue here. This time it meshed the pipe and ELL just fine but it treated it as a single solid object. See here:

So I cannot select any of the two input and output faces for boundary conditions in the simulation.

Question about the CAD though. When reviewing some of the public projects for internal CFD analysis, I noticed that some of the Geometries are shells and some are solids.

What are the actual requirement for the CAD drawings when doing an internal CFD analysis?

I know the geometry has to be closed (i.e. water tight) but does it also have to be hollow as well?

Should I run the shell command in OnShape before importing it to SimScale? I didn’t think it mattered.

Hi @mas985, no need to shell you can have a solid and mesh it with the material point within and IT will be the fluid domain. Hope this clears it up :slight_smile:

Kind regards,