SimScale CAE Forum

Empty boundary condition


Hi, I am doing an FS body simulation where the radiator is modeled as a single stl surface- porous rectangular block, which is not aligned with any of the origin planes. The mass flow rate through the radiator is a parameter that i’d like to monitor and optimize, but since the radiator is one merged stl surface, I can’t select the inlet in a surface data result control method. so:
-Is it possible to create a surface geometry primitive?
-If i merge the existing radiator with its inlet face (multi surface stl), is there an empty boundary condition that i could apply to the inlet surface? would it cause a problem in the solver that two coincident surfaces have different boundary conditions?
any other simpler solutions?


Hi @Naly,

interesting question! I am tagging my colleague @sjesu_rajendra here. He might give you more information about your issue.

All the best,



Hey Jousef, thanks for tagging me in!!

Hello @Naly,

Yes, there is a simple solution for your case. Please try to use the STL surface split geometry operation before proceeding to mesh operation. This way you can make assignment to individual faces. Meanwhile, I’m not exactly sure how you would want the flow to be. Is it something like a bulk flow where the porous region is present non-aligned to the axes? In this case you need to model the bigger domain, or use a larger bounding box to get the necessary mesh. The template projects to define porous region can be found from the public library.

I hope this helps! Please feel free to contact if there are any other queries.



@jousefm @sjesu_rajendra
Hi, thanks for replying
I exported each of the radiator block six surfaces from the cad software separately and merged them with the rest of the car.
I still can’t select the radiator inlet in the surface result control method, since after the meshing, all porous block surface were grouped together as cell zone bounding faces:

The mesh refinement is the same as in the fs tutorial:

NB: I didn’t use the STL split operation, since that would explode all body parts surfaces (which have different mesh refinement levels and different boundary conditions). Unless there a way to split just one surface of a multi surface stl?


Hey @Naly,

Thanks for making it clear. By default the mesher groups all faces into one during cellZone creation. These faces can’t be selected to define result control item on. The alternative would be to create a combination of slice and clip filters in order to obtain the inlet face and then infer results.



To be clear, you mean clipping parallel to the radiator inlet face, approximating the radiator position in that plane, and approximating further the average velocity on that face from the velocity contours.
wouldn’t that be too erroneous for optimization? Is it doable to just create interior type bounded surfaces for results control?


@Naly Yes, that’s exactly what I mean. The ideal workflow would be to create a slice at the inlet point, then add clip filters along the directions to get the size. The area average/integral filter in Paraview can be used to get the value across the complete area. This state can be saved and later used for other simulation setup as well. Unfortunately it is not possible to create a new boundary for this purpose.


Thanks for for your help