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Boundary Condition on Internal Surface?


Boundary Condition on Internal Surface?

I have several scenarios where the end of a flat bar is in contact with the flat side of a plate of steel. There is a convection boundary condition on the side of the plate of steel and on the sides (not the end) of the the flat bar. When running the simulation for heat transfer, I get warnings about the boundary condition (on the side of the plate of steel) is against and internal surface (the end of the flat bar). I'm given the option to ignore it.

What is actually happening when it is ignored?

Does the simulator just disregard that boundary condition over the area of the end of the flat bar and use the material properties of both pieces to calculate the thermal conduction between the two pieces?

Or is the simulator altogether disregarding the thermal conduction between the two pieces?

Is there an interface type that I should define between the two parts that the simulator will recognize and know not to apply the boundary condition at the mating surface?

Many thanks for your help!



Hi Paul,

I'm trying to reproduce the issue that you described and am unable to. I don't see "the option to ignore it". Would you please provide some additional information about how to see the message? Perhaps a screen shot would help.


Tad Doxsee



Good question and I am not exactly sure what Mechanica is doing internally when this happens, if the conduction or convection overrides one or the other, or is some type of interpolation is applied. I could not find anything on the precedence rules of convection conditions versus conduction conditions when this situation happens.

I was able to quickly reproduce the warning and play around with different setups for this issue. There is a simple way to avoid this warning. Just create a surface region on your plate that is the area of contact between your plate and bar. This region will be where you want to simulate only conduction, and the outer portion of this region will be where you apply the convection condition. If you have the model setup as a bonded default, the conduction between bar and plate will be defined, provided your material properties are setup correctly. I compared with and without this surface region and there is a very slight difference. This difference might mean nothing for my case, but could mean something for a different case. I probably don't have all of your boundary conditions shown, but the important region is the contact between the bar and plate. The good thing here is that Mechanica does warn you about this situation.

Hope this helps,




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