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temperature undershoot problem in time dependent simulation near time t = 0

Posted Feb 6, 2022, 10:25 a.m. EST Heat Transfer & Phase Change Version 6.0 2 Replies

Barry Ren Imaging physics
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I am a new user and I have a simple 2D axis-symmetric model to test. In my first run of a time dependent simulation, at time t = 0, I raise temperature of one edge boundary with a delta T of +30 K above the ambient temperature of 300K and see how temperature changes in the nearby tissue volume. The temperature near the edge rises as time t becomes large (as expected). However, for very small time of t <0.4 second, there is a temperature undershoot near the edge of ~4K below the ambient temperature. Why is this happening? The temperature should never go below the ambient temperature in this simulation. What could be wrong in my simulation to get this wrong behavior? I searched old posts and there was only one post of 2010 talking about a undershoot problem, and it was different to my simulation conditions.

Thank you!

2 Replies Last Post Feb 7, 2022, 9:38 a.m. EST
Robert Koslover Certified Consultant
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Posted: 3 years ago Feb 6, 2022, 5:07 p.m. EST

Your mesh is much too coarse in the region of interest. If you use a much finer mesh in the vicinity of your heated boundary, the undershoot disappears.

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Scientific Applications & Research Associates (SARA) Inc.
www.comsol.com/partners-consultants/certified-consultants/sara
Your mesh is much too coarse in the region of interest. If you use a much finer mesh in the vicinity of your heated boundary, the undershoot disappears.
Barry Ren Imaging physics
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Posted: 3 years ago Feb 7, 2022, 9:38 a.m. EST

Dear Robert, thank you very much for your advice! The problem was solved after I used finer mesh around the boundary. I also found that, between normal mesh and extra-fine mesh modes provided by Comsol, the actual mesh sizes along boundary of interest did not change much. I had to manually set smaller mesh size over the boundary to see the problem to disappear. I appreciate your hint, it was a great learning experience!

Dear Robert, thank you very much for your advice! The problem was solved after I used finer mesh around the boundary. I also found that, between normal mesh and extra-fine mesh modes provided by Comsol, the actual mesh sizes along boundary of interest did not change much. I had to manually set smaller mesh size over the boundary to see the problem to disappear. I appreciate your hint, it was a great learning experience!

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