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Kinetic Energy in CFD

Posted Aug 13, 2021, 4:22 p.m. EDT Computational Fluid Dynamics (CFD), Results & Visualization 1 Reply

David Reisman
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Doesnt look like COMSOL outputs kinetic energy automatically. I've had to postprocess the results to get KE. Specifically, I do a volume integration of rho v^2 over all my stored timesteps in Derived Values - Volume integration. This takes a long time! Does anyone have a faster way to get kinetic energy from a transient CFD calculation?

1 Reply Last Post Aug 14, 2021, 6:10 a.m. EDT
Edgar J. Kaiser Certified Consultant
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Posted: 3 years ago Aug 14, 2021, 6:10 a.m. EDT

David,

yes complex post processing in Comsol can be painstakingly slow. One workaround I sometimes use is doing the integration using an intop() operation during solving. I define a respective variable and I define a state variable pointing to this variable.

In my experience such integrations during solving do not noticeably increase the solution time. I assume during solving this runs in optimized C code while the post processing is based on slow interpreted Java code.

Using the state variable is key because only this way this variable becomes a part of the solution. Otherwise, if using the normal variable, the integration will be done in Java again. You don't need to run the solver again, update solution is sufficient after you defined it all.

State variables don't have a unit, you need to take care of that by yourself.

Hope it helps, cheers,

Edgar

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Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
David, yes complex post processing in Comsol can be painstakingly slow. One workaround I sometimes use is doing the integration using an intop() operation during solving. I define a respective variable and I define a state variable pointing to this variable. In my experience such integrations during solving do not noticeably increase the solution time. I assume during solving this runs in optimized C code while the post processing is based on slow interpreted Java code. Using the state variable is key because only this way this variable becomes a part of the solution. Otherwise, if using the normal variable, the integration will be done in Java again. You don't need to run the solver again, update solution is sufficient after you defined it all. State variables don't have a unit, you need to take care of that by yourself. Hope it helps, cheers, Edgar

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