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Laser pulse heating and cooling: Temperature unphysically drops
Posted Jul 14, 2021, 4:27 p.m. EDT Electromagnetic Heating, Heat Transfer & Phase Change, Studies & Solvers Version 5.6 2 Replies
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Hello,
I am simulating the heating of a copper foil by x-ray laser pulses of 50 fs duration spaced by 444 ns.
To approximate this, I am defining a large disk/cylinder with ambient temperature boundaries and in the centre of that disk a smaller cylinder which region is referred to the focal spot. Then, I instantly raise the temperature T of a focal spot by a certain amount DT that relates to the laser pulse energy, absorption efficiency, and number density of absorber. Throughout the simulation, I am watching the temperature initialise, decay, and once the next pulse arrives, replace the temperature T within the focal spot with T+DT: 
as Explicit Event.
This works great for the first pulse, however, for multiple absorption cycles, the temperature between the focal spot and the disk edge goes below ambient. This is unphysical and I think related to the solver or my poor temperature initalisation that causes a clear jump at the focal spot - large disk edge.
Attached is my model where I have also added descriptions, which executes in 5 seconds on a MacBook Pro 2018. Also attached is a plot of the radial temperature, showing the increase of focal spot temperature, the ambient disk edge, and the oddly dropping central temperature.
I googled a lot (which is relative of coruse), but am new to COMSOL, so think it might be a simple thing I'm doing wrong.
I'm happy about any help you could give me, please, starting from some more standard debug steps (tried smaller time steps, finer mesh, some solver tolerance changes, and more) to just solving all my problems and writing a paper for me... Jokes aside, thank you very much in advance for your potential time.
Best regards, Oliver
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Try smaller time steps
Events may not be appropriate.
Attachments:
Please login with a confirmed email address before reporting spam
Try smaller time steps
Events may not be appropriate.
Dear Lin,
Thank you very much for your post. I have varied the time steps (double checked whether I properly did that in the first place), but to no avail. However, I've made a custom mesh, and thereby meshed much much finer than before - with great success! It doesn't fully fix the temperature drop, but at least it makes it arbitrarily close to ambient temperature.
For computational efficiency, it might be better to properly configure the solver though, as the simulations now take very long (related to the fine mesh). Any feedback on this highly appreciated still.
Again, many thanks for your input! Oliver