Discussion Forum

Hello,

I want to model a pulsed laser that hits a solid, excites electrons which "thermalize" then transferring their energy to the lattice (solid, atoms). I think most people do not use COMSOL for this purpose, but I want to, because I started with classical heat conduction and now want to setup my model in greater detail.

The two-temperature model is basically a model where you define two temperatures for electrons and atoms in a material respectively. This is necessary if you take into account that the electrons react much faster on an external excitation (femtosecond laser pulse [1E-15 s]) than the atoms.

The question is: how to realize this in COMSOL . In principal I would expect to be able to add and couple two heat transfer physics with two different dependent variables Te (electronic) and T (atoms) and custom material parameters (thermal conductivity, source terms). But how do I couple these?

Or would it be easier to go directly for custom PDEs and without predefined physics terminals?

Please help me here.

Best regards,
Henning Hollermann

Hi

I am trying to solve a similar problem and I am using PDE (c) interface. Anyway, how did you try to solve it?

Thanks!

Meshing, tolerances and convergence

Meshing is very important for finite-element problems, as it influences convergence and the quality of the solution. For smaller time steps you have to chose a finer mesh to prevent unphysical oscillations of the solution in-between the nodes of the mesh, but you should keep the mesh as simple as possible as the computational complexity grows with the number and type of mesh elements.

Convergence is usually defined by the relative and the absolute tolerances. The smaller they are set, the more accurate the solution will probably be, but also the longer the calculation of each time step may take. If this tolerance criterion cannot be reached within a certain number of Newton iterations, the solver throws an error. To reach it anyway while maintaining a physically correct and accurate solution it is very important to set these values correctly. Sometimes it can be very hard to make a good compromise here.

Additionally, for better computational performance, the dependent variables are scaled so that their values are near to one. The tolerances are usually applied to the scaled variables. Although COMSOL tries to guess the scaling values, it fails especially for complex problems with equations of your own, as we deal with here. Therefore it is very important for the smooth convergence of the problem to manually choose the right scaling value for each variable. This is typically the highest order of magnitude you would expect the variable to reach during simulation.

Dear Henning,

Thanks for your support.
How did you interface the two Coupled PDEs in TTM ?
I am not able to interface these equations.
please help me to solve the problem.

thanks
kiran

Hi, I was also trying to implement TTM to predict ablation in thin film. However, one thing I don't understand is how to removel the material in thin films which would definately change the thermal condition during the process. Is LTNE helpful?

Hello Tong,

If you're wanting to model material removal, you may also find this article helpful: https://www.comsol.com/blogs/modeling-thermal-ablation-for-material-removal/

Best Regards,

Hello,

I am trying right now to implement a e-p thermal coupling in a solid throught the LTNE physics. So far I haven't manage to go through since I want to set the coupling between two solids and not a solid and a fluid.

The problem is that when I set up a "solid" node in the "Heat transfer in Fluid" physics, all the domains in "domain selection" of LTNE are set as "non applicable".

I gonna contact Comsol and let you know the answer if I have one.