Discussion Forum

Hello,

I am having trouble with simulating my model. Here's a rundown of what's going on:

I basically have a 1-D geometry acting as a wire, linked with the AC/DC module using Electrical Circuit and Electric Current modules. I am also using the 1-D heat equation to model the thermal distribution in the wire. The point is pretty much, I have a circuit with this wire in it, and I want to see the effects of Joule heating with a changing current.

Here's where the problems start. I want to simulate the whole system for 4ns. As of right now, In order to study the initial timeframe, I've been simulating it for 20ps at 0.5 and 0.1ps time step and it works fine. Now without changing anything, if I increase the simulation time from 20ps to anywhere between 50-150ps, it doesn't converge. And as far as I can tell, it seems to keep stopping at the same value (around 40ps). I've tried refining the mesh a whole bunch and decreasing the timestep to even smaller, but doing that makes it say it can't find consistent initial values. Lastly, sometimes it will simulate if I pick a time 200-250ps, but it will clearly be an incorrect result (negative temperature in K). And choosing a value such as 1-4ns also results in not being able to find consistent initial conditions.

So what's the deal? I wouldn't think that simply changing the simulation time settings should have this effect on the model. Now I just started using COMSOL last month, so I'm pretty new to this. However, one of my colleagues who is a little more familiar with the software says it doesn't make sense to him either. So here I am! Any information at all about how to solve this problem or where I could find more info about it would be greatly appreciated!

Thank you in advance for your time and responses!

Thomas