Discussion Forum

Hi

sounds to me like a not fully defined BC (boundary condition). Have you tried to check your initial conditions: select in the Solve ... Dependent variables - right click and use "compute / solve to here"

Then you might check the voltage, temperature and initial flux(es) in the postprocessing tab

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Good luck
Ivar

Thanks for the response!! I'm still very new to comsol and appreciate your help. I'm still having the same problem. It seems like my model should be fairly straight forward.I am doing a Time dependent study using the Joule Heating module. I have a cylinder inclosed by a larger block. The cylinder BERYLLIUM COPPER, and the block is WATER, Liquid. I defined one end of the cylinder with an ELECTRIC POTENTIAL (5V), and the other end GROUND. I added a HEAT SOURCE domain condition just to the cylinder domain and chose TOTAL POWER DISSAPATION DENSITY as the source. For the intial values I used 250 K for temp and 0 V for potential. For the time frame of the study I chose 25 frames from 0 to 60 seconds. That is all my model consist of. However, when I run my model to 60 seconds, I have an area all the way around the cylinder that heats up as expected and then right out side of that area the temperature actually decrease by a few degrees. I have attatched post processed images of my results. This should be easier that I am making it...

By the way, to clarify, I simplified the conditions of my model, such as voltage, in order to get things straight.

One other thing,after 60 seconds the range of the temperature in my model is from dark blue=244.19 K to bright red=279.52 K. The intial temp is 250 K.

The heat propagates very slowly in water (is has a low thermal diffusivity), and when it propagates much less than the element size in transient analysis you will get this type of behavior. It seems from your attached figures that the mesh is too coarse in the horizontal direction. Consider a boundary layer mesh on the water side of the water-copper interface.

For more details, check out the discussion in Thread 16808: www.comsol.com/community/forums/general/thread/16808/

Nagi Elabbasi
Veryst Engineering

Hi,

I want to knnow how to load a model .mph using Java code,

My goal is to load a model named "mymodel.mph" build in Comsol 4.2, after that I use the Model object in order to resolve equations,

I did...

Model model = ModelUtil.load("Model", "mymodel.mph");

and I find

exception NullPointerException


Thanks in advance

I read the discussion forum you posted and found the equation that compares element size to time step.
L=sqrt{[(k)(t)]/[(rho)(C)]}. If my calculations are right, using the properties of water and a time step of 120/24, my mesh size needs to be about 8.3e-4. Does that sound right?? That is a VERY small mesh. I tried using the boundry layers(all default values) around the copper rod. I increased my potential to 1000 V and still had temperatures around -8000 K. any thoughts?? Thanks for the great feedback!!