Discussion Forum

Hi,

I think you should try to simplify or decrease your mesh and furthermore your time step in transient mode.

Best regards,

Dinh An

Is your operating system 32 bit? If yes, the soft could only use up to 2GB ram.

Thanks Dhin An,

Even If I take very large time step with simulation time of 500 seconds (in two steps) and time step of 10 seconds , I can not simulate it. Initially Mesh was defined as fine but now I made it normal with the same working setup mentioned above and it has shown me some results.

I am worried about the accuracy of results because of defining a larger mesh, does it affect the results?

Thanks.

Thanks Vincent,

Why it can not go beyond 2GB of memory in 32 bit system? By the way while simulating it was showing the virtual memory in use which never exceeded beyond 500 Mb for this simulation. For other simulations some times it goes up to 1 Gb, is there any connection between the virtual memory in use and the RAM in use?

Secondly, is there any way to increase the memory size for COMSOL in 32 bit system?

Thanks in advance for any feedback!

Hi

In theory a 32 bit word can address 2^32=4G values, hence corresponding 8 bit adress space. But, normally MS limited the total address sapce of their 32 bit version to 2Gb (which was already huge for some years ago ;) Probbaly they needed 1 bit for some context switching or security reasons.

There were some tricks to get older win-32 OS to consider more than 2Gb, but I'm not sure all software can use it.

The easiest is to:
1) turn off all RAM eating unecessesary tasks on your PC
2) to use symmetry, ignore small features, and manually meshed volumes.

In v3.5 one could gain some RAM by running COMSOL in client - server mode, as the Server module, solving your model, used less RAM and could swap the client module out of RAM when solving. But with V4 I'm no sure how it works, particularly with the early 4.0. Optimisation of RAM usage has progressed in the newer V4 versions.

And indeed RF requires fine mesh, if possible 5 or more 2nd order elements per wavelengh IN the material(s)

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Good luck
Ivar

Thanks Ivar,

Can you please elaborate on "if possible 5 or more 2nd order elements per wavelengh IN the material(s)"?

For example how can I calculate the mesh elements required for a particular frequency, say 2.45 GHz?

Thanks in anticipation.

Hi

in vacuum (or air) the speed of light is known something like c = 299,792,458 metres per second (see i.e. en.wikipedia.org/wiki/Speed_of_light ). But in glass or another media the speed is slower. The optical index of refraction gives the ratio to the speed of light in vacuum, typically n=1.5 or therearound for most glasses.

the wavelength is then

lambda[m] = c[m/s] / f[Hz]

in vacuum or

lambda[m] = c[m/s] / f[Hz] / n

in a higher index media.

Therefore you need a 1.5 times denser mesh in glass than in vacuum to reach the 5 mesh elements per wavelength.

If you use linear elements, you would typically need 10 mesh elements per wavelength. The mesh size variable is "h" for COMSOL, you can plot it with a mesh plot

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Good luck
Ivar

I'd better advise you to install a 64bit OS, because your RAM must be 4GB or even more. Using 32bit OS on it is a kind of waste.

Thanks Ivar,

If I run a simulation for maybe one hour in COMSOL, after getting the results if I try to save the file it consumes a lot of space. Is there any efficient way to save the simulation results as COMSOL files?

When I started the simulation, I was having 16 Gb free in C drive, after getting the results I was having almost 7 Gb free in C drive and I was saving my results in D drive where the free space was almost 5.5 Gb.When I tried to save the file, it consumed all the space available in C drive and gave the error of Insufficient Space.The model was not saved at all.

Can you please suggest something to resolve this issue?

Regards,

Hi

normally in v4 the files are "zipped" hence already compressed, that means you have a huge model, and are storing many intermediate results. Check if you have ticked "all" or only specified results in the solver node. You must reduce the model size someway ;)

--
Good luck
Ivar

Thanks Ivar for your response,

I think that the model is not huge in my case as I am just using the dynamic properties of fluid defined as interpolation function at different temperatures. Microwaves heating is coupled with Darcy's flow for this model, total file size of the COMSOL file for this model is just 450 Kb.

I think it makes sense that it would be storing many intermediate results, how can I check that?
Regarding your statement " Check if you have ticked "all" or only specified results in the solver node", I am not sure how to check and define specified results in the solver as I did not see any option addressing this issue, can you please explain in brief?

Please have a look on the attached image and let me know in which section of the solver I can define use of only specified results?

Many Thanks in advance!

Hi

You should search for a "output" tab in the Time-Depedent Solver node (or a Parametric sub node if you do a sweep). There you can specify the times to store ... (or steps given ...)

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Good luck
Ivar