SimScale CAE Forum

Gas diffusion in SimScale?



I’m gathering information how to do a FEM simulation of a gas diffusion in a 3D office room. Is it possible to simulate gas diffusion with SC or any other simulation UI?

For this I assume the room must not have any inlet or outlet. I would like to put a gas source to the desk, and like a good smelling food, the scent would propogate into the room every corner slowly. So after all, it is diffusion.

Thanks for helping.


You can do this through a passive scalar transport simulation, which appears in the Simscale documentation. However, it does not appear as option in the solver selection so I do not know if it is still available.

Keep in mind that diffusion is a very very slow process at room temperature and a room is a massive length for a particle to cover by diffusion. I expect that air current induced mixing (e.g. due to the movement of people, opening/closing of a door, an open window, a computers cooling fan, convection currents from a hot object) will be the dominant process here and not diffusion. You should start with an analytical study to estimate the diffusion length scale and timescales. See the following link to calculate the diffusion time

You need to find the diffusion coefficient for your case. If we take Co2 in atmospheric air, its value is 0.0016 mm^2/s. Using the above calculator, we find that it takes 36 days for a distance of 1 meter and 326 days for 3 meters. Your particles are likely to be larger than CO2 and therefore diffusion will be even smaller.


Thanks for the fast reply.

How is that the diffusion speed would be so slow? For example, putting coffee to a pot, one minute after you are really feel the scent from 2-3 meters. Though the coffee scent has other particles with different coeff.


Because the dominant driving force that causes dispersion in the air is not diffusion, it is air currents. With the example of your coffee, the hot coffee warms the air around it, warm air rises (and quite fast!), taking the particles around the room.


@jousefm I mentioned in my first post that it looked like there was a scalar transport solver in the past and that it has vanished. Well I just saw a project from June of this year which was performed using the scalar transport solver. Why does it not appear for me (not that I currently need it)?


Hi Roy,

Could you show me the project? I can ask the engineers what the reason for that is.




Yeah, that makes sense :smiley: Thanks. I will learn more about how to do this.


Hi @jousefm

These are the choices I get when I choose a solver


Hi Roy!

You can choose the incompressible solver and then choose the number of species as depicted in the picture below:


Let me know if that helped!