# DSMC - Calculate partilces passing through surface

Hi All,
I have a basic hollow cylinder (shown below) simulated in DSMCFoam. I set up a slice plane then a cylinderical clip so a surface would be used at the inlet and later the outlet.

Is it possible to know the number of particles passing through this surface at a given time?

The end goal is compare the number of particles entering at the inlet to those exiting at the outlet.

Any adice would be greatly appreciated.
Kind regards,
Owen.

Computing the number of particles that pass through a surface can be tricky. You have to look at the particles at two time steps, identify which particles are the same in the two time steps, and then identify which are on different sides of the surface.

Can you instead just count the number of particles in the cylinder? (You can do that by just extracting particles in that region of space.) Then, count the number of particles in each time step. If it goes up, more particles went through the inlet than the outlet. If it goes down, then the opposite.

Hi,
Thank you so much for your reply. Would you mind explaining more about extracting the particles please? Like the kind of filters to use etc? I’ve set up a cylinderical slice (I’ve attached the pic) where I could possibly count the number of particles for the inlet and outlet seperately. Do you think this is posssible?

Kind regards,
Owen.