I would like to understand the beaming effect that gets created in the tube: supposedly with a relatively long tube almost all particles should be perpendicular to the tube cross section and hit the opposite wall in an small circular area. Which is more or less what I’m seeing:
To determine the number of particles desorbing in each direction I used the “incident angle distribution” option on a transparent circular face placed between the two zones. And this is the resulting distribution of the particles based on the theta angle:
I’m confused by this graph. If theta=0 is the normal of the plane I’d expect a concentration of particles around this value, while it almost seems like particles around this are almost zero. It looks like most of the particles cross this surface with an angle around 0.4rad = 23°, and thus they should end on the side walls rather than on the front wall.
Am I misinterpreting the results of this graph? Or is there some other parameter I’m not taking into consideration?
Sorry for the very late reply, due to a notification error we only read this today.
It’s not obvious to grasp, but at theta=0 the solid angle from where the particles can originate converges to zero. I made a few plots and explain in detail in section 1.2.1.8.5 (angular profiles, page 22) of my thesis.
I’m not sure what you mean by “incident angle distribution option”. If you used angular profile, then there’s a checkbox to compensate for this:
It divides each angle bin by the sine of the bin centerpoint. If you use other post-processing, you can do this correction yourself.
Angular profile of beaming (L/R=100 pipe) with surface hits sampled, similar to your plot:
Is this the correct option to use to study the angle of all particles through this surface? I didn’t see the “Surface → Volume conversion” option, I don’t know if it’s somewhere else.
Regarding why it converges to zero at theta=0: if I understood correctly, molflow is computing this value based on a hemisphere where areas get smaller as angle gets closer to 90°, thus those values are less detected?
You can use the function you highlighted, see its documentation. The surface->volume conversion is, as pasted in the screenshots above, is in the Profile Plotter, when you use the incident angle profile:
If you read the referenced pages of my thesis, I demonstrate that no particle will hit a surface exactly perpendicularly, as the probability of that converges to zero. This is independent of how Molflow measures and the reason lies in molecular dynamics.
