I have a few questions regarding the velocity distribution in the Profile Plotter of MolFlow. I want to measure the distribution of velocity components of a gas at certain locations of a geometry which contains parts with different temperatures. For this I have created three perpendicular surfaces for the x, y, and z direction with an area of 1 cm^2. Each of the three surfaces I have divided into two facets for the + and - direction of the specific component. After that I set the area to volume conversion to get the actual temperature of a gas. For each of these surfaces I set the Profile as orthogonal velocity.
Is this measurement principle reasonable?
Is it correct that the y-axis is a bin probability and not a bin probability density?
What happens if I set a double-sided facet instead of two single-sided facets, how exactly is the composite distribution calculated? Is this simply an average of the bin probabilities of the two individual distributions in positive and negative directions?
Here you can see two plots with double-sided and single-sided facets and the arrangement of measuring facets.
The bin position (in speed, m/s) is determined as:
ort_speed=ABS(velocity . facet_normal)
where the period is a dot product.
The ABS function means that in case of 2-sided facets, it doesn’t matter which side the particle comes from, just its orthogonal component.
The bin values are normalized, i.e. bin_probability=bin_count/all_facet_hits
I would call this “bin probaility”, I’m not sure what the difference with “bin probability density” would be - what do you mean by that?
Your measurement makes sense - you can create velocity vectors as a special texture to show the vectorial average at each location:
but as you need more detail (distribution), you must indeed use something more advanced, like the profile plotter as you do, or the particle logger (which records the actual hit directions that you could post-process outside Molflow).
As you probably know, in a multi-temperature system (at equilibrium), the impingement rate on the walls is independent from the temperature, meaning your probe surfaces would get the same frequency of hits from all sides. As the hot surfaces emit faster particles, this would mean that you’d see a higher (orthogonal or absolute) velocity on facets that measure particles coming from hot walls.
