Moving parts
This dialog allows you to assign a velocity vector in the lab frame (i.e. where your geometry resides) to parts of your model. It does two things:
- At every hit (desorption, reflection or absorption) the local motion vector
v_motion_localis calculated. It is either fixed (same everywhere) for a translation, or varies based on the axis distance (rotating part). - After the molecule's "natural" outbound velocity vector
v_thermalis calculated, the actual outgoing velocity vector is modified by addingv_motion_local:v_out = v_thermal + v_motion_local
General example:
As in the dialog above, an 1000 m/s translation speed is defined as the global motion vector.
Then, for certain facets the feature is enabled in the advanced parameters:
Then, at every moving facet, a hit on a moving part is displayed as a purple dot (if Hits display is enabled), and the vector [0 , 0, 1000] is added to the reflected thermal velocity.
Cubesat example
Below is a very simple simulation of a cubesat traveling at 1000m/s in space. The space is modeled as a surrounding box generating gas uniformly. As we model in the satellite's frame, the space, represented by the outer box, will be the moving part.
I set outgassing and 100% sticking on the outer box -Z and +Z facets. On the side facets, you would need to calculate the ratio of gas crossing the perpendicular planes of an 1000m/s moving frame. For simplicity (as v_motion > v_thermal), I omit this and set 0. Finally I enable "Moving part" on the outer facets:
We can see...
- purple dots where gas is created (moving parts)
- red dots where gas arrives (mostly on the +Z side, where gas travels to)
- green reflecting dots on the satellite
- a local density maximum in front of the satellite
We can also visualize the direction vectors:
Finally, you will find some leaks on the +Z side:
This is because in this example I've set room temperature for the gas, so the average thermal velocity is ~460m/s. Many particles, even if desorbed to the -Z direction, when adding 1000m/s to their Z component will go against the +Z surface, resulting in a leak.
This also means that for these thermal/movement velocity ratios, you can't simulate in space frame of reference, as the particles that would reflect from the satellite's body would now - with the added motion velocity vector - go against the cube's surface, and enter the satellite.
You can download the example here.