Discussion Forum

It seems as though the instructions in the COMSOL User's Manual (v4.3) may disagree with the logic in the fluid particle tracking module for the wall condition specification. Before I submit it formally as an issue, thought I'd see if anyone else had a similar problem and had come to the same or a different conclusion.

I had been trying to specify how tracer particles (2 um diameter, 0.4 s.g.) with an imposed charge would interact with the electrically grounded walls of a 2-D channel (0.2 m width, 1 m length, air, 1 m/s inlet velocity), and kept getting counter-intuitive results. If you do not want every particle-wall interaction to be identical, COMSOL allows you to specify the type of particle-wall interaction (bounce, freeze, stick) and either a probability or an expression (algebraic, logical, etc). The User's Manual says that if the provided expression (algebraic, logical, etc.) evaluates to 0, then the selected particle-wall interaction will be applied. Otherwise (i.e., expression /= 0), the particles are supposed to bounce.

After entering a variety of algebraic and logical expressions - and not getting the particle-wall behavior I expected, I've found that choosing "stick" or "freeze" as the wall condition and "0" as the expression actually results in the particles bouncing off of the wall. For "0" as the expression, according to the User's Manual, the particles should behave according to the selected wall condition.

Am I misinterpreting this? Anyone else have an issue with specifying the wall condition in fpt?

Thanks for sharing your experiences,

HC