When the Collisions node and subnodes (the reaction set) are used, collisions with a background gas occur randomly and the particle velocity can change discontinuously at discrete times. The friction model, a deterministic force, can be accessed using a dedicated Friction Force node.
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The two collision detection algorithms available from the Collision detection list are vastly different.
If At time steps taken by solver is selected, collisions can only be detected at the beginning of a time step taken by the time-dependent solver, and the collision frequency is assumed to be constant over each step. Usually it is necessary to specify a maximum time step size or fixed step size in the solver settings, to ensure the steps taken by the solver are small relative to the free time between collisions.
If Null collision method, cold gas approximation is selected, collisions can be detected at random times between the time steps taken by the solver. This option allows more accurate collision modeling when the steps taken by the solver are comparable to or larger than the free time between collisions. The tradeoff is that the thermal velocity distribution in the background gas is neglected, so this option is only applicable to highly energetic model particles.
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