Collisions
Use the Collisions node model the interaction between model particles and a rarefied background gas using a Monte Carlo model. Particles have a random chance to be deflected by gas molecules during each time step taken by the solver, based on the gas density and collision cross-section or collision frequency.
By itself, the Collisions node does not cause any collisions to occur. It is necessary to add one or more subnodes, which correspond to different collision types. The following subnodes are available from the context menu (right-click the parent node) or from the Physics toolbar, Attributes menu: Elastic, Excitation, Attachment, Ionization, Resonant Charge Exchange, Nonresonant Charge Exchange, and User Defined.
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.
Fluid Properties
Enter the Background number density Nd (SI unit: 1/m3). The default is 1020 1/m3.
Enter the Background gas molar mass Mg (SI unit: kg/mol). The default is 0.04 kg/mol.
Enter coordinates for the Velocity field u (SI unit: m/s) based on space dimension. If another physics interface is present which computes the velocity field then this can be selected from the list.
To model the case of a stagnant background gas, leave all the velocity components as zero. If a nonzero velocity field is specified, the background gas molecules will follow a drifting Maxwellian velocity distribution.
Enter values or expressions for the Temperature T (SI unit: K) of the background gas. The default is 293.15 K.
Select an option from the Collision detection list: At time steps taken by solver (the default) or Null collision method, cold gas approximation.
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.
Collision Statistics
Select the Count all collisions check box to allocate an auxiliary dependent variable that records the number of times each particle collides with the background gas. If the Collisions node has multiple subnodes, the variable created by the Count all collisions check box is updated for collisions caused by any of these subnodes. In addition, the settings window for each collision type includes a Count collisions check box that can be used to count the number of times a specific type of collision occurs.
New Value of Auxiliary Dependent Variables
This section is available if an Auxiliary Dependent Variable has been added to the model. The settings are the same as described for the Wall node. The auxiliary dependent variables for the particle get updated every time a collision occurs.
Affected Particles
Select an option from the Particles to affect list to apply the force to specific particles: All (the default), Single species, or Logical expression. The settings are described for the Force node.
Collisional Force Theory