collide_modify keyword values ...
vremax values = Nevery startflag Nevery = zero vremax every this many timesteps startflag = yes or no = zero vremax at start of every run remain value = yes or no = hold remaining fraction of collisions over to next timestep nearcp values = choice Nlimit choice = yes or no to turn on/off near collision partners Nlimit = max # of attempts made to find a collision partner ambipolar value = no or yes rotate value = no or smooth vibrate value = no or smooth or discrete
collide_modify vremax 1000 yes collide_modify vremax 0 no remain no collide_modify ambipolar yes
Set parameters that affect how collisions are performed.
The vremax keyword affects how often the Vremax parameter, for collision frequency is re-zeroed during the simulation. This parameter is stored for each grid cell and each pair of collision groups (groups are described by the collide command).
The value of Vremax affects how many events are attempted in each grid cell for a pair of groups, and thus the overall time spent performing collisions. Vremax is continuously set to the largest difference in velocity between a pair of colliding particles. The larger Vremax grows, the more collisions are attempted for the grid cell on each timestep, though this does not affect the number of collisions actually performed. Thus if Vremax grows large, collisions become less efficient, though still accurate.
For non-equilibrium flows, it is typically desirable to reset Vremax to zero fairly frequently (e.g. every 1000 steps) so that it does not become large, due to anomolously fast moving particles. In contrast, when a system is at equilibrium, it is typically desirable to not reset Vremax to zero since it will also stay roughly constant.
If Nevery is specified as 0, Vremax is not zeroed during a run. Otherwise Vremax is zeroed on timesteps that are a multiple of Nevery. Additionally, if startflag is set to yes, Vremax is zeroed at the start of every run. If it is set to no, it is not.
The remain keyword affects how the number of attempted collisions for each grid cell is calculated each timestep. If the value is set to yes, then any fractional collision count (for each grid cell and pair of grgroups) is carried over to the next timestep. E.g. if the computed collision count is 7.3, then 7 attempts are made on this timestep, and 0.3 are carried over to the next timestep, to be added to the computed collision count for that step. If the value is set to no, then no carry-over is made. Instead, in this example, 7 attempts are made and an 8th attempt is made conditionally with a probability of 0.3, using a random number.
The nearcp keyword stands for "near collision partner" and affects how collision partners are selected. If no is specified, which is the default, then collision partner pairs are selected randomly from all particles in the grid cell. In this case the Nlimit parameter is ignored, though it must still be specfied.
If yes is specified, then up to Nlimit collision partners are considered for each collision. The first partner I is chosen randomly from all particles in the grid cell. A distance R that particle I moves in that timestep is calculated, based on its velocity. Nlimit possible collision partners J are examined, starting at a random J. If one of them is within a distance R of particle I, it is immediately selected as the collision partner. If none of the Nlimit particles are within a distance R, the closest J particle to I is selected. An exception to these rules is that a particle J is not considered for a collision if the I,J pair were the most recent collision partners (in the current timestep) for each other. The convergence properties of this near-neighbor algorithm are described in (Gallis11). Note that choosing Nlimit judiciously will avoid costly searches when there are large numbers of particles in some or all grid cells.
If the ambipolar keyword is set to yes, then collisions within a grid cell with use the ambipolar approximation. This requires use of the fix ambipolar command to define which species is an electron and which species are ions. There can be many of the latter. When collisions within a single grid cell are performed, each ambipolar ion is split into two particles, the ion and an associated electron. Collisions between the augmented set of particles are calculated. Ion/electron chemistry can also occur if the react command has been used to read a file of reactions that include such reactions. See the react command doc page. After all collisions in the grid cell have been computed, there is still a one-to-one correspondence between ambipolar ions and electron, and each pair is recombined into a single ambipolar particle.
The rotate keyword determines how rotational energy is treated in particle collisions and stored by particles. If the value is set to no, then rotational energy is not tracked; every particle's rotational energy is 0.0. If the value is set to smooth, a particle's rotational energy is a single continuous value.
The vibrate keyword determines how vibrational energy is treated in particle collisions and stored by particles. If the value is set to no, then vibrational energy is not tracked; every particle's vibrational energy is 0.0. If the value is set to smooth, a particle's vibrational energy is a single continuous value. If the value is set to discrete, each particle's vibrational energy is set to discrete values, namely multiples of kT where k = the Boltzmann constant and T is one or more characteristic vibrational temperatures set for the particle species.
Note that in the discrete case, if any species are defined that have 4,6,8 vibrational degrees of freedom, which correspond to 2,3,4 vibrational modes, then the species command must be used with its optional vibfile keyword to set the vibrational info (temperature, relaxation number, degeneracy) for those species.
Also note that if any such species are defined (with more than one vibrational mode, then use of the discrete option also requires the fix vibmode command be used to allocate storage for the per-particle mode values.
The option defaults are vremax = (0,yes), remain = yes, ambipolar no, nearcp no, rotate smooth, and vibrate = no.
(Gallis11) M. A. Gallis, J. R. Torczynski, "Effect of Collision-Partner Selection Schemes on the Accuracy and Efficiency of the Direct Simulation Monte Carlo Method," International Journal for Numerical Methods in Fluids, 67(8):1057-1072. DOI:10.1002/fld.2409 (2011).