Use the Auxiliary Dependent Variable node to add additional degrees of freedom for each particle. These can be used to solve for the particle size, path length, residence time, number of wall collisions, spin, and so forth.

Enter a Field variable name. The default is rp and can be changed to anything provided it does not conflict with the name of the variables for the position or momentum degrees of freedom. The name should not conflict with other auxiliary dependent variables.

Enter a Source R. The unit of the source depends on the settings in the Units section.

Under Integrate choose whether to integrate the equation you have defined With respect to time or Along particle trajectory. For example, to compute the residence time of a group of particles in a given system, set the Source to 1 and set Integrate to With respect to time. To compute the length of the particle trajectory, set the Source to 1 and set Integrate to Along particle trajectories.

Select a Dependent variable quantity from the list; the default is Dimensionless [1]. To enter a unit, select None from the list and in the Unit field enter a value, for example, K, m/s, or mol/m^3.