For all of the particle tracing physics interfaces, the particle trajectories must be computed using a Time-Dependent Solver. For single-physics particle tracing, this can be set up by using the Time Dependent study.

If a unidirectional coupling between particle trajectories and fields is set up, it is often useful to first compute the fields in the surrounding domains and then to compute the particle trajectories using a Time Dependent study. For example, it is possible to model the motion of particles in a channel containing fluid by first setting up the Laminar Flow interface and computing the fluid velocity and pressure using a Stationary study. Then couple the flow profile to the Particle Tracing for Fluid Flow interface using the Drag Force feature and solve for the particle trajectories in the time domain.

If the particles do significantly perturb the fields in the surrounding domains, it is necessary to create a bidirectional coupling. The most straightforward way to set up a bidirectional coupling is by using one of the Multiphysics Couplings. Alternatively, if the necessary physics interfaces are already present, it is possible to manually add the contributions from the particles to the surrounding fields. For example, if instances of the Charged Particle Tracing and Electrostatics interfaces are present, the contribution of the charged particles to the space charge density can be included by adding the Electric Particle Field Interaction Multiphysics node.

If the fields are not constant, it is necessary for the particle trajectories and fields to be computed together in the time domain. For a charged particle beam, this might mean that the beam is pulsed, and thus the electric potential is time-dependent. Full time-domain calculation of the particles and fields is much more computationally demanding than a unidirectional coupling between particles and fields, because it requires particles to be released at a very large number of time steps. For fully time-domain calculations, Specify release times should be selected from the Particle release specification list in the physics interface Particle Release and Propagation section.

If the fields are stationary, it is possible to significantly reduce the computational cost of the model by using an iterative solver loop in which the particle trajectories are computed using a Time-Dependent solver and the fields are computed using a Stationary solver. To obtain a self-consistent solution, it is necessary to ensure that the result from each of these solvers is used to set the value of variables not solved for by the other solver. The For and End For nodes, when added to a solver sequence, can be used to set up an iterative loop that does the following:

The Bidirectionally Coupled Particle Tracing study step automatically sets up an iterative solver loop, and can be used in place of a Time Dependent study to facilitate the calculation of bidirectionally coupled particle trajectories and fields.

When solving for the fields using a Stationary study, it is possible to considerably reduce the number of particles in the simulation by selecting Specify current or Specify mass flow rate from the Particle release specification list in the physics interface Particle Release and Propagation section. This causes each model particle to represent a number of real particles per unit time, all moving along the same path, and therefore it is only necessary to release particles during the first time step.