The Particle Tracing for Fluid Flow (fpt) interface (), found under the Fluid Flow>Particle Tracing branch () when adding a physics interface, is used to simulate the motion of particles in a background fluid. Particle motion can be driven by a combination of forces including drag, gravity, electric, magnetic, acoustophoretic, and user-defined forces. It is also possible to specify particle size or mass distributions, solve for particle temperature, and model bidirectionally coupled particle-fluid interactions.

When this physics interface is added, these default nodes are also added to the Model Builder: Wall and Particle Properties. Then, from the Physics toolbar, add other nodes that implement, for example, boundary conditions and volume forces. You can also right-click Particle Tracing for Fluid Flow to select physics features from the context menu.

The Label is the default physics interface name.

The Name is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the Name field. The first character must be a letter.

The default Name (for the first physics interface in the model) is fpt.

The Store extra time steps for wall interactions and Maximum number of secondary particles settings are the same as for The Mathematical Particle Tracing Interface. The Relativistic Correction check box is not available.

From the Formulation list, the following options are available:

The Newtonian, ignore inertial terms formulation is unique to the Particle Tracing for Fluid Flow interface. It is a first-order formulation that solves only for the particle position q. Rather than solving for the particle velocity or momentum, this formulation defines the velocity based on the assumption that the drag force perfectly counterbalances all other applied forces on the particle at any instant in time. Therefore, to use this formulation effectively, a Drag Force node must be included in the model. If there are no other applied forces besides the Drag Force, then the particles will simply follow the fluid velocity streamlines.

It is appropriate to use the Newtonian, ignore inertial terms formulation when the time scale over which particles accelerate in the fluid is very small compared to the total simulation time. The time scale for particle acceleration due to Stokes drag is proportional to the square of the particle diameter, and inversely proportional to the viscosity of the surrounding fluid, so the Newtonian, ignore inertial terms formulation is often applicable for small particles in a liquid.

Select an option from the Particle release specification list: Specify release times (the default) or Specify mass flow rate. If Specify release times is selected, then each model particle is treated as the instantaneous position of one or more particles for the purpose of modeling fluid-particle interactions. This means, for example, that if the Volume Force Calculation node is used, the volume force on the fluid is only nonzero in mesh elements that are currently occupied by particles.

If Specify mass flow rate is selected, then for the purpose of modeling fluid-particle interactions, each model particle traces a path that is followed by a number of particles per unit time. This means that the volume force computed by the Volume Force Calculation node is nonzero in all mesh elements that the particle trajectories pass through, not just at the instantaneous positions of the particles. In other words, the model particles leave behind a trail of nonzero force components in the mesh elements they pass through.

The Specify mass flow rate option is primarily used to model streams of particles under steady-state conditions. Changing the particle release specification affects some inputs in the settings windows for release features such as the Release and Inlet nodes. In addition, the Mass Deposition subnode to the Wall node is only available with the Specify release times option, while the Boundary Load and Mass Flux subnodes are only available when Specify mass flow rate is selected.

Select the Include rarefaction effects check box to apply correction factors to the forces defined by the Drag Force and Thermophoretic Force nodes. These correction factors improve the accuracy of the drag and thermophoretic forces when the particle Knudsen number is significantly large. This can be used to model the motion of particles in a rarefied gas flow.

The options Store particle status data and Store particle release statistics are the same as for The Mathematical Particle Tracing Interface. The option Include out-of-plane degrees of freedom, shown in 2D and 2D axisymmetric models only, is the same as for The Charged Particle Tracing Interface.

Select an option from the Particle size distribution list: Uniform size (the default), Specify particle mass, or Specify particle diameter.

When Uniform size is selected, the size of each particle is controlled by the settings for the Particle Properties node. The size of each particle is assumed constant over time, unless its diameter or mass is defined as an explicit function of time. Note that you can still release particles of different sizes in one model by creating multiple instances of the Particle Properties node.

When Specify particle mass (or diameter) is selected, you can specify the initial value of the mass (or diameter) in the settings for particle release features such as Inlet and Release from Grid. Optionally, you may release particles with a nonuniform size distribution. Particles may also grow or shrink over time, after they have been released; in the settings for the Particle Properties node, enter an expression for the Accretion rate R (SI unit: kg/s), which is the time derivative of the particle mass. Because the particle mass (or diameter) is solved for, these settings each use one additional degree of freedom per particle, compared to the Uniform size option.

Select the Compute particle temperature check box to compute particle temperatures (the default is to not compute particle temperatures). When this option is activated the temperature of the particle is computed by solving an additional ordinary differential equation per particle. Thermal properties for the particles can be specified in the Settings window for the Particle Properties node.

Some fluid-particle systems contain such a large number of particles or droplets that modeling each entity individually is not feasible. In such cases, it is often convenient to introduce the concept of a macroparticle, or a single model particle that can represent a larger number of real particles.

Select the Enable macroparticles check box to allocate an auxiliary dependent variable for a dimensionless multiplication factor, allowing the number of real particles represented by each model particle to be stored.

These settings are the same as for The Mathematical Particle Tracing Interface.

The dependent variables (field variables) are the Particle position, Particle position components, Particle velocity, and Particle velocity components. Note that not every dependent variable is needed for every combination of physics interface settings; for example, the Particle velocity and Particle velocity components are only used if Newtonian, first order is selected from the Formulation list. The names can be changed but the names of fields and dependent variables must be unique within a model.