Physics Interfaces
In this section all of the relevant particle tracing physics features are set up. The default boundary condition, Wall, stops particles that touch a boundary. In the following steps, the drag force and the particle release mechanism are specified.
1
In the Model Builder window, under Component 1  click Particle Tracing for Fluid Flow . In the Settings window, locate the Particle Release and Propagation section. From the Formulation list, select Newtonian, ignore inertial terms.
The default Formulation is Newtonian, which is a full inertial treatment of the particles that solves second order differential equations to track them through the fluid. With an inertial treatment, particle trajectories can cross fluid velocity streamlines even if the drag force is the only force in the model, because the particles take a finite amount of time to accelerate in response to a change in the velocity of the surrounding fluid.
When inertial terms are ignored, the particle is velocity is prescribed such that the drag force is always in dynamic equilibrium with all other applied forces. This reduces the second order equations for particle motion to first-order equations. For the Stokes drag law, the relaxation time of a particle is
The Newtonian, ignore inertial terms formulation is appropriate when this time scale is much smaller than the range of solution times you are interested in. This is most likely to be the case for very small particles in a viscous fluid.
2
In the Physics toolbar click Domains and choose Drag Force .
3
Select Domain 1 by clicking on any part of the channel in the Graphics window.
4
In the Settings window for Drag Force locate the Drag Force section.
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From the u list, choose Velocity field (spf) to take the fluid velocity from the Laminar Flow physics interface solved for in the previous study.
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From the μ list, choose Dynamic viscosity (spf/fp1).
Next, set up the release of particles from the Inlet boundary. The goal is to release particles with greater number density where the magnitude of the velocity field is higher. To do this you use the Density option for the particles’ Initial Position.
5
In the Physics toolbar click Boundaries and choose Inlet .
6
Select Boundary 23 only. This is the same surface as the Inlet for the Laminar Flow physics interface.
7
In the Settings window for Inlet locate the Initial Position section.
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From the Initial position list, choose Density.
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In the N text field, type 3000. This is the total number of released particles.
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In the ρ text field, type spf.U. The number density of particles at the Inlet will be proportional to the fluid velocity profile.
For the Newtonian, ignore inertial terms formulation, it is not necessary to enter the initial particle velocity, which is defined implicitly from the drag force.
8
In the Physics toolbar click Boundaries and choose Particle Counter .
Note: The Particle Counter is used to compute the transmission probability of particles. Alternatively, it is possible to compute the transmission probability in postprocessing by assigning a Selection  to the Particle  dataset.
9
Select Boundary 20 only. This was the Outlet boundary for the Laminar Flow physics interface.
10
In the Settings window for Particle Counter locate the Particle Counter section.
11
From the Release feature list, choose Inlet 1. Because this model only includes one inlet, it is not strictly necessary to choose a specific release feature, although doing so can be useful in more complicated models.
12
In the Model Builder under Particle Tracing for Fluid Flow, click Particle Properties 1 .
13
To change the diameter of the particles, in the Settings window for Particle Properties locate the Particle Properties section. From the ρp list, choose User defined. In the dp text field, type 5E-7[m].