Understanding particle tracking simulation

In a particle tracking simulation, the flow solver calculates the particles' positions in the fluid domain at each particle time step. During the analysis, the software solves the equation of motion for each particle once per particle time step.

The random velocity of particles needed for the particle tracking equation is modeled using the Brownian motion and turbulent fluctuation. When turbulence is modeled, the eddy diffusivity is added directly to the diffusivity from the Brownian motion.

Particles which stop at walls are taken into account in the output statistics for the parallel flow solver but not for the serial flow solver.

Because the particle tracking simulation is independent of the flow simulation, the particles do not affect the 3D flow. You can run the following particle tracking simulations:

• Particle tracking simulation in a transient flow simulation

• Particle tracking simulation in a steady state flow simulation

• Particle tracking simulation only

Particle tracking simulation in a transient flow simulation

In a transient flow simulation, the particle time step is the same as the flow time step. At each time step, the particle tracking equation is solved after the fluid equations, using the calculated quantities for the flow field: velocity, pressure, temperature, and turbulence.

Particle tracking simulation in a steady state flow solution

For a steady state simulation, the flow is fully developed, which determines the flow properties used in the particle tracking equation: velocity, pressure, temperature, and turbulence. You must specify the injection duration time, the particle tracking simulation time, and the number of particle tracking time steps. You do this in the Solver Parameters dialog box→3D Flow Solver page→Particle Tracking Controls group. The particle time step, then, defines the iteration at which the software solves the particle tracking equation.

For more information, see Steady state particle tracking.

Particle tracking simulation only

You can solve a steady state particle tracking simulation using the flow field from a solved steady state solution. If you have a solved flow analysis, you can do the following:

• Add Particle Injection simulation objects.

• Modify Flow Surface and Screen simulation objects.

• Modify particle tracking options on the 3D Flow page in the Solution dialog box and on the 3D Flow Solver page in the Solver Parameters dialog box.

After you modify your solution, solve only the particle tracking equation. To do this, in the Solution dialog box→Restart page, select the Perform Restart check box, and under the Options group, select the Solve Steady State Particle Tracking Simulation Only check box.

Particle types

You can model the particles with or without mass. For the massless particles with zero size, there is no dynamic model for the particle trajectory, and the particle follows the flow field. For massless particles of a certain size, for example bubbles, the equation of motion of each particle is solved, although it is assumed that there is no change in the size of particles.

Particle impact types

The flow solver can simulate particles that stick to walls or that rebound. You can set the particle impact type in the following ways:

• Set it individually for each flow surface in the corresponding Flow Surface dialog box. You can define particle tracking options only for the Embedded Flow Surface and Boundary Flow Surface types.

• Set it globally for all flow surfaces in the Solution dialog box.

When you specify the rebound particle impact type, you must define the coefficient of restitution, α. The coefficient of restitution indicates how much energy the particle loses when it hits the wall. The specified value must be: 0 ≤ α ≤ 1.

Particle tracking results

The results of a particle tracking simulation are particle density and particle mass density for each fluid element. After you solve the model, the following results types can appear under the Solution node in the Post Processing Navigator:

• Particle Density - Elemental contains the results for [number of particles in the element volume smeared in time]/[element volume].

• Particle Mass Density - Elemental contains the results for [mass of particles in the element volume smeared in time]/[element volume].

In a transient solution, the two distribution densities are available for each time step. In a steady state solution, the distribution densities are those obtained at the last particle time step. The particle mass density is zero everywhere for massless particles.

Optionally, you can output particle tracking results to the Tecplot or ParaView format. For Tecplot, the software outputs particle tracking information into a file called tecplot.flow for post processing with visualization software from Tecplot, Inc.

Particle impact and pass-through reports

The flow solver generates a ParticleAndImpactTrackingSummary.dat file which contains:

• The Particle Tracking Summary tableThis table lists the number of injected and ejected particles in total for your model and per boundary condition.

• The Particle Surface Impact and Screen Pass-Through Tracking Report tableThis table lists:The number of particles that impact walls in your model per injection boundary condition when you request a particle impact report in the Flow Surface simulation object.The number of particles that pass through screens per injection boundary condition when you request a pass-through report in the Screen simulation object.

The file is generated in the simulation’s run directory.

Optionally, you can output particle impact and pass-through results to the ParaView format.

How do I

Set up a particle tracking simulation

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