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22.12.5 Modeling Turbulent Dispersion of Particles

As mentioned in Section  22.12.4, you can choose for each injection stochastic tracking or cloud tracking as the method for modeling turbulent dispersion of particles.

Stochastic Tracking

For turbulent flows, if you choose to use the stochastic tracking technique, you must enable it and specify the "number of tries''. Stochastic tracking includes the effect of turbulent velocity fluctuations on the particle trajectories using the DRW model described in Section  22.2.2.

1.   Click the Turbulent Dispersion tab in the Set Injection Properties panel.

2.   Enable stochastic tracking by turning on the Stochastic Model under Stochastic Tracking.

3.   Specify the Number Of Tries:

  • An input of zero tells FLUENT to compute the particle trajectory based on the mean continuous phase velocity field (Equation  22.2-1), ignoring the effects of turbulence on the particle trajectories.

  • An input of 1 or greater tells FLUENT to include turbulent velocity fluctuations in the particle force balance as in Equation  22.2-20. The trajectory is computed more than once if your input exceeds 1: two trajectory calculations are performed if you input 2, three trajectory calculations are performed if you input 3, etc. Each trajectory calculation includes a new stochastic representation of the turbulent contributions to the trajectory equation.

    When a sufficient number of tries is requested, the trajectories computed will include a statistical representation of the spread of the particle stream due to turbulence. Note that for unsteady particle tracking, the Number of Tries is set to 1 if Stochastic Tracking is enabled.

If you want the characteristic lifetime of the eddy to be random (Equation  22.2-31), enable the Random Eddy Lifetime option. You will generally not need to change the Time Scale Constant ( $C_L$ in Equation  22.2-22) from its default value of 0.15, unless you are using the Reynolds Stress turbulence model (RSM), in which case a value of 0.3 is recommended.

Figure  22.12.10 illustrates a discrete phase trajectory calculation computed via the "mean'' tracking (number of tries = 0) and Figure  22.12.11 illustrates the "stochastic'' tracking (number of tries $>$ 1) option.

When multiple stochastic trajectory calculations are performed, the momentum and mass defined for the injection are divided evenly among the multiple particle/droplet tracks, and are thus spread out in terms of the interphase momentum, heat, and mass transfer calculations. Including turbulent dispersion in your model can thus have a significant impact on the effect of the particles on the continuous phase when coupled calculations are performed.

Figure 22.12.10: Mean Trajectory in a Turbulent Flow

Figure 22.12.11: Stochastic Trajectories in a Turbulent Flow

Cloud Tracking

For turbulent flows, you can also include the effects of turbulent dispersion on the injection. When cloud tracking is used, the trajectory will be tracked as a cloud of particles about a mean trajectory, as described in Section  22.2.2.

1.   Click the Turbulent Dispersion tab in the Set Injection Properties panel.

2.   Enable cloud tracking by turning on the Cloud Model under Cloud Tracking.

3.   Specify the minimum and maximum cloud diameters. Particles enter the domain with an initial cloud diameter equal to the Min. Cloud Diameter. The particle cloud's maximum allowed diameter is specified by the Max. Cloud Diameter.

You may want to restrict the Max. Cloud Diameter to a relevant length scale for the problem to improve computational efficiency in complex domains where the mean trajectory may become stuck in recirculation regions.

next up previous contents index Previous: 22.12.4 Defining Injection Properties
Up: 22.12 Setting Initial Conditions
Next: 22.12.6 Custom Particle Laws
© Fluent Inc. 2006-09-20