23.3.16 Specifying Turbulent Dispersion of Particles

As mentioned in Section  23.3.15, 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 the Discrete Random Walk Model 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 this section in the separate Theory Guide.

1.   Click the Turbulent Dispersion tab in the Set Injection Properties dialog box.

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

3.   Specify the Number of Tries:

• An input of 1 or greater tells ANSYS FLUENT to include turbulent velocity fluctuations in the particle force balance as in this equation in the separate Theory Guide. 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 using stochastic tracking.

If you want the characteristic lifetime of the eddy to be random ( this equation in the separate Theory Guide), enable the Random Eddy Lifetime option. You will generally not need to change the Time Scale Constant ( $C_L$ in this equation in the separate Theory Guide) 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  23.3.11 illustrates a discrete phase trajectory calculation computed via the "mean'' tracking (number of tries = 0) and Figure  23.3.12 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 23.3.11: Mean Trajectory in a Turbulent Flow

Figure 23.3.12: Stochastic Trajectories in a Turbulent Flow

Cloud Tracking

For turbulent flows, you can also include the effects of turbulent dispersion on the injection. Note that cloud tracking is not available for the massless particle type. When cloud tracking is used, the trajectory will be tracked as a cloud of particles about a mean trajectory, as described in this section in the separate Theory Guide.

1.   Click the Turbulent Dispersion tab in the Set Injection Properties dialog box.

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.

Note that this model is available only in serial or for shared memory tracking!