### 7.8.5. Interphase Turbulent Dispersion Force

Turbulent Dispersion Forces result in additional dispersion of phases from high volume fraction regions to low volume fraction regions due to turbulent fluctuations. This is caused by the combined action of turbulent eddies and interphase drag. For example, in a dispersed two phase flow, dispersed particles get caught up in continuous phase turbulent eddies, and are transported by the effect of interphase drag. The effect is to move particles from areas of high to low concentration. Hence, this effect will usually be important in turbulent flows with significant interphase drag.

Note: If you encounter convergence difficulties when using a turbulence dispersion model in conjunction with `Segregated` volume fraction coupling, then you should consider reducing the physical time scale, or switching to `Coupled` volume fraction coupling. For details, see the “Multiphase Control” section under Advanced Options Tab in the CFX-Pre User's Guide.

#### 7.8.5.1. Favre Averaged Drag Model

This is based on the Favre or Mass weighted average of the interphase drag force. For details, see Interphase Drag in the CFX-Solver Theory Guide. This model has been shown to have a wide range of universality

is the turbulent Schmidt number for continuous phase volume fraction, currently taken to be 0.9.

Expert users may also modify the model by using a CEL expression for the non-dimensional Turbulent Dispersion Coefficient. Its default value is unity.

For theoretical details, see Favre Averaged Drag Model in the CFX-Solver Theory Guide.

#### 7.8.5.2. Lopez de Bertodano Model

This is included for backwards compatibility reasons. It requires a Turbulent Dispersion Coefficient to be specified. Unfortunately however, there does not exist a universally valid value of the non-dimensional Turbulent Dispersion Coefficient. Values of 0.1 - 0.5 have been used successfully for bubbly flow with bubble diameters of order a few millimeters. See Lopez de Bertodano (1998) [21] for a general discussion on recommended values of the turbulent dispersion coefficient.

Note:

- The Lopez de Bertodano turbulence model may not converge well for a coupled volume fraction case that has Turbulent Force discretization. You may be able to overcome the problem by initializing with a segregated volume fraction case then restarting with coupled volume fraction.

- The Lopez de Bertodano turbulence model cannot be set with the LES model. Instead use the Favre Averaged Drag turbulence dispersion model, which can be used in combination with the LES model.

For theoretical details, see Lopez de Bertodano Model in the CFX-Solver Theory Guide.

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