24.5.4 Modeling Turbulence

If you are using the Eulerian model to solve a turbulent flow, you will need to choose one of turbulence models described in this section in the separate Theory Guide in the Viscous Model dialog box (Figure  24.5.3).

Figure 24.5.3: The Viscous Model Dialog Box for an Eulerian Multiphase Calculation

The procedure is as follows:

1.   Select either k-epsilon or Reynolds Stress under Model.

2.   Select the desired k-epsilon Model or RSM Multiphase Model and any other related parameters, as described for single-phase calculations in Section  12.4.

3.   Under Turbulence Multiphase Model or RSM Multiphase Model, indicate the desired multiphase turbulence model (see this section in the separate Theory Guide for details about each):

• Select Mixture to use the mixture turbulence model. This is the default model.

• Select Dispersed to use the dispersed turbulence model. This model is applicable when there is clearly one primary continuous phase and the rest are dispersed dilute secondary phases.

• Select Per Phase to use a $k$- $\epsilon$ turbulence model for each phase. This model is appropriate when the turbulence transfer among the phases plays a dominant role.

You can enable the effect of drift velocity by performing the following:

1.   If it is not already done, set the Turbulence Multiphase Model to Dispersed in the Viscous Model dialog box.

2.   Enter the multiphase-options text command in the console.

define $\rightarrow$ models $\rightarrow$ viscous $\rightarrow$ multiphase-turbulence $\rightarrow$ multiphase-options

/define/models/viscous/multiphase-turbulence> multiphase-options

Enable dispersion force in momentum? [no] yes

Enable interphase turbulence source? [no] yes

The effect of the drift velocity is influenced both by the momentum equation and, to a lesser extent, the turbulence equation. Therefore, you should answer yes to both questions to take into account the effect of drift velocity.

Anisotropic drag applicable to free surface modeling is only compatible with the Mixture turbulence model.

Including Source Terms

By default, the interphase momentum, $k$, and $\epsilon$ sources are not included in the calculation. If you want to include any of these source terms, you can enable them using the multiphase-options command in the define/models/viscous/multiphase-turbulence/ text menu. Note that the inclusion of these terms can slow down convergence noticeably. If you are looking for additional accuracy, you may want to compute a solution first without these sources, and then continue the calculation with these terms included. In most cases these terms can be neglected.

Customizing the $k$- $\epsilon$ Multiphase Turbulent Viscosity

If you are using the $k$- $\epsilon$ multiphase turbulence model, a user-defined function can be used to customize the turbulent viscosity for each phase. This option will enable you to modify $\mu_{\rm t}$ in the $k$- $\epsilon$ model. For more information, see the separate UDF Manual. In the Viscous Model dialog box, under User-Defined Functions, select the appropriate user-defined function in the Turbulent Viscosity drop-down list.