### 7.11.1. Setting up a Polydispersed (MUSIG) Simulation

Important: Before considering the MUSIG model as a modeling option, you may want to read MUSIG Model Advice., which outlines the applicability and limitations of the model.

To begin setting up a MUSIG simulation, select the Polydispersed Fluid option for Fluid and Particle Definitions... > <fluid definition> > Morphology on the Basic Settings tab for the domain, then define at least one Polydispersed Fluid on the Polydispersed Fluids tab. Boundary conditions, initial conditions and sources should then be set as appropriate.

#### 7.11.1.1. Creating a Polydispersed (MUSIG) Fluid

On the Polydispersed Fluid tab, one polydispersed fluid must be added per Polydispersed (MUSIG) Fluid Definition. In future releases, multiple fluids may be selected in order to enable different size groups to move with different velocity fields.

The Size Group Distribution setting can be set to `Equal Mass`, `Equal Diameter`, `Geometric`, or `User Defined`. For details, see Size Group Discretization in the CFX-Solver Theory Guide. When the Size Group Distribution setting is set to `User Defined`, the Size Group Diameter parameter must be defined for every Size Group object with the groups ordered so that the diameters are increasing monotonically. The Reference Density must be set if the density of the polydispersed fluid is not constant; this reference density is used only for assigning the masses of the size groups. When the size group diameter is needed to calculate coalescence and breakup rates, the diameter is calculated from the group mass and the local fluid density.

The group names will have the format `Group <N>`, where `<N>` is an integer denoting the group number (sorted in order of increasing size).

Coalescence and breakup models must be set on the Polydispersed Fluid tab. For details, see Multiple Size Group (MUSIG) Model in the CFX-Solver Theory Guide.

#### 7.11.1.2. Boundary Conditions

Inlet and Opening boundary conditions require the specification of size fractions for each of the size groups. The size fractions may be set to `Value` or `Automatic.` All size fractions set to `Automatic` are calculated to have the same value such that the overall sum of size fractions (including those that are specified by value) is unity. If all size fractions are set to `Value`, you must ensure that the specified size fractions sum to unity.

#### 7.11.1.3. Initial Conditions

Initial conditions can be set to either `Value` or `Automatic` in the same manner as at inlet and opening boundary conditions.

#### 7.11.1.4. Sources

There are two types of sources that are relevant for polydispersed (MUSIG) fluids. The first is a continuity source, which is commonly used to model an inlet without resolving the inlet geometry. The source can be applied to a subdomain, a point, or a boundary. When specifying a continuity source, the size fraction distribution must also be provided. The specified size fractions must sum to unity. By default, if the continuity source is negative (that is, a mass sink), the specified size distribution is ignored and the local size fractions are used instead. If you would like the specified size distribution to be used even if the continuity source is negative, set the setting MUSIG Sink Option to `Specified Size Fractions`.

Sources may also be added directly to the size fraction equations without affecting the continuity equation. Consistency requires that the size fraction sources sum to zero when summed over all size groups.

#### 7.11.1.5. Postprocessing Variables

The MUSIG-specific variables available for postprocessing are:

- `Size Fraction`

- `Cumulative Size Fraction`

- `Mean Diameter` (Sauter mean diameter)

- `Interfacial Area Density`

Release 18.2 - © ANSYS, Inc. All rights reserved.