Multiphase Simulation Concepts
This section presents the key concepts you come across when using the Simcenter STAR-CCM+ multiphase models.
- Phase
- A Phase is matter whose properties continuously change in space. Typically the word properties brings to mind the physical properties. However a generalization of the concept of phases also includes properties like velocity, temperature, or characteristic length scale of the matter. For example, if a problem involves spherical water droplets which are very close to each other and have different diameters, and/or move with different velocities, and/or have different temperatures, they could also be considered different phases.
- Phases Interactions
- A phase interaction is an object that defines the choice of models that are used
to predict the influence of one phase upon another phase within a multiphase
flow simulation. The phase interaction models available for selection depend on
the physics models that are selected in the physics continuum.
The Multiphase Interaction model used to define phase interactions is activated automatically when the Multiphase model is selected. Once created, phase interactions are stored under the Phase Interactions node. Each Phase Interaction node contains a Models node that is used to select phase interaction models for each phase pair. When applicable, these models include interaction terms to model the interchange of mass, momentum, and energy between the phases.
Simcenter STAR-CCM+ provides standard phase interaction models for the flow pattern where one dispersed phase (particles or bubbles) is carried in a continuous phase (liquid or gas). You are advised to define a phase interaction for every possible significant coupling. See Defining Phase Interactions.
- Phase Conditions
- Some physics models expose their own region or boundary conditions for the phase
to which they apply. These conditions are Phase
Conditions. In some circumstances, you need to define different
boundary conditions for a specific phase. To set these conditions, phase
conditions are applied at the boundary level and are used to define
phase-specific attributes. For example, when you select a multi-component liquid
in a Eulerian phase, Simcenter STAR-CCM+ adds the
Species Source Option within a phase node for regions
that are associated with the phase.
In a Lagrangian Multiphase flow simulation, the settings that are used to define a Lagrangian phase in the physics continuum can be overridden at a particular boundary by specifying different settings at the boundary level.
For more information on boundary conditions, and how the conditions may differ between single-phase and multiphase flows, see Flow Boundaries Reference.
- Multiphase Mixture
- A multiphase mixture is a fluid that is composed of multiple phases. The corresponding node () is available for Volume of Fluid (VOF), Mixture Multiphase (MMP) and Two-Phase Thermodynamic Equilibrium simulations. This node allows you to specify the bulk properties of the mixture.
- Volume Fraction
-
The volume fraction of a phase is the ratio of the volume that the phase occupies to the computational cell volume. You specify the volume fraction for each phase under the total fluid volume of all phases that are present. Therefore, the sum of volume fractions of all phases equals 1. See Eqn. (873) in Volume Partitioning.
The initial and boundary conditions of this scalar are defined before running the simulation. When you specify initial and boundary values, there is no restriction on the values that you can set for the volume fractions of the phases. However, Simcenter STAR-CCM+ uses normalized values for each phase volume fraction to ensure that the total volume fraction is 1.0.