Important: Before considering the MUSIG or IMUSIG model as a modeling option, you may want to read MUSIG Modeling Advice, which outlines the applicability and limitations of the model.
A partial procedure for setting up a MUSIG or IMUSIG simulation follows:
Edit the applicable domain that contains the polydispersed fluid(s) and the continuous fluid.
The details view for the domain appears.
On the Basic Settings tab, under Fluid and Particle Definitions, for each fluid in the list that represents a polydispersed fluid, select the fluid in the list then set Morphology > Option to
Polydispersed Fluid.On the Polydispersed Fluids tab, in the list under Polydispersed Fluid, define at least one polydispersed fluid.
Each polydispersed fluid in this list will represent one or more of the polydispersed fluids listed on the Basic Settings tab.
For each polydispersed fluid in the list, (select the fluid in the list then) set Option to either
Homogeneous MUSIGorInhomogeneous MUSIG, as appropriate.For a polydispersed fluid that uses the
Homogeneous MUSIGoption, set Polydispersed Fluid (below Option) to the corresponding (same basic material) polydispersed fluid (which is available due to being listed on the Basic Settings tab).For a polydispersed fluid that uses the
Inhomogeneous MUSIGoption, set Polydispersed Fluids (below Option) to a selection of some or all of the corresponding (same basic material) polydispersed fluids (which are available for selection due to being listed on the Basic Settings tab).Each polydispersed fluid in this selection represents a velocity group to which one or more size groups can be assigned.
The set of polydispersed fluids (each constituting a velocity group) that define a polydispersed fluid should have similar (ideally identical) material properties because they represent the same material. In some cases, a slight difference in material properties is appropriate; for example large bubbles might generally be hotter with a lower density, requiring slightly different thermal properties.
Under Size Group, select each listed size group and then configure that group’s Polydispersed Fluid and Size Group Distribution settings. Note that:
For Homogeneous MUSIG, all size groups are forced to belong to the same polydispersed fluid.
For Inhomogeneous MUSIG, each size group can belong to any one of the available polydispersed fluids (which are available for selection due to being listed under Polydispersed Fluids). All size groups that belong to the same polydispersed fluid are part of the velocity group represented by that polydispersed fluid, and so share the same velocity field.
The Tomiyama diameter may serve as a guide for dividing small bubbles from large bubbles.
Configure the Breakup Model and Coalescence Model settings.
Specify boundary conditions, initial conditions, and sources as appropriate.
Note that you can use the RPI wall boiling model to drive phase change at walls.
The following topics are discussed:
On the Polydispersed Fluid tab, one or more polydispersed fluids can be added per Polydispersed (MUSIG) Fluid Definition. Selecting more than one polydispersed fluid (only for IMUSIG) enables 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 Fluids tab. For details, see Multiple Size Group (MUSIG) Model in the CFX-Solver Theory Guide.
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.
Initial conditions can be set to either Value or
Automatic in the same manner as at inlet and opening
boundary conditions.
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.
The MUSIG-specific variables available for postprocessing are:
Conservative Size FractionFor a given small volume of the domain, the
Conservative Size Fractionis the volume of all bubbles contained in a given size group divided by the volume of all bubbles contained in all size groups of the containing velocity group.Conservative size fractions add to one for all size groups in a given velocity group.
Size FractionSize Fractionis derived from the computed value ofConservative Size Fractionand represents, for a given small volume of the domain, the volume of all bubbles contained in a given size group divided by the volume of all bubbles contained in all size groups (of all velocity groups) for a polydispersed fluid.Size fractions add to one for all size groups (without regard for velocity group).
Cumulative Size Fractionis computed as:cumulative size fraction 1 = size fraction 1
cumulative size fraction 2 = size fraction 1 + size fraction 2
cumulative size fraction 3 = size fraction 1 + size fraction 2 + size fraction 3
...
Mean Diameter(Sauter mean diameter)Interfacial Area Density