The tendency for a flow to cavitate is characterized by the cavitation number, defined as:
 | (5–263) |
where 
is a reference pressure for the
flow (for example, inlet pressure), 
is
the vapor pressure for the liquid, and the denominator represents
the dynamic pressure. Clearly, the tendency for a flow to cavitate
increases as the cavitation number is decreased.
Cavitation is treated separately from thermal phase change, as the cavitation process is typically too rapid for the assumption of thermal equilibrium at the interface to be correct. In the simplest cavitation models, mass transfer is driven by purely mechanical effects, namely liquid-vapor pressure differences, rather than thermal effects. Current research is directed towards models that take both effects into account.
In CFX, the Rayleigh Plesset model is implemented in the multiphase framework as an interphase mass transfer model. User-defined models can also be implemented.
For cavitating flow, the homogeneous multiphase model is typically used.
The Rayleigh-Plesset equation provides the basis for the rate equation controlling vapor generation and condensation. The Rayleigh-Plesset equation describing the growth of a gas bubble in a liquid is given by:
 | (5–264) |
where 
represents the bubble
radius, 
is the pressure in
the bubble (assumed to be the vapor pressure at the liquid temperature), 
is the pressure
in the liquid surrounding the bubble, 
is the liquid density, and 
is the surface
tension coefficient between the liquid and vapor. Note that this is
derived from a mechanical balance, assuming no thermal barriers to
bubble growth. Neglecting the second order terms (which is appropriate
for low oscillation frequencies) and the surface tension, this equation
reduces to:
 | (5–265) |
The rate of change of bubble volume follows as:
 | (5–266) |
and the rate of change of bubble mass is:
 | (5–267) |
If there are 
bubbles per unit volume,
the volume fraction 
may be expressed as:
 | (5–268) |
and the total interphase mass transfer rate per unit volume is:
 | (5–269) |
This expression has been derived assuming bubble growth (vaporization). It can be generalized to include condensation as follows:
 | (5–270) |
where 
is an empirical factor that may
differ for condensation and vaporization, designed to account for
the fact that they may occur at different rates (condensation is usually
much slower than vaporization). For modeling purposes the bubble radius 
will be replaced by the nucleation site radius 
.
Despite the fact that Equation 5–270 has been generalized for vaporization and condensation, it requires further modification in the case of vaporization.
Vaporization is initiated at nucleation sites (most commonly
non-condensable gases). As the vapor volume fraction increases, the
nucleation site density must decrease accordingly, because there is
less liquid. For vaporization, 
in Equation 5–270 is replaced by 
to give:
 | (5–271) |
where 
is the volume fraction of the nucleation sites. Equation 5–270 is maintained in the case
of condensation.
To obtain an interphase mass transfer rate, further assumptions regarding the bubble concentration and radius are required. The Rayleigh-Plesset cavitation model implemented in CFX uses the following defaults for the model parameters:
For an illustration and a validation of the Rayleigh-Plesset cavitation model see Bakir et al. [148]
Additional information on creating a user-defined model is available. For details, see User Defined Cavitation Models in the CFX-Solver Modeling Guide.
When using a user-defined cavitation model, the CFX-Solver will perform generic linearizations for the volume fraction and volume continuity equations to help stability and convergence. The saturation pressure is used by the CFX-Solver in linearizing the cavitation rate against pressure.