Material Property

Material properties are volume conditions in the Cavitation module that refer to selected volumes.

The material properties appear in the Properties panel when you select a fluid domain under Domains in the Flow Analysis Tree.

Schimdt Numbers

The Schmidt numbers Gas Schmidt Number, Vapor Schmidt Number, and Dissolved Gas Schmidt Number are specified for each volume. They are set to a constant value of 1 by default.

Cavitation Property

Parameters under Cavitation Property describe the physical properties of the NCG and the vapor in the mixture flow. They also pertain to the amounts and rates of formation, rates of absorption, and release of the NCG and the vapor in the liquid.

The following properties are specified for a selected volume under Cavitation in the Model Panel:

• Gas Mass Fraction—Allows you to specify the mass fraction of the nondissolved gas (NCG) present in a selected volume.

This is available for a volume under the constant gas model, dissolved gas mass fraction, and equilibrium dissolve gas model. For the Dissolved Gas Model, and Equilibrium Dissolve Gas Model, the value of the Gas Mass Fraction is used as the concentration of nondissolved gas at the boundary. It does not restrict the value of the Gas Mass Fraction within the volume.

This is not available as a volume condition under the Full Gas Model and the Variable Gas Mass Fraction model, where the amount of nondissolved NCG is computed as part of the solution.

• Molecular Weight—For the NCG in the Cavitation module, specify the following:

◦ Gas Molecular Weight—Molecular weight of the NCG, which is required when you use the ideal gas law to calculate NCG density.

◦ Vapor Molecular Weight—Molecular weight for the vapor, which is required when you use the ideal gas law to calculate vapor density.

• Liquid Density—Density of the liquid component in cavitating flows. Liquid density is a constant, compressible liquid, or user-defined function given in the Expression Editor.

• Liquid Bulk Modulus—Bulk modulus of the liquid component of the fluid. The effective bulk modulus of the fluid (locally) is computed on the basis of the Compressible Liquid model using the Liquid Bulk Modulus, Liquid Reference Pressure, and Linear Bulk Modulus(B1). Liquid bulk modulus of the fluid is made a function by using the Expression Editor.

• Liquid Reference Pressure—Used to compute the density based on the Compressible Liquid model.

• Saturation Pressure—Vapor pressure at which the vapor and its liquid are at equilibrium. This is used as a threshold pressure to determine the evaporation and condensation process in cavitation. The saturation pressure is a material property that is strongly temperature-dependent. Saturation pressure is a constant at the given temperature for isothermal flows.

If temperature varies or the Heat module is active, then the Saturation Pressure should be made into a function of temperature by using the Expression Editor to include temperature effects. The effective vapor density is computed as function of the local pressure and Saturation Pressure based on Boyle’s Law.

• Minimum Pressure—Positive lower limit imposed on the flow pressure (minimum pressure) to guarantee physical solutions as well as to enhance numerical stability. Used in cavitation models.

• Dissolved Gas Reference Pressure—Used to determine the equilibrium mass fraction of the dissolved gas at the local pressure in equation 2.233.

It is specified for selected volumes under the Cavitation module when the Equilibrium Dissolve Gas Model, Dissolved Gas Model, or Full Gas Model is active.

• Dissolved Gas Mass Fraction—Mass fraction of the dissolved gas at the Dissolved Gas Reference Pressure that is used to determine the equilibrium mass fraction of the dissolved gas at the local pressure according to equation 2.233.

It is specified for selected volumes under the Cavitation module when Equilibrium Dissolve Gas Model, Dissolved Gas Model, or Full Gas Model is active.

• Dissolved Gas Release Time τ (seconds)—Determines the rate at which NCG is released from the solution based on equation 2.233.

This is specified for selected volumes under Cavitation Property when Dissolved Gas Model and Full Gas Model are active.

• Dissolved Gas Dissolve Time τ (seconds)—Determines the rate at which NCG goes into solution based on equation 2.233.

This is specified for selected volumes under Cavitation Property when Dissolved Gas Model or Full Gas Model is active.