The selection of density in Ansys Fluent is very important. Set the density relationship based on your flow regime.

If you want to define the density of the fluid as a constant, select constant in the Density drop-down list under Properties in the Create/Edit Materials Dialog Box. Enter the value of density for the material.

For the default fluid (air), the density is 1.225 kg/.

To enable the Boussinesq approximation for density, choose boussinesq from the Density drop-down list in the Create/Edit Materials Dialog Box and specify a constant value for Density. You will also need to set the Thermal Expansion Coefficient, as well as relevant operating conditions, as described in The Boussinesq Model.

The compressible liquid treatment enables you to model liquid compressibility under high pressure applications. Fluent models compressible liquids using the Tait equation of state, which establishes a nonlinear relationship between density and pressure under isothermal conditions.

The compressible liquid treatment also helps in reducing unphysical pressure spikes that appear in moving and dynamic mesh applications, especially during solid-fluid interactions.

The Tait equation can be represented in terms of pressure and density using the following relationship:

(8–7)

where, and are coefficients that can be determined by assuming that the bulk modulus is a linear function of pressure. The values of coefficients and are based on the reference state values of pressure, density, and bulk modulus.

The simplified form of the Tait equation can be written as:

(8–8)

where,

(8–9)

and

(8–10)

The density ratio is limited to the range:

(8–11)

The speed of sound, , is calculated as:

(8–12)

 

8.3.4.1. Compressible Liquid Inputs

Select the compressible-liquid under Density in the Create/Edit Materials panel as seen in Figure 8.17: Compressible Liquid Materials Setting.

Figure 8.17: Compressible Liquid Materials Setting

In the Compressible Liquid dialog box you need to specify values for the following settings:

• Reference Pressure

• Reference Density

• Reference Bulk Modulus

• Density Exponent

• Maximum Density Ratio Limit

• Minimum Density Ratio Limit

Figure 8.18: Compressible Liquid Density Settings Panel

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Note:

• Ansys Fluent automatically fills user input values for certain materials based on open literature. If data is not available Fluent will fill in values of zero. Therefore, you must verify that suitable values are entered for your application.

• When Fluent automatically fills in values, the default value of the density exponent is set close to the value for water-liquid.

• The value for the density exponent is generally found by calibrating against data for density, pressure, bulk modulus, and speed of sound. If the value is unknown, it is recommended that you use a value of 1, which corresponds to a linear relationship between density and pressure.

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You can set the compressible-liquid density method using the following text command:

define → materials → change-create

In the text command interface, set the change Density? option to yes, and set new method [constant] to compressible-liquid as shown below:

/define/materials> change-create

material-name> water-liquid
material name [water-liquid]
water-liquid is a fluid
change Density? [no] yes

Density
methods: (constant ideal-gas incompressible-ideal-gas real-gas-soave-redlich-kwong real-gas-peng-robinson 
real-gas-aungier-redlich-kwong real-gas-redlich-kwong boussinesq piecewise-linear piecewise-polynomial 
polynomial compressible-liquid user-defined)
new method [constant] compressible-liquid
Reference Pressure (pascal) [101325] 
Reference Density (kg/m3) [998.2000000000001] 
Reference Bulk Modulus (pascal) [2200000000] 
Density Exponent [7.15] 
Maximum Density Ratio Limit [1.1] 
Minimum Density Ratio Limit [0.9]

If you are modeling a problem that involves heat transfer, you can define the density as a function of temperature. Three types of functions are available:

For one of the these methods, select piecewise-linear, piecewise-polynomial, or polynomial in the Density drop-down list. You can enter the data pairs (), ranges and coefficients, or coefficients that describe these functions using the Create/Edit Materials Dialog Box, as described in Defining Properties Using Temperature-Dependent Functions.

In Ansys Fluent, if you choose to define the density using the ideal gas law for an incompressible flow, the solver will compute the density as

(8–16)

where,
	= the universal gas constant
	= the molecular weight of the gas
	= the operating pressure

In this form, the density depends only on the operating pressure and not on the local relative pressure field.

For compressible flows, the gas law is as following:

(8–17)

where,
	= the local relative (or gauge) pressure predicted by Ansys Fluent
	= the operating pressure

If you are solving species transport equations, set properties for the mixture material and for the constituent fluids (species), as described in detail in Defining Properties for the Mixture and Its Constituent Species. To define a composition-dependent density for a mixture, do the following:

If you are modeling a non-ideal-gas mixture, Ansys Fluent will compute the mixture density as

(8–18)

where is the mass fraction and is the density of species .

For compressible flows, the gas law has the following form:

(8–19)

where,
	= the local relative (or gauge) pressure predicted by Ansys Fluent
	= the universal gas constant
	= the mass fraction of species
	= the molecular weight of species
	= the operating pressure

In Ansys Fluent, if you choose to define the density using the ideal gas law for an incompressible flow, the solver will compute the density as

(8–20)

where,
	= the universal gas constant
	= the mass fraction of species
	= the molecular weight of species
	= the operating pressure