Fluid material models can be used with hydrostatic fluid elements to model compressible fluids. For theoretical background on these materials, see Fluid Material Models in the Mechanical APDL Theory Reference. For more information on using these fluid material models with the hydrostatic fluid elements, see Modeling Hydrostatic Fluids in the Structural Analysis Guide.

There are three ways to define material data for compressible fluids: liquid, gas, or pressure-volume data.

Liquid

Use the TB,FLUID command with TBOPT = LIQUID to define material behavior for a liquid, and specify the following material constants using the TBDATA command:

You can define a temperature dependent liquid material with up to 20 temperatures (NTEMP = 20 max on the TB command) by using the TBTEMP command, as shown in the example below:

TB,FLUID,1,2,,LIQUID ! Activate liquid material model 
TBTEMP,100.0         ! Define first temperature
TBDATA,1,K,α,ρ0f     ! Define material constants at temp 100.0
TBTEMP,200.0         ! Define second temperature
TBDATA,1,K,α,ρ0f     ! Define material constants at temp 200.0

When specifying temperature dependent density values for a liquid, consider that the current density (ρ_f) for hydrostatic fluid elements is computed at each iteration as a function of pressure change (ΔP), bulk modulus (K), coefficient of thermal expansion (α), and temperature change (ΔT). A reference temperature may be input using the TREF or MP,REFT command. For details on how the current density is calculated, refer to Liquid in the Mechanical APDL Theory Reference.

Gas

Use the TB,FLUID command with TBOPT = GAS to define material behavior for a gas, and specify the following material constant using the TBDATA command:

You can define a temperature dependent gas material with up to 20 temperatures (NTEMP = 20 max on the TB command) by using the TBTEMP command, as shown in the example below:

TB,FLUID,1,2,,GAS    ! Activate gas material model 
TBTEMP,100.0         ! Define first temperature
TBDATA,1,ρ0f         ! Define material constants at temp 100.0
TBTEMP,200.0         ! Define second temperature
TBDATA,1,ρ0f         ! Define material constants at temp 200.0

When specifying temperature dependent density values for a gas, consider that the current density (ρ_f ) for hydrostatic fluid elements is computed at each iteration based on the Ideal Gas Law. For details on how the current density is calculated, refer to Gas in the Mechanical APDL Theory Reference.

To use the Ideal Gas Law, you also need to define a reference pressure (input as real constant PREF) and a reference temperature (input with the TREF or MP,REFT command) with temperature offset (input with the TOFFST command).

Pressure-Volume Data

Use the TB,FLUID command with TBOPT = PVDATA to define compressible fluid behavior in terms of a pressure-volume curve. You can specify up to 20 temperature-dependent pressure-volume curves (NTEMP = 20 max on the TB command). The temperature for the first curve is input with TBTEMP, followed by TBPT commands for up to 100 pressure-volume data points. The data points (X, Y) entered on TBPT are:

The pressure-volume data point must be defined in terms of total pressure and total volume of the fluid in the containing vessel.