Mechanical APDL Structural Analysis Guide

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:

Constant Symbol Meaning

C1 ρ0f Initial density

Since the compressible gas is modeled using the Ideal Gas Law, you must specify a reference pressure
(use real constant PREF on the hydrostatic fluid element) and you must specify a reference temperature
(use the TREF or MP,REFT command) with temperature offset (use the TOFFST command).

Use the TB,FLUID command with TBOPT = PVDATA to define compressible fluid behavior in terms of
a pressure-volume curve. Use the TBPT command to enter the data points (X,Y) for the curve:

Constant Meaning

X Pressure value

Y Corresponding volume value

Use the MP,DENS command to define initial density for a fluid with pressure-volume data.

Use the TBTEMP command to define temperature dependent material constants for all of the above
material options.

Use theMP,DENS and MP,ALPX commands to specify density and coefficient of thermal expansion, re-
spectively, for an incompressible fluid that allows thermal expansion.

18.3.2. Loads and Boundary Conditions

The following loads and boundary conditions can be applied to hydrostatic fluid elements:

Load Command Comments

Apply initial fluid pressure at pressure node with

HDSP degree of freedom.

Initial fluid pressure IC

TREF or

MP,REFT

Initial fluid temperature TREF applies to entire model.

MP,REFT applies to a specific material.

Temperature offset (from TOFFST

absolute zero)

Temperature loads BF By default, the temperature is set to TUNIF.

Apply this constraint type to the pressure node.

This constraint is equivalent to applying a surface

Hydrostatic pressure DOF D

constraint

load on the underlying solid surface, as far as the

effect of fluid pressure on the underlying solid is

concerned. The change in hydrostatic pressure value

is assumed to occur as a result of the addition or

removal of fluid mass to or from the containing

vessel.

Apply fluid mass flow rat e at the pressure node. A

positive value adds fluid mass to the containing

Fluid mass flow rate F

vessel, and a negative value results in removal of

fluid mass.

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Modeling Hydrostatic Fluids