Mechanical APDL Structural Analysis Guide

Figure 18.1: Negative and Positive Volumes for Hydrostatic Fluid Elements

(2) Hydrostatic fluid elements having

positive volume

(1) Hydrostatic fluid elements having

negative volume

Hydrostatic fluid elements do not require underlying solid or shell elements in following cases:

• The fluid volume is enclosed by a rigid solid. Instead of adding very stiff underlying solid or shell
  elements, displacement degree-of-freedom constraints may be imposed directly on the hydrostatic
  fluid element nodes. Another alternative is to use a rigid body instead of stiff solid or shell elements
  (see Modeling Rigid Bodies in the Contact Technology Guide).


• Some of the underlying solid or shell elements undergo large displacements while others do not. For
  example, when a piston moves inside a fluid-filled cylinder, the fluid volume in contact with the piston
  and the cylinder may be modeled by a hydrostatic fluid element with one node shared with the solid
  or shell element on the piston, and the other node shared with the solid or shell element on the
  cylinder. Such an arrangement allows the piston to displace without adversely affecting the total
  fluid volume or pressure calculations. (For an example of this type of model, see Example Model Using
  Hydrostatic Fluid Elements (p. 479).)

18.3. Material Definitions and Loading

18.3.1. Fluid Materials

Use the TB,FLUID command to define a material model for compressible hydrostatic fluid elements. All
elements sharing a pressure node must share the same material. There are three ways to define mater-
ial data for compressible fluids: liquid, gas, or pressure-volume data. See Fluids in the Material Reference
for a complete description of these material models.

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:

Constant Symbol Meaning

C1 K Bulk modulus

C2 α Coefficient of thermal expansion

C3 ρ0f Initial density

Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information

Material Definitions and Loading