Mechanical APDL Structural Analysis Guide

Chapter 18: Modeling Hydrostatic Fluids

Hydrostatic fluid elements are used to model fluids that are fully enclosed by solids (containers). The
available elements are:

• HSFLD241 for 2-D models (plane stress, axisymmetric, or plane strain)


• HSFLD242 for 3-D models

These elements are formulated for linear and nonlinear static and transient dynamic applications. For
more information, see HSFLD241 - 2-D Hydrostatic Fluid and HSFLD242 - 3-D Hydrostatic Fluid in the
Mechanical APDL Theory Reference.

The following topics related to using hydrostatic fluid elements are available:

18.1. Hydrostatic Fluid Element Features

18.2. Defining Hydrostatic Fluid Elements

18.3. Material Definitions and Loading

18.4. Example Model Using Hydrostatic Fluid Elements

18.5. Results Output

For an example analysis, see Hydrostatic Fluid Analysis of an Inflating and Rolling Tire in the Technology
Demonstration Guide.

18.1. Hydrostatic Fluid Element Features

The following are characteristics of the hydrostatic fluid elements,HSFLD241 and HSFLD242:

• The fluid volume is fully enclosed within the solid so it has no free surface.


• All hydrostatic fluid elements defining a fluid volume share a pressure node with a hydrostatic pressure
  degree of freedom, so the fluid volume has uniform pressure, temperature and density.


• The pressure node can be located anywhere within the fluid volume. It is automatically moved to
  the geometric center of the fluid volume if there are no displacement degree-of-freedom constraints
  specified. However, if the fluid volume is bounded by one or more symmetry lines or planes, the
  pressure node must be on the symmetry line or plane or the intersecting corner or line of multiple
  symmetry lines or planes, and it must have symmetry boundary conditions.


• For 2-D models, the planar behavior of HSFLD241 elements (plane stress or plain strain) is based on
  the underlying solid element. Use KEYOPT(3) to define axisymmetric behavior.


• The fluid may be modeled as incompressible (use KEYOPT(6) = 1), or it may be modeled as compressible
  by defining fluid material models (TB,FLUID). The effects of fluid viscosity are ignored.


• Fluid flow between two fluid volumes with separat e pressure nodes may be modeled by connecting
  the pressure nodes with FLUID116 coupled thermal-fluid pipe elements. In this case, you must set
  KEYOPT(1) = 1 for the hydrostatic fluid elements to activate the HDSP and PRES (pressure) degrees
  of freedom at the pressure node,and you must set KEYOPT(1) = 3 on the FLUID116 elements. The

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