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 2D 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 2), 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 separate pressure nodes can be modeled by connecting the pressure nodes with FLUID116 coupled thermal-fluid pipe elements. Fluid flow through an orifice between a fluid volume and the atmosphere can also be modeled with FLUID116. For both cases, 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 PRES (pressure) and HDSP (hydrostatic pressure) degrees of freedom are made to be the same at the pressure node.

Inertial effects such as sloshing cannot be modeled due to a uniform pressure assumption. However, fluid mass can be added to the hydrostatic fluid element surface nodes that are shared with underlying solid or shell elements. Use KEYOPT(5) to distribute fluid mass to surface nodes.