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.
18.2. Defining Hydrostatic Fluid Elements
Hydrostatic fluid elements must be associated with solid or shell elements. Use HSFLD241 with 2-D
solid or axisymmetric solid elements; use HSFLD242 with 3-D solid or shell elements.
Follow this general procedure to automatically generat e the hydrostatic fluid elements on the faces of
selected solid or shell elements:
- Build the model using solid or shell elements.
- Select the solid or shell elements and the attached nodes that enclose the fluid volume.
- Define a node to be used as the pressure node for the fluid volume.
- Use the ESURF command with the pressure node ID specified as XNODE to generat e hydrostatic
fluid elements. - Inspect and verify the newly created hydrostatic fluid elements; the elements should be triangle-
shaped (2-D or axisymmetric) or pyramid-shaped (3-D) with a common vertex at the pressure node.
As an alternative to the above method, you can use the E or EN command to manually define hydro-
static fluid elements that share nodes of underlying solid or shell elements. For the 2-D or axisymmetric
case (HSFLD241), nodes I through K (I, J for lower order; I, J, K for higher order) should be shared with
the underlying 2-D or axisymmetric solid element, and node L should be defined as the pressure node.
For the 3-D case (HSFLD242), nodes I through P (I through L for lower order; I through P for higher order)
should be shared with the underlying 3-D solid or shell element, and node Q should be defined as the
pressure node.
In general, the nodes should be ordered in a counter-clockwise manner to get a positive volume. If the
node ordering is not correct, it can be reversed by either using the ESURF command or changing the
order on the E or EN commands.
In some situations, the element volume may need to be negative with nodes ordered in the clockwise
direction. For example, when modeling an inflated tire inner tube, the pressure node will be located
outside of the fluid volume. In this case, some of the hydrostatic fluid elements filling the doughnut-
shaped fluid volume will need to have negative volume (see (1) in the figure below), while others will
have positive volume (see (2) in the figure below) such that the total volume of all the hydrostatic fluid
elements adds up to the volume of the enclosed fluid. For this reason, hydrostatic fluid elements with
negative volume are allowed. However, to avoid errors, a warning message is issued at the beginning
of the analysis for all hydrostatic fluid elements having negative volumes. An internal element component
containing all hydrostatic fluid elements with negative volumes is created and listed in this warning
message.
Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential informationModeling Hydrostatic Fluids