A fluid cavity interaction allows you to define a liquid- or gas-filled volume within a model. You can define a fluid cavity interaction in the initial step of an Abaqus/Standard or Abaqus/Explicit analysis. The fluid cavity interaction cannot be modified or deactivated in subsequent analysis steps. For a detailed discussion, see “Fluid cavity definition,” Section 11.5.2 of the Abaqus Analysis User's Guide.
To define a fluid cavity interaction:
From the main menu bar, select Interaction
Create.
Tip:
You can also create a fluid cavity interaction using the 
tool in the Interaction module toolbox.
In the Create Interaction dialog box that appears, do the following:
Name the interaction. For more information about naming objects, see “Using basic dialog box components,” Section 3.2.1.
Select Initial for the step in which the interaction will be created.
Select the Fluid cavity type of interaction.
Click Continue to close the Create Interaction dialog box.
Select the cavity point.
The cavity point is a reference node used to identify the cavity. It should not be connected to any elements in the model. For symmetrical models the cavity point must lie on the axis or axes of symmetry. You can choose a cavity point from the viewport or from a saved set. The default selection method is based on the selection method you most recently employed. To change methods, click Select in Viewport or Sets on the right side of the prompt area.
If your model contains both mesh and geometry regions, select Geometry or Mesh in the prompt area to specify the region that contains the cavity.
Select the cavity surface.
The cavity surface is made up of all the model faces that enclose the cavity. You can choose the faces of a cavity surface from the viewport or select a saved surface. The default selection method is based on the selection method you most recently employed. To change methods, click Select in Viewport or Surfaces on the right side of the prompt area.
The Edit Interaction dialog box appears.
Select a fluid cavity property. If desired, click 
to create the interaction property; see “Defining a fluid cavity interaction property,” Section 15.14.4, for more information.
If desired, toggle on Specify ambient pressure, and enter a pressure to account for the effects of external ambient pressure on the fluid cavity.
For two-dimensional models, you must specify an out-of-plane thickness.
The out-of-plane thickness is used to define the cavity volume.
For Abaqus/Explicit analyses using a pneumatic fluid cavity interaction property, toggle on Use adiabatic behavior if desired.
If desired, toggle off Check surface normals to prevent Abaqus/CAE from checking that all surface normals bounding the fluid cavity point in toward the cavity.
This check can be computationally expensive for complex cavity geometry.
Click OK to create the interaction and to close the editor.
Symbols that represent the fluid cavity interaction that you just created appear in the viewport. For more information, see “Understanding symbols that represent interactions, constraints, and connectors,” Section 15.10.
