COMSOL 6.3 - Enclosed Cavity

Enclosed Cavity

Add an node to indicate that certain boundaries represent a closed volume. The node defines variables for the cavity volume in the undeformed and deformed configurations.

The cavity can be filled with a compressible or an incompressible fluid by adding a subnode, see Fluid (Enclosed Cavity). In this case, the pressure of the fluid inside the cavity is governed by an equation of state, and it changes when the enclosed volume varies due to external loads. It is also possible to apply a constant pressure, or an initial overpressure using the node, see Prescribed Pressure (Enclosed Cavity).

The node can be used without any subnodes. In this case, variables for the enclosed volume in the undeformed and the deformed configurations are created, and they can be evaluated in the node.

The node is only available with some COMSOL products (see https://www.comsol.com/products/specifications/).


	An example can be found in Hyperelastic Seal: Application Library path Nonlinear_Structural_Materials_Module/Hyperelasticity/hyperelastic_seal


	See also Enclosed Fluids in the Structural Mechanics Theory chapter.

Reference Point Selection

This section is only visible if the is and is . In that case, select one point, representing the reference point Xref.

Sketch

This section is only visible if the is . When the boundary selection forms an open surface, a reference point has to be carefully selected to correctly compute the enclosed volume. The sketch section provides hints for suitable reference points for different configurations.

Volume Definition

When using the node in the Membrane interface, select the — or to indicate on which side of the membrane the cavity is located.

Select the — (default) or . Choose when the selected boundaries do not form a closed volume due to, for instance, mirror and sector symmetries, or a plane end cap.

For , select the — (default) or . The reference point, Xref, must be carefully selected according to the rules defined above.

•

When is selected, select a reference point, Xref, in the section.

•

When is selected, enter the coordinates for Xref.

Enter the fV.


	See also Volume Definition and Symmetry Planes in the Structural Mechanics Theory chapter.

Advanced

Enter a value for Vext, if applicable. If the fluid-enclosing volume can be divided into a deforming volume and a rigid volume (such as a rigid pipe or reservoir), the rigid volume does not need to be modeled explicitly. It will however affect the relative volume change, and thus, the pressure change acting on the elastic walls.

If certain enclosing boundaries are not part of the current physics selection (for instance, because they are considered rigid), they can still be marked as enclosing boundaries by selecting the checkbox.

When selecting this checkbox, boundaries that are not part of the physics become applicable in the section. For all external boundaries, a normal direction has to be specified. By convention, the normal direction must point into the cavity (or fluid domain). Define a suitable normal for all external boundaries by referencing a boundary system in . You can always create a new boundary system specific for this feature. If necessary, use the boundary system’s subnode , so that the normal for all external boundaries points toward the cavity.


	At least one boundary has to be internal to the current physics, when including external boundaries in the Boundary Selection section. In 2D, the out-of-plane thickness of external boundaries is assumed to be equal to the average thickness of selected physics boundaries.


	The Enclosed Cavity node can be used to define a cavity across multiple physics interfaces when the total enclosed volume is defined by boundaries of two interfaces, for instance a Solid Mechanics and a Membrane interface. In this case, add the Enclosed Cavity node to only one of the two interfaces, and select the Include boundaries external to current physics checkbox to add the boundaries of the other physics in the Boundary Selection section. If the Enclosed Cavity node is added to e.g. Membrane, the pressure load must then be manually included in e.g. Solid Mechanics. The pressure variable has a name such as <phys>.enc1.p_rel, where <phys> is the name of the Membrane interface. The volume change of due to external boundaries is only considered if the checkbox Include geometric nonlinearity is selected in the study step. For a geometrically linear study, external boundaries are considered rigid.

Location in User Interface

Context Menus

Ribbon

Physics tab with or selected: