Periodic Flow Condition
The Periodic Flow Condition splits its selection into a source group and a destination group (the Destination Selection section is optional). Fluid that leaves the domain through one of the destination boundaries enters the domain through the corresponding source boundary. This corresponds to a situation where the geometry is a periodic part of a larger geometry. If the boundaries are not parallel to each other, the velocity vector is automatically transformed.
If the boundaries are curved, the orientation of the source must be specified manually (see Orientation of Source).
No input is required when Compressible flow (Ma<0.3) is selected for Compressibility under the Physical Model section for the physics interface. Typically when a periodic boundary condition is used with a compressible flow, the pressure is the same at both boundaries and the flow is driven by a volume force.
Boundary Selection
The software usually automatically identifies the boundaries as either source boundaries or destination boundaries, as indicated in the selection list. This works fine for cases like opposing parallel boundaries. In other cases, right-click Periodic Flow Condition and select Manual Destination Selection to control the destination. By default it contains the selection that COMSOL Multiphysics identifies.
Destination Selection
This section is available for specifying the destination boundaries, if needed, when the Manual Destination Selection option is selected in the context menu for the Periodic Flow Condition node. You can only select destination boundaries from the union of all source and destination boundaries.
Flow Condition
This section is available when Incompressible flow is selected for Compressibility under the Physical Model section for the physics interface.
The Flow Condition at the boundary is specified through a value or expression for either the Pressure difference, , or the Mass flow, . The mass flow option ensures a pressure difference is achieved across the source and destination boundaries. This pressure difference can, for example, drive the fully developed flow in a channel.
To set up a periodic boundary condition, both boundaries must be selected in the Periodic Flow Condition node. COMSOL Multiphysics automatically assigns one boundary as the source and the other as the destination. To manually set the destination selection, a Destination Selection subnode is available from the context menu (by right-clicking the parent node) or from the Physics toolbar, Attributes menu. All destination sides must be connected.
Orientation of Source
To display this section, click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog box. For information about the Orientation of Source section, see Orientation of Source and Destination in the COMSOL Multiphysics Reference Manual.
Orientation of Destination
This section appears if the setting for Transform to intermediate map in the Orientation of Source section is changed from the default value, Automatic, and Advanced Physics Options is selected in the Show More Options dialog box. For information about the Orientation of Destination section, see Orientation of Source and Destination in the COMSOL Multiphysics Reference Manual.
Mapping Between Source and Destination
To display this section, click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog box. For information about the Mapping Between Source and Destination section, see Mapping Between Source and Destination in the COMSOL Multiphysics Reference Manual.