You use a fluid-structure co-simulation interaction to define the interface boundary for the co-simulation. The co-simulation is between Abaqus/CFD and Abaqus/Standard or Abaqus/Explicit, depending on the type of analysis step used in the structural model. Select Interaction
Create from the main menu bar, and select the boundary region for exchanging data; the coupling schemes for the co-simulation are determined automatically by the analysis. For more information, see Chapter 26, “Co-simulation.”
For an Abaqus/CFD analysis involved in a fluid-structure interaction with shells/membranes, a seam defines a zero-thickness shell in the mesh. You can create a double-sided surface that represents the seam and select that surface as the boundary region. For more information, see “Modeling cracks and seams” in “Using the Special menu in the Interaction module,” Section 15.12.14.
Within the fluid model (Abaqus/CFD), the FSI co-simulation interaction can be created only in a flow step. Within the structural model (Abaqus/Standard or Abaqus/Explicit), the FSI co-simulation interaction can be created only in an implicit dynamic, explicit dynamic, or heat transfer step.
To define a fluid-structure co-simulation interaction:
From the main menu bar, select InteractionCreate.
Tip:
You can also create a fluid-structure co-simulation 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 the step in which the interaction will be created. In the Abaqus/CFD fluid model, the step must be a flow step.
Select the Fluid-Structure Co-simulation boundary type of interaction.
Click Continue to close the Create Interaction dialog box.
Use one of the following methods to select the region:
Use an existing surface to define the region. On the right side of the prompt area, click Surfaces. Select an existing surface from the Region Selection dialog box that appears, and click Continue.
Note: The default selection method is based on the selection method you most recently employed. To revert to the other method, click Select in Viewport or Surfaces on the right side of the prompt area.
Use the mouse to select a region in the viewport. For more information, see “Selecting objects within the current viewport,” Section 6.2.
If the model contains a combination of mesh and geometry, click one of the following from the prompt area:
Click Geometry if you want to select the surface from a geometry region.
Click Mesh if you want to select the surface from a native or orphan mesh selection.
Click OK to create the interaction and to close the editor.
