# Input

In this section the general input for any FSI run is discussed.

For detailed information regarding specific SFSI/TFSI input data, refer to the corresponding document. In OptiStruct-AcuSolve FSI, you typically only need to define the fluid-structure interface to run Fluid-Structure Interaction Analysis.

For example, in OptiStruct, if the interface surface has a SURF ID of 10, the input data would be:
FSI,2,10

In this case, all of the other parameters are set to their default values.

In order to reduce run times, it is recommended to set an upper bound on the number of exchanges/staggers, as well as setting higher values for the solution variable convergence tolerances.

For example, in OptiStruct, set the maximum number of exchanges to 5 and the force and displacement convergence tolerances to 0.01, respectively with the data:
FSI,2,10,,,,0.01,0.01

The FSI Bulk Data Entry in OptiStruct should be referenced in the Subcase Information section by a corresponding FSI Subcase Information Entry. Currently, only one FSI Subcase Information Entry can be specified in a model (in other words, within a model containing multiple subcases, only one subcase can have the FSI Subcase Entry). See General Limitations in Run FSI for further information.

## Model Location

The OptiStruct and AcuSolve models must be co-located. In other words, fluid boundary mesh and the corresponding structural damp surface mesh match each other as a physical boundary. The gap between these meshes should be small. To verify the conformity of exchange surfaces in AcuSolve, consult the .log file for the structural mesh information, as shown below. Verify the number of damp surface elements and the minimum and maximum coordinates of the damp surface, as well as for matching bounding box and areas to help identify position, size and orientation mismatch:
acuSolve:   No. external code  nodes =           170
acuSolve:   No. external code  tri3s =             0
acuSolve:   No. external code quad4s =           168
acuSolve:   No. external code  tri6s =             0
acuSolve:   No. external code line2s =             0
acuSolve:            Min/Max x-coord = -2.500000e-001 2.500000e-001
acuSolve:            Min/Max y-coord = 0.000000e+000 2.000000e+000
acuSolve:            Min/Max z-coord = -2.500000e-001 2.500000e-001
acuSolve:         Total surface area = 4.500000e+000
acuSolve:               Total length = 0.000000e+000
acuSolve:   No. external code   mbds =             0
acuSolve:     No. fluid wet surfaces =            18
acuSolve:            Min/Max x-coord = -2.500000e-001 2.500000e-001
acuSolve:            Min/Max y-coord = 0.000000e+000 2.000000e+000
acuSolve:            Min/Max z-coord = -2.500000e-001 2.500000e-001
acuSolve:   Total fluid surface area = 2.250000e+000