Fluid–Structure Interaction

The fsi module couples laminar, single-species flow to a structural solver through preCICE.

Basic Coupled Case

Use a Stream build with preCICE and a structural solver with a preCICE adapter. Load both modules and update these entries in case.vars, using your moving-wall name in place of rod:

FSI settings in case.vars

 loadModule: gridMotion
 loadModule: fsi
 {

 // Boundary conditions
 boundary_conditions: <rod=noslip(adiabatic,moving,fsi=rodSolid)>

 // Time-stepping
 timeStep: 1.0e-2
 numTimeSteps: 10
 convergenceRelativeTolerance: <default=1.0e-4,pressure=1.0e-5>
 maxIterationsPerTimeStep: 200

 // FSI coupling
 meshCouplingMode: delayed
 fsiAdapterBackend: precice
 fsiPreCICEConfigFile: ../precice-config.xml
 }

rodSolid labels the fluid interface. moving enables mesh motion. Use the same label on adjoining FSI boundaries. Stream sends forces after the fluid equations meet their convergence tolerances.

Place precice-config.xml in the parent of the fluid and solid run directories. Configure the structural adapter as participant Solid with mesh SolidSurface. Use coordinates in metres and supply the solid surface connectivity through its adapter.

precice-config.xml

 <precice-configuration xmlns:data="urn:precice:data"
   xmlns:mapping="urn:precice:mapping" xmlns:m2n="urn:precice:m2n"
   xmlns:coupling-scheme="urn:precice:coupling-scheme">
   <data:vector name="Displacement" />
   <data:vector name="Force" />

   <mesh name="StreamNodes" dimensions="3">
     <use-data name="Displacement" />
     <use-data name="Force" />
   </mesh>
   <mesh name="SolidSurface" dimensions="3">
     <use-data name="Displacement" />
     <use-data name="Force" />
   </mesh>

   <participant name="Stream">
     <provide-mesh name="StreamNodes" />
     <read-data name="Displacement" mesh="StreamNodes" />
     <write-data name="Force" mesh="StreamNodes" />
   </participant>
   <participant name="Solid">
     <provide-mesh name="SolidSurface" />
     <receive-mesh name="StreamNodes" from="Stream" />
     <mapping:nearest-projection direction="write" from="SolidSurface"
       to="StreamNodes" constraint="consistent" />
     <mapping:nearest-projection direction="read" from="StreamNodes"
       to="SolidSurface" constraint="conservative" />
     <write-data name="Displacement" mesh="SolidSurface" />
     <read-data name="Force" mesh="SolidSurface" />
   </participant>

   <m2n:sockets acceptor="Stream" connector="Solid" exchange-directory=".." />

   <coupling-scheme:serial-explicit>
     <participants first="Stream" second="Solid" />
     <time-window-size value="1.0e-2" />
     <max-time-windows value="10" />
     <exchange data="Displacement" mesh="StreamNodes"
       from="Solid" to="Stream" initialize="on" />
     <exchange data="Force" mesh="StreamNodes"
       from="Stream" to="Solid" initialize="off" />
   </coupling-scheme:serial-explicit>
 </precice-configuration>

The example uses ten coupling steps of 0.01 s, matching timeStep and numTimeSteps in the fluid file. Displacement is the nodal displacement from the reference mesh in m. Force contains nodal loads in N acting from the fluid onto the solid.

Start the structural solver with this configuration. In a second terminal, run Stream from the fluid case directory:

stream -q solution case

Coupling Options

For loads relative to an ambient pressure of 1 atm, set fsiPressureReference: 101325.0. The value is in Pa. The default 0 uses absolute fluid pressure.

Use these inputs to match a different preCICE configuration:

preCICE names and configuration
Input	Default	Meaning
`fsiParticipantName`	`Stream`	Fluid participant
`fsiPreCICEConfigFile`	`precice-config.xml`	Configuration path
`fsiReadMeshName`	`StreamNodes`	Displacement mesh
`fsiWriteMeshName`	`StreamNodes`	Force mesh
`fsiReadDataName`	`Displacement`	Data read from the solid
`fsiWriteDataName`	`Force`	Data sent to the solid

Prescribed Displacement

For prescribed motion, select debugFile (the default) and set the boundary displacement from the reference mesh:

Prescribed-motion settings in case.vars

 {
 // Boundary conditions
 boundary_conditions: <
   rod=noslip(adiabatic,moving,fsi=rodSolid,
   fsiMockDisplacement=[0 m,1.0e-4 m,0 m])
 >

 // FSI coupling
 fsiAdapterBackend: debugFile
 }

To override individual nodes, write fsi_read_displacement.rankN.dat in fsiAdapterOutputDir. N is the MPI rank, or 0 for a serial run. Use node IDs from that rank’s fsi_interface_nodes.rankN.dat. Each row contains node-id dx dy dz, with displacement in metres. For node 1234:

1234 0 1.0e-4 0

Blank lines and # comments are allowed. Nodes omitted from the file use fsiMockDisplacement, whose default is zero.

Output

Add fsi to diagnostics to write interface and load files:

diagnostics: <fsi>

debugFile writes these files automatically. They use fsiAdapterOutputDir (default output), which also holds prescribed displacement files. Load files append; use a fresh output directory per run.

FSI output files
File	Contents
`fsi_interface_nodes.rankN.dat`	Node IDs and reference positions
`fsi_interface_faces.rankN.dat`	Reference faces and node connectivity
`fsi_write_nodes.rankN.dat`	Nodal positions and forces
`fsi_write_mechanical.rankN.dat`	Face loads, areas and wall velocities