For a strong coupled solution, both the structure and the acoustic fluid interact with each other via coupling boundary conditions. The displacement and pressure degrees of freedom are solved simultaneously in the fluid-structure interaction (FSI) model.

To specify the strongly coupled FSI interface, issue the following command before the solution:

The program automatically detects and specifies the strongly coupled FSI interfaces between the acoustics elements (FLUID30, FLUID220 and FLUID221) and the solid structural elements (SOLID185, SOLID186, SOLID187 and SOLSH190), but not for the shell elements (SHELL181 and SHELL281).

The FSI interface must be defined on the nodes belonging to the coupled acoustic elements (KEYOPT(2) = 0) rather than on the nodes of the structural elements.

et,1,220,,0         ! coupled acoustic element
…
esel,s,type,,1      ! select acoustic element
nsel,s,ext          ! nodes on the exterior surface
sf,all,fsi         ! FSI interface

The unsymmetric matrix equation is raised by the coupled FSI problem, which can be transformed into the symmetrical matrix equation for performance improvement (KEYOPT(1) = 1 and 2 for modal and harmonic analysis, respectively). Meshing the entire acoustic domain with coupled elements (KEYOPT(2) = 0) leads to a larger Jobname.EMAT file, although the displacement degree of freedom is not solved in uncoupled elements.

After defining the FSI interface or solid structural elements, issue the following command for file size optimization:

The ECPCHG command modifies the attributes of acoustic elements so that only acoustic elements with an FSI interface become the coupled elements, and other acoustic elements are uncoupled, pure acoustic elements. Up to 80% reduction in file size may occur for the Jobname.EMAT file.

If the contact element is used between acoustic elements and solid structural elements for a nonconforming mesh, the MPC contact surface (CONTA173 or CONTA174 with KEYOPT(2) = 2, KEYOPT(4) = 3, KEYOPT(8) = 2, and KEYOPT(12) = 5) must be on the fluid side, and the target surface (TARGE170 with KEYOPT(5) = 1) must be on the solid side.

For more information, see Acoustic Fluid-Structural Interaction (FSI) in the Mechanical APDL Theory Reference.