[WUYing]

Hi, I use openfoam to study a 2-way FSI problem, i.e. vortex-induced-vibration (VIV) of cylinder, and my configuration is dynamic mesh + pimpleFoam + sixDoFRigidSolver + Newmark scheme. For users, it mainly involves how to define dynamicMeshDict and give appropriate settings of fluid solver. But I'm still unclear about what exactly FSI I used in my case. And unfortunately, I cannot find very systematic description about the FSI strategy in OF. So I'd like to post my questions here and hope to figure it out with the help of foamers . If I make any mistakes, please point them out.

• For 2-way FSI, there are two categories: monolithic and partitioned. As the name implies, monolithic approaches consider fluid and structure as a whole and solve their governing equation simultaneously with a single solver; while partitioned approaches solve fluid and structure separately with different solvers. As far as I know, OF in present uses partitioned FSI because of lower computational cost and high flexibility.

• For partitioned FSI, fluid and structure must coupled together after solved individually. Here, there are also two coupling methods: explicit and implicit. Explicit coupling (also called weakly-coupled) solves structural motion and moves mesh only once but solves flow field for multiple times. Implicit coupling (also called strongly-coupled) solve structural and mesh motion as well as flow field for multiple times. OF has both explicit and implicit FSI.

Since the cylinder is vibrating, I apply dynamic mesh technique in OF. The dynamicMeshDict file has a template as:

dynamicFvMesh   dynamicMotionSolverFvMesh;

motionSolverLibs ("libsixDoFRigidBodyMotion.so");

motionSolver    sixDoFRigidBodyMotion;

patches           (cylinder);
innerDistance   0.3;
outerDistance   1;
...
solver
 {
     type    Newmark;
     gamma   0.5; 
     beta    0.25;   
 }
...

Here I have a few questions:

(1) For the 2nd and 3rd lines, it specifies sixDoFRigidBodyMotion solver as both a mesh motion solver and a structural motion solver. I can understand the boundary motion of cylinder can be numerically derived from 6-DOF model using Newmark scheme. But what about the deformation of internal mesh between innerDistance and outerDistance? Some literature said it uses SLERP (spherical linear interpolation) to diffuse the internal mesh, but I haven't find any official documents about that.

(2) How to apply strongly-coupled FSI in OF?

• specify the implicit fluid solver, e.g. pimpleFoam

• specify the implicit numerical solver for structural motion equation, i.e. Newmark

• appropriate settings for PIMPLE algorithm in fvSolution to activate strongly-coupled FIS, e.g. nOuterCorrectors > 1, moveMeshOuterCorrectors yes or no?

I am very appreciated for any suggestion!