I want to achieve fluid-structure coupling, while my solver is based on the FEM and DEM. And in my solver, the nodes numbers during calculation are different from the original input data. I don’t know if such fluid-structure coupling can be achieved through preCICE.

There are two cases.
For the first, there are two discrete triangle elements, and they are deformable, which means each of them is a finite element. In my opinion this fluid-structure interaction is similar to CFD-DEM.

For the second case, two discrete elements are connected by a cohesive element, they may separate to produce cracks.

@Chambernan Considering this case, I think this can already be achieved.

The easy way is, your triangles see Fluid forces (Drag + Buoyancy) through the exchange of flow field values & fluid properties, and the fluid sees the triangles as drag, you need not modify the OF solver itself, just use fvOptions to model this drag.

As it seems that you are interested in the collision & deformation of the triangles, I believe this was should suffice.

But if you want to accurately resolve the flow field around the triangular element, you might have to think of Immersed Boundary method, which makes things complicated.

Edit:
I don’t clearly understand the 2nd case, as I don’t understand what kind of flow field will have such opposing velocities. But if such external fluid forces are applied, you should get the expected behavior.