Combination of Moving Mesh and Laminar Flow interfaces

My model consists of a bellow which goes up and down periodically with a square function, and, by that, it moves a fluid which is inside.

I'm trying to attach both interfaces by setting the mesh velocity of the moving mesh to the wall of the fluid. I write 'zt' and 'rt' in the velocity fields of the wall boundary, but it says that "time derivatives are not supported".

So now I'm using two time-dependent steps, one just for the ALE, in which the velocity will be stored, and taken as an initial value for the second step, in which SPF is active. But I'm struggling at this point.

Does anyone know how store (maybe in a variable) the displacement/velocity of the moving mesh after the first time-dependent step?

(I attach my model, in case anyone wants to see how it works)


Thanks,
Nacho