Coupling Physics (Piezoelectric + Laminar Flow)

Good day,

I am currently running a simulation of a SAW device that interacts with a fluid domain.
I have managed to simulate the SAW device with the piezoelectric physics but have yet to be able to couple it with the laminar flow physics to be able to see the interaction between the SAW produced and how it affects the fluid domain on it; as well as if the viscosity of the fluid dampens the SAW.

I have tried defining the wall velocity of the fluid as the time derivative of the displacement from the piezoelectric physics in the Global Definitions > Functions > External. But it proved futile.

Can someone teach me the right way to be able to couple these two physics?

Thanks!