Cacluate power exchanged between solid/fluid and solid/solid

Dear all,

I've solved a coupled linear elasto-acoustic model using COMSOL's Acoustic-Solid Interaction module. During the post-processing I would like to calculate energy balances to better analyze the implemented system. I have 2 questions.

Firstly, so far I've managed to calculate the power exchanged between a solid and a fluid domain using the surface integration function. However, I noticed that using the local velocity components (spatial) "acsl.vx, acsl.vy, acsl.vz" of the fluid domain provides a correct result whereas using the the velocity components of the solid domain (Material) "acsl.u_tX, acsl.u_tY, acsl.u_tZ" results in a deviation. Usually these two velocities should provide the same results due to the continuity of stress vectors and displacements. How come the results are different?

Secondly, I have problems implementing the power exchanged between two solid domains. I would like to implement the "...surface_integral((sigma.n) * conj(...))". I have difficulties to chose the correct tensor of constraints from the list.

Thank you very much in advance.

Kind regards,

Martin