Yeah, I understand why acceleration is happening. Makes sense.

Is the dynamic mesh for the interface or for the bridge?

The thing is that the bridge is a void area, therefore it cannot move. If it is designed like this, is buoyancy still considered? 

In this analysis I just want to get drag and lift coefficients, but since values were varying a lot for a steady state, I don't know if the simulations is performed correctly. (I would understand if values are sinusoidal ish for transient states.