Rob
Ansys Employee

Dynamic mesh would be if the bridge was moving. You can probably assume it's fixed, but may want to google Tacoma Narrows Bridge Collapse for when that's not case! It'll only move if you tell it to, so at present it's not going anywhere. 

We often talk about "dynamic adaption" which refers to automatic mesh refinement & coarsening. That's commonly used around a free surface to refine the mesh as the surface moves. As you've not got any waves it's probably not necessary. 

 

The only issue with the set up (that I can see) is that the bridge supports are effectively continuous so the water can't go around them. I don't know if that's part of the design but in most cases they probably shouldn't be there.

Drag etc will almost certainly vary with time, in the steady solver it can vary with iteration if the solver can't lock the flow down to a single converged solution. You either need to run transient, or take the steady result with some scatter due to inherent transients in the flow: that's something you may want to discuss with your supervisor.