Rob
Ansys Employee

Steady state solver will try and find the single solution to the problem, if there are any transient features in the flow it'll struggle to converge, and typically give a residual plot like you see. However, the scale of error may be small enough that you can ignore it. 


The transient solver will pick up and include all of the small changes if the time step is correct. These changes may be small enough that you don't really notice them, but the changes may be sufficient that there isn't a steady solution. 


Have a look in the tutorials for monitor points: we use those to help understand if a solution is converged.