Unsteady Calculation

Tom

Hello, I try for the first time an unsteady simulation of fluid flow. Now I do not know how large a typical time-step is. My volume has a length of about 300 mm and the typical velocity is about 15 m/s. My calculations are done with fluent, seperated, implicit. I tried a time-step of 0.001s. The solution converged (residuals smaller than 0.001) in one or two iterations each time-step. 1.) Is the time-step too small, or do I have to reduce my convergence-crit. ? I feel, but I don't know, that the convergence-criterion (residuals) from a steady flow cannot taken for an unsteady one. 2.)What influence has the length of the time step to the result at time t ? I heard, that you get different forces at the same time on your monitors (middled wall-shear-stress) with different time-steps. Thanks a lot