Boundary conditions

Glenn

Hi I have a small(I think) problem.Im modeling oscillating flow over a 2D circular cylinder. I have tried 2 sets of boundary conditions. The domain is rectangular and Im am using a structured mesh.

Firstly I tried with a velocity inlet on the lhs and a pressure outlet (set to 0) on the rhs. The velocity is set as Umax*cos(omega*t) where Umax=1. Now when I look at the velocity against time, the lhs gives a max velocity of 0.98 and rhs velocity gives 1.

For the second case I tried a pressure inlet and pressure outlet. The pressure was set according to du/dt=(-1/rho)*dp/dx where u is as above. This gives max velocity of 0.98 for both lhs and rhs.

Im using implicit, Quick, Presto and Piso solvers Can anyone explain why Im getting this slightly lower max velocity?? Thanks (and thanks to anyone who has helped me so far)