[kino]

Hello:
I'm working on my own pressure-based code, using fractional-step method, referring to OpenFlower. Now this code can deal with lid-driven cavity, but when simulating flow around a cylinder(Re=100), pressure oscillation can be seen near the pressure outlet boundary, so unphysical result appears.
For the pressure outlet boundary, in the moment equation, the normal gradient of velocity is set to zero; in the pressure equation, the pressure is set to zero.
What may cause this oscillation? I checked my code carefully but found nothing.
Thank you for your instructions!