pressure oscillation near pressure outlet boundary
Hello:
I'm working on my own pressurebased code, using fractionalstep method, referring to OpenFlower. Now this code can deal with liddriven 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! 
Vortex shedding?

Thank you very much for your reply, but I think the outflow boundary is far enough from the cylinder. I used to enlarge the computational domain but the problem remains.

Hi Kino,
Let me try again. The problem you are experiencing might be an expression of the vortex shedding phenomenon, AKA Von Kármán vortex street (see, for example, http://en.wikipedia.org/wiki/Vortex_shedding and references therein). If my guess is correct, then you cannot solve this inherently unsteady (and also asymmetric) problem with a steadystate solver, no matter how far your outlet is, and if you try  you'll probably notice oscillations or divergence in the iterative process. If this is the case, either try a transient solution, with no symmetry assumptions, far enough outlet and/or appropriate nonreflecting BC, or (easier way) reduce the Re No. far below the instability limit, so that a steady, symmetric solution exists. I hope this helps, Rami 
I think that the problem may be in the setting of your outflow boundary conditions. You are using a pressure based solver and setting constraints on both pressure (fixed condition) and velocity (zero gradient).
I did some work long ago on a finite difference solver and I seem to recall that I used a gradient calculation for the velocity at the outflow (using a backward scheme), not zero gradient. If you are using finite volume, you could achieve similar results by changing how you compute the gradients at the near boundary cell. Let me know if it helps. Sincerely, Julien 
Thank you for your kindly help.
Hello Rami, actually I'm using an unsteady solver, and the pressure oscillation can be seen from beginning, so I think it's not caused by vortex shedding. Hello julien. decharentenay, I checked my code carefully today and found that when dealing with the convection term, the transporting velocity at cell face can't be treated by interpolation, but have to be updated using the pressure correction equation. Primiry results shows that the pressure oscillation can be removed. Thank you all the same for your instruction. 
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