convective bc

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I am trying to implement a convective BC for vortex shedding problem.I use a SIMPLEC algorithm.

The convective BC for u,v is: du/dt+Uc*du/dn=0 dv/dt+Uc*dv/dn=0

Then I made the correction for u based on continuity.

Q1: How to choose Uc? Q2: Shoud I use zero gradient BC for pressure correction on the convective BC or fixed the pressure correction to zero? Q3: I tried several combinations of velocity and pressure correction on the convective BC, all failed. It seems that the convective BC is not numericallly stable. However, it works well for steady case such as 2D channel flow.

If you have some experience on it, please make comments and suggestions. Thanks a lot.