Does anyone know how to specify pressue (pressure correction) BC for incompressible flow on a moving boudary (esp. where the acceleration is not zero)? Is dp/dn = 0 correct? or should it be dp/dn = -an (an is the boundary acceleration in the normal direction of the moving boundary)? Thanks a lot.

The correct condition is the physical one, velocity of the fluid at boundary equals the velocity of the boundary.

How that relates to the pressure depends on the algorithm you've selected.

The normal pressure gradient at the boundary will be proportional to the normal acceleration. You can derive it very easily by taking the normal component of the momentum equation at the boundary.

To addition : 1. if you apply pressure boundary condition (as recommended), check your surface velocity (see recomendation 1) carefuly. 2. If you want to transfer into body coordinate system, do NOT forget about acceleration.