Pressure BC

Mavinakere

Hi,I'm analysing a multiphase flow that is water and air around a automobile tire.I made a 2d rectangular model with a tire inside.my domain is: 1m in front of the tire and 2m in back of the tire.height is 1m.All the dimensions are with the tire axis.the bottom edge is the ground which is just 1MM below the tire. MY QUESTION IS :what is the appropriate BC on the top edge and the edge behind the tire. I'm using pressure outlet for edge behind the tire. If i use far field pressure for the top edge i get sg_mphase.c error when i initialize. Is it advisable to use symmetry condition on the top? The BC i need there is a DPDN zero...pressure in the cell next to the edge is same as it is on the edge. but what kind of BC does this? Anybody plz respond...thanks.Mavinakere.