Pressure outlet boundary condition
 

with pressure outlet: wat should be the order of backflow conditions same as those as inlet or default?

Re: Pressure outlet boundary condition
 

If you're using pressure outlets, you should not be expecting backflow through this outlet - so the backflow conditions are needed mainly to prevent the computation being unrealistic or unstable partway through the convergence, when there may be local unsteadiness causing backflow. It is therefore important to choose appropriate (i.e. realistic) backflow conditions, so you should really be using values that are close to the flow that would be found there physically.