The type of BCs depends on the chosen model, compressible or incompressible.

The incompressible flow problem is well posed if you prescribe your pressure BCs

Thank Filippo. Yes, it us incomprehensible, and now what boundary conditions should I use?

The should use specified pressure using periodic functions along the inlet, and a far field boundary condition for the outlet for incompressible flow. The initiation will take care of the rest so you will have a unique solution for the velocity field.

@Erin

Thanks for your response. When I use periodic boundary for pressure inlet, what should I use for velocity inlet?

if you are using Fluent i don't think you will to need specify bc's for the velocity inlet since you already specified periodic bc for pressure. if you are using your own code you may use second order upwind scheme with zero gradient bc for the inlet velocity condition.