Learning Forum

[Margins]

 Far walls set as open boundaries with a zero normal stress boundary condition, imposing ≈ = − p= f = 0 N/m^2, where f is the boundary stress vector directed in a negative normal orientation. The top of the fluid domain was set as a constant pressure outlet boundary condition (p = 0 Pa ), with backflow suppression.

[Rob]

Pressure boundary, with zero gauge pressure I suspect. Assuming you also want gravity you'll need to be careful with the operating density to avoid problems. Searching for "operating density" in the Fluids section should explain why.