Use the Normal Mass Flow node to set the inward mass flow boundary condition.

For The Linearized Potential Flow, Frequency Domain Interface, the natural boundary condition for the total wave has the meaning of a mass flow through the boundary surface:

For The Linearized Potential Flow, Boundary Mode Interface, the natural edge condition for the total wave has the meaning of normal mass flow.

Enter an Inward mass flow mn (SI unit: kg/(m2·s)).