With an assigned pressure, the pressure is known at the pipe endpoint. The unknown is then the flow rate, which can be obtained by solving the appropriate compatibility equation.

If the pressure is known at the upstream end of the pipe, the negative compatibility equation is used (Equation 2).

Conversely, if the pressure is known at the downstream end, the positive compatibility equation is used (Equation 1).

If the known pressure changes with time, then a current pressure is obtained for the current time step and used in the equations.

Assigned Pressure Vapor Cavitation Theory

Assigned Pressure junctions cannot reach vapor pressure because the pressure is specified and is therefore always above vapor pressure.