Pressure inflow boundaries are in many ways similar to the velocity inflow boundaries described in Velocity or Mass Flow Rate Inflow Boundaries. But there are two main differences:

(1) At a pressure inflow boundary, user does not specify flow velocity. Flow velocity is calculated by the flow solver and is affected by the pressure gradient across the boundary.

(2) Pressure at the inflow boundary must be specified by the user. The specified pressure can be a constant, or a time-varying profile as a function of time or crank angle. User can specify either static pressure or total pressure. To compute density in the ghost cell, if total pressure is specified, it is converted into static pressure using the relations described in either Conversion of Total Quantities to Static Quantities for Gas or Conversion of Total Quantities to Static Quantities for Two-phase Fluid, depending on the actual fluid compositions at the inlet.

At a pressure inflow boundary, pressure in the ghost cell is used as boundary condition when solving the momentum conservation equation (Equation 2–3). As is described in SIMPLE Method, the momentum equation is solved iteratively by the SIMPLE method. If total pressure is specified at the boundary, its conversion into static pressure must be performed in an efficient manner. This is not a concern for gas flows, in which the conversion is given by an algebraic relation (Equation 3–24). But for two-phase fluid, the conversion as indicated by Equation 3–27 and Equation 3–28 requires iterations. Simplification is made to the relation between total pressure () and static pressure (), considering the incompressible flow regime:

(3–29)

in which is the fluid's density in the ghost cell as a function of static pressure and local temperature, calculated by the thermodynamic Equation-of-State (Equation 11–1, Equation 11–3, and Equation 11–5 ). Equation 3–29 is a quadratic equation for the static pressure and can be solved explicitly and quickly.