Considering that the Inflow boundary condition models a virtual channel feeding the inlet, pressure losses are expected between the virtual channel inlet and the boundary where the condition is defined. This explains why the upstream pressure is different from the inlet pressure. While the fluid flows through the channel, it is subject to pressure work that results in a temperature change between the virtual channel inlet and the boundary where the Inflow boundary condition is defined. This is what is described by the pressure-dependent term in Equation 3-1. Note that the viscous dissipation in the virtual channel is not accounted for.