At a velocity or mass flow rate inflow boundary, the flow velocity or mass flow rate is user-specified. Both flow velocity and mass flow rate require a magnitude and a velocity direction. The magnitude can be specified as either a constant or a time-varying profile, and zero or negative magnitude values are allowed. A positive magnitude indicates flow into the computational domain and a negative magnitude indicates flow going out of the computational domain. The velocity direction can be assumed to be normal to the open boundary surface (the surface normal vector points into the computational domain in this context). Alternatively, the u, v, and w velocity components can be specified explicitly as either a constant vector or a time-varying vector profile.

When mass flow rate is specified for an inlet boundary, the mass flow rate will be converted into a velocity based on the boundary surface area and the inflow density at this inlet. Note that the velocity resulted from this conversion is the velocity normal to the boundary surface. If the user-specified velocity direction is different from the local surface normal, the magnitude of the actual flow velocity will be larger than the magnitude of the normal velocity by a factor of , where θ is the angle between the user-specified velocity direction and the surface normal vector.