When only a poor initial guess is available, it may be helpful to first run with a specified mass flow inlet and a static pressure outlet. The outlet pressure in this case is fairly arbitrary and is usually set at, or close to zero to reduce round-off error. The specification of a mass flow inlet may be more robust. However, a mass flow inlet assumes a uniform inlet velocity—which may not be appropriate. Once the overall flow is established, the boundary conditions may then be changed to total pressure at the inlet and mass flow at the outlet.