where ∂un/∂n is the normal derivative of the normal velocity component.
Equation 4-36 is prescribed by the Pressure condition in the Inlet and Outlet features and the Normal stress condition in the Open Boundary and Boundary Stress features.
Equation 4-36 is mathematically more stringent compared to specifying the pressure pointwise and at the same time cannot guarantee that
p obtains the desired value. In practice,
p is close to
Fn, except for low Reynolds number flows where viscous effects are the only effects that balance the pressure. In addition to
Equation 4-36, all but one velocity component must be specified. For low Reynolds numbers, this can be specified by a vanishing tangential stress condition:
where ∂ut/∂n is the normal derivative of the tangential velocity field. It is also possible to prescribe
ut to be zero. The latter option should be used with care since it can have a significant effect on the upstream solution.