Laminar Outflow
In order to prescribe an outlet velocity profile, this boundary condition adds a weak form contribution and constraints corresponding to unidirectional flow perpendicular to the boundary. The applied condition corresponds to the situation shown in Figure 3-3: assume that a fictitious domain of length Lexit is attached to the outlet of the computational domain. The domain is an extrusion of the outlet boundary, which means that laminar outflow requires the outlet to be flat. The boundary condition uses the assumption that the flow in this fictitious domain is fully developed laminar flow. The “wall” boundary conditions for the fictitious domain is inherited from the real domain, Ω, unless the option to constrain outer edges or endpoints to zero is selected in which case the fictitious “walls” are no-slip walls.
Figure 3-3: An example of the physical situation simulated when using the Laminar outflow boundary condition. Ω is the actual computational domain while the dashed domain is a fictitious domain.
If the average outlet velocity or outlet volume flow is specified instead of the pressure, the software adds an ODE that calculates pexit such that the desired outlet velocity or volume flow is obtained.