Laminar Inflow
In order to prescribe a fully developed inlet 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 3-2: a fictitious domain of length Lentr is assumed to be attached to the inlet of the computational domain. The domain is an extrusion of the inlet boundary, which means that laminar inflow requires the inlet 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 3-2: An example of the physical situation simulated when using the Laminar inflow boundary condition. Ω is the actual computational domain while the dashed domain is a fictitious domain.
If an average inlet velocity or inlet volume flow is specified instead of the pressure, COMSOL Multiphysics adds an ODE that calculates a pressure, pentr, such that the desired inlet velocity or volume flow is obtained.