The inlet-outlet boundary condition is the most basic example of the mixed ﬁxed value/gradient type, described in Sec. 4.9 . The condition sets the reference gradient and uses a speciﬁed reference value . The value fraction is then set to
At the free boundary, the inlet-outlet condition enables to be speciﬁed where inﬂow occurs. The inlet value in the example is set to ; the image shows mixing of ﬂuids at diﬀerent temperatures, from the inlet and entrained at the free boundary.
Boundaries may be described “inlet” and “outlet” based on the expectation of the ﬂow direction during a simulation. But the ﬂow direction may not always happen as expected.
In the case of an outlet, for example, inﬂow might occur during a simulation. For example, at the start of a simulation, the initial conditions may induce inﬂow before the internal ﬂow is established. Localised inﬂow can also occur when rotating ﬂow structures pass through an outlet boundary, e.g. when a bluﬀ body sheds vortices, as shown below.
Where inﬂow occurs, the inlet-outlet condition can switch to the ﬁxed value type to ensure stability, as discussed in Sec. 4.5 . The inlet-outlet condition is therefore commonly applied to scalar ﬁelds (except ), at a boundary which is notionally an outlet, to avoid numerical instability associated with unexpected inﬂow.