The **most** common boundary condition condition for fluid
mechanics is to know the concentration at the boundaries. This
arises primarily as the *no slip condition*. The no slip
condition means that the fluid velocity at the boundary is equal to
the boundary's velocity. This holds true both for fluid-solid
boundaries (which might be zero like at point(s) 1) as well as
fluid-fluid boundaries. (where the two fluids have the same
velocity, like at point 2).

We can specify a constant *flux* at the boundary when we
know the value of the *stress* there (i.e., knowing the
momentum flux). The most common place that this would arise would
be as a continuity of stress condition at the boundary between two
fluids. (At point 2 notice that the slopes of the velocity profiles
have the same *sign*, but not necessarily the same value of
the slope, because the viscosities may be different)

A special case of this is for the case of where we assume that

Not common in fluid mechanics.

Identify reasonable boundary conditions in a fluid mechanics problems (explain when each is most useful)