1.8.2 Parabolic Flows
The boundary layer approximation described briefly above leads to a set of
equations that have essentially parabolic character. Information travels only
downstream in these equations and they may be solved using methods that
are appropriate for parabolic equations.
Note, however, that the boundary layer equations require specification of
a pressure that is usually obtained by solving a potential flow problem. Sub-
sonic potential flows are governed by elliptic equations (in the incompressible
limit the Laplace equation sufiices) so the overall problem actually has a
mixed parabolic-elliptic character.