1.8.1 Hyperbolic Flows
To begin, consider the case of unsteady inviscid compressible flow. A com-
pressible fluid can support sound and shock waves and it is not surprising
that these equations have essentially hyperbolic character. Most of the meth~
ods used to solve these equations are based on the idea that the equations
are hyperbolic and, given sufficient care, they work quite well; these are the
methods referred to briefly above.
For steady compressible flows, the character depends on the speed of the
flow. If the flow is supersonic, the equations are hyperbolic while the equations
for subsonic flow are essentially elliptic. This leads to a dmiculty that we shall
discuss further below.
It should be noted however, that the equations for a viscous compressible
flow are still more complicated. Their character is a mixture of elements of
all of the types mentioned above; they do not fit well into the classification
scheme and numerical methods for them are difficult to construct.