Up to this point we have considered only continuous flows, flow systems in which
state changes occur continuously and thus whose processes can easily be identified
and plotted. Recall from Section 4.3 that infinitesimal pressure disturbances are called
sound waves and these travel at a characteristic velocity that is determined by the
medium and its thermodynamic state. In Chapters 6 and 7 we turn our attention
to some finite pressure disturbances which are frequently encountered. Although
incorporating large changes in fluid properties, the thickness of these disturbances
is extremely small. Typical thicknesses are on the order of a few mean free molecular
paths and thus they appear as discontinuities in the flow and are called shock waves.
Due to the complex interactions involved, analysis of the changes within a shock
wave is beyond the scope of this book. Thus we deal only with the properties that
exist on each side of the discontinuity. We first consider a standing normal shock, a
stationary wave front that is perpendicular to the direction of flow. We will discover
that this phenomenon is found only when supersonic flow exists and that it is basically
a form of compression process. We apply the basic concepts of gas dynamics to
analyze a shock wave in an arbitrary fluid and then develop working equations for a
perfect gas. This procedure leads naturally to the compilation of tabular information
which greatly simplifies problem solution. The chapter closes with a discussion of
shocks found in the diverging portion of supersonic nozzles