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Next: Issues Related to the Up: Oblique Shock Previous: Maximum Value of Oblique   Index

# Detached Shock

When the mathematical quantity becomes positive, for large deflection angle, there isn't a physical solution to an oblique shock. Since the flow sees'' the obstacle, the only possible reaction is by a normal shock which occurs at some distance from the body. This shock is referred to as the detach shock. The detached shock's distance from the body is a complex analysis and should be left to graduate class and researchers in this area. Nevertheless, a graph and a general explanation to engineers is provided. Even though this topic has few applications, some might be used in certain situations which the author isn't aware of.
Analysis of the detached shock can be carried out by looking at a body with a round section moving in a supersonic flow (the absolute velocity isn't important for this discussion). Figure 13.12 exhibits a round-tip bullet with a detached shock. The distance of the detachment is determined to a large degree by the upstream Mach number. The zone A is zone where the flow must be subsonic because at the body the velocity must be zero (the no-slip condition). In such a case, the gas must go through a shock. While at zone C the flow must be supersonic. The weak oblique shock is predicted to flow around the cone. The flow in zone A has to go through some acceleration to became supersonic flow. The explanation to such a phenomenon is above the level of this book (where is the throat'' area question13.25. Yet, it can be explained as the subsonic is sucked'' into gas in zone C. Regardless of the explanation, these calculations can be summarized by the flowing equation

where is a function of the upstream Mach number which tabulated in the literature.

The constant and the function are different for different geometries. As a general rule, the increase in the upstream Mach results in a decrease of the detachment distance. Larger shock results in a smaller detachment distance, or, alternatively, the flow becomes blinder'' to obstacles. Thus, this phenomenon has a larger impact for a relatively smaller supersonic flow.

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Next: Issues Related to the Up: Oblique Shock Previous: Maximum Value of Oblique   Index
Created by:Genick Bar-Meir, Ph.D.
On: 2007-11-21