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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.
Figure:
The schematic for a roundtip bullet
in a supersonic flow.

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 roundtip 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 noslip 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 question
^{13.25}.
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.
Subsections
Next: Issues Related to the
Up: Oblique Shock
Previous: Maximum Value of Oblique
Index
Created by:Genick BarMeir, Ph.D.
On:
20071121