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Shock or Wave Drag Result from a Moving Shock
Figure:
The diagram that reexplains the shock drag effect of a moving shock.

In section (5.2.4)
it was shown that there is no shock drag in stationary shock.
However, the shock or wave drag is very significant so much so
that at one point it was considered the sound barrier .
Consider the figure (5.7) where the stream lines
are moving with the object speed.
The other boundaries are stationary but the velocity at right boundary
is not zero.
The same arguments, as discussed before in the stationary case,
are applied.
What is different in the present case (as oppose to the stationary
shock), one side has increase the momentum of the control volume.
This increase momentum in the control volume causes the shock drag.
In way, it can be view as continuous acceleration of the gas around
the body from zero.
Note this drag is only applicable to a moving shock (unsteady shock).
The moving shock is either results from a body that moves in gas or
from a sudden imposed boundary like close or open valve^{5.5}
In the first case, the forces/energy flows from body to gas and there for
there is a need for large force to accelerate the gas over extremely short
distance (shock thickness).
In the second case, the gas contains the energy (as high pressure,
for example in the open valve case) and the energy potential is lost
in the shock process (like shock drag).
For some strange reasons, this topic has several misconceptions
that even appear in many popular and good textbooks^{5.6}.
Consider the following example taken from such a book.
Figure:
The diagram for the common explanation for shock or wave drag
effect a shock. Please notice the strange notations (e.g. V and
not U) and they result from a verbatim copy.

Solution
Neglecting the mistake around the contact of the stream lines with
the oblique shock(see for retouch in the oblique chapter), the control
volume
suggested is stretched with time.
However, the common explanation fall to notice that when the isentropic
explanation occurs the width of the area change. Thus, the simple
explanation
in a change only in momentum (velocity) is not appropriate.
Moreover, in an expanding control volume this simple explanation is not
appropriate.
Notice that the relative velocity at the front of the control volume
U_{1}
is actually zero.
Hence, the claim of
U_{1} > U_{2}
is actually the opposite,
U_{1} < U_{2}.

Next: Shock Result from a
Up: The Moving Shocks
Previous: The Moving Shocks
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Created by:Genick BarMeir, Ph.D.
On:
20071121