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Next: Shock Result from a Up: The Moving Shocks Previous: The Moving Shocks   Index

Shock or Wave Drag Result from 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 valve5.5 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 textbooks5.6. Consider the following example taken from such a book.

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 U1 is actually zero. Hence, the claim of U1 > U2 is actually the opposite, U1 < U2.

Next: Shock Result from a Up: The Moving Shocks Previous: The Moving Shocks   Index
Created by:Genick Bar-Meir, Ph.D.
On: 2007-11-21