"xerj" wrote in message
...
On this page, there's an animated diagram (aptly done in shockwave) about
normal shockwave formations:-

http://selair.selkirk.bc.ca/aerodyna...ed/Page2c.html

Something confuses me though. As diagrammed, the red pressure wave at the
rear has a low frequency at the front and a high frequency at the back.
Why would this happen (if indeed it does)? Why is the frequency HIGHER in
the direction that the airflow is moving?

Following the custom of this ng, I shall now post even though I have no deep
knowledge of the topic and no qualifications to comment on cartoons.

The airflow forward of the critical point is supersonic, faster than the
speed of sound. Therefore, the pressure wave propogates rearward relative to
its point of generation. Aft of the critical point, airflow is subsonic,
slower than the speed of sound. Sound pressure waves propogate forward of
their point of generation. At the critical point, the pressure wave is a
standing wave, propogating such that the wave front remains tanget to a line
through the critical point. Each successive wave cycle at the critical point
reinforces the preceding waves.

Sponge Bob Squarepants then goes on to say: As to the wave frequencies, the
effect is similar to Doppler shift. The pressure wave center moves relative
to the airfoil. Forward of the critical point, the center of pressure moves
rearward away from their point of generation. Versa for points rear of the
critical point, in subsonic flow.

It really does become very clear when they present it this way, in primary
colors.