ORIGINAL: Flyboy Dave
Bernoulli's Principle states that as the speed of a moving fluid increases, the
pressure within the fluid decreases.
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Our friend Mr. Bernoulli experiments pertain to fluids, which of course do not
compress....and unfortunately....he passed away long before the invention of
the airplane.
I suggest those interested take a look at the following links for more information (and note that air *is* a fluid that is often considered incompressible at subsonic speeds)
http://www.bbc.co.uk/dna/h2g2/A517169
http://142.26.194.131/aerodynamics1/Basics/Page3.html (or why Jumbo jets fly)
http://www.answers.com/topic/bernoulli-s-principle
Therefore -- where the gasflows are fastest in an exhaust system, the pressure will be lowest.
Since the exhaust stack is usually one of the narrowest parts of an exhaust, there will be far less pressure at that point than at a point where the gases have slowed.
It's just science!