The Non-Mathematician's Guide To
Basic Valveless Pulsejet Theory
© 2008 Larry Cottrill
by Larry Cottrill 25 Sep 2008
You Gotta Know the Rules
From the observations made so far, I have derived and now offer the following rules for
air moving (laymen and some beginners would find some of these quite surprising; experienced
designers will probably think them utterly trivial). Anyway, at a minimum, these are the
facts you should understand after studying this Subtopic:
Rule 1: You cannot get air moving (or change its already established
motion) without changing its pressure.
Rule 2: You cannot alter the pressure of air without moving it (or
changing its established motion).
These two rules are completely general: they apply not only to the pressure wave described above,
but in fact cover everything from a kid blowing a paper wad out of a straw to a "breeze box"
window fan to a vacuum cleaner to a shop compressor to a turbojet engine to the interior of
a supersonic ramjet powering a guided missile. The remainder are more specific to pressure-induced
flow in air constrained by some rigid container (e.g. pipe, pressure tank, musical instrument,
Rule 3: Pressure is applied to air in the form of a wave that moves
through the air mass at a relative speed equal to the speed of sound at the local condition
(temperature and pressure) of that air mass.
Rule 4: The rising and falling "fronts" of a pressure wave represent
the mechanical work of changing the motion of the air mass they pass through.
Rule 5: A rising pressure wave front influences motion of the air
mass in the direction of the wave motion; a falling pressure wave front influences motion of
the air in the opposite direction to the wave motion.
Rule 6: Passage of a pressure wave front alters the local conditon
(pressure, temperature and density) of the air mass in the region it passes through;
density and temperature increase with rising pressure and decrease with falling pressure.
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