Topic 3 - Subtopic 3 - Section 6 ...

The  Non-Mathematician's  Guide  To  Basic  Valveless  Pulsejet  Theory
2008 Larry Cottrill                                                                   by Larry Cottrill   25 Sep 2008


Section 03.03.06
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, pulsejet):

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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