flow through an orifice drops significantly

huangyhg

flow through an orifice drops significantly
i remember porsche having issues in the 1998 racing season due to the size of the air restrictor for there turbo engine and also due to were they could place this restrictor.  the reason was that the airflow approached the speed of sound.  i would love someone to explain to me in laymen’s terms why the following happens.  flow through an orifice drops significantly at 0.5 times the speed of sound, i.e. mach index z=0.5?
paul
i think the pervading principle is that, as you start to get above mach .40, compressibility effects start to matter.  think of it in terms of a bunch of people trying to get out of a doorway.  under normal conditions and walking speed, the doorway isn't much of an impediment and everybody just walks through.  set off the buildings fire alarm though, and everybody wants to run out the building as fast as they can.  as a result, they start banging into each other and congregating in front of the door as everyone tries to push through.  it's kind of like that with the air molecules.  up to about mach .40, everythings cool.  much higher than that and air molecules start colliding into each other and congregating at the orifice in the form of a standing pressure wall.
much of this will depend on the actual geometry of the orifice.  also, your intake system could have a pressure wave traveling back and forth due to the piston engine (but i'm not an automotive guy).
anyway, that's the general idea.
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joseph k. mooney
director, airframe structures - faa der