Re: Elastic air and inertia AD V7 #877

[bearbvd@domain.elided (Greg Hermann)]

but in fact the highest pressure area at the front of the vehicle
>is at the base of the windshield (an ideal place to put the intake for the
>ventilation system for the interior!) so Pontiac knew what they were doing.
>When the flap opened the engine started breathing from the high pressure
>area at the base of the windshield, a mild supercharging effect at speed.
>The amusing part is that the benefit is present all the time the car is
>moving forward so why the flap? So the driver could see how much power he
>was getting! Weird science.

Not at all--the cause of the weirdness was the emission regs of the
day--they wanted temperature regulated air under idle and light throttle
conditions (as abtained from mixing air from the exhaust manifold stove and
a cooler source--but at unregulated, wide open throttle conditions, the
scoop got opened wide, so as to get the coolest possible air. I think the
Chevelle SS was the first of the old muscle cars to grab the air from the
base of the windshield.

Of course--the older Alfa Veloces had this figured out far earlier, even
though their air source was under the left side eyebrow. One very
interesting aspect of the aerodynamics of these older Alfas is that the air
pressure against the firewall _under_ the hood is actually HIGHER than it
is at the base of the windshield on the outside of the hood!! This speaks
pretty eloquently to the effectiveness of the front radiator and eyebrow
intakes on these cars!

If you do not believe what I am saying about which pressure is higher, try
driving one of these cars with broken (or missing) hood hinges faster than
about 85 or 90 mph, and watch the back edge of the hood lift up!!

Regards, Greg
>
>I don't think air is elastic exactly.  For airflow at subsonic speeds it is
>treated as incompressible, but it clearly is compressible even below the
>speed of sound, if not highly compressible. While logic dictates that the
>intake is only open on any given cylinder for just over 25 % of the time, I
>don't see this as meaning the airflow actually "stops". At high rpm's the
>time intervals between successive valve opening events are pretty short.
>
Air is most definitely a compressible fluid. If you are dealing with any
flow situation with air where the (absolute) pressure ratio across the flow
restriction exceeds about 1.1 : 1, you will get seriously bogus results if
you simplify the calculation by assuming an incompressible flow.

An absolute pressure ratio of somewhat less than 2: 1 (with air) will give
a flow velocity of Mach 1 (choked flow) through the smallest, most
constricted area of a flow restriction. Higher pressure ratios than what it
takes to get Mach 1 flow at the choke point WILL NOT force the choked point
of the flow to exceed Mach 1. The only thing that can be done to increase
the MASS flow (not the velocity) through a choked orifice is to increase
the density of the fluid upstream of the orifice. PERIOD. (Of course, one
of the outhouse lawyers out there may pipe up here and point out that
increasing the temperature of the upstream fluid will increase Mach 1 for
that fluid, and would be correct -Mach 1 for any gas is a function of
molecular weight and the square root of its absolute temperature, and not
much else!)

Under some conditions, a convergent-divergent nozzle will give SUPERsonic
flow DOWN stream of the narrowest (choke) point of the nozzle, but the flow
velocity at the choke point STILL does not exceed Mach 1. There are a LOT
of SERIOUSLY counterintuitive (flat backwards) things that happen in
supersonic flows--for instance: Adding heat will lower the temperature, and
cause the flow speed to be reduced, and increasing the area of the flow
will cause the velocity to INCREASE!! Yeee haww!

Regards, Greg

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