fluid, shmuid (turbo bypass)

[pjrose@servtech.com (Phil & Judy Rose)]

Hey all,
I detect some beating around the bush on this one. The point in addressing
the question of air density doesn't seem to be whether to call air a fluid
or a gas. Of course it's a gas (and a fluid), but the essential point
surely is that gases (air, nitrogen, etc.) are _compressible_, whereas
_liquids_ are essentially incompressible. Both gases _and_ liquids will
expand with increasing temperature, but with gas expansion (contraction) is
enormously greater (than in liquids) for the same temperature change.

Perhaps the following physical chemistry tidbit may bring back some
twitches of recollection and might even be pertinent to the discussion. If
not, well I tried ;-)

The state behavior of gases can be precisely described by very complicated
equations (equations of state) as one lister pointed out, but for many
applications, the gas properties (as in the case for air at engine
conditions)  probably are quite adequately handled by the so-called "Ideal
Gas Law". This simple equation is:

PV = nRT

where P is pressure, V is volume, n is the number of moles (proportional to
the number of gas molecules within "V"); R is a constant, and T is the
Kelvin (absolute) temperature (deg K = deg C + 273).
[Here it's useful to note that since 30 deg C =303 deg Kelvin, changing the
gas temp by ,say, +/- 30 Celsius will change its absolute temp by only
10%.]

Anyway, rearranging the gas law equation lets you can write:

P/RT = n/V (note: the ratio "n/V" is proportional to the gas _density-)

If the system's volume and T(temp) stay essentially constant, but P is
increased (i.e., by turbo boost), then to maintain the above equality, the
value of "n" (the _amount_ of gas) must increase. This is equivalent to
saying the gas _density_ increases. Note that it changes in direct
proportion to the pressure change.

Full disclosure statement: I know even less about turbo operation, or even
if the ski boot can be used to enhance rear-seat sexual activity. And on
the question of turbulent flow (or its significance) in turbo-driven gases
I know nil. If all this gas stuff has already been covered in the volumes
of posting on turbo boost  over the past month, I apologize for having
merely, uh, "skimmed" that material ;-)

Phil

              THIS SIG LINE INTENTIONALLY LEFT BLANK

>Date: Thu, 13 Nov 1997 15:29:39 GMT
>From: paul.heneghan@bbc.co.uk (Paul Heneghan)
>Subject: Air Flow in a bypass valve
>
>Date: Wed, 12 Nov 1997 18:41:32 -0800
>From: Wolff <wolff_b@ix.netcom.com>
>Subject: Re: Air Flow in a bypass valve
>
>Scott said:
>>>Air is a fluid by definition.
>
>Wolff said:
>>Ummm, I really don't want to get stuck in this whole thing, but "air is
>>a fluid by definition" is wrong.
>
>I say:
>At my physics class at school I was told that "a fluid is a liquid or a
>gas".  Before you all jump in and say "bxxxxxxs", I just checked it in the
>Oxford Dictionary and that confirms it.
>
>I fully agree that air is a gas (or to be more precise consists of a number
>of gasses), but by definition, that also makes it a fluid.
>
>Unfortunately, I am not brainy enough to comment on the rest of the
>discussions.