?Threshold pressure for supersonic gas escaping?
Clark Magnuson
escapes from behind the exhaust value or from behind the bullet, I
assume that there is a threshold of pressure below which the escapement
is much quieter.
What is that pressure?
TIA
--
A society that teaches evolution as fact will breed a generation of atheists that will destroy the society. It is Darwinian.
Cliff Knight
function of pressure) that is the turning point--at sea level it's 1115.7
feet per second. Above that velocity is where rifle bullets develop the
"crack" sound.
"Clark Magnuson" <c.mag...@comcast.net> wrote in message
news:HcZOb.101325$I06.457179@attbi_s01...
Clark Magnuson
CB Longs are around 400 fps, and the only thing supersonic is that gas ball escaping from behind the bullet when it leaves the muzzle of a 22" barrel. The 24" barrel provides more expansion ratio, and so has less pressure.
An engine with no muffler makes a coughing sound when turned over, until it fires, then there is a "BLAM!" sound.
Jon Elson
Clark Magnuson wrote:
> When an engine without a muffler or a rifle makes noise when the gas
> escapes from behind the exhaust value or from behind the bullet, I
> assume that there is a threshold of pressure below which the escapement
> is much quieter.
>
> What is that pressure?
I read somewhere that the theoretical pressure required for air to reach
Mach 1 when exhausting into standard atmosphere was only 13 PSI gauge.
Presumably, even with less than optimum nozzle geometry, it wouldn't
take a lot greater pressure to guarantee that part of the stream is
then supersonic.
That is amazingly low, but I guess air molecules are amazingly light, too.
Certainly, air escaping at lower pressure can still make noise.
Jon
Vince Iorio
I think some wind tunnels that operate above the speed of sound actually
use a vaccum tank. I don't remember where I read it, but it does
support the 13 psi number you are quoting
Vince
Clark Magnuson
explosion with a super sonic gas ball emanating until it slows down to
the speed of sound and a wave starts. That sphere of expanding gas is
three dimensional, but maybe it is the same.
The maximum sound wave amplitude is limited by cavitation, and that
would be a vacuum = - 14 psi .
Vince Iorio wrote:
--
Glenn Ashmore
to explore it further there are a whole bunch of interactive
pressure/velocity/sound calculators on this page:
http://www.grc.nasa.gov/WWW/K-12/airplane/shortc.html
Clark Magnuson wrote:
> When an engine without a muffler or a rifle makes noise when the gas
> escapes from behind the exhaust value or from behind the bullet, I
> assume that there is a threshold of pressure below which the escapement
> is much quieter.
>
> What is that pressure?
> TIA
>
--
Glenn Ashmore
I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com
Murray Peterson
> The maximum sound wave amplitude is limited by cavitation, and that
> would be a vacuum = - 14 psi .
That's only the limit if you assume symmetric variations in the pressure
wave. You can't go below (or even quite get to) a vacuum, but the high
pressure end of the wave can be as high as you want.
dave martin
> I am not sure there is an absolute answer to that question. If you want
> to explore it further there are a whole bunch of interactive
> pressure/velocity/sound calculators on this page:
> http://www.grc.nasa.gov/WWW/K-12/airplane/shortc.html
>
> Clark Magnuson wrote:
> > When an engine without a muffler or a rifle makes noise when the gas
I'm pretty sure that the pressure ratio across a Mach 1 shock in air
is 1.78.
That corresponds to a pressure drop across an orifice at the speed of
sound.
For air at 14.7 psia downstream, the upstream pressure would be
1.78*14.7 psia=26.16psia or 11.47 psig.
Not a very high pressure. I'm not sure there's much of a change in
sound quality or quantity as one goes through Mach 1 for an orifice.
Most of the noise is due to vortexes etc. in the deccelerating air &
that depends on the details of the orifice.
Maybe someone with direct experience can comment.
Or do an experiment with a small orifice. Start with a gage pressure
below 8 psi & increase it to 15 or 20 psi. For sure you'll cross mach
1 in the process.
Do you hear a distinctive change in sound quality or quantity at a
particular pressure? If so, that's probably at mach 1 when the nozzle
is said to be "choked".
Let us know what you hear please!
pragmatist
> When an engine without a muffler or a rifle makes noise when the gas
> escapes from behind the exhaust value or from behind the bullet, I
> assume that there is a threshold of pressure below which the escapement
> is much quieter.
>
> What is that pressure?
> TIA
Pressure is only one part of the answer.
A trumpet is damn loud, but the musician can only produce maybe 2.5 -
3 PSI.
Sound is pressure waves in the air. Cavity shape, (resonance), has a
good deal to do with the sound produced by the movement of air.
Supersonic velocities are not a neccessity for the production of loud
noises, a small vibration that matches the frequency of its cavity can
build to a very impressive volume.
Pragmatist
"Where is Madame DeFarge now, when we really need her?"
Clark Magnuson
dave martin wrote:
>
>Do you hear a distinctive change in sound quality or quantity at a
>particular pressure? If so, that's probably at mach 1 when the nozzle
>is said to be "choked".
>
>Let us know what you hear please!
>
The difference between a 22" and 24" barrel can be ~ ~ 40db.
The pressure difference of escaping gas must be 9%, and the sound wave
amplitude changes ~ ~ 100000%.
dave martin
> dave martin wrote:
>
> >
> >Do you hear a distinctive change in sound quality or quantity at a
> >particular pressure? If so, that's probably at mach 1 when the nozzle
> >is said to be "choked".
> >
> >Let us know what you hear please!
> >
>
> The difference between a 22" and 24" barrel can be ~ ~ 40db.
> The pressure difference of escaping gas must be 9%, and the sound wave
> amplitude changes ~ ~ 100000%.
It makes sense that this is the transition between a sonic and
subsonic burst of gas out of the muzzle. I don't think I hear a
similar difference when a continuous flow through an orifice crosses
the sonic barrier. I'll try an experiment & let you know.
Jon Elson
Vince Iorio wrote:
> FWIW
>
> I think some wind tunnels that operate above the speed of sound
> actually use a vaccum tank. I don't remember where I read it, but it
> does support the 13 psi number you are quoting
>
Yup, I saw one of these about 40 years ago, at the David Taylor Model
Basin, a Navy
research center outside Washington D.C. They had a wind tunnel with
about a 4" square
test section and two huge vacuum spheres out back that must have been 30
feet in
diameter, each. The pumps looked like V-8 engines with huge electric
motors mounted
on them. They could do a test of about 5 seconds every twenty minutes.
Jon
Theron King
that produces a velocity of Mach 1. Select the pressure ratio as the
entry variable and enter values until the calculator gives Mach 1 as the
result.
I came up with a pressure ratio of 0.528 (p/pt) to produce a Mach 1
velocity. The inverse of this gives a ratio of pt/p of 1.894 (rounded to
4 digits). This would give a pressure of 27.84 psia or 13.14 psig.
TDK
Andrew Case
>If you use the calculator on the following page you can back into the dP
>that produces a velocity of Mach 1. Select the pressure ratio as the
>entry variable and enter values until the calculator gives Mach 1 as the
>result.
>
>I came up with a pressure ratio of 0.528 (p/pt) to produce a Mach 1
>velocity.
Typically for air the pressure ratio needed to form a sonic shock is
0.53 - this is the point at which the air in the narrowest cross section
of whatever channel you're dealing with reaches the speed of
sound. Increasing the pressure difference (i.e. p/pt<0.53) won't increase
the flow speed. The density still increases with pressure, so mass flow
goes up, but the velocity peaks at the speed of sound unless you do
something to increase it. This is choked flow, which you'll run into if
you have to deal with valves switching gasses. To get gas flowing above
the speed of sound you need a nozzle (like on a rocket). A simple
supersonic nozzle uses a divergence angle of 15 degrees, more
sophisticated nozzles use more complicated bell shapes.
There's a fair amount of analysis that goes into deriving that 0.53, but
for most purposes you can take it as a given (the number is different for
gasses other than air - it has to do with the ratio of the specific heats
at constant volume and constant pressure, which vary from gas to
gas). I've noticed that a lot of valve designers use 0.5 as a rough
estimate, which is close enough for most purposes.
Hope this isn't too rambling,
......Andrew
--
--
Andrew Case |
ac...@plasma.umd.edu |
?mund Breivik
> Increasing the pressure difference (i.e. p/pt<0.53) won't increase
> the flow speed. The density still increases with pressure, so mass flow
> goes up, but the velocity peaks at the speed of sound unless you do
> something to increase it.
Won't the speed of sound increase with pressure? Mach 1 is not an absolute velocity.
--
Aamund Breivik
Glenn Ashmore
?mund Breivik wrote:
Yes but the pressure that determines the speed is the "free stream" side
of the outlet. There is a lot going on inside a supersonic nozzle
basically, the diverging sides of the nozzle restrict the expansion of
the escaping gas so that it sees a higher free stream pressure and has
time to accelerate past the atmospheric speed of sound. I remember
from my pre-Ertl rocketry days that you could determine the mach number
of the exhaust by counting the shockwave diamonds in the stream.
Here is a nozzle simulator you can play with to see the effect of the
divirging side geometry and pressure.
http://www.grc.nasa.gov/WWW/K-12/airplane/ienzl.html
Larry Jaques
<gash...@mindspring.com> brought forth from the murky depths:
>--
>Glenn Ashmore
>
>I'm building a 45' cutter in strip/composite. Watch my progress (or lack
>there of) at: http://www.rutuonline.com
I've been meaning to ask you what the cruising speed will be for
this supersonic yacht you're building, Glenn. Got in-air pics yet?
;)
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Andrew Case
Speed of sound depends only on the temperature of the gas, the ratio of
specific heats, and its molecular mass. Pressure doesn't come into it.
This is kind of handy, since it lets you control mass flow in a simple
way by tweaking pressure at constant temperature. Mass flow is then
directly proportional to pressure.
There's refinements I'm leaving out, but they are higher order corrections
that really don't have a major effect in most situations.
Hope this helps,
Glenn Ashmore
Larry Jaques wrote:
> On Thu, 22 Jan 2004 08:56:13 -0500, Glenn Ashmore
> <gash...@mindspring.com> brought forth from the murky depths:
>
>
>>--
>>Glenn Ashmore
>>
>>I'm building a 45' cutter in strip/composite. Watch my progress (or lack
>>there of) at: http://www.rutuonline.com
>
>
> I've been meaning to ask you what the cruising speed will be for
> this supersonic yacht you're building, Glenn. Got in-air pics yet?
> ;)
About 9 knots with both JATOs blasting. :-)
There actually will eventually be one in-air picture and that is the one
I really want to take. It will be when a big crane lifts her out of the
shed, over over the cedar tree and onto the yacht transit trailer. :-)
--
Glenn Ashmore
I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Larry Jaques
brought forth from the murky depths:
>Larry Jaques wrote:
>> I've been meaning to ask you what the cruising speed will be for
>> this supersonic yacht you're building, Glenn. Got in-air pics yet?
>> ;)
>
>About 9 knots with both JATOs blasting. :-)
I thought they were supposed to be set up in sets of 3, one set
for each side. Even so, a pair of JATOs should get you over 35
knots in a streamlined cutter profile. What's up?
>There actually will eventually be one in-air picture and that is the one
>I really want to take. It will be when a big crane lifts her out of the
>shed, over over the cedar tree and onto the yacht transit trailer. :-)
ETA for that, sir?
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Gary Coffman
The speed of sound is proportional to temperature. Pressure has relatively
little effect as long as the gas is under conditions where the ideal gas law
is more or less a correct model of gas behavior.
This is why the hot gas inside the barrel of a gun can push a bullet to
way more than the speed of sound in the ambient air outside the barrel.
In other words, the compression wave in the barrel is travelling at sonic
velocity (by definition of a compression wave), but because the gas is
very hot, that can be much faster than the speed of sound in the normal
temperature air outside the barrel.
This is also why a muffler works on a gas engine. Its primary purpose
is to slow and expand, and hence cool, the hot exhaust so that the
compression waves in it will be down near the speed of sound in
ambient air by the time the exhaust reaches ambient air. Otherwise
you get a supersonic shock each time a hot impulse hits the cool
ambient air.
Gary
Backlash
screw pipe and put it temporarily on a small engine in place of the muffler.
Support it on the outer end, and fire it up. We did this once as kids, as a
hoot.
RJ
"Gary Coffman" <ke...@bellsouth.net> wrote in message
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