?Threshold pressure for supersonic gas escaping?

It's not pressure by itself, but rather the velocity of the gasses (a 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.

atheists that will destroy the society. It is Darwinian.

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Cliff, a 22 rim fire with 22 inch barrel makes a huge "BLAM!" when shooting a wimpy ammo called "CB Longs". A rifle with a 24 inch barrel makes three separate small sounds with the same ammo; click of the firing pin, the bang of the muzzle, and the ping of the target being hit. 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.

Cliff Knight wrote:

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

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

V>

Clark Magnuson wrote in news:u73Pb.105057$na.64858@attbi_s04:

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.

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!

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?"

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.

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

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. 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

Glenn Ashmore wrote:

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

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.

On Thu, 22 Jan 2004 08:56:13 -0500, Glenn Ashmore brought forth from the murky depths:

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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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, ......Andrew

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. :-)