gasses & speed of sound

[Guest Stalin]

one of my quations looks weird. And I found out that it's because that at certain conditions the gas is travelling at nearly the speed of sound but I can't really figure out why.

 

is it because that for the gas to break the sound barrier, it needs to liquefy. but there's too much energy, so it can't?..or is it...well I don't know...and my head hurts.

 

so why can't gas travel faster then the speed of sound?

 

'have it nicely'

[Yggdrasil]

You're thinking about it the wrong way. The speed of sound is defined by the speed of the gas molecules in the air. Sound travels at the speed of the gas molecules because sound is propagated as pressure waves through the gas molecules. So, if you increase the speed of the gas moleules (i.e. by using a lighter gas such as helium), you increase the speed of sound. That's why talking with helium gives a higher pitched sound.

[DQW]

one of my quations looks weird.

Please write down what this "quation" says.

[Guest Stalin]

sorry quations = equations

 

if near soundvelocity:

Kv = (Qmax/(257*inlet P)) * sqroot(density*gas temp.)

 

if not:

Kv = (Qmax/514) * sqroot((density*gas temp.)/((P1-P2)*P2)))

 

but it's not so important. more important i the theoretical aspect

[Guest Stalin]

let's say I'am flying in a fighter jet. and in the nose theres a nozzle connected to a valve and a flask containing some sort of gas at high pressure.

let's say that I'am now flying supersonic and I open for the gas then the gas would be shot out in the same direction as I'am flying. Then the gas would have a speed (in relation to the surrounding gas (air) and to the observer on the ground) greater than the speed of sound.

 

but my textbook says that it's impossible. But it doesn't explain why...

[Yggdrasil]

Well, first we need to address the definition of the speed of a gas. Lets say you have a liter of air (for simplicity, we'll say that the air is 100% N₂) at standard conditions (273K [zero celcius] and 1 atm). Within this liter you have about 2.69x10²² individual molecules of nitrogen gas. Not all of these gas molecules are moving at the same speed. If these gas molecules are at thermal equilibrium, the distribution of the speeds of the individual gas molecules can be described by the Maxwell-Boltzmann distribution.

 

Therefore, when describing the speed of a gas, one must measure an average speed of the gas molecules (I think, though I'm not completely sure, that the root mean squared average is used). The formulas you describe calculate the average speed of the gas molecules in a gas. At 20 deg Celcius and 1 atm pressure, the average speed of the gas molecules is the speed of sound. However, note that half of the molecules in the gas are traveling above the speed of sound and half o fthe molecules in the gas are traveling below the speed of sound.

 

So, in your example, the molecules of gas released from your jet would be traveling at a velocity faster than the speed of sound. However, the kinetic energy of these gas molecules would be transfered to other gas molecules in the air as they collided with other gas molecules. After the air reached thermal equilibrium again, the extra kinetic energy of these molecules would have a minimal effect on the total population of gas molecules, and the average speed of the gas molecules would still be near, if not at. the speed of sound.

 

So yes, it is possible for gas molecules to travel above the speed of sound. However, to increase the average speed of a population of gas molecules you would need to change some of the surrounding conditions -- for example, by raising the temperature.

[YT2095]

(i.e. by using a lighter gas such as helium), you increase the speed of sound. That's why talking with helium gives a higher pitched sound.

 

actualy helium works because it rises so fast past the vocal chords they vibrate faster, it doesn`t work if you`re upside down

[fuel pod]

i dont understant the guys orignal question..cause as far I know u can atain a speed of gas higher than the speed of sound for that gas at that temperature...

I recently took fluid mechanics course in whcih we studied basic Da-laval nozzle..

Its possible to attain a supersonic speed for the gas if the nozzle is operating a third critical condition..

[jdurg]

actualy helium works because it rises so fast past the vocal chords they vibrate faster, it doesn`t work if you`re upside down

 

Correct. In fact, because the density of helium is so small, sound travels less efficiently through helium and does not appear to be as 'loud'. Popping a ballon in an atmosphere of helium sounds a lot quieter than popping a balloon in an atmosphere of carbon dioxide.

[robotochan]

You cant be right about that helium thing i want proof. Surely the speed of the waves do not determing the pitch, the frequency does, by having the waves more faster just means it will reach your ear quicker, thing about it. If your voice box is vibrating at 30Hz (for example) it will make the changes in pressure of the helium be at 30Hz also. I have always been told that the reason helium makes your voice go funny is because it constricts the voice box (due to its lower desity) making it unable to produce the right sounds.