8 ms·

The speed of sound does not depend on pressure, only temperature. I suspect this is occurring because the (sonic) flow through the nozzle cools as it expands,

The speed of sound does not depend on pressure, only temperature. I suspect this is occurring because the (sonic) flow through the nozzle cools as it expands, therefore the speed of sound drops, making the same flow now supersonic in the cooled gas.

sand500 3y ago
This was interesting so I looked it up >For a given ideal gas the molecular composition is fixed, and thus the speed of sound depends only on its temperature. At a constant temperature, the gas pressure has no effect on the speed of sound, since the density will increase, and since pressure and density (also proportional to pressure) have equal but opposite effects on the speed of sound, and the two contributions cancel out exactly. https://en.m.wikipedia.org/wiki/Speed_of_sound#Dependence_on_the_properties_of_the_medium https://en.m.wikipedia.org/wiki/Speed_of_sound#Dependence_on...
- bongodongobob 3y ago
  I never knew this either. This makes sense as when you tap on a propane tank they always sound empty, different than a metal tank of water. Even though it's a liquid and under extreme pressure, it's only the temperature that matters.
  - thfuran 3y ago
    Liquid propane is definitely not an ideal gas.
    - bongodongobob 3y ago
      Not saying it is, but it's very volatile. It really doesn't want to be a liquid at room temperature. I can't tell if it's empty or full when a tap on it, but definitely can with with water. It's an impedance thing.
      - thfuran 3y ago
        I'm just saying a fact about ideal gases doesn't really imply anything about LP.
        bongodongobob 3y ago
        It does, vapor pressure.
        deleted 3y ago
        [deleted]
        nautilius 3y ago
        That's got nothing to do with some fluid behaving like an ideal gas or not. Also, except for very low pressures, vapor does not behave like an ideal gas.
        bongodongobob 3y ago
        Sure it does, it's riding the border of a phase change between gas and liquid. It's in a super compressed equilibrium so it's going to behave similarly to an ideal gas.
        nautilius 3y ago
        That's - not how that works. I suggest you look at any fluid data `riding' that border. "Super compressed" and "ideal gas" are mutually exclusive. "Equilibrium" has even less to do with any of that.
  - jldl805 3y ago
    That's because propane tanks are only 1/4 full even when filled to max capacity.
AnotherGoodName 3y ago
Whoa that's something I didn't realize. I always had the intuition the speed was faster in denser materials but you're right that pressure doesn't matter. The temperature difference does make sense for the same reasons here though!
- carabiner 3y ago
  It does depend on density because density, pressure, and temp are all related. https://en.m.wikipedia.org/wiki/Ideal_gas_law https://en.m.wikipedia.org/wiki/Ideal_gas_law
- thsksbd 3y ago
  For an ideal gas. Get close to the triple point of the gas and you better upgrade your equation of state
foxyv 3y ago
I think it's just relativity. After all, the gas in the Earth's atmosphere is traveling 29.78km/s relative to the sun (~Mach 85). Inside of the nozzle it doesn't really matter that it's traveling supersonic relative to the air outside the nozzle until it leaves the nozzle. I don't think the difference in speed of sound will have much effect in any case.
thsksbd 3y ago
"The speed of sound does not depend on pressure, only temperature" Of an ideal gas

sand500 3y ago

This was interesting so I looked it up >For a given ideal gas the molecular composition is fixed, and thus the speed of sound depends only on its temperature. At a constant temperature, the gas pressure has no effect on the speed of sound, since the density will increase, and since pressure and density (also proportional to pressure) have equal but opposite effects on the speed of sound, and the two contributions cancel out exactly. https://en.m.wikipedia.org/wiki/Speed_of_sound#Dependence_on_the_properties_of_the_medium https://en.m.wikipedia.org/wiki/Speed_of_sound#Dependence_on...

bongodongobob 3y ago
I never knew this either. This makes sense as when you tap on a propane tank they always sound empty, different than a metal tank of water. Even though it's a liquid and under extreme pressure, it's only the temperature that matters.
- thfuran 3y ago
  Liquid propane is definitely not an ideal gas.
  - bongodongobob 3y ago
    Not saying it is, but it's very volatile. It really doesn't want to be a liquid at room temperature. I can't tell if it's empty or full when a tap on it, but definitely can with with water. It's an impedance thing.
    - thfuran 3y ago
      I'm just saying a fact about ideal gases doesn't really imply anything about LP.
      - bongodongobob 3y ago
        It does, vapor pressure.
        deleted 3y ago
        [deleted]
        nautilius 3y ago
        That's got nothing to do with some fluid behaving like an ideal gas or not. Also, except for very low pressures, vapor does not behave like an ideal gas.
        bongodongobob 3y ago
        Sure it does, it's riding the border of a phase change between gas and liquid. It's in a super compressed equilibrium so it's going to behave similarly to an ideal gas.
        nautilius 3y ago
        That's - not how that works. I suggest you look at any fluid data `riding' that border. "Super compressed" and "ideal gas" are mutually exclusive. "Equilibrium" has even less to do with any of that.
- jldl805 3y ago
  That's because propane tanks are only 1/4 full even when filled to max capacity.

AnotherGoodName 3y ago

Whoa that's something I didn't realize. I always had the intuition the speed was faster in denser materials but you're right that pressure doesn't matter. The temperature difference does make sense for the same reasons here though!

carabiner 3y ago
It does depend on density because density, pressure, and temp are all related. https://en.m.wikipedia.org/wiki/Ideal_gas_law https://en.m.wikipedia.org/wiki/Ideal_gas_law
thsksbd 3y ago
For an ideal gas. Get close to the triple point of the gas and you better upgrade your equation of state

foxyv 3y ago

I think it's just relativity. After all, the gas in the Earth's atmosphere is traveling 29.78km/s relative to the sun (~Mach 85). Inside of the nozzle it doesn't really matter that it's traveling supersonic relative to the air outside the nozzle until it leaves the nozzle. I don't think the difference in speed of sound will have much effect in any case.

thsksbd 3y ago

"The speed of sound does not depend on pressure, only temperature" Of an ideal gas