Simviation Forum - mach 1 queston

Reply #15 - Aug 4^th, 2006 at 2:24pm

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An object on the ground is not always measured relative to the ground. Let's say you're on one of those fancy European high speed trains traveling 150mph. You drop your pen, it rolls across the aisle. The person across the aisle tosses it back to you at 10mph. Do you duck and cover because the pen is traveling at 160mph and you don't want it to hit you?

You're complicating things, like all academics. No wonder they're typically so impractical. Tongue

The hypothetical question is about a man standing on terra firma. He is not on a train or ship. Any movement is measured relative to the ground he's standing on.

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A ship is coming in to the harbor against a five knot current. There are two speeds which are critical to the pilot: the wind speed, and the speed the ship is making way through the water. The pilot doesn't care how fast the ship is moving relative to land until he's right next to the dock.

A ship's speed is measured relative to the water it's floating in, not the air or the ground.

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Stationary object break the sound barrier in wind tunnels all around the world every day.

See my previous reply. The air might be travelling at supersonic speed but the model remains stationary.

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Reply #16 - Aug 4^th, 2006 at 2:43pm

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See my previous reply. The air might be travelling at supersonic speed but the model remains stationary.

I think what he's trying to say, is not that the object is travelling at the speed of sound, but more that it is just breaking the sound barrier. There's a difference between the two.

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Reply #17 - Aug 4^th, 2006 at 2:49pm

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I think what he's trying to say, is not that the object is travelling at the speed of sound, but more that it is just breaking the sound barrier. There's a difference between the two.

Ah, but is it? That is the question.

The actual question is this: Quote:

i figured so like if i was standing in a 600 mph wind or how ever fast it is i would thereticaly break the sound barrear or the wind would

This is the quote I was referring to: Quote: "The Mach number is commonly used both with objects travelling at high speed in a fluid, and with high-speed fluid flows inside channels such as nozzles, diffusers or wind tunnels."

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Reply #18 - Aug 4^th, 2006 at 3:02pm

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See my previous reply. The air might be travelling at supersonic speed but the model remains stationary.

Is it just semantics we're talking about or physical phenomena? If it's just semantics then I'll conceede that we say a flow through a stationary oriface is refered to as a supersonic flow, we don't say the oriface itself is supersonic. It's just the way we use the term.

But the physical phenomena is that sound which would travel through that moving air is moving slower than the flow relative to the oriface.

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Reply #19 - Aug 4^th, 2006 at 3:17pm

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The way I see it is that the Mach number is a convenient way of measuring the speed of an object or fluid flow. Sound actually has very little to do with it.

As a matter of interest I must have been one of the youngest people ever to break the so-called "sound barrier" in 1959 at the tender age of 16. I can confirm that the only way of telling that we'd reached the magic speed was by watching the needle on the Mach meter.

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Reply #20 - Aug 4^th, 2006 at 3:36pm

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The way I see it is that the Mach number is a convenient way of measuring the speed of an object or fluid flow. Sound actually has very little to do with it.

As a matter of interest I must have been one of the youngest people ever to break the so-called "sound barrier" in 1959 at the tender age of 16. I can confirm that the only way of telling that we'd reached the magic speed was by watching the needle on the Mach meter.

Sound does have little to do with it, the sound barrier is broken whether it is done in quiet air or noisy air. Speed is always a measure of the motion of one object relative to another. We usually use the Earth since it's a convenient object that everyone can relate to, but that's not always the case.

The Mach number is of interest at the boundry between air and some object. The skin of an airplane, the surface of an oriface, etc. The reason it is important is because the characteristics of how the air behaves relative to the object changes dramatically at this speed.

Relative to the fluid flow, there really is no importance of the speed of either the fluid or the object relative to the earth. Nobody really concerns themselves with that. That's why we don't say that subsonic airliners fly faster than the speed of sound even though their ground speed is often faster than the speed that sound travels on the ground, as up where the plane is the speed of sound is greater and the plane is subsonic.

If there was a supersonic wind at ground level then the flow of air around every object would occur just like any other supersonic flow. The objects would be moving supersonic relative to the air. The air would be traveling supersonic relative to the objects.

BTW, what are the circumstances behind your breaking of the sound barrier at 16?

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Reply #21 - Aug 4^th, 2006 at 3:50pm

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I've enjoyed this discussion but I still think you're overcomplicating things. All Richard wanted was an answer to a simple question. I'm not convinced that any of us have answered it satifactorily. The question remains; in a strong (very strong in this case) wind, is it the air or the objects that are moving with regard to stationary objects on the ground? I say it's the air.

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BTW, what are the circumstances behind your breaking of the sound barrier at 16?

I was in the ATC (Air Training Corps) & was lucky enough to be chosen for a flight in a Hawker Hunter T.7 from RAF Chivenor in Devon while on summer camp in August 1959. That's a long time ago now but I still recall every second. I believe one cadet was chosen from each squadron during the summer of that year. That would make me one of a very select few. The Hunter is supersonic in a dive.

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Reply #22 - Aug 4^th, 2006 at 4:10pm

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I've enjoyed this discussion but I still think you're overcomplicating things. All Richard wanted was an answer to a simple question. I'm not convinced that any of us have answered it satifactorily. The question remains; in a strong (very strong in this case) wind, is it the air or the objects that are moving with regard to stationary objects on the ground? I say it's the air.

Well, the objects on the ground are stationary with respect to the ground regardless of how fast the wind is going. I don't think that was the question. The question was whether, when faced with that wind, the objects on the ground would break the sound barrier. Which they would.

Cool story BTW.

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