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What exact speed point does shock wave start to appear...

by taregg
Tags: appears, exact, point, shock, speed, start, wave
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AlephZero
#19
Dec7-13, 09:25 PM
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I'm not sure another "anecdote" is going to convince you, but in a high bypass turbofan, flow through the fan blade tips is transonic, and we certainly worry about the location of shock waves.

But the overall pressure increase from inlet to outlet is only about 2 psi. (If that number seems unfeasibly small, a pressure of 2psi over an area over a 100 inch diameter circle generates quite a lot of thrust.).

So where are your alleged pressure increases of more than 15psi in the shocks coming from? Nowhere. They don't exist.

The basic cause of shocks is very simple. If the flow speed is faster than the speed of sound, any pressure variations downstream can't propagate back upstream to "even out" the global flow pattern. The place where they "get stuck" trying to go upstream is the shock. That's all there is to it. There is no "minimum pressure difference" required. The rest of the subject is just explanations of what the shock wave patterns around particular objects look like (with no offense meant to CFD gurus, of course!)
rcgldr
#20
Dec8-13, 12:06 AM
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Quote Quote by AlephZero View Post
So where are your alleged pressure increases of more than 15psi in the shocks coming from? Nowhere. They don't exist.
The 2 psi jump is in the direction of flow that is nearly perpendicular to the direction of the rotating blades. The pressure at the stagnation zones on the leading edges of the blades would be higher.

Another article with a reference to shock wave and their assymetry, in this case for a focused sound field, unrelated to the tables linked to in previous posts:

535_1.pdf
boneh3ad
#21
Dec8-13, 02:14 AM
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Quote Quote by rcgldr
Another article with a reference to shock wave and their assymetry, in this case for a focused sound field, unrelated to the tables linked to in previous posts:

535_1.pdf
This article is in reference to focused sound beams, where an apparatus is essentially producing a continuous sinusoidal wave train, and as soon as the wave start to form shocks, they unsurprisingly become nonlinear. This is not the same situation as a lone shock or finite system of shocks produced by a projectile or a blast.
Aero_UoP
#22
Dec8-13, 07:32 AM
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rcgldr, don't take it too personally but I'm just wondering why you prefer wiki over a whole lot of good textbooks on compressible flow that say what boneh3ad and AlephZero say (and I agree)...? I mean since when wiki is more credible than Anderson J.D. for example?
rcgldr
#23
Dec8-13, 12:36 PM
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Quote Quote by Aero_UoP View Post
rcgldr, don't take it too personally but I'm just wondering why you prefer wiki over a whole lot of good textbooks on compressible flow that say what boneh3ad and AlephZero say (and I agree)...? I mean since when wiki is more credible than Anderson J.D. for example?
I seem to recall a similar statement (at sea level, sound waves > 194 db are "shock waves" and clipped at zero pressure) in an old physics textbook, but that was from decades ago and I no longer have the book. Again, it wasn't just wiki that makes that makes this statement, some of those decible tables are from universities.
boneh3ad
#24
Dec8-13, 03:34 PM
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Some are from universities and copied from Wikipedia. None of them cite a source. None of them are aerospace or physics or any other flavor of fluid mechanics or in background. In other words, those tables do not have any cited sources and they go against the longstanding and time-tested definitions of and relations across shock waves. They therefore are not reliable (at least not the portion about shocks).
voko
#25
Dec8-13, 04:33 PM
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Landau & Lifschitz, Hydrodynamics, defines a shock wave as a discontinuity in pressure, density or velocity, or any other quantity related to these thermodynamically. The 1 atm thing is bogus.

Astrophysics studies shock waves of interstellar matter, where pressures are less than minuscule, both sides of the shock wave.
rcgldr
#26
Dec9-13, 01:44 AM
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Quote Quote by voko View Post
Landau & Lifschitz, Hydrodynamics, defines a shock wave as a discontinuity in pressure, density or velocity, or any other quantity related to these thermodynamically. The 1 atm thing is bogus.
The 1 atm thing is related to the values in the decible table, which assumes a sea level ambient pressure of 1 atm, and which corresponds to about 194 db. I think all that being stated there is 194+ db sound waves are clipped at zero pressure and are a form of shock waves, not that all shock waves are clipped at zero pressure.
cjl
#27
Dec9-13, 11:36 AM
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Quote Quote by boneh3ad View Post
A shock definitely has to increase the static pressure as it passes. The opposite (a so-called expansion shock) is mathematically admissible but violates the second law of thermodynamics, and so never actually occurs.
I seem to remember coming across a paper documenting the existence of expansion shocks under certain specific conditions, but I can't remember where I found it. I'll dig around and see if I can find it, but I could just be misremembering here...

As for the main discussion here? I agree with boneh3ad - everything he says agrees with what I've learned in all of my fluid dynamics courses. A shock is a sudden, sharp (very nearly discontinuous) change in the flow characteristics.
boneh3ad
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Dec9-13, 11:47 AM
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Unless you could find a way to decrease the system entropy you won't have an expansion shock. Someone may have therefore figured out how to artificially create one, but it doesn't happen in nature to my knowledge. The nonlinear wave equation that governs sound in air would seem to preclude the formation of expansion shocks as well.
cjl
#29
Dec9-13, 12:10 PM
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Quote Quote by boneh3ad View Post
Unless you could find a way to decrease the system entropy you won't have an expansion shock. Someone may have therefore figured out how to artificially create one, but it doesn't happen in nature to my knowledge. The nonlinear wave equation that governs sound in air would seem to preclude the formation of expansion shocks as well.
Oh - I absolutely agree with this. If I remember right (though I still haven't found the paper), it was under some sort of specific artificial laboratory conditions with a dense gas, but you would never see it in nature (or in air). It was mostly just a fascinating curiosity.
boneh3ad
#30
Dec9-13, 12:24 PM
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Yeah, the curious thing is that you would seem to need a gas whose speed of sound decreases with temperature as opposed to the opposite, otherwise I am not sure there is any way for the wave to break in the first place if the condensation side of the wave doesn't catch up with itself. Now you've got me thinking and distracted from work.
cjl
#31
Dec9-13, 02:21 PM
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Ahh, I seem to have found the paper I had remembered (and it's from one of my professors too, which explains why I saw it in the first place):

http://rd.springer.com/article/10.1007%2FBF02511381
Aero_UoP
#32
Dec9-13, 02:33 PM
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and right on time. I was studying about shock tubes just now and I needed extra examples! xD
Thanks!
boneh3ad
#33
Dec9-13, 03:26 PM
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And there we go: Van der Waals gas. I'm not even familiar with that sort of equation of state, but that seems to be about the sort of tomfoolery required to create an expansion shock.


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