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taregg
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what exactly speed point does shock wave will start to appears..
boneh3ad said:A shock wave isn't just formed when "passing through the sound barrier". It forms and is constantly maintained any time air is moving greater than the speed of sound relative to the body/vehicle.
rcgldr said:A shock wave occurs when the decible level reaches the point that the peak pressure is greater than 1 atm, producing a non-sinusoidal save, since the minimum pressure is 0 atm.
rcgldr said:A shock wave will transition into a regular sound wave over time, and this is the sonic boom that is heard. If the shock wave is heard, it sounds like a loud crack instead of a boom.
rcgldr said:Perhaps there are two definitions of a shock wave, or one describes the physics and the other describes how they are created:
Wiki articles and quotes from those articles:
Shockwave (distorted sound waves > 1 atm; waveform valleys are clipped at zero pressure)
wiki_sound_pressure_levels.htm
rcgldr said:Over longer distances a shock wave can change from a nonlinear wave into a linear wave, degenerating into a conventional sound wave as it heats the air and loses energy. The sound wave is heard as the familiar "thud" or "thump" of a sonic boom, commonly created by the supersonic flight of aircraft.
http://en.wikipedia.org/wiki/Shock_wave
rcgldr said:The F-14 in this video is about the same size as an F-15 and it's clearly making a crack, due to the closeness of the F-14 to the observers:
http://rcgldr.net/real/f14flyby.wmv
taregg said:ok..what about the blast wave that created from the shock waves..
I meant the local ambient pressure (the pressure at current altitude), not the absolute pressure constant for sea level air.boneh3ad said:peak pressure greater than 1 atm
rcgldr said:I meant the local ambient pressure (the pressure at current altitude), not the absolute pressure constant for sea level air.
taregg said:but what i mean exactly...what is difference between shock waves that from supersonic jet and from high explosive bombs...
I think the key point of the wiki statement is that clipping of what would otherwise be a sine wave that occurs at zero pressure, which coexists when peak pressure is greater than double the ambient pressure, and that's considered to be a shock wave. Perhaps there are other forms of shock waves, but that is description used in that wiki table.boneh3ad said:What you originally typed was that the peak pressure reaches 1 atm while the minimum is 0 atm, "producing a non-sinusoidal wave".
It's not just wiki:boneh3ad said:That Wiki explanation makes no sense though.
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.AlephZero said:So where are your alleged pressure increases of more than 15psi in the shocks coming from? Nowhere. They don't exist.
rcgldr said: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:
http://www.icacommission.org/Proceedings/ICA1998Seattle/pdfs/vol_1/535_1.pdf
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.Aero_UoP said: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?
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.voko said: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.
boneh3ad said: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.
boneh3ad said: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.
A shock wave is a type of pressure wave that occurs when an object moves faster than the speed of sound. It is characterized by a sudden increase in pressure and temperature.
The exact speed at which a shock wave appears depends on various factors such as the medium through which the object is traveling and its shape. However, in general, shock waves start to appear when an object reaches speeds greater than Mach 1, which is the speed of sound.
The speed of a shock wave is typically measured in Mach numbers, which represent the ratio of the object's speed to the speed of sound. For example, if an object is traveling at Mach 2, it is moving at twice the speed of sound.
Some common examples of shock waves include sonic booms produced by supersonic aircraft, the sound of a whip cracking, and the shock waves created by explosions or meteorite impacts.
Shock waves can be dangerous, especially at high speeds, as they can cause significant damage to structures and living organisms. They can also result in hearing damage and other health issues for individuals in close proximity to the source of the shock wave.