Shock wave formation.
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Shock wave formation.
I understand how and why the air above a wing can go supersonic when the aircraft is subsonic, but what exactly causes the actual shockwave to form where it does? Is it caused by the decreasing camber of the wing further aft causing the air to once again decrease to subsonic speeds?
I understand why the air increases above Mach 1, but what causes the associated slowdown and shockwave?
I understand why the air increases above Mach 1, but what causes the associated slowdown and shockwave?
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what exactly causes the actual shockwave to form where it does?
A good reference for all of this gumph is 'Aerodynamics for the Naval Aviator', its ancient and new copies look like they are photocopies of photocopies, but the content is very good.
When I was in Uni looking at all this, someone once mentioned that a B747 wing creates supersonic airflow near the wing root at high speed cruise - can anyone confirm or scupper this rumor?
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Actually, the shock forms when the airflow starts to slow down again, not when it first goes supersonic. Check out Brian's post in the "I spy" thread that just popped up on the front page again. He's got a good explaination and a cool link with some animations.
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Thanks...although to be honest, I'm still not sure that I understand the cause of the air slowing down to below supersonic speeds. I know it does, and I know that is what causes the shockwave, but what is the cause of it slowing down?
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The fastest flow (lowest static pressure) is over the first 10-20% of the upper surface.
But the shock wave forms, I believe, from the area rule effect (cross-section of the wing + fuselage increasing around the center section). When sitting on the shady side of the fuselage, you can often see the shadow of the shock wave on the wing surface. It dances around a bit in light turbulence. I think there are photos of same somewhere in pprune.
But the shock wave forms, I believe, from the area rule effect (cross-section of the wing + fuselage increasing around the center section). When sitting on the shady side of the fuselage, you can often see the shadow of the shock wave on the wing surface. It dances around a bit in light turbulence. I think there are photos of same somewhere in pprune.
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Thanks...although to be honest, I'm still not sure that I understand the cause of the air slowing down to below supersonic speeds. I know it does, and I know that is what causes the shockwave, but what is the cause of it slowing down?
If you want to get slightly more technical, you can think of a venturi tube. The mass flow through the venturi has to be equal at all points along the tube. Mass flow can be described as the density times the area of the cross section of the tube times the velocity of the flow (rho*A*V). So in incompressible flow, if the area of the tube is decreasing, the velocity must increase to keep the mass flow constant, and vice versa. Now think of expanding this venturi wider and wider, until at some point, the air passing through the center doesn't even know it's flowing through a tube yet the air near the wall still has to accelerate / decelerate to keep the average mass flow constant. At this point, it doesn't even matter if the other side is there, right? Well that's the case with your wing.
Phew, that's a lot of setup. Anyway, keeping the venturi in mind, air is accelerating over the wing whenever the area is decreasing (i.e. the distance between the surface and the chord line is getting larger) and slowing when the area is increasing. Now things get more complicated as you go faster because the density can change and that's where the shockwaves come in. However, the air eventually has to return to the freestream condition and hence ultimately be going slower than it was over the top of the wing.
I hope that made some kind of sense to somebody. If you want to know more, pick up a copy of "Aerodynamics for Naval Aviators." It was written so that even a squid can understand so it's probably much easier to digest than what I posted.
Edit: Oops. Looks like lovejoy already suggested AfNA. In that case, I give that notion a hearty second.
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twentysixpointfive - wiki comes in for a lot of flak from some people, but take a look here and see if it answers your question satisfactorily. If not, come back and I'll try harder.
http://en.wikipedia.org/wiki/Shock_wave
http://en.wikipedia.org/wiki/Shock_wave
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Ah! The example with the venturi makes it clear as day. Thanks for taking the time to explain it all.
I've got a copy of Aerodynamics for Naval Aviators, but I couldn't really find a good explanation for my exact question in there, unless I missed it. I did however, realize where the author of Fly The Wing got all of his illustrations!
I've got a copy of Aerodynamics for Naval Aviators, but I couldn't really find a good explanation for my exact question in there, unless I missed it. I did however, realize where the author of Fly The Wing got all of his illustrations!
Hurt's text is a true classic but his wording--is very sophisticated and one topic build upon the other---it's not really the type of text you go to for quick answers---but if you read chapter one 'basic aerodynamic--he goes over EAS and compressibilty as well as the the relationship between dyanamic and static pressure--without that his later chapter on high speed aerodynamics is nearly incomprehensible---but with out his satbility and control chapter---those aspects of high speed aerodynamics is also nearly incomprehensible---you MUST READ it beginning to end--or you wont gwet it---when I say basic I don't mean superficial---I mean foundational---later courses--will give you all the Blade element theroy you want---the book is very helpful for begining aerodynamics students but---you don't get easy answers--H. H Hurt can really make it HURT
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Oooh a Mach cone
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Well I havent got my protractor or any shock tables handy but I am estimating the angle is around 60 degrees and conclude the speed must have been around M1.15 - how'd I do? If I am right then I learned it here in the Tech Forum on PPRuNe
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Hey slip
I learned almost everything I know about airflow listening, not reading, so what follows is highly suspect. First my brother was present during that pass and swears it was subsonic.
Everything about airflow comes from area ruling, as suggested above. And Bernoulli and his "inviscid flow". There is a very substantial eddy at wing root and fuselage, it can be impenetrable dependent on available thrust. When the F-102 was launched as a supersonic fighter, it disappointed. Something was keeping it from Mach 1 despite all the calculations. Turns out a "slice" through the A/C at this barrier of air was representative of more cross section than 24 very obese opera singers. What the Tunnel taught the builders of the F-106 (the new 102), was to pinch the body at the wing root (coke bottle). This allowed the "dam" of viscid air to reduce and allow the supersonic region of airflow past.
Back to the Hornet. The Wave you see is most likely the "tail event" or the first of two potential "Booms" passing through Mach1. The larger pic would likely show the "nose event" wave or #2 Boom. When there are no shock waves of note attached to A/C it is assuredly smokin' nuttin'.
Bernoulli's work suggests that air in an infinite enclosure is "Incompressible" below a speed of 267(?) knots. Above that speed and it becomes compressible, both against itself and solids (wings, tails,etc). I think it is the leading edge of this "packed" air that slows the faster air down and creates those beautiful cones.
If this is accurate, It is due to memory of Hoser and Coon talking after several beers. If it's not I'll track them down and wring their necks.
Airfoil
Everything about airflow comes from area ruling, as suggested above. And Bernoulli and his "inviscid flow". There is a very substantial eddy at wing root and fuselage, it can be impenetrable dependent on available thrust. When the F-102 was launched as a supersonic fighter, it disappointed. Something was keeping it from Mach 1 despite all the calculations. Turns out a "slice" through the A/C at this barrier of air was representative of more cross section than 24 very obese opera singers. What the Tunnel taught the builders of the F-106 (the new 102), was to pinch the body at the wing root (coke bottle). This allowed the "dam" of viscid air to reduce and allow the supersonic region of airflow past.
Back to the Hornet. The Wave you see is most likely the "tail event" or the first of two potential "Booms" passing through Mach1. The larger pic would likely show the "nose event" wave or #2 Boom. When there are no shock waves of note attached to A/C it is assuredly smokin' nuttin'.
Bernoulli's work suggests that air in an infinite enclosure is "Incompressible" below a speed of 267(?) knots. Above that speed and it becomes compressible, both against itself and solids (wings, tails,etc). I think it is the leading edge of this "packed" air that slows the faster air down and creates those beautiful cones.
If this is accurate, It is due to memory of Hoser and Coon talking after several beers. If it's not I'll track them down and wring their necks.
Airfoil
Gr8shandini,---
Ok, Ok, I forgot how rough the crowd here can be---no I'm not going to derive it---can't do good math on Pprune
but it's simply---M=CSC[mu].....mu= angular Mach come deflection simple vector math---now may I?
any book in advanced aero eng has this though---so don't accuse me of wiki/google---I like McGraw Hill
PA
Ok, Ok, I forgot how rough the crowd here can be
---no I'm not going to derive it---can't do good math on Pprunebut it's simply---M=CSC[mu].....mu= angular Mach come deflection simple vector math---now may I?
any book in advanced aero eng has this though---so don't accuse me of wiki/google---I like McGraw Hill
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Just to follow up on airfoilmod's post -
The area rule effect has been credited to Dr. Richard Whitcomb, although others paved the way. It was Whitcomb's work that solved the XF-102 transonic drag problem - implemented on production versions of the F-102 (and F-106, of course, although a bigger airframe). I had a close colleague who worked on these.
The area rule effect has been credited to Dr. Richard Whitcomb, although others paved the way. It was Whitcomb's work that solved the XF-102 transonic drag problem - implemented on production versions of the F-102 (and F-106, of course, although a bigger airframe). I had a close colleague who worked on these.
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Oooh a Mach cone
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And
As I recall, the optical effect at the boundary of Hi/Lo pressure isn't to do with relative pressure, But HEAT. Also, consider this manouver is happening @ ~600knots in GROUND EFFECT (Bay effect?). At that speed and 28 feet altitude, one PUSHES on the Stick to get down to that level.The Blues are a cut above.
Airfoil
Airfoil