View Full Version : I spy with my little eye something beginning with 'B' ...Boeing? Nope Boundary Layer!

slip and turn
21st Apr 2008, 19:52
Was sitting adjacent to the wing on a recent flight and it being a perfect morning with great visibility was looking out a lot.

Up in the cruise at FL380 I thought I noticed a very narrow vortex dancing approximately along the centreline of the wing more or less from root to tip (winglet actually!)

It was about the thickness of a transparent washing line and was in places veined like the stem of a leaf, or like the lifeline on the palm of a hand.

I have never seen this phenomenum before and was fascinated to see where it settled in smooth air in the cruise, and how it moved forward and was apparently lost off the leading edge after we had got settled into descent and were throttled back.

Before that, it appeared to get driven backward partially onto the speedbrake surfaces as we seemed to speed up in the first few moments of descent.

Is this something that I can expect to see on most flights? Or is it more to do with the angle of sunlight (the sun was behind me as I viewed it). I guess it is some kind of refraction effect caused by a superdense shockwave where the air over the wing has gone supersonic?

21st Apr 2008, 20:31
slip and turn,

your along the right lines....its an occurence when a certain part of the a/c reaches transonic speeds (or critical mach number) hence forming a high pressure shockwave boundary....pretty sight! :cool:

21st Apr 2008, 21:21
I've seen this a couple of times too - it's an amazing sight; the thing twists and writhes like its alive.

21st Apr 2008, 21:27
Transonic wing - designed to be very efficient near (but not too close) to "coffin corner" in the cruise.

slip and turn
21st Apr 2008, 21:30
Thanks link_142 and you's others :ok:

I knew I wasn't dreaming ... I was quite surprised to see it fluttering backwards and forwards maybe in the range of 100mm at constant speed / level flight / perfectly smooth air.

I said it was broadly out from the wing root along the centreline of the wing but of course it wasn't exactly ... it followed a curved path from a point on the inboard trailing edge initially and then in towards the centre and then out towards the winglet:

It seemed to leave the inboard trailing edge somewhere behind the engine pod (it was a 738 wing). The shockwave, if I can call it that, then traced a curve (bow shaped) well forward of the front of the speedbrakes, but not parallel to them, and then tapered at a more shallow angle in front of the ailerons so that it was more or less parallel to the forward edge of the aileron. The forwardmost point in the curve was ahead of the line of the outboard speedbrake/flap edge.

Out towards the winglet is where I noticed more divergent 'feathering' (or veining) of the shockwave.

When we adjusted course in the cruise, I noticed the shockwave on the uppermost wing moved forward maybe 100 or 200mm. Being many moons from any study of the subject, I am fighting to work out why that would be, because I would have thought the airflow over the uppermost wing would be slightly accelerated in the turn :p

Tartare, I vaguely remember seeing separation point diagrams ... is this exactly that? Or is this something different? Separation point diagrams left me thinking that the air behind the separation point would be kind of fluffy (turbulent / reduced lift resulting), but didn't see any fluffy bits :p ... and if the separation point moved forwards and disappeared off the leading edge wouldn't that mean a stall?

I need to study again to get my head fully around what I was seeing today:8

21st Apr 2008, 23:17
I think you may be right.
I assumed it was the separation point when i saw it... I was SLF on an SQ 744 in the cruise over the Pacific; quite a sight, but I think from what the other guys are saying it may have been a shockwave.

Dan Winterland
22nd Apr 2008, 01:11
I don't think the aircraft will be going past enough to generate a shockwave. And if you did see it at cruise speed, it would't be where you saw it. I think you saw indeed the 'separation point', the point at which the pressure gradient becomes adverse. What you were observing was a line of very low pressure . The point at which the adverse gradient starts has stagnant airflow and is very turbulent. This turbulence creates the low pressure which condenses the moisture in the air which can make it visible. I have attached a diagram from my CFS notes which I drew some years ago ilustrating it.


There is also one on the bottom of the wing. This can generate even more drag and it's elimination has been the focus of aerodynamisists for some time. The Airbus types use an 'inverse cusp'. It's a reflex camber which is towards the trailing edge and which accelerates the air delaying the separation point. This is alo seen on gliders, which also employ other methods. A line of 'knobbly tape' is stuck along this point to act as a mini vortex generator to encourage a 'cleaner' separation point. On glider type has a series of small holes which feed higher pressure air from a plenum chamber in the wing which is pressurised by a pito on the leading edge. These holes feed air into the separation point to act as a vortex generator and as a pressure equaliser.

Brian Abraham
22nd Apr 2008, 02:19
From http://yarchive.net/air/airliners/shock_waves.html

J. Gordon Leishman
Associate Professor of Aerospace Engineering,
University of Maryland at College Park

Yes, indeed! What you were seeing was, in fact, a shadowgram (or shadowgraph) of the shockwave on the wing. I have seen this many many many times from the windows of various aircraft including B-757, DC-10, B-747 etc.

You need to know what to look for, but if the sun is in the right location (preferably above, so about mid-day is a good time), the sunlight is refracted (bent) out of its original path as it passes through the high density gradients at the the shockwave, and a shadow (usually a fairly fine dark line) is cast onto the wing surface. There may be several finer dark/bright lines since the shock is three-dimensional, and there will be multiple light paths that undergo refraction. You may also see a region of "distortion" off the wing surface which again indicates refraction through the density gradients in the flow near the shock, although this latter phenomenon is harder to see under most lighting conditions.

If you are really lucky, you will see the shockwave shadow all the way along the wing to near the tip and you will see how much more three-dimensional the flow becomes in the vicinity of the engines, such as on a DC-10. As the aircraft flies though turbulence you will also see the shadow move as the shock wave repositions itself under the (very mild!) unsteady flow conditions.

In my experience, the B-757 seems to show a fairly pronounced shadow compared with say a 767 or a DC-10 but this may also be related to the lighting conditions, viewing angle etc. For most modern transport aircraft with supercritical airfoils the shock is quite far back from the leading edge and you can certainly see the shockwave shadow under a wide variety of conditions if your seat is over the wing and if you look very carefully. I also have a couple of textbooks and reports that document this observation. One even has a photo of a shadowgram of the shockwave on a B-707 wing.

I will admit however, that although I have seen the phenomena many times, making a decent photograph has been difficult. Now, only if we could convince Boeing to cover one wing of a 757 with 3M Scotchlite retroreflective film, then we could have some real interesting stuff to talk about!

Real science from your airplane window! Have fun!

The observation of flowfields containing shocks and other density variations are routinely examined by means of a class of density gradient flow visualization methods known, in general, as schlieren methods. A simple schlieren system is direct shadowgraphy - which is essentially what is being described by the various observers of shockwave images on transport aircraft wings.

Note that the refractive index varies if the density in the flow changes. For practical purposes, the refractive index, n, is related to the density, rho, by the equation

n-1 = k * rho

where k is a constant for a particular gas and wavelength of light. This equation can be written as

n-1 =(n_0-1)(rho/rho_0)

where _0 indicates the quantities at a reference temperature and pressure. For air, n_0=1.000292 at 0 deg C and 760 mm Hg and for 5893A.

Consider a beam of light (could be from the sun) passing through a flow with a density variation (a shockwave being a good example), and this beam of light eventually falls on a viewing screen (the wing of an airplane, say). If the density changes (at the shock, for example) then the time of arrival of a particular point on the screen on a light wave will change because the velocity of light, c, is related to the refractive index, n, by the equation

c=(1/n) c*

where c* is the velocity of light in a vacuum.

If there is a gradient in refractive index normal to the light rays, then the rays will be deflected because the light travels more slowly where the refractive index is larger according to the above equation. The deflection of these light rays is a measure of the first derivative of the density with respect to distance, that is the density gradient, and can be observed using various schlieren techniques (which require lenses or mirrors and a knife edge or graduated filter for a cut-off). If the refractive index gradient normal to the light rays varies, then deflection of adjacent rays will differ, so they will converge or diverge giving regions of increased or decreased illumination on a viewing screen (dark or bright bands). This is the basis of the direct shadowgraph method. It requires no lenses or mirrors and is essentially a measure of the second derivative of the density field.

These schlieren methods are routinely used in the laboratory when examining high speed flows containing shockwaves. Turbulence and vortices can also be observed, such as those behind propellers and helicopter rotors. In the field, obviously it is much more difficult to visualize such flows, but the example of the "natural" shadowgraph of the shockwave on a transport wing has been cited in the literature for many years. It is indeed interesting to me that so many of our friends on the internet have also observed such phenomena.

slip and turn
22nd Apr 2008, 09:52
Hey thanks guys ! You've made my day ! Now I hope to be able to show the kids en route to Spain and/or back in the summer. Must choose our seats carefully :ok: - might even be worth Speedy Boarding to be sure :p

This is indeed a great science lesson from the window. I was privileged to be able to watch it for quite some time and couldn't help thinking that the data I was merely observing would be very valuable to a wing designer. I wondered how complete wind tunnel data actually is? Despite the undoubted investment in wind tunnel technology, there's surely limits on what you can simulate versus real world dynamics?

When I saw this "thing", we would have been heading approximately 240 at around 1000Z, latitude 51N/52N or thereabouts, so I guess we were headed approximately perpendicular to the sun, and the view I had was along the wing furthest from the sun. Azimuth (is that the right word for the angle of elevation of the sun in the sky?) was about 45 degrees I think.

Nerdy but brilliant :8

22nd Apr 2008, 15:47
Just for the sake of clarity, as others have said, what you saw was the standing shockwave which is common (but not commonly visible) in transonic cruise. It's pretty cool that you were able to witness that. And you were right in your observation about the turn. When you increase load factor, you're generating excess lift, which means you're also increasing the acceleration of air over the upper surface of the wing. That will change both the position and height of the standing shock.

Now for the inaccuracies:

The boundary layer is the usually small distance (~ 0 - 3") above a surface where the velocity of the air transitions from zero (the air "stuck" to the surface) to free stream. Since there is a shear foce in the fluid in this area, flow is almost always turbulent in the boundary layer. Since turbulent flow creates more drag than laminar flow, great lengths are taken to reduce the size of the layer.

Separation point refers to the point where the air can no longer conform to the shape of the wing and a bubble of recirculating air is formed. This is usually a low speed phenomenon and moves from the trailing edge forward. If you were seeing separation at the point on the wing you mentioned, you'd be in a stall.

OK, end of science lesson. I'm glad that you were lucky enough to witness and recognize something that 99% of the folks in back probably never noticed.

slip and turn
22nd Apr 2008, 17:17
Thanks for the revision/definite corrections gr8shandini ... very much appreciated. I knew that 'Boundary layer' wasn't really what I saw, but I needed something beginning with 'B' for my thread title :p ... and I thought it might be the boundary of something, even if not the layer!

I am still smiling that I actually saw this thing - we all know that wing vortices are two a penny in certain conditions, and all kinds of strange effects come off fast jets at airshows of course, but this standing wave (or the shadow of it) was as revealing as the first X-Rays of a barium meal in a digestive system! A very privileged look into exactly how that transonic wing did its job of work, but this time no X-rays required, just a Mk I eyeball and that little bit of science knowledge that tempted me to blink, shift my position a little to be sure it wasn't refraction in the window, and to look again and wonder!

The thing most definitely did exit steadily via the leading edge, so I guess that is just how it is when the speed reduces slowly from a transonic state (if that's the right words).

As you and others have said I think, if the separation point exited that way then it'd be a stall, and because we didn't plummet, it wasn't :p, and more to the point, I note what you say about separation point sitting anywhere near the centreline ... that'd already be a stall in most cases :eek:

Oh and about the movement of the shockwave during the turn, I am now struggling with the direction it moved ... one part of my brain could cope with the idea that accelerated airflow over the top of the uppermost wing might mean that the hi speed generated shockwave occurs earlier i.e. appears to move forward ... but that conflicts with the other part that saw it moving forward and right off the front when I thought we had slowed in the descent :confused:

22nd Apr 2008, 17:54
That part about exiting off the leading edge is the only thing that strikes me as odd. It must have been some kind of illusion based on the multiple reflections / refractions that are required to make such a thing visible.

Usually, these shocks form around 2/3 chord and move forward with increasing airspeed. If it reached the leading edge, that means that airflow over the entire wing was supersonic and thus your airspeed would be Mach 1.0. Obviously, that wasn't the case in a 737 so something else must have been going on.

22nd Apr 2008, 18:00
I think the shock wave moves aft with increasing speed, correct me if wrong.

slip and turn
22nd Apr 2008, 18:01
Yep, that's what I was just thinking gr8s, - in a truly supersonic aircraft the shockwave indeed occurs earlier - in front of the airframe! (or does it??)

2/3 chord is indeed about where it sat in the cruise. It was quite leisurely the way it just moved forward in the descent and was lost forever, over the front, so to speak. Now remembering that what I saw was a shadow of the shock, I don't suppose that the shockwave itself is any significant distance above the surface of the wing such that when we moved from level flight into a few degrees down, that would have caused the shadow to move?

airfoilmod - yes and I was thinking the same for a while ... and it may yet be so --- something in my head says that a sonic boom, for example, occurs behind the wing / airframe?

And remember, I did indeed see the shockwave move aft a little in the first part of the descent when I thought we were going faster for a few seconds or tens of seconds...

gr8s? Help us here ... :8

22nd Apr 2008, 18:21
counterintuitive to think that any disturbance is gaining ground on a supersonic and accelarating airframe. The Wave is left behind above mach 1, not in front. As the aircraft decelerates, the wave gains on the airframe, "accelarating" relative to the leading edge. I'm sticking to that, again, correct if I'm missing something. Faster than sound means by definition, in front of sound, in this universe. Otherwise the slowest runner would win every race, Yes? Related: Bow Wave, Pressure Wave, Sonic Boom, delayed onset, etc.

22nd Apr 2008, 19:37
Well, you're sort of right, airfoil. In supersonic flight, there's a shock at both the leading and trailing edges of an airfoil (that's why if you've ever heard a sonic boom, it has a "dual thud" kind of sound. Sort of like running over an expansion joint on a bridge). But at precisely Mach 1.0, there's just the one normal to the stagnation point of the airfoil.

If you think about the airstream moving past the aircraft (as in a wind tunnel) instead of the airplane moving through the air, it might make a little more sense. When the air first encounters the wing at the stagnation point, it's got to slow down to zero (air molecules "stick" to the wing due to viscosity) and any time you slow supersonic flow, you get a shock. Now, of course, this doesn't happen instantaneously, so there's a small area akin to the boundary layer where this deceleration occurs. So above M 1.0, the shock actually detatches from the wing and hovers somewhere out in front. That's the bow shock that you mentioned.

And, of course, we still need to generate lift, so you have to accelerate flow over the wing just like in subsonic flight. And since the air has to meet up in the same place at the trailing edge, it decelerates again causing a shock there as well.

But that's all "supersonic" flow, which usually refers to flight above M 1.2. In the "transonic" region (~M 0.8 - 1.2) where airliners operate, the flow starts off subsonic and then hits supersonic speed while being accelerated over the wing. Once again, it needs to slow to meet the flow at the trailing edge, so a shock forms at the back end of the supersonic region. Obviously, this all depends on the airfoil, but modern "supercritical" wings are designed to shift this point as far aft as possible and that's usually around 2/3 chord. Now, as the airspeed increases, the amount the flow needs to be accelerated to hit M 1.0 decreases, so the supersonic region and the shock will move forward.

And, of course this is all for straight and level flight. Any time you increase load factor, you're increasing the amount that flow is accelerated (shock moves forward) and any time you unload the wing, it's the opposite (shock moves aft as in when you started the descent).

As you can see, it all gets rather complicated and I'm trying to remember stuff from my school days 10 years ago, so I may not be 100% accurate. If you want to do further research, I recommend "Aerodynamics for Naval Aviators." It's a US government publication, so it's cheap and is probably on the web somewhere for free. Plus, it had to be written so that Sailors could understand it, so I'm sure y'all won't get lost in technobabble.

22nd Apr 2008, 22:47
It is a bit more clear. My memory is dependent on info far older than ten years, my first flight was an A/C without brakes, sometimes like my posts.

slip and turn
23rd Apr 2008, 00:51
My memory also reliant on earlier decades, and first flight was on an aircraft with brake pedals but it is still debateababble as to whether what they were fixed to qualified as brakes :p

Brian Abraham
23rd Apr 2008, 02:17
Usually, these shocks form around 2/3 chord and move forward with increasing airspeed

Afraid not. The first shock will form on the upper surface when the upper part of the airfoil reaches its Mcrit. The airflow at that point has reached M 1.0 although the aircraft will be at a speed less than M 1.0 As airspeed increases the shock moves aft and at some point the lower surface of the airfoil will reach its Mcrit and a shock will form. Both the shocks at this point are referred to as “normal shocks” since they stand at 90° to the respective wing surface. As the aircraft accelerates further the two shocks continue to move aft until the aircaft reaches an airspeed of M 1.0, when both shocks are positioned at the trailing edge. On accelerating further (slightly above an airspeed of M 1.0) a bow wave shock forms at a position slightly in front of the leading edge. Between this shock and the leading edge is the stagnation point, which is an area of extremely high pressure, and it is this that is responsible for the large increase in drag. Beyond M 1.0 the leading and trailing edge shocks become known as “oblique shocks”, since they start to form an ever increasing acute angle “arrowhead”. At M 2.0 the angle is 30° and at M 3.0 is 20°. This angle is known as the “Mach angle” and is equal to 1/Sin(Mach number). You will find a little movie here http://selair.selkirk.bc.ca/aerodynamics1/High-Speed/Page2c.html which demonstrates what happens (at the bottom of the page).

23rd Apr 2008, 04:19
Brian Abraham,

Beat me to it, good explanation, it's been a while since I've had access to some texts but that sounds right. Oblique shocks are a sonic or supersonic phenomenon, Normal shocks are initially a transonic phenomeneon. (If vapour is apparent, sharp cone shape versus contact lens shape around the aircraft.)

Also If you're lucky enough to see it, the normal shock moves forward in descent as the mach speed decreases but the indicated speed increases.

(Anyone in the know, does the 747 hold a higher mach speed in descent than other types?)

23rd Apr 2008, 11:34
I saw something interesting on takeoff at BCN on Monday morning... as the 757 was increasing thrust, fog suddenly appeared in the nacelle in front of the compressor. It would surge up, disappear, surge up again. Once it was brakes off, it all disappeared (for obvious reasons). It was an interesting phenomenon. Anyone?


Brian Abraham
23rd Apr 2008, 12:46
fog suddenly appeared in the nacelle in front of the compressor
Quite often seen, just as you often see the vortices from wing tips, end of flaps and even from the tips of the horizontal stabiliser at times. Caused by the low pressure which makes moisture dissolved in the air condense out to form a "cloud". Also often seen as corkscrew of vapour from the tips of propellers.
contact lens shape around the aircraft
Called a Prandtl-Glauert condensation cloud.

23rd Apr 2008, 14:59
Thanks for the correction Brian. As I said, it's been few years since I worried about anything actually transitioning to supersonic speeds. Somehow I still have a distinct memory of a single shock at the leading edge, but it's quite possible that I was severely hung over during that lesson.

This is a fun thread. Reminds me of why I got into this line of work to begin with.

Brian Abraham
24th Apr 2008, 01:11
gr8shandini - you and me both. I've attached a photo below to show a visible shock, you will notice no condensation, its purely because of the change in refractive index of the air brought about by changes in the pressure field. Notice too, from the disturbed water that this must be the first area on a F-18 to produce a strong shock. As an aside, 747 crews tell of hearing a "rumble" just behind the top of the cockpit when the air flow over the hump reaches its Mcrit and a shock forms. Further, the design speed of a super sonic aircraft can be ball parked some times by measuring the angle made by a line drawn from the nose to the wing tip and using the sin formula mentioned earlier to calculate the Mach. Thats because you dont want the wing operating ahead of the shock cone formed by the nose.

slip and turn
24th Apr 2008, 14:43
What a superb photograph ...

A visible shock indeed to all those yachties lucky enough to be looking the right way :), but quite another type of shock for those tipping gin down their throats at the critical moment :E

slip and turn
29th May 2008, 13:05
I've found another shockwave shadow on the other side :8

Nah, it isn't what you think it is :p, ' cos this one is on the sunny side of the aircraft, and its evidence of a different shockwave I think :)

It is linear, and in high speed cruise appears to run just 2.5m from centre wingroot adjacent to the rearmost 737NG wing emergency exit, diagonally forward to where the engine pod and leading edge meet.

I reckon that means there are actually two roughly parallel shockwaves about 1.5m apart on the broad part of the wing inboard of the engine - can anyone confirm? :ok:

29th May 2008, 13:29
Brian Abraham, thanks for the explanation. I found the phenomenon quite fascinating. :)

I neglected to thank you before. So thank you :)


Brian Abraham
29th May 2008, 13:48
VAFFPAX - Tis a pleasure to help. I learn a lot myself in finding the answers, so I have to thank you for asking the question. :ok:

slip and turn
30th May 2008, 13:10
I have been puzzling again as to whether my surmise that there must be two roughly parallel diagonal standing shockwaves on the 737NG inboard section can be true.

So I have been revisiting those links that Brian so very kindly provided earlier in the thread, including that brilliant 'fish in the river' illustration.

There is the possibility that what I have seen is some kind of break in the standing shockwave that occurs exactly at the chord where the power unit is mounted i.e. that the somehow the presence of the engine stuck right out in front at that point somehow accelerates the outboard air more than the inboard air that goes around it. That doesn't sit too well in my mind.

I can however imagine that there might be a contact lens type shockwave around the front of power unit, out ahead of the wing, on the basis that the engine cannot possibly 'eat' all the air it confronts even at 39000 where I was sat watching it.

And further to that idea I can then imagine that near the engine, there might be two 'funnels' of fast air resulting which the wing then encounters and causes to go supersonic over the top:
1. inboard between power unit and fuselage
2. outboard but quickly tucking in diagonally towards the fuselage.

Maybe I should try to draw it, scan it and photobucket what I mean, or am I letting my imagination run too far already?

PS Brian, have you seen that Blue Angels video on YouTube? http://www.youtube.com/watch?v=K7rAUu8djZ4