At 1:02 AM -0800 12/30/99, Ken Norton wrote:
>Date: Thu, 30 Dec 1999 01:35:35 -0600
>From: Ken Norton <image66@xxxxxxx>
>Subject: Re: OT!! RE: [OM] Notes on the road
>
>Various people commented on the "wrinkle"...
>>I have seen this on other wings of highspeed, subsonic aircraft. I
>>believe it is where the airflow over the wing suddenly goes from laminar
>>flow to turbulent flow. The sudden change will cause a small difference
>>in refraction and produce your "wrinkle in the air."
>
>...and...
>>The phenomenon described sounds more like a sudden
>>change in the refractive index of the air over the wing such as might
>>occur at a supersonic transition boundry or the place where laminar flow
>>changes abruptly to turbulent flow. I believe it is the latter
>>phenomenon as it occurs over the top of the wing about a third or more
>>back from the leading edge where turbulent flow is expected to begin.
>
>The "wrinkle" that I had seen was not vapor condensation over the wing at
>low altitude, but occurred at 35,000 feet and at sustained cruise. This
>wrinkle was a true defraction of my vision and when lined up with a
>straight line such as the leading edge of the wing would cause a noticable
>wiggle in the vision, similar to that of looking through an old window
>pane. This anomaly was verticle and extended up above the cabin window.
>It was positioned at the CG (center of gravity) point of the plane and
>would move forward and back maybe an inch or so as light turbulance was
>encountered. When the angle of the sun was perpendicular to the "wrinkles"
>(there were several) they would cast shadows on the wing. The wrinkles are
>only visible at straight and level cruise.
>
>I thought about the supersonic transition boundry theory for a bit and that
>is a possibility. Since the plane is cruising at a speed relatively close
>to the speed of sound, the increased airflow speed over the wing can
>approach, if not exceed the sonic limit, thereby creating either a caviting
>of the atmosphere or a shockwave that is maintained at that point. I do
>not believe it to be a separation of the laminar flow as that would occur
>near the wing itself.
It's unlike to be a sonic transition because airliners really don't like
operating in the transonic regime (drag and stuff) but it probably does
mark some kind of standing wave in the airflow around the plane. These kinds
of boundaries in temp/density/refractive index are the basis of schlieren
photography, which used to be used a lot to visualize flow, and are also
what give you the shimmer behind the engine (or over a heat source). (A
bbq grill will cast a nice shadow from heated air on a sunny day.)
Although this kind of stuff appears as a "wrinkle" it's more of a lensing
effect, so ascribing a position to it ain't easy (sort of like trying to
reconstruct a water surface from the patterns of light and dark on the bottom
of a pool). In general, the sharp shadows (or bright lines) are called
caustics;
I recall this only because there was a piece some years back in Scientific
American's Amateur Scientist column describing the various configurations...
paul
Paul Wallich pw@xxxxxxxxx
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