7/25/1998 Steven J Forsberg
Most readers here are no doubt aware that one of the primary pieces of
evidence given for the existence of a supersecret superplane (Aurora) is
the picture that was taken of a contrail/exhaust trail that looks like
"doughnuts-on-a-rope". I and others argued that this was just an unusual
contrail/exhaust pattern and that it did not portend the existence of any
postulated "pulse" engine.
Others, however, argued that the pattern pictured was exceedingly rare --
a good point. And they challenged "us" to come up with more solid evidence
that such a pattern could be formed by a 'normal' aircraft. I'll admit
that at the time I didn't have enough knowledge of the science to make a
strong showing on this counterargument. But, now things have changed....... I've just read "Effects of Stratification on 3-D Trailing Vortex
Evolution", a paper by R. Robins and D. Delisi of the NASA Langley Research
Center (May 13, 1997). While the paper addresses more serious issues than
the existence of 'Aurora', it seems pretty plain that they explain the
whole thing.
The pattern observed is the result of something called "Crow instability"
(named after its discoverer). But why is the "doughnuts on a rope" pattern
seen so rarely? It has to do with the stratification of the atmosphere and
something called the Froude number. The Froude number is calculated using
things like the vortex spacing (similar to wingspan), temperature gradient,
etc. To make a long story short, in order to see the effect there must be
a very high Froude number. I'm no expert, but it seems that such a
situation wouldn't happen very often.To quote the abstract:
"Stratification is seen to accelerate the onset of linking due to Crow
instability and to suppress the downward migration of the vortices. It is
also seen that for low stratification, such that F > ~8, Crow instability
results in the formation of downward propagating vortex rings."
In the conclusions they state: "Low (or no) stratification (F > ~8)
results in formation and propagation of vortex rings."