Pat,
Happy to discuss the physics off line but the local increase in
pressure as described seems feasible assuming the flock wing beats
are reasonably co-ordinated.
The co-ordination would not surprise me as most birds are sensitive
to the slightest aerodynamic nuance/pressure variation, the thermal
soarers being the obvious ones.
Another example is the classic vee (or 1/2 vee) formation that allows
the trailing bird to ride the pressure wave of the one in front.
But to attribute flocking as a response to this phenomena seems to be
drawing a very long bow.
Re injury risk, birds do fly into each other occasionally but their
reflexes & 3 dimensional perception are streets ahead of mere humans
with their taught dumbed-down conversion to 2D & long nerve paths.
Chris
Chris Charles
0412 911 184
33deg 47'30"S
151deg10'09"E
On 14/11/2011, at 8:55 PM, Pat OMalley wrote:
Hi all,
I've been sent the following for comment. can anyone enlighten me
(and thus my physicist friend)?
Best
Pat
It occurred to me while watching a flock of birds take off that they
were doing it in a manner conducive to injury: they were all very
close together, and three dimensionally so. Why would they do this?
Here is the answer, I think.
There has been, of course a lot of study and modelling of flocking and
schooling (fish) and so on, but in my brief perusal of the literature,
viz., I googled <bird flocking>, I saw no reference to the following:
Birds expend a lot of energy taking off. Some big birds have adopted
take-off strategies like running to increase lift and so on. So,
let's first model a bird wing very badly, but sufficiently for our
purposes, by an umbrella: on the up stroke the umbrella collapses, and
although there will be drag, and a slight increase in pressure above
the umbrella, compared to the down stroke, wherein the umbrella opens
up, this will be a lot less. Hence the lift. Now, one bird taking
off (vertically, for simplicity) will be flapping downwards against
normal atmospheric pressure, and the pressure will locally increase
during the downstroke. But because the air is free to move sideways,
the increase in pressure will be limited. If, however, we have a
hundred birds tightly arranged in a circular area, the pressure
increase, particularly towards the center of the flock, will be
greatly increased, since the air can only move sideways at the
perimeter of the circle. This will result in increased lift for most
of the birds (actually for all the birds, since the ones at the
perimeter will still benefit from some pressure increase) for the same
energy expenditure.
The above argument works a fortiori for a three dimensional flock.
Let me know what you think.
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