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VFFman is walking down the street when
he spots his arch-rival, HBman, master of
the counterintuitive. HBman is cleaning
his car.
On closer inspection, the car seems
rather
odd. It is shaped like an oil-drum on its
side, mounted on four wheels. Besides
that, the wheels are obviously
go-cart in
style: there is no propulsion to the
wheels.
Nor is there any sign of a propellor, nor
of
rockets or jets.
VFFman points out this lack of
propulsion,
mockingly (as he always does) to HBman.
"Au contraire", replies HBman "it has a
3.6
litre V8 and runs very nicely, thank you."
"But," comments VFFman, "your
engine isn't connected to the wheels, nor
to any other propulsion system."
Walking to the front-end of the
car,
HBman says "But you have neglected
THIS,
my Bernouilli Disc!"
On closer
inspection, VFFman notes that the flat,
circular face of the car is actually free to
spin, rather like a propellor but with no
blades. There is only a minute gap
between this disc and the front bulkhead
of the car, and this gap appears to be
filled with lubricating banana
oil.
"So?" says VFFman "Having a
spinning circular face is not going to get
you anywhere! You have invented a
zero-
efficiency propellor!"
"Oh yeah?" Replies HBman,
climbing
into his car and starting the engine. The
disc begins to spin and slowly, but with
dignity, his Bernouillimobile starts to
glide
forward, neatly running over VFFman's
toes as it inches forward.
"What
the
dot dot dot?" mutters VFFman to himself,
and then a thought crosses his mind.
"Could it be" he mutters to himself "that
the front surface of the spinning disc,
being in motion relative to the air,
experienced a reduced pressure due to
the
Bernoulli effect? And could it also be that
the rear face of the disc, being seperated
from the front bulkhead by only a thin
film
of banana oil, experienced no such force?
And could this Bernouillification result in
a
net force driving the vehicle forward?
Egads, I think that may be it! Outbaked
again!"
A page on how airofoils work
http://www.av8n.com...s.html#sec-spinners
[zen_tom, Apr 03 2005]
A page on the Bernoulli effect
http://scienceworld...ernoulliEffect.html
Bernoulli effect is real, even though its importance in aerofoil lift is often exaggerated. [Basepair, Apr 03 2005]
Bernoulli meets Magnus, to sail a ship
http://www.grc.nasa...2/airplane/cyl.html
"the propulsion force generated was less than the motor would have generated if it had been connected to a standard marine propeller!" [AbsintheWithoutLeave, Apr 04 2005]
[link]
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+ just for the banana oil. |
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Why, thank you, [DesertFox] :-) Looks
like I could have saved myself many
words and one fishbone if I'd limited
the idea to banana oil alone.... |
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I do enjoy the continued adventures of VFFman and HBman - but am not accustomed to this Earth thing you call the Bernoulli effect. |
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Normally I'd look it up myself, but I'm not feeling very researchy today, would you mind providing us with some explanationary links (preferably with nice animated graphics) please? |
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i'm not familiar with the bernoulli effect, but it sounds remarkably inefficient. doesn't get much more half-baked than that. croissant. |
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Animated graphics are way out of my
league. I am barely animated
myself.
However, the Bernouilli
effect basically says that if air is moving
over a surface, it exerts less pressure
than static air. This is one way to
explain why an aerofoil wing works (the
air going over the curved top has
further to travel, so has to move faster,
so exerts less pressure above the wing
than that below). If you want a simple
demo, take a sheet of paper and hold
the edge just on top of your bottom lip,
leaving the paper drooping downwards.
Blow hard (over the top of the paper)
and it'll rise up towards the
airstream.
I'm assuming here that
any small square you imagine on the
face of the disc will experience a
relative airflow (since that 'square' is
moving as the disc rotates), and hence
feel a reduced air pressure, effectively
drawing the disc forward, oddly. |
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[Cthulu] - wildly inefficient, I would
imagine. HBmans's BernoulliCopter has
yet to leave the ground, sadly.... |
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OK, I'll cope without the pretty graphics. However, I am going to put my intuition to the test and say that the difference between the curved surface of the aerofoil and the flat surface of HBman's device is that the air is moving in one example and static in the other. |
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i.e. it is the velocity of the gas or liquid under question that causes the pressure drop, rather than it's 'relative' motion to some other object. |
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Since static air is in evidence both in the front and at the back of the machine, there should be no difference in pressures, and the car will remain in situ, only to be engulfed by ants, bees and other roving insects attracted by the sweet, sweet aroma of banana oil... |
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Alas, I fear VFFman may be victorious this episode. |
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Ah, but Zen, the air *is* moving over
the surface of the disc as it is over a
wing. Imagine yourself ant-like
standing on the surface of a 747 wing
in flight - you feel a 350mph wind,
apparently coming from the front of the
plane. Now imagine yourself standing
on any point on this disc (except the
dead centre) - you will likewise feel a
wind as the disc spins. The direction of
the wind makes no difference (the lift it
produces is at right-angle to the
surface in either case).
The basic
point is that the front surface of the
disc, like the surface of a wing,
experiences an air flow across it. |
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By the way, the curvature of the wing is
not directly responsible for the Bernoulli
effect (although, confusingly, it *does*
add another cause of lift by a different
mechanism) - in this context, the
curvature is just a means of speeding
up the airflow over the top of the wing
(compared to that underneath it). |
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And this is the question, is it relative motion that causes the effect, or absolute motion? |
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I would land on the absolute side (which is a very rare thing for me to do) and say that the pressure drop is a property of the atoms in the gas being more rarefied when at speed, than having anything to do with nearby spinning objects. |
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Ah, so is the curve a Bernoulli thing or not? If not then I need to go and do some research... |
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It's relative motion that counts.
Basically, air pressure is the effect of
momentum transfer from gas molecules
to the surface as they rebound. When
the air is moving over a surface, the
"average" gas molecules hit the surface
at a glancing angle, and there is less
transfer of momentum. It makes no
difference whether it is the air that
moves or the surface. (If it did,
relativity would have come to a grinding
halt long before Einstein.) |
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"There is a widely-held misconception that it is the velocity relative to the skin of the wing that produces lift. This causes no end of confusion."
This quote taken from the link provided. There are some good bits further up the page (I linked to a part mid-way-through) that describe all manner of airofoilery. |
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A non-curvey-surface demonstration of
the Bernoulli effect: place a drinking
straw in a glass of your chosen
beverage and blow across the top of it -
the liquid will rise. (If you blow slightly
downwards, however, you're blowing
into the straw and will have the
opposite effect. The Bernoulli effect
*is* fairly weak, and it's difficult to
demonstrate it in its 'pure' form. But it
is real - I'll add links). |
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[zen] thanks for the link. The problem
with aerofoils is that they acheive lift
through a whole slew of mechanisms,
including 'reaction lift' (overall, the wing
displaces air downwards as it moves
through it, and hence displaces itself
upwards). Although the Bernoulli effect
is often cited as the cause of aerofoil
life, it is a minor component. But,
nevertheless, the Bernoulli effect per se
is well established. |
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I think the question here is 'what is causing the pressure drop'? |
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i.e. is it relative motion? or is it the property of a gas/liquid in motion? |
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I don't buy the atoms bouncing off at tangents. If that was the case, then a tennis ball dropped on a spinning record-player wouldn't bounce as high as one bounced on the ground. Experiment anyone? |
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[Zen] I take your point about the tennis-
ball (though the real experiment would
be to see if the record-player were
displaced downward as much, not
whether the ball bounced as high). My
physics isn't good enough to
understand why 'glancing' blows
transfer less momentum than 'right-
angle' blows (given that the component
of the velocity at right-angles to the
surface is the same). However, I believe
that Bernoulli's law (apparently there's a
law!) is well established.
This
leaves the question of whether it is
caused by relative or absolute motion of
the air, and I would argue that it
*cannot* be caused by absolute motion
of the air. All of my (inadequate)
knowledge of physics tells me that, if a
certain apparatus behaves one way
when it is held against a wind of
100mph, it will behave exactly the same
if it is on the back of a truck driving
through still air at 100mph. It can only
be the velocity of the air relative to the
surface that matters. (For that matter,
the whole of the earth's atmosphere is
moving at huge velocities relative to the
sun, and even faster relative to the
distant stars, but we never have to
worry about this in aerodynamic
calculations!) |
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OK, I'm going to imagine a hole bunch of air travelling westward at 100mph. |
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At the same-time, underground at Cern or wherever, there's a guy driving circuits at 100mph in his specially designed zero-turbulence car. |
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So both systems are equivalent (not sure about the zero turb car, but just for the argument) |
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The two air systems are essentially the same. If you point-measured the air-pressure at 1m intervals, you'd get a 3d matrix of equal values. |
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Ok, now we add an airofoil. If we point-test the airpressures now, we find a lower pressure above the wing - note, it's not at the surface of the wing, but in the area immediately above it. |
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If we set our point-pressure sensors to velocity mode we also see that the air is going faster above the wing than below it. |
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So the speed of the air has been changed by the addition of the airofoil, but the pressure drop doesn't happen at the surface, but in the region above the surface. The airofoil has sculpted the airflow around it to make this change. |
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The aerofoil does indeed sculpt the air
(a very nice phrase, by the way), as a
means of increasing the velocity of air
over its surface. I guess an aerofoil is a
nasty messy thing, and perhaps not the
cleanest example of the Bernoulli effect.
(Having said that, I note that the
aerofoil works the same on the car as in
the wind.)
But now, try a second
experiment. Instead of an aerofoil, we
have a drinking straw standing dead
vertically in a glass of water. In both
cases (in the wind, and on the car) the
liquid in the straw rises because the air
moving (relatively!) over the top of the
straw exerts a lower pressure. |
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This makes me think of a special pipe tool that blows air (not vacuum) to lift up delicate silicon discs when the velocity is correct. This should theoretically mean that a tire-driven “bernoullimobile” with motionless disc, going fast enough would suddenly feel less resistence and a leap in speed. |
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Cool.
As for the tennis ball dropped on a spinning record player, the ball will not bounce as high as on a stationary surface because some of the energy will be imparted in spinning the ball and some of the energy will be spent knocking it away from plum. |
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Lift Force: A force produced perpendicular to the flow of a fluid. It is predicted by Bernoulli's law for any obstacle which compresses streamlines at one boundary and compresses them on the opposite one, resulting in a difference in pressures which causes the lift force |
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Lift Force = one half of the Lift Coefficient times the area of the wing times the angle of attack (in radians) times ...wait...we don't need to go any further!! |
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The disc on the bernoullimobile has a zero angle of attack, so the Lift Force is also zero. |
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Very creative thinking however [Basepair] , so there's no way I'm boning you. |
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[2 Fries] I disagree; the vertical component of motion will be identical for the ball dropped on the hard surface and for the ball dropped on the record player. The vertical component is independent of the horizontal impulse imparted by the player. |
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I suppose that makes sense. The turn table is adding its own energy to the equation. |
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My intuition tells me that this thing is going nowhere. Because there is no downdraft created, and nowhere for the air to displace to, there is no net force generated. |
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Bernoulli states that for a constant TOTAL PRESSURE, a fast-moving fluid will have lower STATIC pressure than a slow-moving fluid. |
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Now, we have your spinning disk at the front of the HB-mobile. When measured from a point on the disk, the air in front will have the same static pressure as anywhere else. It will have a higher dynamic pressure, however. Unfortunately, since it's the static pressure that would do the work, the HB-mobile will not move forward or backward. |
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There is another effect that would come into play, though. Since the interface between the disk and the air is not frictionless, some of the air would be carried along with the disk. Centrifugal force (yes, I know there's no such thing, as it's simply an observed manifestiation of momentum in the presence of CENTRIPETAL acceleration) will move the air outward. Since some air has now been displaced away from the front of the HB-mobile, there will be a low-pressure region in front, which will initially pull the HB-mobile forward. Shortly thereafter, a circulation will develop from the disk rim back towards the center, and you'll once again have zero net thrust, if you're lucky. If you're unlucky, the forward-flowing component of the flowfield will entrain outside air, causing a net REVERSE thrust. |
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Sorry, HB-man loses again. |
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I disagree. I see this as using the exact principle that centrifugal pumps use, but replacing vanes with friction. Think of your record player having ridges (molecular structure that causes friction) - the balls won't bounce very high upwards. I think it will be a very inefficient centrifugal pump, but will have a forward force nonetheless. Yes, there will be a circular current that will make this even more inefficient, but if it moves forward at all, then air will have been displaced backwards and the circular current will not form a perfect circuit. |
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If you'd like this to work much more efficiently, put a flat plate in front of the disk that's wider than the disk, with a hole in the center for air intake. Curve the outside edge around the front of your vehicle. Then, for much more efficiency, add vanes. |
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[Worldgineer] that would be a fundamental redesign, turning it into a type of laminar flow pump, with the propulsive force reliant on the backwards deflection of the airflow from the 'pump', rather than any Bernouilli effect. |
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I'm assuming the backside of the spinning disc is hidden from the wind. |
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Blimey! I go away for one day, and all
thjis annotation happens! Wayyyy out
of my depth with some of it, but: |
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re tennis-ball on turntable, I agree with
David S and 2fries - the vertical
component of the bounce should be
unaffected by the turning of the
turntable (I think). But this is an aside,
sort of. |
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[ConsulF] //Lift Force = one half of the
Lift Coefficient times the area of the
wing times the angle of attack// - I'd
always understood that the Bernoulli lift
is independent from (though much less
than) the reaction lift (which arises
largely from the angle of attack), and
that you can get aerofoil lift with zero
angle of attack. In fact, this is why
aerofoils are used rather than just flat
slabs - to generate additional life
beyond the a-of-a lift (I thought). |
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[Freefall] - I can't comment on your
anno on Bernouilli effect, it's out of my
depth. I suspect you're right as it's
stated, but I think it still leaves a
paradox. I guess the questions one
would need to ask of a Bernoulliist are: |
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1) If I have a flat surface still air, and I
then blow across that surface (ie, on
one side only), does the
surface experience a lift? My reading of
Bernoulli says 'yes', and this fits with
the experiences of (a) blowing across
the top a straw (reduced pressure does
lift liquid in the straw) and also (b)
blowing across the top of a sheet of
paper (it does get lifted 'into' the
airstream). |
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2) If the answer to (1) is 'yes', then is
there any difference between the
surface moving through the air and the
air moving across the surface? The
answer to this can only be 'no', or
relativity goes to pieces. |
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3) If the answers to 1 and 2 are 'yes'
and 'no' respectively, then can a section
of one face of the disc be considered
equivalent to the flat surface described?
The answer to this must be 'yes' (I'm
setting aside the centrifugal/petal
effects - they'll be real but a seperate
matter). |
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4) If the answers to 1,2,3 are
'yes','no','yes', then does a disc spinning
in free air experience a Bernoulli force
on each face, tending to pull its faces
apart? (Yes, given the answers to
1,2,3). |
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5) If the rear face of the disc is not
exposed to the air, should there
therefore be a net force acting forwards
(effectively caused by the difference
between the reduced pressure at the
front of the disc and the 'normal'
pressure at the back of the whole car).
Presumably 'yes', if the answers to 1-4
are yes/no/yes/yes. |
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Now, the only one of these questions
I'm not sure about is [1] - I don't
understand the theoretical
underpinnings of the Bernoulli effect
well enough to answer from first
principles. But all the sites I checked
seem to say that a moving fluid exerts
less lateral pressure than a static fluid,
and the drinking-straw and paper-
blowing experiments seem to agree
with that. |
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I guess the point is that I *don't* feel
that the car should work, but I can't see
the flaw in the logic that says it should. |
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[AWOL] Re the link - eppur si muove!
(However, the Magnus effect is acting
sideways on a rotating cylinder, which
is not the same effect I was thinking of
here. But interesting link anyway!) |
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You're all forgetting one fundamental piece of this system: The banana oil. Since the (fluid) oil is in contact with the backside of the plate, it will also be subject to the bernoulli and what-have-you effects. Even as the plate creates low pressure at the air side, sucking the plate forwards, it *also* creates low pressure on the oil side, sucking the plate backwards. Intuition tells me the net force will be zero. |
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It is also worth noting, as a side comment, that the Magnus Effect only works if the cylinder is already in translational motion relative to the air. Therefore it is only suitable for redirecting preexisting forces and cannot actually generate any lift or other thrust by itself. In a sentence, the Magnus Effect can only serve to partially redirect a force from its original direction. |
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I thought the preexisting force was friction causing air to flow in a spiraling outward direction across the face of the disc. |
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Aside from the (brilliant) use of banana oil, this is a silly example of yet another mis-understood physics principle. |
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[5th Earth] //Even as the plate creates
low pressure at the air side, sucking the
plate forwards, it *also* creates low
pressure on the oil side, sucking the
plate backwards.// Ah, but, the force
on the back of the disc will simply be
pulling the plate and the front bulkhead
of the car together, and the layer of
banana oil (supplemented if necessary
with some good old fashioned bearings)
keep them just seperated. Hence,
there's still a net force between the
front of the car and the back of the car. |
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//as a side comment, that the Magnus
Effect only works if the cylinder is
already in translational motion// I agree
comletely, but we're not using the
Magnus effect (acting at right angles to
the long axis of a cylinder) but a
Bernoulli effect acting (putatively!) at
right angles to the face of a disc. The
Magnus effect was sort of a red
herring... |
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[moPuddin] //this is a silly example of
yet another mis-understood physics
principle// It may well be, and I sort of
suspect it is. *BUT* I couldn't see why.
Can you help clarify? The only moot
point seems to be Question 1 in my
earlier annotation. If I've got the
answer to this one wrong (and if you
can help me understand why) then the
problem is solved and HBman goes
away sadder but wiser. |
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Yes, air moving past a stationary object will exibit the same behavior as stationary air with an object moving through it. |
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No, simply moving air past an object does NOT reduce the pressure. |
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Unconstrained air (ie air flowing around a vehicle) tends to have a constant TOTAL pressure. Total pressure is the sum of static pressure (what you'd feel from a non-moving reference) and dynamic pressure (the result of the momentum of the air). |
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Example: If you take a big flat plate (at sea level) and stick a pressure sensor in (not on) the surface, you will measure 14.7 psi. If you then took this plate and blew across the surface, you would still measure 14.7 psi. If you mounted it to the top of a car so that it was perfectly level and drove at 100 mph (ignore the disturbance caused by the presence of the car), you would STILL measure 14.7 psi. |
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There will be no change in static pressure caused by moving a flat plate through the air. |
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Read up on "pitot-static probes". These are used on aircraft to measure speed. Their function depends on the fact that simply moving an object past a body of air does not change the pressure, when measured from a surface that is parallel to the flowfield. |
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This flat plate will have a very inefficient centrifugal pump effect, but without some secondary constraint designed to force the airflow out of the toroidal circulation which will develop, you will get zero thrust from it. |
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Also: regarding the "molecules meeting up at the trailing edge" theory and the "longer path over the top" theory, I could show you water-tunnel photos that disprove both. Imagine a hypothetical airfoil which is a smooth curve on top, but which is very wavy on the bottom. The bottom would be longer, but the airfoil would still produce lift. |
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[Frefall] //you would STILL measure
14.7 psi// Damn. |
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If that's the case, then all is lost, and I
will join HBman for a beer so that we
can console ourselves. I guess I don't
understand Mr. Bernoulli after all, and
this probably stems from a non-
understanding of dynamic vs. static
pressure. Woe is me.
HBman 2: VFFman 1. |
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[FF], what effect causes a piece of paper, when I blow on one side, to fly in that direction? How is this different than your plate on top of a car? |
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The air has someplace to go. With the spinning plate, the only displacement would be from the 'centrifugal effect.' |
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Worldgineer, take a look back at what I initially said about Bernoulli. An unconstrained airstream will have a fixed TOTAL pressure, which is the sum of STATIC and DYNAMIC pressure. If you make the air faster (by blowing it through a nozzle), the dynamic pressure goes up and the static pressure goes down. |
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When you blow across the top of the paper, your lips are acting as a nozzle to speed up the air. Static pressure goes down, dynamic pressure goes up, but total pressure stays the same. Since the flow is parallel to the surface (thanks to effects discovered and described by a man named "Coanda"), the surface only "feels" the static pressure. Note, that since the airflow must curve downwards in order to follow the paper, there will be an additional reduction in pressure due to the momentum of the air. I won't go into that now, but you can find out more by looking up Coanda. |
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Since the air below is not moving (zero dynamic pressure), it has full atmospheric static pressure. Since the air above is moving, it has some dynamic pressure, and therefore less than full atmospheric static pressure. Hence, the paper rises. |
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Looking back at my car example, I'll admit that I was a bit off. If you blow across the top, you will measure less than full atmospheric pressure. |
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However, with this disk, you're still not changing the properties of the air in front of the HB-mobile. The air itself is not moving, hence it will have the same pressure as the air behind the HB-mobile. |
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I really wish teachers would stop teaching "Bernoulli" when describing why an airfoil produces lift. Reaction would be much more intuitive, and more accurate too. |
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[freefall] //Looking back at my car
example, I'll admit that I was a bit off. If
you blow across the top, you will
measure less than full atmospheric
pressure.//
Oh no! Now I'm
more confused than ever. This means
that the answer to Question 1 in my
earlier annotation *was* yes after all,
and I can't find any flaw in my reasoning
on questions 2-5, which means that the
car should move.
At the same
time, though, I agree utterly withyour
statement that we're not changing the
properties of the air in front (ignoring
any centrifugal effect) and hence the car
shouldn't move.
So, help!
Which one of steps 2-5 is wrong? I'm
not arguing that the car has to move,
I'm just trying to figure out where the
reasoning is wrong. |
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I've been trying to work-out a way to describe the paper-blowing effect - and will now attempt to do so by describing a shallow, but wide bowl/vat containing a high-viscosity liquid. |
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The liquid starts off in a static mode....
(Oh, for fixed-width annotations)
+++++++++++++++
+++++++++++++++
+++++++++++++++
+++++++++++++++
+++++++++++++++
+++++++++++++++
+++++++++++++++
[top-down view]
Until a spoon sweeps along the middle, creating a 'stream' of liquid moving in a particular direction. Bottom to top.
+++//|||||\\+++
+++//|||||\\+++
+++//|||||\\+++
+++//|||||\\+++
+++//|||||\\+++
+++//|||||\\+++
+++//|||||\\+++
Notice how the static parts of the liquid(+) blend into the flowing part of the liquid (/\) because the flowing part is 'getting out of the way' (||)
So anything (a raisin for example) that is sitting at the edge of the stream will get sucked into it because the flow getting out of the way will have an apparent lower pressure than the static liquid behind it. |
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Likewise, a piece of paper separating a region of flow and a region of no flow, will experience a similar pull, suck, push or whatever because of the speedier air being blown out of the way. |
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[Zen Tom] That is interesting, and
thanks. It makes sense to me, I think.
However, I'm still not sure where that
leaves us. It would suggest to me that
the Bernoulli effect is sort of as I had
imagined, and that the plate on the top
of Freefall's moving car would indeed
experience a measurable reduction in
pressure (as he said in his last anno).
But if that's the case, then the static
plate in the 100mph wind would
likewise experience the reduction in
pressure, which means that any small
square on the face of the spinning disc
would too, which means the car ought
to move.
Damn my brain. What am I
missing here (apart from common
sense)? |
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Perhaps I was unclear earlier. Blowing over the plate on the car (moving air past the plate) will show a reduced pressure, but driving the car (moving the plate through the air) will show full atmospheric static pressure. |
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By simply spinning the disk, you're not doing anything to the air. (viscous effects aside; that's been covered by the recirculating flow discussion). |
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If you put a pitot-static probe on your spinning disk, pointed into the direction of rotation, you would find that the probe does indeed measure higher dynamic pressure (the pitot part of pitot-static) because any part of the disk except for the exact center is moving relative to the air, but the static pressure would still be atmospheric. |
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Perhaps if there was a goldfish involved... |
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[Freefall]//Perhaps I was unclear earlier.
Blowing over the plate on the car
(moving air past the plate) will show a
reduced pressure, but driving the car
(moving the plate through the air) will
show full atmospheric static
pressure."
Now that IS weird,
and I have to admit that I frankly don't
believe it as stated- there is no
difference in the two situations. For
one thing, this means that you would
measure a different pressure depending
on whether you were driving east (with
the Earth's rotation) or west (against it).
There's something screwey here, or else
I'm mis-reading your statement. Both
are possible! |
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I can't really say more than I've said already. I'd just be repeating myself. I have no idea whatsoever how driving with or against the rotation of the Earth has anything to do with this. |
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Aerodynamics is not simple, and is often non-intuitive. Some of this has to do with the various incorrect stated-as-fact oversimplifications that are presented early on, and a couple-paragraph statement on my part is just not sufficient to break through these preconceived notions of how things really work. |
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I'm certainly not perfect, and I readily admit my mistakes. That's evident in this discussion if you read through it. I'm not saying to take my word as fact, but I've given a couple pointers in the right direction if you want to go learn for yourself. |
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This thing will not work. It will move air around, but it will not provide any thrust in the configuration described. |
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Here is the difference you seek: |
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With the paper, there is both the moving air, and the static air surrounding it. This is key, because the static air provides the static pressure baseline and gives the dynamic pressure some value to drop off from, causing the pressure differential and lift. |
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With the rotating plate, the entire air mass is still static with respect to itself, regardless of the plate's spin. |
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To differentiate, try this thought experiment: make the plate infinitely smooth, so that the air is not disturbed by the spinning plate. You can easily see why this would have no effect. Static air up against a plate which it can't in any way tell if it's rotating or not simply won't care. |
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However, if you were to blow across an infinitely smooth piece of paper, no doubt the effect would still work. |
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Another difference to think about: to the air, the spinning plate represents a wing with an infinitely long chord. Such a wing would never fly, because it would not allow the upper air to displace downwards and cause any action / reaction downdraft. |
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[freefall] I wasn't getting at you, just
saying that I thought 'plate moving
through air' and 'air moving past plate'
had to be the same, or simple relativity
goes out the window and you'd have to
worry about some "absolute" frame of
reference for all motion (and hence
have to take into account the rotation of
the earth, the motion of the sun
through space, and all that - sort of an
aerodynamic equivalent of Michelson-
Morley).
[rayfordsteele] your point about the
*differential* movement of the air
makes more sense to me, and I agree
with your excellent thought experiment
of the infinitely smooth disc. I also
agree (from long-held gut feeling) that
the car shouldn't move - but couldn't
see the flaw in the reasoning. I get the
feeling that the conventional
explanation of the Bernoulli effect is
wrong or missing something, since
taking it at its face value leads to these
contradictions.
Either way, it's been fun. |
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Relativity still holds, but you have to be sure to include the entire system in the analysis, which includes the air at the rear of the vehicle. If there's no net pressure drop between the air at the rear and the air at the front, (which would be doing the work), then it's not going anywhere. |
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[Rayfordsteele] Agreed - it's just that
there's still some logical link broken
between the Bernoulli law as stated and
the car not moving. But, I suspect that
this is not a problem we humble half-
bakers can solve.... |
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BP, send me an email. There's a halfbakery yahoo group that we want to know if you want to join. |
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At my stage in life, any invitation is a
surprise and a flattery. Email sent, if
I've deciphered you're email address
correctly. |
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