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[Phys-L] BP or CE?



I checked many (not all) of my paper textbooks, and none of them the
Coanda effect.
I only heard about the CE from phys-l in the past few years. So if there
is much confusion
here, then why aren't these effects clearly spelled out in physics books?

Many of us do classroom demos to show BP, but I think some of them might
show the CE instead.
What about these, which I sort of put into categories? Which effect
dominates?

perfume atomizer
blow between two hanging cans

spin on baseball pitch
dimples on golf ball
Wiffle ball

shape of plane wing
blow over paper
boomerang

vacuum cleaner and beach ball
toy blow ball pipe
vertical Dyson fan and balloon
air blower and toilet paper roll


Phys-L@Phys-L.org writes:
On 12/21/2014 06:58 PM, Anthony Lapinski wrote:

So what's the difference between Bern Prin and the Coanda effect?

Gaack. In my book the Coanda effect is defined to be
"curvature enhanced turbulent mixing". That's how
Henri Coandă thought of it. (Beware that various folks
have applied the Coanda name to almost anything you can
think of.)

This effect occurs when a narrow high-velocity jet impinges
on a curved surface. That means if you look at it in polar
coordinates, the velocity is a rapidly decreasing function
of r. A proper analysis is horribly complicated. The bottom
line is that it creates something roughly analogous to a
Rayleigh-Bénard instability. That means various small bits
of fluid want to change places with each other and mix.

And do any demos illustrate both effects (BP and CE)?

Well, yes and no. The chaos associated with the Coanda
effect means you'll never get a clean application of Bernoulli's
principle, but you might get a qualitative approximation to it.

The famous example is a certain type of perfume atomizer.
A narrow jet of air flows through a sharply-curved restriction
which (a) serves as a venturi, sucking a small amount of perfume
into the air, and (b) promotes Coanda-style curvature-enhanced
turbulent mixing, thoroughly mixing the liquid into the air.

In case you are wondering, no, this effect does not occur in
an ordinary carburetor. It would be helpful if it did, but
the velocity profile is all wrong. Carburetors were invented
many decades before Coanda came along.

The Coanda effect can also be used in aviation, for exotic
high-lift devices, but this is extremely rare. Most aeronautical
engineers go their entire career without ever having any contact
with Coanda effects.

What demos illustrate each?

Reasonable Bernoulli examples include:
-- ordinary airfoils.
-- Pitot-static system.
-- Magnus effect (spinning cylinder + true airspeed).
-- Venturi geometry (probably).
-- levitating disk, as previously discussed.

In contrast, if it looks like a narrow high-velocity jet impinging
on a curved surface, it's probably Coanda. If the same jet
impinging on a flat surface doesn't produce the effect, it's a
dead giveaway that Coanda is involved.

This has happened in the past where
the BP demos mentioned were actually CE demos.

Yes indeed.

Also, to answer the question that wasn't asked: There are
cases where the demo is /neither/ Bernoulli nor Coanda.

A notorious example is a stream of water in air, hitting the
back side of a spoon. That's mostly surface tension. If you
do the same thing under water, where there's no surface
involved and therefore no surface tension, you get completely
different behavior. There are other experiments one can do
to prove that most of what is usually said about this demo
is BS.

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