Re: [Phys-l] question about Bernoulli

[John Denker <jsd@av8n.com>]

On 11/23/2010 12:14 PM, chuck britton wrote:
> How about these examples:
> 	Blowing across the top of a strip of paper held against your lower lip.

1) That's not a good example of the Bernoulli effect.  The rules
forbid comparing the pressure between two different parcels 
unless you know that they started out with the same stagnation
pressure ... and the blown air has a very different stagnation
pressure than the ambient air.

The invalidity of this example is well demonstrated by laying
the paper on the table and blowing on it, as Bob L. suggested.

> 	Blowing between two balloons that are floating (or suspended) 
> side-by-side,
> 		close but not touching.

2) Same as (1).

> 	Fabric top of a convertible billowing upwards.

3) Hard to say.  Depends on many as-yet unspecified details.

> 	Plastic sheet covering side of double-wide being hauled on 
> highway billowing outwards.

4) Same as (3).

> 	Two inch square of cardstock with straight-pin thru center,
> 		pin dropped thru hole in spool, blow thru spool to 
> hold card in place
> 		as you turn the spool over to be blowing downward.

5) There is some Bernoulli going on here, but the analysis is 
quite tricky.

> 	Or perhaps even air flowing faster over the top of an airfoil?????

6) That works, if done properly.  Details matter.

> 	Spinning balls curve due to the Magnus Effect.

7) That works, if done properly.  Details matter.

On 11/23/2010 01:52 PM, Stefan Jeglinski wrote:
> > I thought it had long been established, despite textbook treatments, 
> > that this has nothing to do with planes flying. Not that Bernoulli 
> > doesn't exist for various parts of a plane in flight, just that it is 
> > not the correct physics to explain why a plane can continuously stay 
> > in the air (flying upside down, other things equal, being a leading 
> > thought experiment as to the denial).

Gaaaack!

Bernoulli's principle is a consequence of the laws of motion.
Always has been, always will be.  It can be misapplied, just
like any other tool, but this does *not* invalidate it.  We
do not invalidate Newton's laws when students misapply them.

There are many misconceptions about how wings work.  Almost
everywhere you look you find wrong explanations.

Some of these misconceptions involve misapplying Bernoulli's
principle, but that has zero significance.  It is straightforward
to apply Bernoulli's principle correctly.  Trying to remedy the
misconceptions by invalidating Bernoulli's principle is like
fighting a fire with gasoline.  It only makes the problem worse.

Ordinary wings fly just fine upside down.  Bernoulli's principle
applies just fine in this case, as it must.

For details on all of this, see
  http://www.av8n.com/how/htm/airfoils.html