Re: Bernoulli's relationship

["Kilmer, Skip" <kilmers@GREENHILL.ORG>]

Of course, airplane manufacturers go to some pains to place the alitmeter
port in an area where the air remains static even while the plane is moving.

Skip

> -----Original Message-----
> From: John Denker [SMTP:jsd@MONMOUTH.COM]
> Sent: Wednesday, November 08, 2000 8:38 PM
> To:   PHYS-L@lists.nau.edu
> Subject:      Re: Bernoulli's relationship
>
> At 05:34 PM 11/8/00 -0800, Leigh Palmer wrote:
>
> > > [Argument I haven't read which leads to contrary result, followed by:]
>
> It would be more polite to read what I wrote before contradicting me.
>
> > > Remarks:  The foregoing answer is the obvious answer, and the only
> answer
> > > consistent with relativity and symmetry and other basic principles.
> >
> > It sure ain't obvious to me!
>
> It remains obvious to me.  The last time I went flying, altimeters (which
> essentially measure static pressure) were known to indicate the same value
> regardless of airspeed.  This is important: when somebody asks Tower for
> permission to overfly the field at 1500 feet, Tower would like some
> assurance that that represents 1500 real feet, not something that is off
> by
> hundreds of feet depending on airspeed.
>
> The situation Professor Cohen asked about is a thinly disguised variation
> of the same physics.
>
> > I think you forgot about continuity, John.
>
> I didn't.
>
> > You've got water accelerating in a trough under the influence of
> viscosity,
>
> I indicated the pattern of pressure, taking viscosity into account.  I
> didn't mention velocity at all, because it didn't matter.  If you want to
> worry about velocity, you will find a velocity pattern that is consistent
> with this pressure pattern.  And with continuity.
>
> > and then slowing down as it goes through a propeller.
>
> I idealized the propeller as a pump.  It increases the pressure in just
> the
> way I indicated.  My idealization removes all kinds of complicated jets
> and
> countercurrents.  I don't think anybody on this list wants to see the
> non-idealized version.
>
> High velocity at pump inlet and low velocity at pump outlet is not at all
> unusual.