Message-Id: <v01540b10b1250ece54d2@[192.174.2.173]>
Date: Thu, 5 Mar 1998 18:33:50 -0800
To: pinhole@exploratorium.edu
From: pauld@exploratorium.edu (Paul Doherty)
Subject: Re: bernoulli
Pinhole
Sorry about the delay answering the bernoulli question, I was away skiing
in Canada and then came back to host the eclipse broadcast.
There is a Bernoilli effect but it is most often mis-stated.
The correct statement is:
Air pressure decreases when air speeds up and no work is done on the air by
external forces. (There are several other restrictive limits but let's
ignore them for now.)
The incorrect statement is that high speed air has lower pressure.
Bernoulli's statement is about changes in pressure due to changes in speed,
not about pressure and speed.
We can't use the bernoulli effect to calculate the pressure in the
airstream because the airstream was accelerated in a blower mechanism which
did work on the air. (And also decelerated by turbulent mixing with
surrounding air.)
However there is a way to use the bernoulli effect correctly in the
explanation of the bernoulli blower.
As the air stream is squeezed around the ball the air stream speeds up due
to conservation of mass flow. The speeding up of the air as it goes past
the ball causes a decrease in pressure on the airstream side of the ball
and the difference in pressure between the still atmosphjere outside the
ball and the bernoulli reduced pressure on he airstream side pushes the
ball into the airstream.
(Note that the pressure in the airstream is not necessarily lower until the
ball comes into the airstream and causes it to accelerate.)
Hope this helps.
Paul D