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On Nov 18, 2010, at 6:41 PM, brian whatcott wrote:Where pressure recovers in a duct, it is natural to look for a diverging section.
On 11/18/2010 7:15 PM, John Mallinckrodt wrote:Speed is never negative (and even if it were it wouldn't matter for Bernoulli's eqn. I don't understand the "pressure in the sub-sonic divergent section is rising" thing.
Another question about Bernoulli:I expect John noticed that in the rest frame of the fluid at point A,
The Bernoulli equation (for incompressible fluids) says that the sum of the kinetic energy density, the gravitational potential energy density, and the pressure is constant along a streamline so that if the speed DECREASES from point A to point B along a horizontal streamline, the pressure must be HIGHER at point B than at point A.
But in the rest frame of the fluid at point A, the speed is zero at point A and, thus, necessarily is higher at point B so that, in THAT frame, the pressure must be LOWER at point B than at point A.
What's up with that?
John Mallinckrodt
Cal Poly Pomona
the fluid speed is lower, ( even negative!)
at point B where the pressure in the sub-sonic divergent section is rising?
Anyway the explanation is much simpler, but I dare say nicely subtle.
John Mallinckrodt
Cal Poly Pomona