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Old 11-13-2008, 07:00 AM   #40 (permalink)
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WARNING! Fluid dynamics post!

Originally Posted by ConnClark View Post
If this is so then please explain the attached picture.

Once again I disagree. Any drop in pressure would be due to air flow restriction compounded by differences in velocity of the air flow at he points
I just woke up and will address many of your points later (you made some good ones, but mny are still incorrect) - but first, that picture.

That, that...thing , does not measure what you think it does. Note that the pressure appears to be LOWER in the area with high velocity air. But this is exactly the opposite of what you claim should happen when velocity pressure adds to static pressure, so how can that be? It is because the apparatus actually demonstrates the Bernoulli effect, where high-velocity air moving across an opening creates a localized low pressure area proportional to its velocity.

Those manometers are not measuring the static pressure along the tube, but the relative strengths of the three localized low pressure areas created by the Bernoulli effect. It is not possible to even estimate the static pressures along the tube with that poorly designed apparatus (who would ever connect three manometers in series like that - you can't tell which is measuring what). That contraption is a perfect example of how somone who doesn't understand the principles (not you, the people who put it together) can design an "experiment" or demonstration that they only think illustrates their point (unless the caption to that picture is "Demonstration of Bernoulli Efect" or "World's Worst Design of a Pressure Measuring Apparatus Using Manometers").

A general comment about your interpretations of the relationships between velocity and pressure: Be careful not to confuse cause and effect. Air movement (velocity) is caused by pressure differentials, but the inverse is not true - air pressure is not changed by a parcel of air simply being in motion. This is true in air systems ranging from continental-sized weather systems, to air being pulled through a duct, to the air moving around in your living room.

Last edited by instarx; 11-13-2008 at 10:28 AM..
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