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Old 11-14-2008, 03:00 AM   #48 (permalink)
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Originally Posted by instarx View Post
WARNING! Fluid dynamics post!

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.
I never claimed velocity added to static pressure. I claimed that variing velocities in the intake piping would effect the measured pressure drop across the intercooler depending upon the points the pressure readings were taken.

If you read up on bernoulli's equation air moving across an opening has nothing to do with it. It is directly related to the velocity of the air and nothing to do with an opening.

This is also a mechanism that produces lift on an airfoil. The air moving faster over the top of the airfoil causes a drop in static pressure in reference to the slower moving air below the airfoil.

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").
Actually that is a commercial science kit sold to and used by universities. Yes it is used to demonstrate Bernoulli's principle. It also demonstrate that just like in plumbing of an intake system with an intercooler static pressure rises and drops according to the velocity of the fluid at different points in it, hence the difference in the fluid levels.
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.
If what you say is true would you care to explain why the pressure increases again after flowing through the constriction.

As the experiment in the picture demonstrates the static pressure in the wide part of the pipe before the constriction is the highest because it has the lowest water level. As the pipe narrows the velocity increases and static pressure drops thus raising the water level in reference to the water level before the constriction. As the narrow pipe widens again the velocity drops and static pressure rises again. This is proven by the water level drop in reference to the water level at the constriction.
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