Hood splitter for my vw

[3-Wheeler]

Originally Posted by winkosmosis

That's was my whole point. The inside of the tube that points into airflow is higher pressure than the surrounding air, not lower.

Pressure - Wikipedia, the free encyclopedia

Stagnation pressure is the pressure a fluid exerts when it is forced to stop moving. Consequently, although a fluid moving at higher speed will have a lower static pressure, it may have a higher stagnation pressure when forced to a standstill. Static pressure and stagnation pressure are related by the Mach number of the fluid. In addition, there can be differences in pressure due to differences in the elevation (height) of the fluid. See Bernoulli's equation (note: Bernoulli's equation only applies for incompressible, inviscid flow).

The pressure of a moving fluid can be measured using a Pitot tube, or one of its variations such as a Kiel probe or Cobra probe, connected to a manometer. Depending on where the inlet holes are located on the probe, it can measure static pressure or stagnation pressures.

Hi Wink,

Well, reading what it says from Wikipedia versus actually working with pitot tubes and calibrating them for air flow are two different things.

Yes, you are correct about the stagnation pressure, but if you take a close look *inside* the pitot tube you will be presented with a series of small drilled holes that measure the *dynamic* pressure, which is the velocity component of the air flow.

And like I've mentioned already, proper usage and calibration of pitot tubes requires a differential sensor designed to measure both pressure taps.

If I must tell you, we have such devices at work, and yes, I have calibrated them with precision sub-sonic nozzles, so everything is very straight-forward on the theory of the device and actual usage (1).

By the way, the pitot tube is only good at measuring air velocity, and is not a good *mass flow* device. It does not measure air molecules and their relative mass, only the velocity of the particles.

Pitot tubes are of a family of devices that output a signal velocity-squared to the actual air velocity. For example, if you flow the air at 100 feet per second, you will measure a differential pressure of let's say 10 inches water across the two pressure taps. If you increase the air stream to 200 feet per second, the differential pressure does not double, but instead goes up by the square of the air velocity, or to 40 inches water difference. Because of this, pitot tubes have only a 10:1 usable range for differential pressure transducers measuring a range of 100:1.

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(1) One of our precision sub-sonic nozzles is an ASTM designed and custom built nozzle. And guess what, it has two pressure taps on it. One for upstream inlet pressure and what else, but a series of .040 inch diameter holes spaced 90° apart and located at the throat of the nozzle by design. These four holes are communicated to a annular rubber hose that then connects to a 3 foot tall classical water manometer.

When using the nozzle, all calibrated measurements are accomplished by taking the *differential* pressure between the inlet pressure and the throat vacuum. At full tilt, the nozzle generates almost 30 inches of vacuum at the throat! Pretty impressive to watch and demonstrate.

Remember that any time the air is moving quickly, the air pressure around the object is reduced due to the velocity component. If one is fixed on only the stagnation pressure, then you deserve a trip to a good calibration lab to witness first hand the pressure reducing effects of high air velocity.

The Bernoulli Effect is at work here and can be demonstrated with a straw and a piece of paper. Hold the paper on one edge and let it hang towards the floor under it's own weight. Now bring the straw close to the paper. About 1/8 of an inch should do it. Now blow smoothly through the straw and notice which way the paper moves. Does it move away or closer?

If you said closer then blow a little harder. If done properly the paper will be pulled toward the paper almost every time. The change in pressure due to the high velocity air causes the paper to move toward the straw.

Airplane wings create lift using the same phenomena.

There is *stagnation pressure* (read high) at the front of the wing and *vacuum pressure* on the top (read low).

Hope this helps.

Jim.

[winkosmosis]

Originally Posted by 3-Wheeler

Hi Wink,

Well, reading what it says from Wikipedia versus actually working with pitot tubes and calibrating them for air flow are two different things.

Yes, you are correct about the stagnation pressure, but if you take a close look *inside* the pitot tube you will be presented with a series of small drilled holes that measure the *dynamic* pressure, which is the velocity component of the air flow.

And like I've mentioned already, proper usage and calibration of pitot tubes requires a differential sensor designed to measure both pressure taps.

If I must tell you, we have such devices at work, and yes, I have calibrated them with precision sub-sonic nozzles, so everything is very straight-forward on the theory of the device and actual usage.

By the way, the pitot tube is only good at measuring air velocity, and is not a good *mass flow* device. It does not measure air molecules and their relative mass, only the velocity of the particles.

Pitot tubes are of a family of devices that output a signal velocity-squared to the actual air velocity. For example, if you flow the air at 100 feet per second, you will measure a differential pressure of let's say 10 inches water across the two pressure taps. If you increase the air stream to 200 feet per second, the differential pressure does not double, but instead goes up by the square of the air velocity, or to 40 inches water difference. Because of this, pitot tubes have only a 10:1 usable range for differential pressure transducers measuring a range of 100:1.

Jim.

Is this statement true or false?

The pressure inside the pitot tube is higher than atmospheric pressure because of the force of airflow pushing on the "bubble".

How about this statement?

The high pressure bubble inside the pitot tube exerts a rearward force on the device.

[3-Wheeler]

Originally Posted by winkosmosis

Is this statement true or false?

The pressure inside the pitot tube is higher than atmospheric pressure because of the force of airflow pushing on the "bubble".

How about this statement?

The high pressure bubble inside the pitot tube exerts a rearward force on the device.

Hi Wink,

It seems that only way you will see the whole picture is to find a local university and ask to visit the flow lab. You deserve a good first hand demonstration. You're obviously a "hands on" personality type and really should see it in person. If you were closer in proximity an invitation would be in order.

We've had many young engineers and coop students drop their jaws when we fire up the flow bench and demonstrate the effects I discussed above.

Jim.

[winkosmosis]

Originally Posted by 3-Wheeler

Hi Wink,

It seems that only way you will see the whole picture is to find a local university and ask to visit the flow lab. You deserve a good first hand demonstration. You're obviously a "hands on" personality type and really should see it in person. If you were closer in proximity an invitation would be in order.

We've had many young engineers and coop students drop their jaws when we fire up the flow bench and demonstrate the effects I discussed above.

Jim.

Is there a high pressure bubble inside the pitot tube created by the forward-facing inlet or not? Jesus Christ.... that's all I'm trying to establish, because aerohead swears up and down that the bubble at the base of a sharp hood/windshield transition doesn't apply rearward force to the car, and that somehow the front of a car experiences no resistance from the atmosphere.

[winkosmosis]

By the way, airplane wings don't create lift with the Bernoulli effect. That's one of those persistent myths. It sounds good when you read it in a textbook and when you use the paper strip demonstration, but it doesn't stand up to scrutiny.

Google "bernoulli vs newton" or "bernoulli myth" and you'll find a lot of articles and discussions. Planes generate lift by angling the airfoil.

Edit: Here's a link. http://www.johndcook.com/blog/2009/0...irplane-wings/

[aerohead]

Originally Posted by winkosmosis

This thread is turning into walls of text so I want to discuss you arguments one by one



Think of a simple example, the wing of a bird. How does a bird fly? By pushing against the air on the downstroke. The air is exerting a force opposite to the direction of motion because it has viscosity and resists movement through it.

How about the propeller? Picture in your mind a simple flat propeller blade. The leading edge slices into the viscous air, and pulls the airplane through the air. Why? Because the air is exerting a force on the blade. That force is both forward (relative to the airplane) and against the motion of the blade. Think of the leading surface of a car as the the leading surface of the propeller.


Similar to the propeller blade is the airplane wing... No, planes don't fly because of Bernoulli's Principle, that's one of those myths that gets repeated even in textbooks.

75% of the lift on a wing comes from the top, and 25% on the bottom according to the FAA. If you were right about no force being applied by air to the leading sloped surfaces of a car, then the bottom of the wing would provide 0% of the lift.

How an Airplane Wing REALLY Generates Lift - Associated Content from Yahoo! - associatedcontent.com

I attended the lecture/seminar on Newtonian Aerodynamics by Craig(?) at Oshkosh in 1997 and own his book.He makes some strong arguments.
As to Daniel Bernoulli,his physics is quite adequate to design perfectly serviceable aircraft.All CFD code incorporates his theorem.Wind tunnels are now being supplanted by CFD because of these codes.The Navier-Stokes Equations of 3-D spherical coordinate system used today in CFD fully embrace Bernoulli and I'd be reluctant to just write him off.
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The thing about aeronautics and wings,is that they do not apply to automobiles.It's improper to reference them.

[aerohead]

Originally Posted by winkosmosis

That's was my whole point. The inside of the tube that points into airflow is higher pressure than the surrounding air, not lower.

Pressure - Wikipedia, the free encyclopedia

Stagnation pressure is the pressure a fluid exerts when it is forced to stop moving. Consequently, although a fluid moving at higher speed will have a lower static pressure, it may have a higher stagnation pressure when forced to a standstill. Static pressure and stagnation pressure are related by the Mach number of the fluid. In addition, there can be differences in pressure due to differences in the elevation (height) of the fluid. See Bernoulli's equation (note: Bernoulli's equation only applies for incompressible, inviscid flow).

The pressure of a moving fluid can be measured using a Pitot tube, or one of its variations such as a Kiel probe or Cobra probe, connected to a manometer. Depending on where the inlet holes are located on the probe, it can measure static pressure or stagnation pressures.

*Stagnation pressure is a consequence of the vehicle attacking a stationary air mass or vis-versa in a wind tunnel.
*The stagnation pressure contains the local atmospheric component and the 'ram' component.
*The 'static' portion from the probe,when subtracted from ( hooked to the other side of the manometer) leaves the 'velocity' pressure,the indication of airspeed.
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* Inviscid,incompressible flow is what we have outside the boundary layer of road vehicles.

[aerohead]

Originally Posted by winkosmosis

Is this statement true or false?

The pressure inside the pitot tube is higher than atmospheric pressure because of the force of airflow pushing on the "bubble".

How about this statement?

The high pressure bubble inside the pitot tube exerts a rearward force on the device.

*The pressure inside the ram portion of the Pitot-tube is the atmospheric pressure plus the dynamic pressure.
* The dynamic pressure applies the force which lifts the working fluid inside the manometer or deforms the helix within the magnehelic.
*At 420 mph,Goldenrod was indicating around 11.22 "Hg inside its Bauman scoops.

[winkosmosis]

Originally Posted by aerohead

*The pressure inside the ram portion of the Pitot-tube is the atmospheric pressure plus the dynamic pressure.
* The dynamic pressure applies the force which lifts the working fluid inside the manometer or deforms the helix within the magnehelic.
*At 420 mph,Goldenrod was indicating around 11.22 "Hg inside its Bauman scoops.

Exactly. The airflow is applying a force to the bubble in the tube, just like airflow applies a force to the front of the vehicle and the windshield bubble.

[aerohead]

Originally Posted by winkosmosis

Exactly. The airflow is applying a force to the bubble in the tube, just like airflow applies a force to the front of the vehicle and the windshield bubble.

*The vehicle is attacking the air mass in front of it and this onset flow is impinging upon the air captured within the pitot and connecting tubing to create the dynamic head.
* While it is true that the the onset flow creates the stagnation pressure at nose and windscreen,it is the pressure differential between the forward stagnation point,and the base pressure of the wake which creates the 'force' the vehicle must overcome.
* The bubble ( which is barely distinguishable over the windshield) can be thought of as a solid structure as far as the air is concerned.
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* With respect to the 1999 GM PNGV Precept,the 'features-drag' which included wipers,wheel covers,mirrors,door handles,and cut lines added a total 0.007 to the Cd.The wiper drag was zero.