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Popular Delusions about Bernoulli's Principle

Demonstrations Illustrate Radial Momentum Instead

 

by Ed Seykota, 1999

 

Textbooks and Internet Bernoulli sites present many experiments to demonstrate Bernoulli's Principle. These experiments rarely demonstrate Bernoulli's Principle at all. They demonstrate Radial Momentum.

As high school students must learn, the standard explanation for lift is Bernoulli's Principle; high velocity means low pressure. While Bernoulli's Principle is fundamentally correct, the application of it to explain lift is fundamentally incorrect. The real principle behind lift is Radial Momentum. Ironically, Bernoulli's principle does not even explain the devices commonly used to demonstrate the Bernoulli Principle itself.

The Radial Momentum theory explains all of the experimental observations of pressure decrease in open systems with flow motivation. It explains atomizers, airfoils, airplanes, curve balls, lift, cavitation, eddy currents and a full range of behavior involving pressure decrease.

Bernoulli's Principle does not explain the experimental observations, particularly those differentiating between radial flow and non-radial flow in open systems with flow motivation. Bernoulli's Principle is widely misapplied. Ironically, Bernoulli's Principle does not explain the experiments commonly used to demonstrate the Bernoulli Principle.

 

Flying Paper

  

Note: This experiment comes, complete with Bernoulli misapplication, from NASA.

Flying Paper

Materials: 2" x 8" strip of paper & paper clips

Experiment: Place the strip of paper so that one end is resting just under your lower lip and the other end is hanging down. Holding it in place, blow over the top of the paper. The end of the strip of paper hanging down will lift up so that it is "flying." Try placing paper clips on the hanging end of the paper and see if it has enough lift to overcome their weight.

Explanation: With the increased air speed over the top of the paper, the air pressure over the top of the paper is decreased (Bernoulli's Principle). This makes it so that the paper has higher air pressure underneath it, lifting it into the air.

Bernoulli Faster flowing air over the top of the strip of paper causes lift and the paper rises
Radial Momentum I hold a strip of paper against my bottom lip and blow air across the top. Radial Momentum induces a region of low pressure above the strip and the higher pressure under the strip elevates it.

 

 

Floating Tennis Ball

  

Bernoulli Faster air inside the cone keeps the ball inside the cone
Radial Momentum The effluent stream from a vacuum cleaner is seen to hold a ball aloft, even if the air stream is tilted somewhat off true vertical, in which case the ball may spin rapidly. Radial Momentum induces a region of low pressure inside the cone that keeps the ball inside the cone. Higher air velocity at the center of the cone spins the ball.

 

 

Atomizer

  

Bernoulli Faster air over the nozzle sucks the perfume up the tube.
Radial Momentum In a perfume atomizer, the air supply from a squeeze ball, flows over a delivery tube. Radial Momentum induces a cone of low pressure that acts to elevate the perfume into the flow. Atomizers work with an air supply (that expands radially) and do not work with a liquid supply (that does not). An atomizer nozzle that entrains flat and expanding flow is more efficient.

 

 

Airplane Wing

Bernoulli

Classic Misconception

This wing cross-section diagram appears in numerous textbooks and on countless web sites. It accompanies the claim that Bernoulli's Principle explains lift, in terms of higher velocity over the top of the wing causing lower pressure. 

Does Bernoulli's Principle explain lift? Not at all! The real reason for lift (aside from angle of attack) is Radial Momentum. The classic drawing to the left is inapplicable to lift and is downright misleading.

Faster air over the top of the wing induces lift. The airplane wing receives extensive misapplication of Bernoulli's Principle to explain lift as a function of high air velocity. This is simply not the case; Radial Momentum, not velocity, induces low pressure. To try to make Bernoulli's Principle work, the literature is replete with a wide range of rather inventive add-ons, patches and upgrades such as circulation and induced lift. These rely on unsupported hypotheses about the behavior of air such as (1) air circulates around a wing against the wind and (2) air speeds up to reunify with partner molecules separated by the leading edge of the wing.
Radial Momentum An airplane wing may develop a cone of Radial Momentum over the shoulder of the wing. The lower pressure may induce eddy currents, cavitation, separation and turbulence. Lift is primarily a function of angle attack, not of so-called Bernoulli lift or synthetic circulation. The main effect of curvature is to entrain laminar flow, reduce turbulence, and conserve energy which indirectly contributes to lift.

Radial Momentum ... The Real Explanation 

Air, striking the front of the wing, deflects upward, actually pushing the front of the wing down. It then continues upward by momentum. When it encounters the curvature on the top of the wing, it fans out. This results in lower air density, and lower pressure ... however, this effect occurs only behind the crest of the wing ... and it has very little to do with lift.

In actual practice, the net effect of wing curvature on lift is small; most of the lift comes from angle of attack; the curvature mostly helps train the air off the back edge of the wing ... so that less energy disappears into turbulence and more energy goes to propulsion.

Airplane Wings

Airplane wings do not need to be curved on top to create lift. Lift has to do with the angle of attack as you can easily verify with your hand out a car window. The curvature of a wing is useful, however, in training the flow of air around the wing so it remains laminar and does not become turbulent at the trailing edge. Laminar flow is more efficient and conserves power. In this way, it provides more net power to the wing and hence, more power to lift the plane. It also provides more net power to the bathroom lights. So in that regard, curved wings also cause more photons.

Balsa wood gliders work very well with flat, warped and twisty wings. Some helicopter blades have equal curvature on both sides. Some planes have flat wings. Some planes fly upside-down. This all indicates that wings do not need humpy curvature to have lift.

To the extent that a wing has a curvature, the leading edge has a downward angle of attack and the trailing edge has an upward angle of attack. This creates a net forward rolling torque. In effect, an airplane with a curved wing would like to roll forwards and fly forward with the nose down, with the curved part of the wing into the wind.

Still, some scientists hold on to the notion of curvature = lift. Some even add curious theoretical patches to, well, to try to make it fly better. These include the principle of equal transit times, starting vortex, induction, circulation, and induced angle of attack. Gale M. Craig has a nice web site documenting induced induction and circulation of crazy theories.

Turbulence

Turbulence, an effect observed at high Reynolds numbers, may be induced by Radial Momentum. For example, the cone of vacuum induced at the shoulder of an airplane wing can induce eddy currents and provoke turbulence.

 

 

 

Baseball

Bernoulli The spin of the ball creates a relatively faster air flow over the top. Therefore the ball is supposed to rise ... actually it falls.
Radial Momentum The spin of the ball sets up a small region of Radial Momentum at about 7:00 on the diagram ... this pulls the ball downward. The spin also creates a pressure front at about 1:00 and this also acts to push the ball downward.