bernoulli's principal

[Roj]

Imagine the following hypothetical device:

Take 2 "frisbees" (toy flying discs).

Stack them on top of one another.
The two discs touch - there is no air between them.
However, you have places a perfect lubricant between the 2 hence there is no friction between them.

To keep things simple, lets omit gravity also - so you allow the device to hang weightless in the air.

Spin the top disc only. ie there is now airflow over one surface of the device but no airflow on its other surface.

Now according to Bernoulli, will this device generate lift?
I am not sure what the answer is to this one and would be keen for anyone's thoughts.

[vintage1]

No.

The airflow is radial, and the air will not be deviated from its normal stat as there is no 'hump' in the direction of flow.

[Brandano]

I agree with vintage, but in a non idealized fluid the viscosity will drag the air along the disk, which will move sideways due to centrifugal forces. Other air will move in the low pressure area created in the middle to replace the amount that was displaced, and this will generate a small amount of lift. Naturally, the disk on the bottom, having no anti-torque provisions will spin in a direction opposite to the one on the top, and will generate opposite lift, so unless the disk is stuck onto something you won't be able to measure what lift is produced. It's in any case a very small amount. Essentially the rotating disks would act like the rotors in a Tesla turbine.

[Roj]

No.

The airflow is radial, and the air will not be deviated from its normal stat as there is no 'hump' in the direction of flow.

What if we moulded ripples onto the disk, airfoil shaped for the direction of spin, fanning out radially?

[Brandano]

you don't get lift just because air is moving over a surface, it has to be forced to curve. I know you want to debunk Bernoulli by proving that the pressure differences don't make lift but the airflow deflection does, but the problem is that the pressure difference causes the airflow to deflect and that the airflow deflection leaves a pressure difference... they are al a single system that can explain lift in several ways. at the end of the day, air moves down, the plane moves up (ok, stays up, but only because there's lift keeping it down) so there you have it. It's not Bernoulli, it's dear old Newton... ah, if life was so simple. Anyway, no, even if the disk was covered with ripples, and assuming the air won't accelerate away to the sides radially as I earlier said it will, the low pressure areas on the top of the ripples will be balanced by the higher pressure areas underneath the ripples and you won't have lift. Now, if we opened some holes where the high pressure is so that it will move underneath the disk... oh, that's a prop!

[Roj]

you don't get lift just because air is moving over a surface, it has to be forced to curve. I know you want to debunk Bernoulli by proving that the pressure differences don't make lift but the airflow deflection does, but the problem is that the pressure difference causes the airflow to deflect and that the airflow deflection leaves a pressure difference... they are al a single system that can explain lift in several ways. at the end of the day, air moves down, the plane moves up (ok, stays up, but only because there's lift keeping it down) so there you have it. It's not Bernoulli, it's dear old Newton... ah, if life was so simple. Anyway, no, even if the disk was covered with ripples, and assuming the air won't accelerate away to the sides radially as I earlier said it will, the low pressure areas on the top of the ripples will be balanced by the higher pressure areas underneath the ripples and you won't have lift. Now, if we opened some holes where the high pressure is so that it will move underneath the disk... oh, that's a prop!

Sure, i agree.
I think the whole Bernoulli thing is mistaught.
I never recall it being taught with the caveat that airflow must follow a concave curve but this makes sense.

[Brandano]

It's not that it has to "follow a curve" as such, but the Bernoulli principle is just a principle of conservation of energy applied to a gas. If you force a gas through a restriction, the pressure lowers and the speed increases, if you force it through a widening the pressure raises and the speed lowers. There is no argument whether this happens or not, it has been proved to happen. The attempt to explain lift through Bernoulli considers the top portion of the wing as a restriction to the airflow. This is however a flawed oversimplification, for the purposes of explaining lift through the Bernoulli principle alone you shouldn't just consider the wing shape, but also a large mass of air in the immediate surrounding of the wing that acts as a restriction to the airflow. also, the distribution of speed and pressure around the wing is too complex for the schoolbook "lie to children" approach showing the arrows flowing above and below a flat bottom airfoil.

[HX3D014]

I would have pointed out that the rotations of the Frisbee has a primary effect of maintaining attitude through gyroscopic precision.

the Frisbee as mentioned needs to have a Velocity through the air. and in the case of a Frisbee. it may need a AoA.

I like the point that the Bernoulli's principal has been miss taught to some degrees. there are facts about the principal. but the thing is. the pressure difference is the reason why the wing creates lift.

there are two main types of pressure. Static and Kinetic. if you want to use the static pressure. you need to remove its effect from one side. thus creating an imbalance.

So
without being exact or anywhere near exact. I will just make a guess at stuff as far as perfect # are concerned.

the air has a static pressure of 1013 units at sea level
if we increase the pressure at the bottom of the wing to say 1113 and reduced the pressure above the wing to 913 units then there is a difference of 200 units.

Now, the pressure units I used are wrong. I know they are wrong because I pulled them from my a^%. and the ratio is also wrong for the same reason.

but it highlights the fact that its not just the pressure drop and its not just the pressure increase, its the difference between the two.

Bryce.

PS, anyone know the exact # top and bottom ?
Oops :D

[Yukonho]

Well, I cant resist.... Bernoulli's Principal was probably an older man who's name remains forgotten. It was his father who taught young Daniel mathematics, but I am reasonably sure that Daniel Bernoulli came up with "Bernoulli's Principle" on his own.
Colin

[Texas Buzzard]

Ah yes, who was Berounlli's Principal? I don't thimk we know. :)

[podavis]

Going back to the original simplistic thought experiment the only air velocity is velocity relative to the spinning disc. The air above the disc has no relative velocity to the global air mass since the experiment does not specify viscous friction between the air and disk. Bernoulli's equations deals with the relationship between pressure and velocity in a continous air mass system and there is no change of velocity in the air mass system.

If the experiment assumes viscous friction then there might be an action acting on the air similar to a radial flow turbine disc. There would be suction above the center of the disc located at the impeller's 'inlet' which would pull upward on the top disc, but the downwardly moving inlet airflow hits the disc to change its direction to a mixed radial and tangental direction and this causes a static pressure which causes a downward directed force. So now the question is 'do the two forces cancel out'?

[macboffin]

Bernoulli was born in 1700, died 1782. His theories and principle were concerned with the flow of liquids through pipes of varying diameters at varying pressures and velocities. Flying, and theories about the flow over airfoils, came somewhat later!.

[Wendi Smol]

Bernoulli was born in 1700, died 1782. His theories and principle were concerned with the flow of liquids through pipes of varying diameters at varying pressures and velocities.

Bernoulli formulated some basic and important laws on flow of gases in general terms. He invented the streamline and the stream tube for the purpose of visualisation and explanation. The laws also are applicable to flow in pipes and parts thereof, like bends venturies and diffusers.
The guy who dealt with flow through pipes exclusively and discovered laminar and turbulent flow was Osborne Reynolds, a US citizen (1842 -1912).

According to the laws of fluid dynamics there are basically only two ways to produce lift: 1. by wings 2. by fluid jets. All other ideas have been shown not to be suitable for that job.

W.

[Wendi Smol]

sorry, double posting

W.

[VeeOneRotate]

oops, sorry

[nmasters]

As I understand it Bernoulli's Principle is just a statement that the sum of potential energy (pressure) and kinetic energy (speed) remains the same unless acted upon by an outside force. Since both the math and the actual physics are reversible it doesn't really help with a cause and effect explanation of lift. All this endless argument about poor ol Danial Bernoulli is the result of misinformed ignorance not informed doubt. I blame Theodore von Kármán (http://www.rcgroups.com/forums/showthread.php?p=11344822&highlight=+K%E1rm%E1n#post11344822)

--Norm, donning his flame proof suit