When an aerofoil starts from rest, flow at the trailing edge apparently needs to double back round it to join the rear stagnation point. However the atmosphere is not a superfluid, and so instead there is a flow separation at the trailing edge and a starting vortex is dumped in the flow. Vorticity of opposite rotation is bound up with the aerofoil by the principal of conservation of vorticity, and this generates lift by the Magnus effect. The starting vortex can be visualised, and a stopping vortex can be seen as well. If we alternately start and stop an aerofoil, a vortex street can be generated. The conservation of vorticity, or Kelvin's circulation theorem, is a topological principle. Fluid mechanics is a linear subject when we are talking about configurations of vorticity, so we can easily move back and forth between the Magnus effect and the Kutta-Joukowski circulation theorem. A starting or stopping vortex is shed whenever the flow is not complying with the Kutta condition. Explanations based on Bernoulli or Newton are merely wisdom after the event of the starting vortex being formed. They would not work with an inclined hydrofoil moving slowly through liquid helium.

@aerospacedoctor

Ай бұрын

Almost David, but this is not the Magnus effect, as nothing is rotating. It is just the result of a viscous fluid. Not sure why we have the need to give something a name. That is, the Kutta condition is added to inviscid flows to produce the same effect as having viscosity. Love the last statement!

@david_porthouse

Ай бұрын

@@aerospacedoctor Bits of the fluid in the aerofoil's boundary layers are rotating.

@aerospacedoctor

Ай бұрын

​@@david_porthouse David, that is still not the magnus effect. The aerofoil would need to be rotating for this to be the magnus effect (see the KFC bucket wing RC aircraft here on KZread). Hence why the magnus effect is applicable to spinning round artillery shells or spherical balls, and result in the curve ball effect seen in baseball etc. That is a viscous effect where the rotating body drags the fluid with it, diverging from the trajectory expected due to projectile motion alone.

Thank you for this! It's very easy to understand :)

@StickScience

2 жыл бұрын

Glad it was helpful!

I would point out wings are not “sucked” upwards by low pressure but pushed up (from below) by higher pressure.

Hi Guys - great video style - can you please tell me what software you used? I am interested in creating as well and looking for the best solution. THANKS

Bernoulli applies to the flow and pressure of a fluid that travel though a pipe of varying diameters. A sail is an airfoil that produces lift, yet it has exactly the same length on each side. Newton's laws of motion fully explain the pressure differential between the two side of the sail without any help from Bernoulli. And there is no "constriction" on the lee side of a sail.

@aerospacedoctor

Ай бұрын

Jeffrey you must have accidentally deleted your comment... oh no, let me reply again. Bernoulli is also applicable in external flows, it is literally derivable from Euler or the Navier-Stokes, people just don't understand when you can use it. Outside the boundary layer, and if gravity is not applicable, you can use it. If the flow starts with the same total pressure, and it does not experience viscous losses, you can track the change in static and dynamic pressure, and hence always relate a lower pressure to a faster flow speed, and a higher pressure to a lower flow speed. So, yes, if there is a pressure difference on the two sides of a sail, there is a difference in flow speed.

@annoyingbstard9407

12 күн бұрын

@@aerospacedoctor. Those are definitely words.

Wow. These are really good videos. Very Fascinating. Thanks for making them!

Hi there, I agree with the content of this video. Bernoulli and Newton are not in conflict and equal transit time is the fallacy. But I'd like to make a side point about 'pinching a garden hose'. It turns out the increased velocity is not an example of the Bernoulli Principle. I only found this out quite recently, and clearly the statement requires an explanation to support it, so here goes. Torricelli's Theorem (a form of the Bernoulli Principle) states the fluid velocity at the outlet of a barrel = root 2gh. The usual simplifying assumptions of inviscid, laminar flow apply. According to this equation, the only variable is “h”, so if you put a hose on the outlet and pinch the end, it makes no difference to velocity and that is the undeniable result of the Bernoulli Equation in this scenario - the velocity varies only with height (mass flow rate will nonetheless vary with outlet area). And yet we know the velocity from the hose actually increases when you pinch it. This inconsistency between theory and observation goes away when the real effect of turbulence in the outlet is accounted for. The bottom line is the increase of velocity from a pinched hose is not due to the Bernoulli effect but rather the opposite - it is due to what Bernoulli does not account for, namely turbulence, specifically the variation of turbulent losses as you pinch the hose and vary the mass flow rate. A brief explanation for this behavior is that the outlet, be it tap or flange, introduces turbulence which is a de facto 'head loss' so velocity out of the hose is less than predicted using Bernoulli alone (an empirical coefficient is applied to estimate real velocity). But turbulence would go to zero if mass flow went to zero. So, turbulence varies with flow rate hence jet velocity varies with pinching.

@SuperZardo

2 ай бұрын

You incorrectly apply Torricelli's Theorem to a garden hose. Torricelli's Theorem is about a thin-walled vessel with sharp edge holes in it, from where liquid sprouds, and assesses the velocity of the resulting jet coming out of those holes. Attaching a hose to such a hole totally invalidates Torricelli's Theorem. Its formula cannot be applied to the velocity of the water jet coming out of a hose.

@XPLAlN

2 ай бұрын

@@SuperZardo …sure it can. The only difference with a short length of hose is the additional small viscous loss in the hose section. But that is also the point of my original comment in that the reason the velocity speeds up as the hose is pinched is because losses in the outlet go down as flow rate goes down. Hence Bernoulli (or Toricelli) does not explain that behaviour.

Excellent! Thank you!

Well, you showed the area of low pressure, but forgot that the air travels further and expands back right behind this area (which should slow down the flow according the same bernoulli presumption). This is still happening above the wing surface and should oppositely create an area of higher pressure pushing the wing downward as well. Bernoulli's principle is not a good explanation of the lift. Theoretically if the bottom area had a small jet exhaust to make the wind below travel at the same speed as the flow above, the lift should not be produced anymore according the bernoulli's explanation. But is that true? I highly doubt. The main source of the lift produced above the airfoil is not the speed difference but the direction change caused by coanda effect or by pressure equalization followed by breaking the air by an obstacle... Theoretically an airfoil in long: (__________ shape should also create a lift (of course with a bad efficiency)...

@aerospacedoctor

Ай бұрын

No, the direction change is not responsible for main source of the lift. In 2D flow in a wind tunnel (like NASA used in the 30's and 40's) the flow returns to horizontal as required by the continuity equation (divergent free velocity). Since the front stagnation point is under the LE and the rear stagnation point is at the TE, the net effect is a pressure force pushing up. However, you are correct, and hence why aerofoils have a negative pitching moment, where the nose wants to point down. But we integrate the pressure coefficient from the LE to the TE, between the upper and lower surface to get the net force. Also, yes, if you invented a wing, that actively blew air from the bottom surface, you would destroy the lift. This is opposite the boundary layer control device used on the F-104 Starfighter, where you blow air from the top surface. So your doubt is incorrect. I wish people would stop using the Coanda effect, it is not involve in simple conventional lift (it is for the BLC device of the F-104).

@gravitomagneticpower

Ай бұрын

@@aerospacedoctor Mmmhm, I think the direction change is the only source of the lift especially for anything that has wings. It doesn't matter how you do it (if using deflection, bernoulli forces, any other pressure difference, coanda etc... or even magnetohydrodynamics, ion wind, rocket propulsion..., it doesn't matter) The only thing that creates the upward force is the principle of action and reaction. You need to accelerate somehow some mass of the air traveling downward to get a force that fights gravity upward. Of course the airships and baloons use buoyancy.

@aerospacedoctor

Ай бұрын

​@@gravitomagneticpower You use Newton's 2nd law when there is an acceleration. This is what the Navier-Stoke is, F=ma where F is the sum of forces, the pressure and viscosity, and a is the temporal and spatial acceleration. So, the effect of viscosity is to create a flow asymmetry due to the shed trailing edge vortex and the associated bound vortex (circulation). When you look at the remaining balance, the velocity field results in a pressure difference which acts directly on the surface of the wing. That is lift! There is no downwash for 2D flow, all good 2D wind tunnel flow visualisations will show flow that is returning to horizontal. The streamlines into the wind tunnel are horizontal and they must leave horizontal and colinear with themselves at the exit of the wind tunnel.

@gravitomagneticpower

Ай бұрын

@@aerospacedoctor But in an air tunnel. I would assume that the lines of course get back because they are kinda "springed" back by the boundaries of the tunnel no? An example could be a wing of a helicopter. It is the same kind of wing like an airplane has but doesn't travel linearly but it rotates. There could be no lift if there would be no stream of the air from up to down. If the helicopter only moved the air somehow to the sides, I would not argue but there is a strong blow underneath, obviously. That is the 3rd newtons law. When the airplane flies the stream is also curved down but in such a high speed the curvature is barely visible.

@aerospacedoctor

Ай бұрын

​@@gravitomagneticpower it is a misconception that the aerodynamics of fixed and rotor wings are "identical". Clearly they are related, and if you want to work exclusively in terms of pressure, the lowest pressure is in front of the rotor risk, and the highest pressure is behind, and the pressure difference times the area gives the lift (thrust for a prop). But working exclusively in terms of momentum is where there is a difference. Defining a control volume for a rotor is easy, and the momentum flux in and out is easy, but for an aerofoil, you cannot do this, there will always be some pressure or momentum term you cannot convert into the other to make it exclusively a pressure or momentum term for the flow. However, if you look at the surface of the wing (where the "reaction" force is acting), you can exclusively work in terms of pressure. Technically for an aircraft in the atmosphere you can also remove the momentum term and look at the over pressure on the earth's surface, if you have an infinite horizontal expanse, like we do for a spherical surface.

The bernulli principle doesn't cause lift. A pressure differential doesn't cause lift. A differential is lift. It's cause is deflected air. No deflection equals no lift.

transit time theory is also false in a tube like a pipe or hose the friction of the surface of the pipe induces a reverse current study chaos theory

If you haven't already check out how you can explain lift using Newton's 3rd Law of Motion. Right here: kzread.info/dash/bejne/aIajpNFsltjLZLQ.html