Explaining the Viscosity and Friction Effect

[username]

Did some research on the net and found most information saying it is caused by viscosity and friction, I can understand that in stuff like water but I can't really see how that would hold true for air, could somebody explain it to me or point me to a definitive explanation of the effect?

 

[Tyger]

It's caused by laminar flow

which gases display too. The viscosity of a gas is (almost) independent of the pressure, which is why they are compressed for sending through piplelines. J. C. Maxwell first proved that fact.

 

[ccrummer]

Airfoil lift is caused by a pressure differential between the top and bottom of the airfoil. There are various ways to accomplish this but they fall into two basic classes: 1) increasing the pressure on the bottom of the airfoil, and 2) decreasing the pressure on the top of the airfoil. The Coanda effect is produced as a result of the second. As the viscous air shears past the stationary boundary layer of a surface curving away from the main flow, the pressure at the surface drops and a smoke streamer is "attracted" toward the surface. I have a paper on ArXiV that may shed some light on this. http://arxiv.org/abs/nlin.CD/0507032 . Beware mathematical models that assume zero viscosity for air. That assumption may be valid under some conditions but not when the flow is moving past a surface. Read about "steady" flow and the Navier-Stokes equations, Bernoulli's equation. The concepts are subtle but very interesting. A good reference is Fluid Dynamics by Landau and Lifgarbagez.

 

[FireBones]

Airfoil lift is caused by a pressure differential between the top and bottom of the airfoil. There are various ways to accomplish this but they fall into two basic classes: 1) increasing the pressure on the bottom of the airfoil, and 2) decreasing the pressure on the top of the airfoil. The Coanda effect is produced as a result of the second. As the viscous air shears past the stationary boundary layer of a surface curving away from the main flow, the pressure at the surface drops and a smoke streamer is "attracted" toward the surface. I have a paper on ArXiV that may shed some light on this. http://arxiv.org/abs/nlin.CD/0507032 . Beware mathematical models that assume zero viscosity for air. That assumption may be valid under some conditions but not when the flow is moving past a surface. Read about "steady" flow and the Navier-Stokes equations, Bernoulli's equation. The concepts are subtle but very interesting. A good reference is Fluid Dynamics by Landau and Lifgarbagez.

I appreciate the link to your paper on ArXiV...I'm a fan of trying to understand fluid dynamics from particle models.

I have one question, though...you mention that the curving happens when "viscous air shears past the stationary boundary layer of a surface curving away..." but how does the viscosity factor into this? In principle, there is a wake caused by this curvature regardless of the viscosity of the fluid...and this would (naively) cause a low pressure area that would cause the streamline to deflect regardless.

So, how does the viscosity itself help air to bend around the curve? I'm actually just as interested in the bending of the boundary layer, and I've asked a problem recently on this exact topic in the physics forum: https://www.physicsforums.com/showthread.php?t=364065" .

I would appreciate it if you could shine some light on how viscosity (thought of as diffusion of momentum between boundary layers) actually causes this effect (or, equivalently, the pressure gradient, etc.). In that thread I have a .jpeg posted comparing viscous and inviscid flow, and I would really appreciate better understanding how the viscosity allows laminar deflection (or the pressure gradient which causes the same). I've posted one idea I had in that thread in an effort at making the connection.

 

[ccrummer]

Viscosity is a number that is supposed to capture on the macro level the behavior of a fluid as it interacts at the micro level with itself, another fluid or a surface. In fact, of course, the behavior of the fluid is really very complex, with eddies and turbulence, as its particles scatter off one another and/or the surface. There are diagrams, Figures 2, 3 and 6, in http://arxiv.org/abs/nlin.CD/0507032 that illustrate the mechanism causing viscosity and some of its effects.

Figure 3 shows the "blowing of the boundary layer" that causes the low pressure on the surface. Particles of the main flow that encounter the boundary layer particles, which are more or less dragged along by the surface, strike those particles; they interact. No viscosity, no blowing of the boundary layer and no Coanda effect. Figure 6 shows the particle-particle interaction that results in a vortex behind a barrier.

The Coanda effect is not the only thing connected with lift, just for small angles of attack the the most important. For high angles of attack the impingement of the fluid on the bottom of the air foil is more important and indeed as the wing stalls the Coanda effect can morph into an increase of the pressure on the top of the wing.

When viscosity is important, Bernoulli's equation doesn't hold. I'm saying that if there is a wake it is due to particles interacting with each other and with the surface; viscosity. Look long and hard at some of Prandtl's photographs, e.g. in Gersten & Schlichting.

Thanks for your questions.

 

[ScienceGuyOrg]

No doubt you have seen the classic simple demo of the Coanda Effect of holding the back of a spoon under running water and feeling the back of the spoon pull towards the running water.

Bill Kuhl

 

[ccrummer]

The water stream adhering to the back of a spoon is cited as an example of the Coanda effect but when a liquid is in contact with a surface, Van der Waals forces account for the attraction. In the case of a stream of gas, however, the Van der Waals forces play no role in the attraction. It is the interaction of the gas with itself and with the surface that causes the pressure there to drop.

 

[ScienceGuyOrg]

Interesting, I just did a Google search on Van der Waals. I often find there is so much conflicting information relating to aerodynamics.

 

[ccrummer]

I was amazed at the confusion too. I've written a paper about this at http://arxiv.org/abs/nlin/0507032 . I think part of the problem is that the beautiful mathematics that describes steady flow makes it hard to give up for regimes of non-steady flow. (Just a conjecture.) Another problem is that in the absence of a computer model of the physics of the situation, in contrast to a computer computation model, "authorities" arise who are forceful in their dogmatic statements but who really don't understand the physics at the level of Newton's laws that govern the particle behavior. I'm not saying that I do completely but I think it is helpful to admit it. I would enjoy discussing this with you.

 

[ScienceGuyOrg]

I have started reading your paper and will continue to read it. My only qualifications in aerodynamics are from building model airplanes and reading "Model Aircraft Aerodynamics" by Martin Simmons.

Bill Kuhl

 

[Firefox123]

I was amazed at the confusion too. I've written a paper about this at http://arxiv.org/abs/nlin/0507032 . I think part of the problem is that the beautiful mathematics that describes steady flow makes it hard to give up for regimes of non-steady flow. (Just a conjecture.) Another problem is that in the absence of a computer model of the physics of the situation, in contrast to a computer computation model, "authorities" arise who are forceful in their dogmatic statements but who really don't understand the physics at the level of Newton's laws that govern the particle behavior. I'm not saying that I do completely but I think it is helpful to admit it. I would enjoy discussing this with you.

I wish more people were like you...admitting that they do not completely understand something, but are making an honest attempt at explaining it...

I browsed the paper you referenced...it looks like an excellent read...I will be reading it later on this week.

Thanks for the link.

 

[ccrummer]

Thanks for reading the paper. The lift calculations are purely formal so don't worry about deciphering them. They are mainly to show that from first principles it's not easy. If you have questions, please let me know.

 

[Firefox123]

Thanks for reading the paper. The lift calculations are purely formal so don't worry about deciphering them. They are mainly to show that from first principles it's not easy. If you have questions, please let me know.

Well my background is in electrical engineering so it will take me considerable time to go through the paper in depth...I likely will not understand all of the equations on the first read through.

But I will ask you whatever I do not understand after a few times through it...

I like the fact that you are attempting to take a particle approach with it...