Boundary Layer
06-22-2011, 06:25 AM
#11
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Joined APC: Oct 2008
Position: CFI
Posts: 32
Originally Posted by Cubdriver
Aerodynamics for Naval Aviators is a good starting point without getting overly mathy. US Navy originally published it, and it is currently available in reprint by ASA. I'll come back and give your other questions some thought in a week or so if no one does.
If these three statements are not replaceable because their logic is so faulty then don't bother, I'll understand. I imagine it'd take quite a bit of time to set them straight.
Originally Posted by nciflyer
*When the air first meets the wing leading edge it must accelerate. When it reaches it's maximum velocity the boundary layer is thinned and hence the smallest restriction to go through. This can sometimes be about 1/3 to 1/2 of the chord back from the leading edge.
*After this max velocity point above the wing the boundary layer will get thicker in general and the air must slow down because there is less of a restriction for the air.
*So this decrease in velocity means kinetic energy is reduced and potential is increased. We don't want this because we want to keep as much kinetic energy as close to the wing as possible....
*After this max velocity point above the wing the boundary layer will get thicker in general and the air must slow down because there is less of a restriction for the air.
*So this decrease in velocity means kinetic energy is reduced and potential is increased. We don't want this because we want to keep as much kinetic energy as close to the wing as possible....
06-22-2011, 05:11 PM
#12
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Joined APC: May 2006
Position: ATP, CFI etc.
Posts: 6,056
Well in all honesty, the first two make no sense. The third one could be construed to make sense possibly, depending on what you actually mean.
Let's back up a bit. I would rather see you show a basic understanding of lift without boundary layer theory before tackling boundary layer theory. Let's talk about lift first. Here's a simple article I wrote a few years ago.
Lift
Part One
by Cubdriver
What causes lift? Most will quickly say it is due to Bernoulli’s Law. But can we just add up the pressure differences from top to bottom on our wing arrive at the correct lift? Well sure but what really caused those pressure differences? Bernoulli’s Law alone, maybe because the air parcels need to meet back up at the back of the wing at the same time? Action and reaction per Newton, perhaps like little bullets impacting the bottom of the wing? There’s more to the story than this and by the way, both of these theories are wrong, as is the one about the wing being a huge one-sided venturi. Yes, the air above the wing goes faster and Bernoulli is part of the reason, but Bernoulli’s law comes no where near to the whole story because it can be shown that Bernoulli’s Law alone cannot cause the pressure difference we know are there. So what else is it? Bernoulli plus Newton’s Law, maybe? Newtonian physics might say air is being thrown down as it passes the wing and this means you have action and reaction. But why is it being thrown anywhere? Pressure maybe? That’s all certainly true, but again you have a gaping hole in your theory. Newton and Bernoulli alone cannot explain lift.
“Circulation” is what you are missing. Without it you will never be able to account for the lift. So what is circulation? Think of it as an additional flow added to the linear (straight) air flow we already know is there. It’s an imaginary flow on the theoretical level, a way to account for what we know is happening around the wing, what we can measure and if there is smoke, see. And maybe that’s why most people tend to dismiss it. They want real, tangible, simple facts. But the effects of circulation are real. So it does exist. Circulation is an abstract concept that says if you have lift, then it is proportional to the amount of circulation around the wing. What do you mean, circulation around a wing? It is both a mathematical idea and a physical idea. Mathematically we must add a clockwise flow to the linear flow, one plus the other, to get the effect of both parts. The circulation “kicks” the top air along a little faster and it slows down the air on the bottom side of the wing a little bit too, since it only turns on one direction. Namely, clockwise if you are looking at the left wing of an airplane traveling to the left. It also gives drag through the action of the vortex spirals that must leave the wing due to its presence. So we know circulation exists and we know it works.
Air must leave the trailing edge of a wing because it cannot come around the bottom at the rear. Simply can’t do it. This is why the top air is faster and the bottom air is slower. Little packets or parcels of air must decide whether they are going over top or under bottom but they do not have to both arrive at the trailing edge at the same time. The top air gets there first correct, and although they do not meet parcel for parcel, when the bottom air gets there it is going slower and the combination of these two speeds produces a vortex or spinning twirl. It’s just like water going down a tub drain. Slower mixes with faster and spinning is introduced at the drain. This spin is circulation around a moving central axis.
But what about the physical aspects of circulation? Can we see them? Yes you can, and not just in the vortex trails. This is where circulation meets Bernoulli meets Newton. All 3 aspects happen at the same time, they are one and the same physical process. It is a complex thing to grasp. Let’s talk about the Newton part a little more though. When an air parcel decides whether to go under or over a wing, the decision is made based on how much pressure is felt by the air parcel. It wants to go to the low pressure zone, high to low. But when the wing sets up a low pressure zone more or less in front of the wing because of angle of attack, circulation is established because it suddenly has to lunge forward around the leading edge of the wing to get to the top. To do this, it absorbs energy in the form of centrifugal force. It sucks forward against the wing and the wing pulls backward. The air turns toward the front of the wing to go around the leading edge while seeking low pressure. Bear in mind we are talking about 3 simultaneous effects that make up one complete, continuous behavior, and no particular one can be identified as being the cause and no particular one can be named as the effect of the other two. They all happen at one time, they are one and the same thing, and there is no “first this, then that” chain of causality.
We are not quite done explaining the physical action of circulation. Now the air is on top and the additive part of the clockwise flow when viewed on an airplane that is traveling to our left occurs. You might say ok, fine the air circulates around the front of the wing then why does it not just circulate around the back side and add back to the loss of speed felt on the bottom side? Why do we need these vortices shedding from the back? This is because the friction of the air is such that a sharp trailing edge will not allow the kind of energy levels air needs to make the turn back to the bottom around the edge. A vortex is born and the wing sheds the vortex to the rear. The vortex then drags the wing from behind.
Finally, you can’t say it is the momentum only, the pressure only, or the circulation only. It is possible to calculate lift based on any of these things, but it depends on where you measure these non-separated aspects. For example, at the surface of the wing you feel only pressure differences, right? There is no flow “through” a wing, only around it. But if you do the math a few feet away from the wing, you will find momentum and circulation are movers and shakers. So if you want to measure using only pressure you can do so, but you will not be able to find the right number using only Bernoulli.
Let's back up a bit. I would rather see you show a basic understanding of lift without boundary layer theory before tackling boundary layer theory. Let's talk about lift first. Here's a simple article I wrote a few years ago.
Lift
Part One
by Cubdriver
What causes lift? Most will quickly say it is due to Bernoulli’s Law. But can we just add up the pressure differences from top to bottom on our wing arrive at the correct lift? Well sure but what really caused those pressure differences? Bernoulli’s Law alone, maybe because the air parcels need to meet back up at the back of the wing at the same time? Action and reaction per Newton, perhaps like little bullets impacting the bottom of the wing? There’s more to the story than this and by the way, both of these theories are wrong, as is the one about the wing being a huge one-sided venturi. Yes, the air above the wing goes faster and Bernoulli is part of the reason, but Bernoulli’s law comes no where near to the whole story because it can be shown that Bernoulli’s Law alone cannot cause the pressure difference we know are there. So what else is it? Bernoulli plus Newton’s Law, maybe? Newtonian physics might say air is being thrown down as it passes the wing and this means you have action and reaction. But why is it being thrown anywhere? Pressure maybe? That’s all certainly true, but again you have a gaping hole in your theory. Newton and Bernoulli alone cannot explain lift.
“Circulation” is what you are missing. Without it you will never be able to account for the lift. So what is circulation? Think of it as an additional flow added to the linear (straight) air flow we already know is there. It’s an imaginary flow on the theoretical level, a way to account for what we know is happening around the wing, what we can measure and if there is smoke, see. And maybe that’s why most people tend to dismiss it. They want real, tangible, simple facts. But the effects of circulation are real. So it does exist. Circulation is an abstract concept that says if you have lift, then it is proportional to the amount of circulation around the wing. What do you mean, circulation around a wing? It is both a mathematical idea and a physical idea. Mathematically we must add a clockwise flow to the linear flow, one plus the other, to get the effect of both parts. The circulation “kicks” the top air along a little faster and it slows down the air on the bottom side of the wing a little bit too, since it only turns on one direction. Namely, clockwise if you are looking at the left wing of an airplane traveling to the left. It also gives drag through the action of the vortex spirals that must leave the wing due to its presence. So we know circulation exists and we know it works.
Air must leave the trailing edge of a wing because it cannot come around the bottom at the rear. Simply can’t do it. This is why the top air is faster and the bottom air is slower. Little packets or parcels of air must decide whether they are going over top or under bottom but they do not have to both arrive at the trailing edge at the same time. The top air gets there first correct, and although they do not meet parcel for parcel, when the bottom air gets there it is going slower and the combination of these two speeds produces a vortex or spinning twirl. It’s just like water going down a tub drain. Slower mixes with faster and spinning is introduced at the drain. This spin is circulation around a moving central axis.
But what about the physical aspects of circulation? Can we see them? Yes you can, and not just in the vortex trails. This is where circulation meets Bernoulli meets Newton. All 3 aspects happen at the same time, they are one and the same physical process. It is a complex thing to grasp. Let’s talk about the Newton part a little more though. When an air parcel decides whether to go under or over a wing, the decision is made based on how much pressure is felt by the air parcel. It wants to go to the low pressure zone, high to low. But when the wing sets up a low pressure zone more or less in front of the wing because of angle of attack, circulation is established because it suddenly has to lunge forward around the leading edge of the wing to get to the top. To do this, it absorbs energy in the form of centrifugal force. It sucks forward against the wing and the wing pulls backward. The air turns toward the front of the wing to go around the leading edge while seeking low pressure. Bear in mind we are talking about 3 simultaneous effects that make up one complete, continuous behavior, and no particular one can be identified as being the cause and no particular one can be named as the effect of the other two. They all happen at one time, they are one and the same thing, and there is no “first this, then that” chain of causality.
We are not quite done explaining the physical action of circulation. Now the air is on top and the additive part of the clockwise flow when viewed on an airplane that is traveling to our left occurs. You might say ok, fine the air circulates around the front of the wing then why does it not just circulate around the back side and add back to the loss of speed felt on the bottom side? Why do we need these vortices shedding from the back? This is because the friction of the air is such that a sharp trailing edge will not allow the kind of energy levels air needs to make the turn back to the bottom around the edge. A vortex is born and the wing sheds the vortex to the rear. The vortex then drags the wing from behind.
- angle of attack makes the air have to make the turn around the front of the wing
- circulation is set up, which adds to the speeds on top and subtracts from the bottom speeds
- this leads to the pressure differences top to bottom
- this connects with the acceleration of the mass or momentum changes in the flow, inasmuch as flow leaving the wing at the back goes downward creating a force balance on the wing through the momentum of the air as it changes direction
Finally, you can’t say it is the momentum only, the pressure only, or the circulation only. It is possible to calculate lift based on any of these things, but it depends on where you measure these non-separated aspects. For example, at the surface of the wing you feel only pressure differences, right? There is no flow “through” a wing, only around it. But if you do the math a few feet away from the wing, you will find momentum and circulation are movers and shakers. So if you want to measure using only pressure you can do so, but you will not be able to find the right number using only Bernoulli.
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