Boundary Layer
 

Could anyone please explain to me what the boundary layer of a wing is and what factors effect it?

thanks in advance

The wiki article is pretty good for a short summary but this is always a painfully abstract subject for those who do not worry about aerodynamics and prefer to leave it to the engineers. Boundary layers affect drag in a variety of ways. To bring home how significant this can be, think of a golf ball without dimples. It will not go half as far as one with, simply because the dimples force the air on the ball to remain turbulent which in turn energizes the flow field around the ball enough to affect its Reynolds number. Reynold's number is the ratio of viscous (syrupy) properties to inertial (mass times speed) in the flow. The higher the Re, the better the ability of the field to avoid separation from the body, thus minimizing drag.

It is sort of like "greasing" the air flow. Grease as we know is literally a viscous fluid, it sticks to things, but it acts like grease when placed in a wheel bearing which has the opposite effect. Same thing with vortex generators- they make the boundary layer turbulent on a tiny scale so the large scale flow is eased around the aerodynamic body. This makes the larger flow able to stay close to the aerodynamic body and do more useful work there.

An example. I was riding on a jungle jet yesterday near the wing in the passenger zone, and I saw it has VGs located upstream of the ailerons. The swept wing tends to experience lateral flow in addition to the normal (= 90 degrees) flow, and this lateral path tends to negate the usefulness of the ailerons. In turn, it is important to get the most out of the normal flow component and retain more aileron authority.

VGs are used a a tool for boundary layer transition management, but there are others. When I was in school I designed a heavy-lift STOL airplane that used boundary layer air pumps to retain flow at high angles of attack. It was a novel idea and not mine by the way, but one which can do amazing things for STOL aircraft.

You are too smart to be a pilot. . .I'll stick with my wiki brain.

another good site
 

Another good site:
Paragliding aerodynamics | Sci Fix

After reading your post I decided to read up on it myself. I liked the above site because it brought back most of an introductory aerodynamics lecture I've had before.

I'll make some feeble attempts at identifying some key points though.

*The boundary layer thickness describes the distance above the wing where the velocity of the airflow goes from zero (at the surface) to near 99% free-stream velocity.

*Free-stream velocity is the velocity of the fluid/air far away from the influences of the airfoil.

* Bernoulli's principle states that as a moving fluid meets a restriction the velocity must increase and to obey conservation of energy the pressure must decrease. For example I like to think of putting my thumb over a water hose. A certain mass flow rate has to exit the end of the hose because the pressure driving it out. When I put my thumb over the hose the same mass flow rate must go through this tiny restriction so it must accelerate. Since it must accelerate the pressure must also drop.

*For a wing, think of the wing as a restriction and the free-stream air above it as another restriction. A certain mass flow rate of air must flow through this restriction that is the wing and free-stream air above. Sometimes I like to think of a wing as half a venturi where the air above replaces the missing half.

*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.

*Eventually farther back on the wing the layer will become thick enough and the air low energy enough such that higher energy air outside the boundary layer will overcome and begin to push air backwards over the wing near the surface. This is where turbulation occurs which takes a lot of energy to do, hence drag.

*Also a larger turbulation area means less of the wing is able to do it's job (make lift).

*When there is more separation and turbulation the overall flow shape of air flow over the wing is much more different than what's required for adequate performance. It virtually changes the shape of the airfoil, so careful airfoil design, use of VGs, suction systems, and zig-zag tape are all efforts to control separation.

*When you see VGs and zig-zag tape it is in effort to take high energy air from the leading edge and transport it to the trailing edge with little vortices. This is to prevent the boundary layer air near the trailing edge from loosing too much energy and getting easily separated and turbulated. (I'm still slightly confused by this myself)

*These devices make drag themselves but the benefit is the boundary layer is made thinner than before and separation/turbulation is delayed until much further back toward the tailing edge which reduces drag a lot.

Awesome, thanks for the useful help everyone

http://imgs.xkcd.com/comics/extended_mind.png

:p

Couldn't resist. I now return you to your regularly scheduled topic...

Agree as a loose definition.

Agree.

Sort of. For incompressible fluids like water and slow speed air we can basically say that speed trades with pressure. More speed, less pressure and vice versa. It gets a lot more complicated when you introduce variables like temperature and compressibility.

No, this will not do. I get tired of people thinking I am busting their balls so please refer to NASA:

NASA on the Venturi Fallacy

Whoa, we are in wild places here academically. Boundary layers involve viscosity and Thin Airfoil Theory, which is the basis of modern aerodynamic theory, does not deal with viscosity at all. It is a basic premise of Thin Airfoil Theory that it applies only to inviscid fluids. To deal with the whole thing we have the Navier Stokes equations and the Euler equations. You are speculating about tradeoffs that supercomputers struggle with. You are familiar with any of this? Thin Airfoil theory is given in low-speed aerodynamics for engineers. You got to pay some dues to play that game and I am remiss if I do not mention it. Prandtl would toss a rock from from his grave if I did not ding you here. You know who I am talking about right?

Ok, but you need to talk in terms of pressure gradients and flow adhesion although you are on the right track. When a laminar flow encounters an adverse pressure gradient in excess of potential energy it has available to maintain that flow, it separates from the body and a turbulent, lower energy flow is substituted from that point on. Drag goes way up with turbulent flow and useful lift goes down, so we are not fond of turbulent flows behind wings.

The term is turbulent but when you consider that only so much energy is available and one type of flow is more useful than another, it makes perfect sense. Round bricks roll and square ones do not. Same energy state but totally different usefulness in terms of rolling. The same thing happens with air flows at low speed. Another analogy is cars on a multi-lane highway. Let's imagine a 4-lane highway with many on-ramps and off-ramps. What happens to the speed of the traffic flow? It goes down. Too much turbulence. The cars take up too much space swerving around like that. In contrast the highway with no ramps goes full speed ahead. It is the same thing with molecules of air, they act like little cars on a multi lane highway.

Well, sort of. You have the basic idea. The dimples on a golf ball destroy the laminar flow somewhat but the larger laminar flow is maintained by sacrificing some of the energy that would have been used to maintain a smooth flow down to the surface of the ball. In some cases it is actually a good idea to polish the surface for this very reason. For example, the famous laminar flow airfoil used on the P-51 Mustang was a failure because the surface of the wing could not be maintained in a theatre of war to the standard that was required to make the airfoil work as advertised. It turns out the same airfoil had other advantages in terms of drag divergence at high subsonic speeds which made it useful in the kind of flight it encountered in dives and high speed cruise, but as far as the so called "drag bucket" went which was its original reason for being, it was a practical failure. Bugs and rivets destroyed the smoothness needed for perfect laminar flow.

Cubdriver, what are some good sources for boundary layer reading that we could understand?

Good to know I'm wrong here... I'm liking that NASA page...

No, I'm not familiar with who you're talking about... I took an introductory class years ago that was designed for pilots not engineers, so we discussed the concepts without getting deep into the math and theory. We only spent a day on boundary layer and mostly talked about where energy is gained and lost and velocity gradients and pressure gradients. [Edit add] So to say the least, I definitely don't understand the subject fully nor have I before.

What statements could replace my false ones above?

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.

Cool, thanks. I'm gonna check out that Navy book. I've seen it around before but haven't read it.

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.

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.

• 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.