weekendflyer

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

thanks in advance

Rickce7

Boundary layer - Wikipedia, the free encyclopedia

Cubdriver

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.

Rickce7

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

nciflyer

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.

weekendflyer

Awesome, thanks for the useful help everyone

FlyerJosh

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

Cubdriver

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.

nciflyer

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?

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.