Originally Posted by
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...
Agree as a loose definition.
*Free-stream velocity is the velocity of the fluid/air far away from the influences of the airfoil.
Agree.
* 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.
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.
*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.
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
*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....
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?
*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 [sic] 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.
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.
*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)
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.
*These devices make drag themselves but the benefit is the boundary layer is made thinner than before and separation/turbulation [sic] is delayed until much further back toward the tailing edge which reduces drag a lot.
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.