Dear fellow aviators,
I am learning about Aerodynamics right now, So I stumbled upon few question, Can somebody please explain me the concept of boundary layer, and Why does it get bigger as you move aft on an Airfoil? 2. Why does laminar airflow create less friction Drag?
Thank you for all of your help, It is appreciated,
Sandesh P.
The concept of the boundary layer is basically the concept of friction applied to a fluid... In the case of an airplane wing the fluid is air. (By the way, the term fluid is used to describe both liquids and gases, not just liquids).
Imagine first what would happen to a solid block (like a brick) if you slid it along a flat plate, like the surface of a flat glass table. The solid would start out at a particular speed, and do to friction between the plate and the block it would begin to be slowed, until it eventually came to a stop.
But because we are talking about a SOLID the whole object would slow at the same time. In other words, the friction force from the bottom surface would affect the entire brick simultaneously, and slow the entire brick simultaneously.
Now imagine doing the same thing to something that isn't quite a solid, like a block of jello. Unlike the brick, as the block of jello slid along the table, the bottom of the jello would slow faster than the top of the jello, because jello is a little bit flexible. Because the bottom is slowing faster than the top, the block of jello would start to deform and sort of "smear out" a little. But jello is still basically a solid, so it can't "smear out" completely like a liquid or a gas could. So the bottom is going to be dragged along a little by the motion of the rest of the cube of jello. If it weren't, the cube of jello would fall apart completely.
Now think about what happened to the jello and apply that to an imaginary "block" of air. The air is even more "flexible" than the jello, so it is going to smear out even more than the jello did. In fact, because it is a fluid (which just means it is a liquid or a gas), it can smear out completely. The bottom of the "cube" of air can come to a complete stop, and as you move up the cube the rest of the air will be affected less and less. And in fact this is exactly what happens. The bottom of the air comes to a complete stop, and the speed increases as you move farther up from the surface of the plate. When you get high enough above the surface of the plate, the air is moving along affected very little by the friction forces that are acting on the air molecules below them. The boundary layer is the area that is slowed by friction.
So that is what a boundary layer is, now why does it grow as you move aft along the plate?
Ok, let's go back to our mental model of the block of jello. Imagine the jello is moving along at a constant speed through space with no table below it. It is shaped like a perfect cube, just flinging along. All of a sudden, it comes to the table in such a way as to be perfectly aligned so that the bottom of the cube of jello is just skimming the surface of the glass table.
Now imagine the shape of the jello right as it encounters the table. Try to picture it in your mind in slow motion. Just before it encounters the table, the whole thing is moving at the same speed and it's a perfect cube. As the bottom just barely encounters the table, it is slowed by friction, but most of the cube just continues along at the same speed. So only a very thin sliver of jello is slowed down.
A few fractions of a second later, though, the jello has moved forward some, but also, the slowing from the bottom has been transfered up the cube of jello just a little bit higher, and the jello is less cube shaped. The jello is now less cube shaped, as more of the bottom of the jello is "smeared" backwards. At this time maybe only the top half of the cube is still moving along at its original speed and only the top half of the cube is still "cube-shaped". The bottom half is kind of starting to smear backwards. So now instead of just the very bottom surface of the cube being "smeared", the bottom half of the cube is "smeared". Our boundary layer (the amount of the fluid that is affected by friction forces) has grown in thickness.
The exact same thing would happen with a cube of air, only more so, due to the fact that air is even more flexible than jello. After all, jello behaves sort of like a fluid, but it is really still solid. For air or water or any other fluid, the effect would be even more pronounced.
Hope this helps. Just realize that the above analogy is good, but it isn't 100% exact. I modeled a block of air as being similar to a block of jello, but they aren't similar enough to treat them exactly the SAME the way I just did. It's a convenient mental model, but there are some more complexities that I sort of glossed over. For example, a block of jello CAN'T become turbulent.
