What is this????

[shdw]

I was basing my question off this page from "Introduction to Fluid Mechanics," Pritchard, Philip J, 8th ed (pp. 423)

Well that explains it, I didn't realize you meant the boundary layer at the leading edge. I guessed you to mean the air in front of (far in front of) the wing.

Is that fluids book any good? I've got one that I inherited from a mechanical engineer at my airport: Fluid Mechanics Seventh Edition by Victor L. Streeter and E. Benjamine Wylie. Haven't looked at it much yet though.

Also highly recommended is "Aircraft Design: A Conceptual Approach" by Daniel Raymer, or pretty much any of the other AIAA published textbooks. (I haven't even read this one all the way through yet, but it's fantastic.)

Have it, only about half way through myself though. I'll gladly take any other book suggestions you may have though, I'm always looking to expand the library. Two I'd recommend are Mechanic of Flight and Airplane Performance, Stability, and Control.

I haven't finished reading either of them yet, only a few hundred pages from each. The first one is a 1,000 pages book that covers virtually anything and everything you could ever want to know about how an airplane flies. Combine with Raymers text and you're ready to go and design an airplane. If you can understand it all :-| eek

The second book is a classic. It is referenced in both Raymer and Andersons books as well as a reference for the mechanics of flight book. Very well written and one I highly recommend you pick up if you don't already have it.

Thanks again for the hand-holding through the Boundary Layer questions. This would be an appropriate time for the :beer: smiley, but it's gone.

Not a problem, I enjoyed it. I'm a self study, so I'm sure you already know quite a bit more than myself or soon will. In other words, you can teach me some stuff later. :-D



--

Maurus, I'll get you a picture next week. And I realize stall strips are used unconventionally at times, such as one wing and not the other. My main point through this thread was to make it known that they are designed to force a stall, opposite the purpose of vortex generators. Though I never suspected you'd see them on the outboard wing section like this arrow. Mainly because every text I have speaks to the advantage of ensuring a wing stall begins at the root.

My only guess for outboard stall strips is to prevent the aircraft from having enough lift to deeply stall the inboard section. Oh well, I think I'll be left to always ponder this perplexity. Should be fun!

 

[inigo88]

Not a problem, I enjoyed it. I'm a self study, so I'm sure you already know quite a bit more than myself or soon will. In other words, you can teach me some stuff later. :-D

Thought I'd necropost since I still think about this thread. I'm now in advanced fluid mechanics, and still don't know jack. But we're covering boundary layer flow, flow separation, etc. It's incredible that this topic really is obscure enough that you can't really jump directly into aerodynamics without studying some basic fluid mechanics first, simply because it's such a specific discipline that it's really easy to get lost in and confused by the nomenclature (as I did, earlier in this thread).

Here's a cool idea to ponder though. Vortex generators have the same end result as the dimples on a golf ball. Both designs, while slightly different in how they work, result in inducing a turbulent boundary layer before it would otherwise naturally occur, and delay flow separation.

I watched these videos in class, and they were a HUGE help. Highly recommended:

Fluid Mechanics of Drag

Part 1:



Part 2:



Part 3 (only goes to 1:01):



Part 4: How to Reduce Drag.



However, this skips the most important part, which is the conclusion where he compares pressure drag to viscous drag on blunt vs. streamlined objects. If anyone can find the full version, please post!

Synopsis (of what was skipped over in the last video):

In Laminar flow (glycerin) at low Reynold's Number with no separation, the airfoil falls slower due to increased surface area and thus increased viscous drag. The blunt object falls faster due to less viscous drag. This is a flow visualization of strictly viscous drag in laminar flow with no flow separation from either object.

For the wind tunnel tests things start to get interesting.

For his streamlined object (airfoil or "bomb") he assumes no flow separation, and thus viscous drag is the sole contributor. At low Re laminar flow the viscous drag is low, and as we increase Re through transition to turbulent flow, the viscous drag increases accordingly.

Things get weird for the blunt object (ball). At low Re laminar flow, boundary layer flow separation occurs at the very top and very bottom of the circle, leaving a huge low pressure wake behind. The ball has less viscous drag in laminar flow, but the pressure drag from this wake contributes the majority of the drag (basically the ball is stalled!). When he increases the speed of the flow to turbulent (higher Re), drag actually decreases! This is because the adverse pressure gradient from the back of the ball is pushed back by the greater momentum of the oncoming turbulent flow, which pushes the separation point rearwards on the ball, creating a smaller wake. Less wake = Less pressure drag. Thus even though viscous drag increases due to the higher speed, pressure drag was decreased so much that total drag went DOWN!

Surface roughness (like the dimples on a golf ball) induces a turbulent boundary layer sooner, because it actually lowers the transition Reynolds Number. This is called "tripping the boundary layer", and is what was shown in the video when he had the smooth and rough balls on the balance beam, and the beam reversed at 125 mph (where transition occurred).

While I'm sure I'm admitting my dorkiness by saying this... that result blew my mind.

Anyway, I hope we can talk more aerodynamics in future threads now that I'm kinda sorta learning how it works. I wish tgrayson was still around, then the three of us could hash it out.

 

[CoffeeIcePapers]

Thought I'd necropost since I still think about this thread. I'm now in advanced fluid mechanics, and still don't know jack. But we're covering boundary layer flow, flow separation, etc. It's incredible that this topic really is obscure enough that you can't really jump directly into aerodynamics without studying some basic fluid mechanics first, simply because it's such a specific discipline that it's really easy to get lost in and confused by the nomenclature (as I did, earlier in this thread).

Here's a cool idea to ponder though. Vortex generators have the same end result as the dimples on a golf ball. Both designs, while slightly different in how they work, result in inducing a turbulent boundary layer before it would otherwise naturally occur, and delay flow separation.

I watched these videos in class, and they were a HUGE help. Highly recommended:

Fluid Mechanics of Drag

Part 1:



Part 2:



Part 3 (only goes to 1:01):



Part 4: How to Reduce Drag.



However, this skips the most important part, which is the conclusion where he compares pressure drag to viscous drag on blunt vs. streamlined objects. If anyone can find the full version, please post!

Synopsis (of what was skipped over in the last video):

In Laminar flow (glycerin) at low Reynold's Number with no separation, the airfoil falls slower due to increased surface area and thus increased viscous drag. The blunt object falls faster due to less viscous drag. This is a flow visualization of strictly viscous drag in laminar flow with no flow separation from either object.

For the wind tunnel tests things start to get interesting.

For his streamlined object (airfoil or "bomb") he assumes no flow separation, and thus viscous drag is the sole contributor. At low Re laminar flow the viscous drag is low, and as we increase Re through transition to turbulent flow, the viscous drag increases accordingly.

Things get weird for the blunt object (ball). At low Re laminar flow, boundary layer flow separation occurs at the very top and very bottom of the circle, leaving a huge low pressure wake behind. The ball has less viscous drag in laminar flow, but the pressure drag from this wake contributes the majority of the drag (basically the ball is stalled!). When he increases the speed of the flow to turbulent (higher Re), drag actually decreases! This is because the adverse pressure gradient from the back of the ball is pushed back by the greater momentum of the oncoming turbulent flow, which pushes the separation point rearwards on the ball, creating a smaller wake. Less wake = Less pressure drag. Thus even though viscous drag increases due to the higher speed, pressure drag was decreased so much that total drag went DOWN!

Surface roughness (like the dimples on a golf ball) induces a turbulent boundary layer sooner, because it actually lowers the transition Reynolds Number. This is called "tripping the boundary layer", and is what was shown in the video when he had the smooth and rough balls on the balance beam, and the beam reversed at 125 mph (where transition occurred).

While I'm sure I'm admitting my dorkiness by saying this... that result blew my mind.

Anyway, I hope we can talk more aerodynamics in future threads now that I'm kinda sorta learning how it works. I wish tgrayson was still around, then the three of us could hash it out.

He is fairly active on facebook.