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Offline kalayzor

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Laminar flow
« on: 04/12/2007 03:15:59 »
Here's something a little off the beaten path of quantum, thermo, and relativity that seem to pervade this forum...

I was wondering today while driving home in a decent crosswind (at least 15kts of wind about 40 degrees to the direction of travel), how does laminar flow work over an wing?  Why does air "stick" to an airfoil?  I've read that there is a boundary layer between the airflow and the airfoil, and that the boundary layer has some amount of viscosity that keeps it on the airfoil, but nothing that I've found goes into more detail than that...and I really don't want to wait for my Aerodynamics 101 class to find out.

Is it some sort of surface tension in the air (since gases act as fluids at high speeds), or something like that?  Any help is appreciated!


 

Offline JimBob

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Laminar flow
« Reply #1 on: 04/12/2007 04:01:42 »
Laminar flow is the exception to the norm in flui9d dynamics. The boundary layer on an air foil is laminar, not turbulent. Were it turbulent, no lift would occur. Lift occurs because the laminar flow over the airfoil is faster over the top than under the bottom of the foil. This is what creates lift and lets the airplane fly. Laminar flow in pipe (what I am most accustomed to dealing with) is extremely difficult to achieve since there are so many imperfections in steel as well as at the (threaded) connections. What is striven for in the oil industry is the lowest Reynolds number for the flow. The Reynolds Number - a dimensionless number, is a measure of how coherent the fluid flow through the pipe is.

See http://en.wikipedia.org/wiki/Reynolds_number for Reynolds Number explanation and more info
 

Offline Mr Andrew

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Laminar flow
« Reply #2 on: 04/12/2007 21:53:05 »
I'm gonna guess that the boundary layer is formed because air molecules stick in the imperfections in the wing/pipe wall and smooth it out.  If the bumps and ridges were large enough the flow would turn turbulent because laminar flow would require large groups of molecules to 'stick' in the grooves which simply won't happen (on the small scale, EM forces can reduce the flow along the surface, but on a larger scale, these forces play less of a part in the behavior of the system).  The boundary layer is kind of like wetting down a plastic slide.  The water fills in the dips in the surface, reducing friction and allowing little kids to slide down it at high speeds.
 

Offline kalayzor

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Laminar flow
« Reply #3 on: 04/12/2007 22:08:24 »
I like the explanation about the boundary layer providing a sort of "slick" layer, but I have some reservations about it.

In order to have the best airfoil, it needs to be as clean (minimizes surface drag, and I'd think maximizes lift as well) as possible...but then you can also bring in the concept of turbulator tape -- essentially breaking up the boundary layer near the trailing edge of a surface in order to get it to reattach itself.  I'm not sure if anyone really knows how that works, though.  But I digress...what's holding on the boundary layer?

http://www.leichtwerk.de/eta/en/news/index.html [nofollow] -- This is the website of the eta project, the most technologically advanced sailplane in the world (and the largest!!!).  There isn't much to go from on the site, but perhaps there might be some more resources out there related to it.

Secondly -- why does the air over the top of the airfoil move faster than that on the bottom?  I've seen a pictorial explanation than an airfoil is like half a venturi with the rarefied air further above the wing's surface acting as the upper half of the venturi...is there approaching correct at all?  If not, what's right?
 

Offline kalayzor

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Laminar flow
« Reply #4 on: 05/12/2007 03:54:28 »
So, after talking to a mechanical engineer, it apparently is the surface imperfections that keep the boundary layer stuck onto an airfoil.  The reason why they're taken out as much as possible is for reasons of decreasing drag since not nearly as much lift is lost in the process.  Other than that...the viscosity is just intermolecular interactions dependent on the type of fluid.

Could I get a taker on the second question, however?
 

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Laminar flow
« Reply #4 on: 05/12/2007 03:54:28 »

 

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