Mean Local Flow?

[Spaghettiohead]

Hello,
Can anybody clue me in as to what mean local flow around an airfoil is? I believe it has something to do with the downwash created by the wing...?
Thanks!
~Andrew

 

[CESSNA1]

SPAGHETTIOHEAD: It probably refers to the circulation. As an airfoil moved through the air there is a circulation of air around the airfoil. The circulating air moves forward under the wing, up around the leading edge and back over the top of the airfoil to the trailing edge then around the trailing edge to move forward again. It is this circulation that causes the air to travel faster over the wing and slower under it. It also causes the leading edge upwash and the trailing edge downwash. Lift is caused by the low pressure on top of the airfoil due to the higher airpseed and higher pressure under the airfoil due to the lower airspeed.

Regards
Dave

 

[Spaghettiohead]

Circulation...that makes sense. So, on a cross-section of an airfoil, how could I draw the mean local flow. Would it be a line representing the "average" circulation?

~Andrew

 

[Spaghettiohead]

Also, do I understand correctly that the mean local flow is perpendicular to the Aerodynamic Force?

 

[CESSNA1]

SPAGHHETTIOHEAD: You can draw a line around the airfoil to represent the circulation. By aerodynamic force I believe you mean "lift". The lift is perpendicular to the chord line and the center of lift is at approximately the 25% chord line from the leading edge. Since the top of the airfoil is curved the circulation line is not quite (only closely) perpendicular to the lift vector.

Regards
Dave

 

[GregLocock]

and drag? Consider the mean local flow around a circular airfoil.

Cheers

Greg Locock

 

[Spaghettiohead]

I was looking over an aerodynamics book...what I think I got out of it was that the Aerodynamic Force, or the resultant of Lift and Drag vectors, is perpendicular to the mean local flow. That seems to agree with what you, CESSNA1 and GregLocock are saying...

Thanks again,
~Andrew

 

[RaduP]

Only the lift is perpenndicular to uniform mean flow (taken as the opposite from that of wing/airfoil, and away from it), while drag is in the same direction as the mean uniform flow. You end up with two perpendicular forces.

Lift can be related to the ciculation around an airfoil (and therefore potential theories can deal with it), while drag can be classified as form (or pressure) drag and friction drag, and usually needs separate treatment, depending on the Reynolds number and wake characteristics.

Cheers,
Radu