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This
photograph is of the same airfoil in the same speed flow, however the angle of
attack has been increased to 180. The flow some distance from the
foil is similar to that at the 100 angle of attack, but that passing
close to the top of the airfoil has separated from the airfoil surface
much closer to the leading edge and the "wake" region behind this separation
point is much larger. For these flow conditions, this airfoil is said
to have stalled. Its lift has decreased and the drag forces have increased.
These
photographs were taken using air as the fluid but similar behavior
would have been observed with other fluids provided the "Reynolds
Number" was kept the same in the two experiments. The Reynolds Number,
Re = UDr/m
= UD/n , where U is the speed of the
undisturbed incoming flow, D some characteristic length of the object in the
flow, r the density of the fluid, and
m its viscosity. A kinematic viscosity,
n = (m/r) has also been introduced. The
Reynolds number is dimensionless and is a parameter used to characterize fluid
flows. |
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