Talk:Lift (force)/Comments

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This is an attack on the use of the Bernoulli equation as a partial explanation of aerodynamic lift.

Bernoulli's equation for the conservation of energy is commonly used to explain aerodynamic lift, however the equation assumes a steady flow, i.e. flow in which particles follow velocity streamlines. A stream line is a line of constant velocity in the flow. When the particles themselves do not follow the streamlines, they are changing velocities, that is, they must cross the streamline bundle. Shear flow is an example of non-steady flow. The flow near the airfoil surface is shear flow because of the existence of the boundary layer, a layer of air in which the velocity changes across the flow; the layers of air shear past one another, faster layers toward the ambient flow and slower layers near the airfoil. This is non-steady flow and Bernoulli's equation is therefore not applicable in this region. (See pg. 9ff. Fluid Mechanics. Landau, L. D. and E. M. Lifshitz. (1959.) Addison-Wesley: Paris.}

Here is the beginning of a correct explanation of this part of lift:

Upwash (from the bottom of the wing at angle of attack) joins with the main flow at the leading edge to envelop the top of the wing. The combined flow interacts with the boundary layer on the curved part of the top of the wing and, for sufficiently small angles of attack, depletes the boundary layer population there, i.e. the pressure on the top is decreased, by the "blowing" of particles away from the wing's surface as it curves away from the flow. It is the curvature of the wing that is crucial in this component of lift. This decrease in pressure on the top of the wing adds to the lift. It can be seen in the Coanda effect, i.e. a smoke stream near the curved surface is deflected toward the surface by the higher pressure in the flow (the ambient pressure). The difference between the lower pressure at the top surface and the higher pressure under the wing results in lift. —Preceding unsigned comment added by Ccrummer on 27 January 2007.

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I have just found these comments published by Ccrummer in January 2007. There is universal agreement with Ccrummer that Bernoulli's principle is not applicable in the boundary layer because BP is stated to be applicable to inviscid flow (or flow that closely resembles inviscid flow) and the flow in the boundary layer is clearly not inviscid flow. However, the great majority of air affected by an airfoil is not part of the boundary layer and its flow closely resembles inviscid flow.

Ccrummer’s "correct explation of this part of lift" looks like original research. There is no reference or citation to indicate where the ideas come from, or whether these ideas have ever been published by someone. Wikipedia is not the place for our original research or our personal views on what we believe is true. See Wikipedia:No original research.

Ccrummer’s stated views are at odds with statements by most (all?) specialist authors in the fields of fluid dynamics and aerodynamics. For example, in the excellent book Aerodynamics, L.J. Clancy has written “When a stream of air flows past an airfoil, there are local changes in velocity round the airfoil, and consequently changes in static pressure, in accordance with Bernoulli’s Theorem. The distribution of pressure determines the lift, pitching moment and form drag of the airfoil, and the position of its centre of pressure.”^[1]

Ccrummer has written “steady flow, i.e. flow in which particles follow velocity streamlines. A stream line is a line of constant velocity in the flow.” I have read many books on fluid dynamics, but I haven’t ever read of a “velocity streamline”. Also, when Ccrummer writes “A stream line is a line of constant velocity in the flow” he is at odds with all the authors I have read in this field.

The bottom line is - where did this information come from? The threshold for inclusion in Wikipedia is verifiability, not truth. See Wikipedia:Verifiability. See also Wikipedia:Wikipedia is an encyclopedia.

In my view, as "an attack on the use of the Bernoulli equation as a partial explanation of aerodynamic lift" these comments have failed completely. Dolphin51 (talk) 12:43, 25 May 2008 (UTC)

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