måndag 20 augusti 2012

Rebuttal of AIAA Referee Criticism

AIAA editor Greg Blaisdell has reported that he will respond in a week or so to our rebuttal of the referee reports for our article New Theory of Flight. Here are some key points of our rebuttal:

1. Our Criticism of 2d Kutta-Zhukovsky Circulation Theory of Lift.

Classical Kutta-Zhukovsky circulation theory of lift, as a 2d theory for a 3d real phenomenon, is unphysical. The Kutta condition of specifying the velocity to be zero at a point on the boundary of inviscid flow where the flow is not entering (including the trailing edge), is mathematically meaningless and physically impossible.

The classical proof of Kelvin's theorem is incorrect from wellposedness, since the vorticity equation can exhibit exponential growth of perturbations, and perturbation is part of wellposedness.

2. Our Criticism of Prandtl's Boundary Layer Theory of Drag

Prandtl's boundary layer theory attributes drag to the presence of boundary layers.  We compute drag in  slightly viscous flow accordance with observation by solving the Navier-Stokes equations with slip boundary condition without presence of boundary layers. We conclude that the major part of drag (form drag) in slightly viscous flow does not originate from any boundary layer, and thus that Prandtl's theory does not cover the major part of drag in slightly viscous flow. 

3. Our New Theory of Flight

The reviewers do not question that our New Theory of Flight describes real 3d flow. 

What they question is our criticism of classical 2d theory: Instead of frankly admitting that it is unphysical and thus incorrect, as we do, the reviewers want to describe classical theory as correct in principle as a 2d theory, even if this 2d theory does not really describe real 3d flow. This is a common way of handling the unphysical aspect of classical 2d theory; admitting that it is 2d and thus in a sense unphysical as any model (no model is perfect) but insisting that anyway it is correct in some sense as a 2d flow model, which somehow "represents real 3d flow" without describing the actual 3d physics. Thus correct even if incorrect, as any model (no model is perfect). This is the split between theory and practice which has troubled fluid mechanics starting with d'Alembert's paradox in 1752.

The reviewers claim that a slip boundary condition does not describe the physics of slightly viscous flow. This is not correct because the skin friction of slightly viscous flow is small and slip models small skin friction. Slip is also a mathematical meaningful (and possible) boundary condition. The reviewers are stuck to a Prandtl dictate to use no-slip with lacks both mathematics and physics rationale. 

Altogether, the criticism of the New Theory is weak, and the defense of the Old Theory is also weak.

Reviewer 2 offers the following starting point for the continued discussion with AIAA: 
  •  I believe that serious issues of substantial public interest are involved.

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