torsdag 16 augusti 2012

Analysis of AIAA Referee Reports: Reviewer 1

I now turn to referee report AIAAreview1 of the New Theory of Flight submitted to AIAA Journal.

The punch line is
  • The authors provide no documented scientific evidence to discredit the current state of the art.
Yes, we do and we are not alone: Very substantial criticism of the Kutta-Zhukovsky-Prandtl theory of lift has been expressed by many scientists ever since this theory was conceived 100 years ago.

The reviewer then branches out into a sequence of incoherent statements without meaning:
  • First of all, the concept of circulation is not necessary for the physical explanation of lift-the physical Kutta condition leads to the correct (as verified by experiment) solution to the potential flow problem. Circulation enters the mathematical problem for the incompressible potential flow past an airfoil since the problem is non-unique without its specification.
  • More importantly, Prandtl’s boundary-layer theory is not a viscous theory for drag but an 
  • asymptotic theory for the solution to the Navier-Stokes equations at large Reynolds number.
  • Potential flow is not presented as the solution for lift but as the first term in an asymptotic expansion - the potential flow and boundary-layer theories are connected through the matching process. 
  • The versions of potential theory and boundary-layer theory the authors present are only the first terms in the expansion.
  • Their claim that the theories of Kutta-Joukowski and Prandtl are both incorrect at “separation” (undefined by the authors but apparently only considered at the trailing edge) does not take into account the extensive research into the potential flow-boundary-layer coupling. 
  • In fact, the inclusion of the effect of the displacement thickness in the second-order potential flow solution renders arguments associated with a trailing-edge stagnation point moot. (The trailing edge stagnation point does not appear for a cusped trailing edge). 
  • In addition, problems arising with the calculation of the boundary layer past the trailing edge or a separation point are addressed with a strong-interaction version of the boundary-layer equations (see the discussion in Chapter 14 of Katz and Plotkin which also  includes a detailed discussion of the matching process referred to above).
After this excursion into terra incognita the reviewer returns to our article:
  • The authors’ new approach is to solve the Navier-Stokes equations numerically with an unphysical slip boundary condition. 
  • The authors must demonstrate that the slip boundary condition somehow matches the physics of viscous flow near a solid boundary. They do not do this. 
Yes, this is precisely what we do. We show that slip models the small skin friction of slightly viscous flow and that solution of Navier-Stokes with slip matches observation. This is a key point of our article.

The reviewer concludes with
  • In summary, the authors have not presented us with an aerodynamic theory alternative to the modern boundary-layer theory in the literature which addresses lift and drag. 
  • At most, perhaps they present a numerical model of the governing equations which avoids the need to discretize the boundary layer. 
  • They would however need to demonstrate how drag is calculated with such a model and that the results match with detailed experiments.
The reviewer has understood one of our major points: Discretization of the boundary layer is not necessary, which makes it possible to solve the Navier-Stokes equations with millions of mesh points, instead of the impossible quadrillions required by state-of-the-art. But the reviewer then falls back to misunderstanding: We compute both lift and drag for all angles of attack in close correspondence to observation.

Altogether, this is a very poor report by a reviewer who misrepresents key aspects of the article. The report does not meet the standards of AIAA as The World's Forum for Aerospace Leadership. 


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