## torsdag 26 juni 2014

### Perspective on the Prandtl Medal: Model vs Reality

Prandtl with his model of reality in the form of a little water tank in Göttingen in 1904.

The Prandtl Medal to be given to me (and my coworkers) on July 21 in Barcelona connects to the basic question in science and engineering of model vs reality: As far as I can understand the award expresses a recognition of our work showing that the basic idea of Prandtl of the boundary layer as the origin of lift and drag of an airplane wing, is incorrect.

Our evidence is computed solutions of the Navier-Stokes equations with slip boundary condition, which does not give rise to any boundary layers, but yet fit closely with observations. The agreement improves with increasing Reynolds number beyond $10^5-10^6$, because slip models a turbulent boundary layer. For a jumbojet the Reynolds number is $10^8$ or larger.

More precisely, the computations with slip show somewhat better lift and stall characteristics (higher lift and delayed stall) than experiments.

There are two basic approaches to handle a difference between model and reality: Change the model or change the reality. The second option represents an engineering approach with the goal of constructing a wing so as to realize slip also for moderately large Reynolds numbers (of size $10^5$). One way of doing that is to use rotating cylinders at leading and trailing edge and/or a moving upper wing surface, which indeed shows to improve both lift and stall.

The Prandtl Medal thus invites to redesign of wings according to the model, rather than changing the model to represent existing wings constructed in the spirit of Prandtl as the Father of Modern Aerodynamics.