torsdag 20 september 2012

Ahlborn: Early Critic of Prandtl

Friedrich Ahlborn (1858-1937), a today forgotten pioneer of fluid dynamics, performed and photographed experiments in Hamburg similar to those performed by Prandtl in Hanover described in his 1904 article representing the birth of modern fluid mechanics with the all-important role of the boundary layer as the novelty, as described in The Dawn of Fluid Dynamics.

Ahlborn's motto was "Per experimentum et induction omnia" (everything is to be obtained inductively from experiments).

Ahlborn compared the boundary layer to a ball bearing, "like frictional rolls between the solid surface and the surrounding free flow", which he referred to as a "balanus layer" from the Latin word "balanus" meaning barnacle. Ahlborn further identified in his photographs "tender cycloidal serpentines" supporting his view that the transitional space to the free stream at some distance from the surface was "filled with a long chain of vortices".

Ahlborn thus identified the same structures experimentally as we have identified as 3d rotational separation in computational simulations 100 years later, as described on The Secret of Flight.

We cite from The Dawn of Fluid Dynamics:
  • In the 1920s Ahlborn and Prandl became engaged in sometimes harsh scientific quarrels. Despite their common interest in the nature of the boundary layer and the mechanism of skin friction, their mentalities  as researchers had little in common. "Per experimentum et induction omni" Ahlborn's leitmotif, was not Prandtl's.... 

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