söndag 15 juli 2012

Making of the Prandtl Myth by John D. Anderson

Continuation of Making of the Prandtl Myth, now with quotes from Fundamentals of Aerodynamics by John D. Anderson, 1984:

Anderson: The modern science of aerodynamics rests on a strong fundamental foundation, a large percentage of which was established in one place by one man, at the University of Göttingen by Ludwig Prandtl. Prandtl never received a Noble Prize, although his contributions to aerodynamics and fluid mechanics are felt to be of that caliber.

Comment: The myth is supported by a Nobel Prize which Prandtl did not receive.

Anderson: Prandtl was considered a tedious lecturer because he could hardly make a statement without qualifying it.

Comment: This can be read as a sign that Prandtl's science was unclear.

Anderson: By the 1930s, Prandtl was recognized worldwide as the "elder statesman" of fluid dynamics…his "Nobel-Prize-level" contributions had all been made….He was clearly the father of modern aerodynamics - - a monumental figure in fluid dynamics. His impact will be felt for centuries to come.

Comment: Prandtl's 1904 paper with its suggestion that drag originates from a vanishingly thin boundary layer was long ignored, in particular by the leading UK aerodynamicist Lanchester, who was not at all convinced by Prandtl's sketchy speculations.

Further quotes form Prandtl's Boundary Layer by John D. Anderson:

Anderson: Despite the important work by Blasius and the sub- sequent publication of several papers on boundary-layer theory by Prandtl’s research group at Göttingen, the aero- dynamics community paid little attention, especially out- side of Germany. Finally in 1921, Theodore von Kármán, a former student of Prandtl’s and a professor at the University of Aachen, obtained a momentum-integral equa- tion through the simple expedient of integrating the boundary-layer equations across the boundary layer. That equation proved to be directly applicable to a large number of practical engineering problems, and with it, the boundary-layer theory finally began to receive more attention and acceptance in the technical community.

Anderson: The delayed acceptance of the boundary-layer concept is illustrated by the fifth and sixth editions of Horace Lamb’s classic text Hydrodynamics.6 The fifth edition, published in 1924, devoted only one paragraph to the boundary-layer concept and described Prandtl’s work as follows: “The calculations are necessarily elaborate, but the results, which are represented graphically, are interesting.” In contrast, the sixth edition, published in 1932, had an entire section on boundary-layer theory and the governing equations.

Comment: The Prandtl myth was carefully drafted by Prandtl's clever students von Karman and Schlichting. Science needs myths and heroes, and Prandtl showed to be useful.  

Anderson: Prandtl’s boundary-layer idea revolutionized how scientists conceptualized fluid dynamics. Before Prandtl, there was much confusion about the role of viscosity in a fluid flow. After Prandtl’s paper, the picture was made clear; in most cases, viscosity only played a role in the thin layer of flow immediately adjacent to a surface. What a breakthrough in the analysis and understanding of a vis- cous flow! Before Prandtl, there was no mathematically based, quantitative means to calculate the drag due to friction on a surface immersed in a fluid flow. After Prandtl’s paper, the fluid dynamicist could quantitatively calculate the skin-friction drag. Before Prandtl, there was no un- derstanding of the physical mechanism that caused a flow to separate from a surface. After Prandtl’s paper, the physics of separated flow became clear and the understanding of fluid dynamics underwent a revolutionary change.

Comment: This is a summary of the myth: Notice the shift from drag to skin-friction drag claimed to be computable by Prandtl's theory. But skin-friction drag is small compared to the pressure drag and can be set to zero in most applications. 

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