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Viscosity effects at high Reynolds numbers

When the Reynolds number is large you might think that the viscosity could be ignored altogether, in which case we return to nonviscous fluid mechanics. However, we once again encounter d'Alembert's paradox--that a nonviscous fluid exerts no drag on a solid body--so we are at a loss when it comes to explaining aerodynamic drag. The important insight in resolving this paradox is due to L. Prandtl, who in 1904 suggested that the viscosity could be ignored everywhere except in a thin layer close to the surface of a body. Understanding the behavior of this boundary layer  has been crucial to the development of modern fluid mechanics and aerodynamics.

Vittorio Celli
Sun Sep 28 22:13:11 EDT 1997