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.

Sun Sep 28 22:13:11 EDT 1997