The importance of being thin

Abstract

This article outlines a family of approximations for solutions to the incompressible Navier–Stokes equations valid for flow domains that have one small dimension. The approach is applicable to two- or three-dimensional systems, steady or unsteady flows, viscous or inviscid fluids, high or low Reynolds numbers, and internal or external domains. Among the methods in this class are lubrication theories, slender-body theories, shallow-water theories, Hele–Shaw flows, and boundary layers. By displaying the commonalities in these, one sees a systematic approach to many fluid- flow problems as well as those in other fields.

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