Ludwig Prandtl (1875–1953) was a German physicist and engineer who is often referred to as the father of modern fluid mechanics. He made significant contributions to the understanding of fluid flow, particularly in the areas of boundary layer theory and viscous flow. Prandtl formulated the concept of the boundary layer in 1904, which describes the layer of fluid in the immediate vicinity of a bounding surface where the effects of viscosity are significant.
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Ludwig Prandtl explained why a fluid with very small viscosity can still experience substantial wall effects. In a thin boundary layer, rapid transverse changes of velocity make viscous stresses important even when they are negligible in the outer flow. Matching these regions reconciles an almost inviscid exterior with the wall's no-slip boundary condition. The approach also connects separation of the near-wall flow with large changes in the forces on a body.[ref]
Motion of fluids with very little viscosity by L. Prandtl. NACA Technical Memorandum 452, translated paper: small-viscosity approximation and boundary layer
Portrait of Ludwig Prandtl
. DLR, CC-BY 3.0; photographer unknown. CC BY 3.0, via Wikimedia Commons. Source.