Boundary Layer

A boundary layer is the thin region of fluid immediately adjacent to a solid surface, where viscous effects dominate and the fluid velocity transitions from zero at the wall to the free-stream value. First conceptualized by Ludwig Prandtl in 1904, the boundary layer transformed fluid dynamics from a failed attempt to solve the Navier-Stokes equations exactly into an engineering science of approximations that work. The layer's structure — laminar near the leading edge, turbulent downstream — determines drag, heat transfer, and flow separation. The boundary layer is not merely a mathematical artifact; it is the physical location where the fluid 'remembers' the surface, and its thickness is the measure of how far that memory extends into the flow.