Grenselaget

Grenselaget, in fluid dynamics, refers to the thin region of fluid in contact with a solid boundary where viscous forces are significant. Within this layer, velocity goes from zero at the wall to freestream away from the surface. The concept, introduced by Ludwig Prandtl in 1904, separates viscous effects from inviscid flow.

The boundary layer is typically classified as laminar or turbulent. The laminar boundary layer features smooth flow with a velocity profile described by the Blasius solution for a flat plate; the turbulent layer shows chaotic fluctuations and higher dissipation.

Thickness increases with distance from the leading edge and is influenced by the Reynolds number Re = rho U x / mu. In high-Reynolds-number flows, the laminar layer may transition to turbulence; turbulent boundary layer grows faster and adheres longer, affecting drag and heat transfer.

Boundary-layer separation occurs when flow reverses near the surface, often causing stall on wings or increased drag.

Applications include aircraft design, automotive aerodynamics, and meteorology, where the atmospheric boundary layer is the lowest part of the atmosphere affected by surface heating and roughness.

Methods to study the boundary layer include experimental techniques like hot-wire anemometry and particle image velocimetry, and numerical approaches based on Prandtl's boundary-layer equations.

See also: Navier–Stokes equations, Prandtl boundary layer, Reynolds number, boundary-layer separation, turbulence.