In fluid mechanics , a thin layer of flowing gas or liquid in contact with a surface (e.g., of an airplane wing or the inside of a pipe).
The fluid in the boundary layer is subjected to shear forces . A range of velocities is established across the boundary layer, from zero (provided the fluid is in contact with the surface) to maximum. Flow in boundary layers is more easily described mathematically than is flow in the free stream. Boundary layers are thinner at the leading edge of an aircraft wing and thicker toward the trailing edge; such boundary layers generally have laminar flow in the leading (upstream) portion and turbulent flow in the trailing (downstream) portion. See also drag .