Boundary layer

The boundary layer is a thin region of fluid—like air or water—next to an object’s surface where flow velocity changes from zero (due to friction at the surface) to the free stream velocity away from the object. Within this layer, the fluid's speed gradually increases from the surface to the outer flow, and forces like friction and heat transfer are most significant. Understanding the boundary layer is important for designing efficient aircraft, ships, and other structures, as it influences drag, heat exchange, and overall performance.