Boundary layer

A boundary layer is a thin region of fluid next to a surface where viscosity strongly affects motion. Within it, velocity typically changes from the surface value to the surrounding flow.

No-slip condition: A fluid boundary condition stating that fluid velocity matches the velocity of a solid surface in contact with it. It sets the near-surface velocity that the boundary layer must connect to the outer flow.
Viscosity: A fluid property that measures resistance to deformation and internal flow. Viscosity diffuses momentum from the wall into adjacent fluid.
Skin friction drag: Fluid resistance caused by tangential shear stresses acting along a surface. It is the direct drag contribution produced by boundary-layer wall shear.
Ludwig Prandtl: A German physicist who founded modern aerodynamics and introduced boundary-layer theory. Prandtl formulated the theory that separates thin viscous layers from outer flow.
Potential flow: An idealized, irrotational fluid flow represented by a velocity potential, often neglecting viscosity. It describes the outer flow well but cannot enforce the no-slip condition at a wall.
Boundary-layer thickness: A measure of the distance from a surface over which boundary-layer velocity approaches the outer-flow velocity. Its growth tracks how far viscous effects extend away from the surface.
Reynolds number: A dimensionless ratio of inertial forces to viscous forces in fluid flow. It helps predict boundary-layer development and whether transition to turbulence is likely.
Lift coefficient: A dimensionless measure of lift produced by a body moving through a fluid. Boundary-layer state and separation strongly affect attainable lift.
Boundary-layer equations: Simplified fluid-flow equations derived for thin layers where viscous effects dominate across the surface-normal direction. They turn Prandtl’s physical picture into equations for velocity and pressure.
Stokes flow: A viscous flow regime in which inertial forces are negligible compared with viscous forces. Unlike thin boundary-layer flow, Stokes flow is viscosity-dominated throughout the fluid.

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