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Boundary layer thickness
This page describes some parameters used to measure the properties of boundary layers. Consider a stationary body with a turbulent flow moving around it, like the semi-infinite flat plate with air flowing over the top of the plate. At the solid walls of the body the fluid satisfies a no-slip boundary condition and has zero velocity, but as you move away from the wall, the velocity of the flow asymptotically approaches the free stream mean velocity. Therefore it is impossible to define a sharp point at which the boundary layer becomes the free stream, yet this layer has a well-defined characteristic thickness. The parameters below provide a useful definition of this characteristic, measurable thickness.
== Motivations for boundary layers ==
(詳細はNavier-Stokes equations that describe the motion of fluids are not solved in the general case, simplifying assumptions are used to describe approximate behaviour. For a fluid of low viscosity (compared to the geometrical configuration of the problem), the Reynolds number is large and the viscous term in the Navier-Stokes equation is small compared to the inertial term in much of the space. However, this is not true near the boundaries of the fluid and this description fails to capture some important behavior in a lot of real systems, for instance the Kármán vortex street or the drag force on an airplane wing.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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