Thin layer of
fluid surrounding a
body. It can be
laminar or
turbulent. A
laminar boundary layer is one in which the
fluid velocity is well-ordered and
regular throughout the boundary layer. The
speed of the
fluid is taken to be
zero at the
surface of the body. A
turbulent boundary layer is one in which the
fluid velocity shows a great deal of
randomness throughout the layer. A
turbulent boundary layer is responsible for
increased drag on the body, since the mean
velocity profile of the boundary layer is 'fuller' than that of a
laminar boundary layer, resulting in increased
shear stress at the body's
surface.
Laminar to
turbulent transition occurs on, for example, the
raised red
stitches on a
baseball. This drag transition is responsible for a certain
portion of the
behavior of
knuckleballs,
curve balls,
sliders, etc.
Aircraft whose value is determined by the cost of operation per passenger-mile typically require a great deal of thought and planning be put into where, when, and if boundary layer transition occurs on their wings. When a boundary layer is forced to turn through an excessively high angle on the surface of the body, it detaches and forms the boundary of a wake. On an airfoil, boundary layer detachment results in a stall. A laminar boundary layer will detach more readily than a turbulent one. This is why a smooth ping pong ball curves quite a lot when struck hard through the air. It would curve less if it had raised stitches, if memory serves.
If the fluid in question is air, the boundary layer is typically the only location in a flowfield where fluid viscosity cannot be neglected. Many calculations ignore the effects of the boundary layer since it is generally very thin with respect to the size of the body it surrounds. The boundary layer is responsible for the bulk of the heat transfer which occurs at the surface of a hypersonic vehicle (like the space shuttle).
In a pipe, the boundary layer's thickness grows until the thickness of the boundary layer equals the radius of the pipe. Using this knowledge, and formulae for the speed of boundary layer growth, calculations of pipe pressure loss per linear foot are possible.
Basically, a critical concept in fluid mechanics.