When a viscous fluid flows along a fixed impermeable wall, or past the rigid surface
of an immersed body, an essential condition is that the velocity at any point on the
wall or other xed surface is zero. The extent to which this condition modies the
general character of the
ow depends upon the value of the viscosity. If the body is of
streamlined shape and if the viscosity is small without being negligible, the modifying
effect appears to be conned within narrow regions adjacent to the solid surfaces; these
are called boundary layers