This describes a region of a flow where there is a significant velocity gradient, and consequently the viscous shear stresses defined by
are important. The most common example of a shear layer arises when a fluid passes over a solid boundary (Figure 1a) to form what is commonly termed a Boundary Layer. In this case, the velocity distribution is approximated by a Universal Velocity Profile. Another example involving a free shear layer (or one which is not attached to a solid boundary) arises in the lee of a structure placed within a flow. In this case the shear layer develops between the free stream velocity (U0) and the near zero velocity occurring within the wake region (Figure 1b). (See also Cross Flow.)
Laboratory investigations of shear flows are often based upon Couette Flow in which a layer of fluid is located between two plates, one of which is moving with a given velocity. This produces a region of strongly sheared flow which may be either laminar or turbulent.