English: Fluid streams past a solid boundary from right to left. As it does so it experiences friction with the boundary resulting in the application of a shear stress. According to the no-slip condition, the speed of the flow at the boundary is equal to zero, but the speed increases from 0 to v (v = the speed of the fluid) at some height above the boundary. The region between the boundary and this height is called the boundary layer and it is in this layer that we can see the formation of counter-rotating vortices in the flow. Magnification ×100
Shear flow vortices form as ethanol is injected into a viscous glycerol medium from the right side, and the ethanol streams along the solid arched boundary of an air cavity. (Note, the small circular air cavity is not in the flow path.)