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 <a href="https://en.wikipedia.org/wiki/boundary_layer" class="extiw" title="w:boundary layer">boundary layer</a> and it is in this layer that we can see the formation of counter-rotating vortices in the flow. Magnification ×100 <br><a href="https://en.wikipedia.org/wiki/Shear_flow" class="extiw" title="w:Shear flow">Shear flow</a> vortices form as <a href="https://en.wikipedia.org/wiki/ethanol" class="extiw" title="w:ethanol">ethanol</a> is injected into a viscous <a href="https://en.wikipedia.org/wiki/glycerol" class="extiw" title="w:glycerol">glycerol</a> 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.)
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