When fluid flows past the obstacle, boundary layers of viscous, slowmoving
fluid are formed along the outer surface. Because the obstacle is not streamlined,
the flow cannot follow the contours of the body on the downstream side, and the
separate layers become detached and roll into eddies or vortices in the low-pressure
region behind the obstacle. The shedding frequency of these alternately shed vortices
is proportional to the fluid velocity past the body.