Theory

When a flowing medium strikes a non-streamlined object or obstruction, it separates and moves around the object and passes on downstream. At the point of contact with the object, vortex swirls or eddy currents separate from the object on alternating sides. When this occurs, the separating or shedding causes a local increase in pressure and a decrease in velocity on one side of the object, and a local decrease in pressure with corresponding increase in velocity on the other side of the object. Vortex swirls are shed continuously ?180 degrees out of phase with each other. The frequency of the shedding process is proportional to the velocity of the material flowing past the object.