An unsteady and oscillatory flow behaviour of a moving fluid around a body in which vortices are formed at certain velocities. The vortices are formed on the downstream side of the body and become detached from either side of the body known as a von Kármán vortex street. The body will tend to move towards the region of low pressure and forms the basis of a vortex-shedding flow meter. The vibrating oscillations are also responsible for the whistling sound of overhead power lines. Tall stacks and chimneys are protected from the potentially destructive oscillatory effects by the use of helical fences known as strakes or spoilers.