Flow Induced Noise - Overview of Phenomenon

Since ancient times, it has been known that wind causes vortex induced vibration of the wires of an Aeolian harp. According to Rabbinic records, King David hung his kinnor (kithara) over his bed at night where it sounded in the midnight breeze. In the fifteenth century, Leonardo da Vinci sketched a row of vortices in the wake of a piling in a stream. In 1878, Strouhal found that the Aeolian tones generated by a wire in the wind were proportional to the wind speed divided by the wire thickness. He noticed that the sound greatly increased when the natural tones of the wire coincided with the Aeolian tones. In 1879, Lord Raleigh found that a violin string in a chimney draft vibrated primarily across the flow, rather than with the flow. The periodicity of the wake of a cylinder was associated with vortex formation by Benard in 1908 and with the formation of a stable street of staggered vortices by von Karman in 1912. (ref.: Flow-Induced Vibration, Robert D.Blevins).

As a fluid particle flows toward the leading edge of a cylinder, the pressure in the fluid particle rises from the free stream pressure to the stagnation pressure. The high fluid pressure near the leading edge impels flow about the cylinder as boundary layers develop about both sides. On the other hand, the high pressure is not sufficient to force the flow about the back of the cylinder at high Reynolds numbers. Near the widest section of the cylinder, the boundary layers separate from each side of the cylinder surface and form two shear layers that trail aft in the flow and bound the wake. Since the innermost portion of the shear layers, which is in contact with the cylinder, moves much more slowly than the outermost portion of the shear layers, which is in contact with the free flow, the shear layers roll into the near wake, where they fold on each other and coalesce into discrete swirling vortices. A regular pattern of vortices, called a vortex street, trails aft in the wake, according to the figure above. The vortices interact with the cylinder and they are the source of the effects called vortex induced vibration. (ref.: Flow-Induced Vibration, Robert D.Blevins).