VORTEX NOISE FRO & ROTATING CYLINDRICAL RODS

A series of round rods of the same diameter were ro-tated indivfdually about the mid-point of each rod. Vor-tices are shed from the rods when in motion, giving rise to the emisskon of sound. With the rotating system placed--in the open air, the distribution of sound in space, the acoustical power output, and the spectral distribution have been studies. The frequency of emission of vortices m 4 from any ooint on the rod is shown to be given by the for-mula of van K&m&. From the spectrum estimates are made of the distributlon.of acoustical paver along the rQd, the---.. amount of air concerned in sound productioil, the "equiva--:. lent size " of the vortices, and the acoustical energy con- c tent for each vortex. c 1 IBTRODQGTION The flow of air past stretched wires has been known for many centuries to produce musical tones, and this phenomenon forms the basis of the ancient Aeolian harp. This instrument consisted of a nunber of taut wires of graduated lengths fastened to a frame. The device was placed in a location subject to strong drafts and a 5uc-cession of pure tones resulted as the air spee-d changed in.-magnitude.-_ The first investigator to exanine this-phenomenon wag Strouhal (reference 1). He attached vertical wires to a drum in such a may that they coulrl be rotated about an ax---is parallel to their length. He. found the goneratod fre-quencies to be independent of the m&teridl',~len~~th, and---tension of the wire, and that they depended only on the-. speed of rotation and the diameter of the sire.-‘The gen--erated freouency proved to be _._. _--l a f = 0.185 f cycles.