M. H. Krane
P. H. Bent
D. A. Quinlan
AT&T Bell Labs., 600 Mountain Ave., Murray Hill, NJ 07974
Previous investigations have shown the presence of narrow-band peaks in the acoustic spectra of ducted axial fans at frequencies other than harmonics of the blade passage frequency. This phenomenon is of practical importance because these peaks are strongest when the fan is operating near its best efficiency point. Measurements of the fan flowfield were conducted in the rotating frame using hot-wires and surface-mounted hot-film sensors in order to identify the source of the narrow-band tones. These measurements have shown a rotating wavelike motion composed of several discrete modes. The wave pattern was strongest on the pressure side of the blade in the region between the tip and the midspan. One of the modes of the rotating pattern was found to be an integer multiple of the number of fan blades, resulting in efficient sound generation at that frequency. [sup a)]Present address: Acoustics Technology, McDonnell Douglas Aerospace, Mail Code 71-35, 1510 Hughes Way, Long Beach, CA 90810-1864.