Discrete Frequency Noise Due to Irregularity in Blade Row of Axial Fan Rotor

It is experimentally made clear in this paper that discrete tones appear in the frequency band lower than the blade passing frequency, when a rotor has manufacturing errors of stagger angle, pitch, camber or chord length, which is at variance with the general understanding that the frequency of discrete noise corresponds to the blade passing frequency and its harmonics. The relations between the magnitude of the deviation of the blading parameter from a design value and the induced sound pressure level of discrete tone and the resulted change of fluid dynamic characteristics of the fan are also examined. analytic considerations of measured wave patterns of pressure fluctuation indicate that this type of discrete noise originates in a distortion of circumferential distribution of mean flow rates through the rotor from the uniform distribution and the consequent circumferential variation of mean pressure level, arising from the unequal blade loading of each blade of a rotor due to the error in manufacturing the rotor.