SOUND RADIATION FROM A HIGH-SPEED AXIAL FLOW FAN DUE TO THE INLET TURBULENCE QUADRUPOLE INTERACTION

摘要

One of the dominant sources of broadband noise (and possibly also of pure tones) in aircraft en¬gine fans is the turbulence drawn into the fan inlet. A number of investigations have analyzed this noise source by using a dipole model. However, at the high subsonic Mach numbers at which some current engine fans operate, the quadrupole mechanism may be dominant over the dipole mechanism. In such a case, analyses based on the dipole model are unsatisfactory. No analysis of this noise source which is based on the quadrupole model and is suitable for high subsonic Mach numbers has been given. The present report is written to fill this gap. A for¬mula is obtained for the total acoustic power spectra radiated out the front of the fan as a func¬tion of frequency. The formula involves the design parameters of the fan as well as the statis¬tical properties of the incident turbulence. Numerical results are calculated for values of the parameters in the range of interest for quiet fans tested at the Lewis Research Center. As in the dipole analysis, when the turbulence correlation lengths become equal to the interblade spacing, the predicted spectra exhibit peaks around the blade passing frequency and its harmonics. There has recently been considerable conjecture about whether the stretching of turbulent eddies as they enter a stationary fan could result in the inlet turbulence being the dominant source of pure tones from nontranslating fans (i. e., fans in nonmoving aircraft). The results of the current analysis show that, unless the turbulent eddies become quite elongated, this noise source con-tributes predominantly to the broadband spectrum.