Effect of Turbulence Structure on Broadband Fan Noise

Abstract

The effects of turbulence energy spectra on fan broadband noise are examined. The acoustic power density (APD) spectra scattered from the fan are calculated using our recently developed code BB3D. A parallelization scheme based on the master-slave paradigm is implemented on BB3D to reduce computational time. We use the geometry and inflow conditions of NASA STD low-count fan stator. The effects of the popular Liepmann, von Karman and Gaussian isotropic turbulence energy models are analyzed. The difference between the Liepmann and von Karman models on the APD is much smaller than that indicated by the turbulence energy models, particularly at higher fan-relevant frequencies. The Gaussian model yields markedly different APD spectra. At low and moderate frequencies, reducing the turbulence integral length scale strongly reduces the APD level. At higher frequencies, however, larger turbulence length scales yield slightly lower APD level. At higher Mach number the effect of reducing the turbulence length scales is even stronger at low frequencies but much reduced at higher frequencies. Finally, the effect of increasing the Mach number is strongly frequency-dependent. It generally increases the APD level, however, this increase is much stronger at high frequency. This suggests that fan broadband noise has no simple dependence on the Mach number.