• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.15 no.6 s.99
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• pp.687-696
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• 2005

Acoustic power spectrum of the upstream and downstream sound field due to an isotropic frozen turbulent gust impinging on a cascade of flat plate airfoils are computed by using a analytic formulation derived from Smith's method, and Whitehead's LINSUB codes. A parametric study of the effects on sound power of the number of blades and turbulence length scale is performed with an emphasis on analyzing the characteristics of sound power spectrum. Through the comparison of the computed results of sound power, it is found that acoustic power spectrum from the 2-D cascade subject to a ingested turbulence can be categorized into two distinct regions. one is lower frequency region where some spectral components of turbulence do not contribute to the cut-on acoustic modes and therefore the effect of the cascade geometry is more dominant ; the other is higher frequency region where all of spectral components of turbulence make contributions to cut-on acoustic modes and thus acoustic power is approximately proportional to the blade number.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.15 no.10 s.103
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• pp.1177-1185
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• 2005

This paper investigates the noise radiated by a cascade of flat-plate airfoils interacting with homogeneous, isotropic turbulence. At frequencies above the critical frequency, all wavenumber components of turbulence excite propagating cascade modes, and cascade effects are shown to be relatively weak. In this frequency range, acoustic power was shown to be approximately proportional to the number of blades. Based on this finding at high frequencies, an approximate expression is derived for the power spectrum that is valid above the critical frequency and which is in excellent agreement with the exact expression for the broadband power spectrum. The approximate expression shows explicitly that the acoustic Power above the critical frequency is proportional to the blade number, independent of the solidity, and varies with frequency as ${\phi}_{ww}(\omega/W$), where ${\phi}_{ww}$ is the wavenumber spectrum of the turbulence velocity and W is mean-flow speed. The formulation is used to perform a parametric study on the effects on the power spectrum of the blade number stagger angle, gap-chord ratio and Mach number. The theory is also shown to provide a close fit to the measured spectrum of rotor-stator interaction when the mean square turbulence velocity and length-scale are chosen appropriately.