Investigation of Fan-Wake/Outlet-Guide-Vane Interaction Broadband Noise

Fan-wake/outlet-guide-vane interaction broadband noise in turbofan jet engines is studied. The mechanism and some issues are first discussed using a two-dimensional gust-prediction model. An oblique gust-prediction model is then developed. Quasi-three-dimensional unsteady lift is calculated using a two-dimensional equivalence method. It is coupled with annular duct modes to obtain the sound power spectrum density. Spanwise turbulence integral length scales and their impact on power spectrum density predictions are investigated. A spanwise integration limit suitable for the complete frequency range is proposed. The model is validated using the NASA Source Diagnostic Test data. Sound power scaling with vane count B is examined. If solidity is maintained, the cascade response does not converge on the single-airfoil response, even for low vane counts. The sound power varies inversely with B at low frequency; it scales with B at very high frequency. The power spectrum density trend with the fan tip Mach number M-T is also identified. It scales with M-T(5) if turbulence intensity in the fan wake scales "ideally" with M-T. At offdesign conditions, fan wakes are not ideal; therefore, different speed trends apply. M-T(3.3) scaling is found to best fit the Source Diagnostic Test data and the prediction.