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Abstract:The term "open rotor" refers to unducted counter-rotating dual rotors or propellers used for propulsion. The noise generated by an open rotor is very complicated and requires special techniques for its analysis. The determination of its tone and broadband components is vital for properly assessing the noise control parameters and also for validating open rotor noise prediction codes. The data analysis technique developed by Sree for processing raw acoustic data of open rotors has been modified to yield much better results of tone and broadband separation particularly for the case when the two rotor speeds are approximately the same. The modified algorithm is found to eliminate most or all of the "spikes" previously observed in the broadband spectra computed from the original algorithm. A full description of the modified algorithm and examples of improved results from its application are presented in this paper.

The self noise generated by a low-speed, isolated, ducted rotor was investigated experimentally. The apparatus provided inflow conditions to the rotor that were circumferentially uniform and steady, such that the noise generated by the rotor was primarily a result of unsteady surface pressures convecting past the blade trailing-edges. The volume flow rate through the rotor was manipulated in order to vary the blade-relative incidence angle, or "blade loading." The rotor RPM was also varied in order to study the speed scaling of the radiated sound. The resulting spectra were found to exhibit a wide range of features including blade-rate tones, broadband sound, and stall noise. The broadband sound was predicted using a model based on wall pressures. The wall pressures, in turn, were measured using a microphone mounted to an airfoil, and were also modeled using an empirical spectrum for boundary layers. Both methods of predicting wall pressures were found to result in reasonable broadband noise estimates. NOMENCLATURE B Number of rotor blades BPF Blade passing frequency, V tip B/2πr tip c Speed of sound, m/s C Rotor blade chord, m D Duct diameter, m f Frequency, Hz k Acoustic wave number, 2πf/c r tip Rotor tip radius, m

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An open rotor experiment was conducted at cruise Mach numbers and the unsteady pressure in the nearfield was measured. The system included extensive performance measurements, which can help provide insight into the noise generating mechanisms in the absence of flow measurements. A set of data acquired at a constant blade pitch angle but various rotor speeds was examined. The tone levels generated by the front and rear rotor were found to be nearly equal when the thrust was evenly balanced between rotors.