Semi-empirical prediction of tone noise due to counter-rotating open rotors
Counter-rotating open rotors (CROR) were extensively studied to power medium-size aircraft in the 80s aft er the first increases in fuel costs. Indeed, their efficiency i s greater than that of turbofans and of single prop fans. They have again become a topical subject due to the new incre ase of fuel costs and to the risk of oil shortage. They however raise a serious acoustic issue because noise is gen erated by the two rotors and by their interactions, and due to the lack of any shielding nacelle. An original and fast semi-empirical method is proposed to predict radia ted sound levels. The objective is to rapidly assess if certification rules can be fulfilled, and to estimate the possib le impact of a future fleet on noise contours around airports. This work is thus focused on takeoff and approach conditions (advancing Mach number lower than 0.3). It is shown that direc tivity of a tone is mainly determined by a Bessel fu nction, and a parabolic pattern is suggested for overall sound pr essure levels. The shape of third-octave spectra is based on the large number of interaction tones which are present in each frequency band. Finally, some information is given on overall sound levels.
Experimental observations of an installed-on-pylon contra-rotating open rotor with equal blade number in pusher and tractor configuration
Noise from contra-rotating open rotors is a major obstacle to the adoption of this fuel efficient technology as a viable aircraft propulsion system. A better understanding of both contra-rotating open rotor noise generation and reduction has been achieved due to ongoing extensive research. One of the most recent research activities is the WENEMOR (wind tunnel tests for the evaluation of the installation effects of Noise EMissions of an open rotor advanced regional aircraft) project, which has been developed in response to the requirements described in the Clean Sky-Integrated Technology Demonstrators under the heading of Green Regional Aircraft. The project investigates the airframe installation effects of a 1 / 7 th scale model of a regional aircraft equipped with two contra-rotating open rotors of the same rotor diameter, the same rotational speed and equal blade number. For this case, the blade passing frequency of rotor-alone tones and the frequency of relevant interaction tones cannot be distinguished due to the equal blade count of the two rotors. This study presents the tone directivity plots up to 4 × blade passing frequency of the isolated WENEMOR single pylon contra-rotating open rotor engine in both pusher and tractor configurations at various angles of incidence and flow velocities. A linear array of 13 microphones is deployed for the far field sound measurements. The tone directivity trends show the efficient on-axis acoustic radiation at all blade passing frequency tones with the contra-rotating open rotor tone at 2 × blade passing frequency dominating in the vast majority of the tests. The main objective is to compare the acoustic emission of pusher and tractor configurations tested under the same flow velocities and angles of incidence. The results suggest that the pusher configuration of the isolated contra-rotating open rotor tends to be slightly louder than the tractor at 2 × blade passing frequency. However, it is shown that the acoustic performance of the isolated contra-rotating open rotor is complicated and sensitive to any change in the flow velocity and the angle of incidence. The increasing flow velocity and the increasing angle of incidence show limited consistency in proportional trends in the directivity plots of sound pressure levels. It is anticipated that the findings will be different for a more realistic case of installed-on-model contra-rotating open rotor.
Prediction - and its validation - of the acoustics of multiblade
A newly developed high resolution 3-D Panel method to predict the unsteady aerodynamics of multiblade helicopter rotors has been used to generate the input data for an also new Acoustics code based on Farassat's Formulation 1A of the FWH/equation. The combination of this aerodynamics/aeroacoustics codes is used to predict the acoustics of multiblade rotors in hover and forward flight. Of special interest are cases of low speed descent where severe BVI is known to occur. The use of panel method excludes the treatment of shocks in the rotor flow field. No restrictions are placed on the blade profile, planform or other geometry characteristics as well as blade motions. The acoustic pressure-time history, mid-frequency sound pressure levels and other predictions are validated with flight test and wind tunnel data for two and four blade rotors in hover and forward flight modes. Some comparisons with the predictions of the well known WOPWOP code have also been performed.