Aerodynamic broadband noise from contra-rotating open rotors

Abstract

In recent years, there has been growing interest in contra-rotating open rotors (CRORs) for use as the power plants on aircraft, since they are estimated to burn 20% to 30% less fuel than equivalent turbofan engines for short-haul flights. However, one of the main challenges for their introduction is their very high levels of noise emissions. There is therefore a need for schemes by which the noise from CRORs can be predicted and hence reduced. Nearly all of the research effort on the prediction and reduction of CROR noise has been so far focused on the tonal component, whereas broadband noise emissions remain poorly understood. Nevertheless, it is shown that broadband noise emissions from CRORs can be significant, hence the necessity of the present research.
In this thesis, broadband noise emissions from uninstalled CRORs are investigated for the first time. It is assumed that the two most significant sources of broadband noise in uninstalled CRORs are the broadband rotor-wake/rotor interaction noise (BRWI) and the broadband rotor trailing edge noise (BRTE). Fast semi-analytical prediction schemes are developed for these sources of broadband noise, which exhibit good agreement with noise measurements from a scaled model CROR. The relative importance of the BRWI and BRTE noise sources is investigated for a realistic CROR configuration at assumed take-off, cruise and approach-type conditions. It is predicted that both broadband noise sources are significant at assumed take-off, whereas only the BRTE noise contributes to the total broadband noise emissions at assumed cruise and approach. A parameter study is conducted to investigate the effects on CROR broadband noise emissions of variations in rotor-rotor separation distance, rotor speed and blade number at constant engine power, torque split and solidity. The validity of two widely used approximations for fan broadband noise predictions is also studied: Amiet's approximate BRTE model and the use of isolated airfoil theory for turbulence-cascade interaction noise. Criteria for the validity of Amiet's approximate model are established by comparing it to the general BRTE noise model developed in this work. The two models are shown to differ in the low and high frequency limits, but excellent agreement is observed for realistic rotor configurations over most of the audible frequency range. In addition, sound power predictions from isolated airfoils and blade cascades in a turbulent flow are compared. Excellent agreement is observed between an isolated airfoil model and a cascade model for frequencies higher than a critical frequency. Below this critical frequency, agreement is poor for high solidity cascades but is reasonable for low solidity cascades, typical of CRORs, thus validating the use of isolated airfoil theory in the BRWI model.

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