Aerodynamic broadband noise from contra-rtating open rotors

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Description/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 signifcant,
hence the necessity of the present research.
In this thesis, broadband noise emissions from uninstalled CRORs are investigated for
the frst time. It is assumed that the two most signifcant 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 confguration at assumed take-off,
cruise and approach-type conditions. It is predicted that both broadband noise sources
are signifcant 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.