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Research Papers

Contrarotating Open Rotor Operation for Improved Aerodynamics and Noise at Takeoff

[+] Author and Article Information
Alexios Zachariadis

e-mail: alexios.zachariadis@cantab.net

Cesare Hall

e-mail: cah1003@cam.ac.uk
Whittle Laboratory,
Department of Engineering,
University of Cambridge,
Cambridge CB3 0DY, United Kingdom

Anthony B. Parry

Rolls-Royce plc,
Moor Lane,
Derby DE23 8BJ, UK
e-mail: anthony.parry@rolls-royce.com

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received January 11, 2012; final manuscript received February 27, 2012; published online March 25, 2013. Editor: David Wisler.

J. Turbomach 135(3), 031010 (Mar 25, 2013) (10 pages) Paper No: TURBO-12-1005; doi: 10.1115/1.4006778 History: Received January 11, 2012; Revised February 27, 2012

The contrarotating open rotor is, once again, being considered as an alternative to the advanced turbofan to address the growing pressure to cut aviation fuel consumption and carbon dioxide emissions. One of the key challenges is meeting community noise targets at takeoff. Previous open rotor designs are subject to poor efficiency at takeoff due to the presence of large regions of separated flow on the blades as a result of the high incidence needed to achieve the required thrust. This is a consequence of the fixed rotor rotational speed constraint typical of variable pitch propellers. Within the study described in this paper, an improved operation is proposed to improve performance and reduce rotor-rotor interaction noise at takeoff. Three-dimensional computational fluid dynamics (CFD) calculations have been performed on an open rotor rig at a range of takeoff operating conditions. These have been complemented by analytical tone noise predictions to quantify the noise benefits of the approach. The results presented show that for a given thrust, a combination of reduced rotor pitch and increased rotor rotational speed can be used to reduce the incidence onto the front rotor blades. This is shown to eliminate regions of flow separation, reduce the front rotor tip loss and reduce the downstream stream tube contraction. The wakes from the front rotor are also made wider with lower velocity defect, which is found to lead to reduced interaction tone noise. Unfortunately, the necessary increase in blade speed leads to higher relative Mach numbers, which can increase rotor alone noise. In summary, the combined CFD and aeroacoustic analysis in this paper shows how careful operation of an open rotor at takeoff, with moderate levels of repitch and speed increase, can lead to improved front rotor efficiency as well as appreciably lower overall noise across all directivities.

Copyright © 2013 by ASME
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Figures

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Fig. 1

Overview of the Rig-140 straight blade geometry

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Fig. 2

Summary of blade geometrical characteristics

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Fig. 3

Experimental and calculated distributions of pressure coefficient (CP) on the rig bullet surface at takeoff, from [10]

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Fig. 4

Measured and calculated cruise absolute stagnation pressure ratio (TPR) downstream of the contrarotating rotors at cruise

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Fig. 5

Comparison of the front rotor-alone noise directivities at blade passing frequency in the very near and moderate near field at takeoff

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Fig. 6

Very near-field noise directivities of the (2,1) rotor-rotor interaction tone for straight blades at takeoff and (1,2) interaction tone for another blade design at a takeoff condition

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Fig. 7

Radial distribution of the front and rear rotor incidence at the nominal takeoff condition

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Fig. 8

Contours of entropy function (exp-(s/R)) for Rig-140 at the nominal takeoff condition

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Fig. 9

Rotor face velocity triangles before and after repitching and speeding up at takeoff: (a) thrust versus incidence and (b) propeller efficiency versus incidence

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Fig. 10

Performance tradeoffs due to rotor repitch and rotational speed increase

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Fig. 11

Contours of entropy function downstream of the front rotor due to combined blade repitch and speed up

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Fig. 12

Effect of repitching and speeding up on the rotor suction surface limiting streamlines at takeoff

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Fig. 13

Radial distribution of front rotor stagnation pressure loss coefficient (YP) due to operating point variation at takeoff

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Fig. 14

Calculated front rotor tip vortex contraction due to operating point variation at takeoff

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Fig. 15

Wake profiles at r/R = 0.70 downstream of the front rotor trailing edge

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Fig. 16

Orientation of noise prediction locations: (a) front rotor and (b) rear rotor

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Fig. 17

Comparison of the predicted front and rear rotor-alone tones at 1 and 2 BPF

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Fig. 18

Comparison of the predicted (1,1) and (1,2) interaction tones

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Fig. 19

Predicted sound pressure level (dBA) reduction relative to the nominal condition at takeoff

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