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Research Papers

# Rotor Interaction Noise in Counter-Rotating Propfan Propulsion Systems

[+] Author and Article Information
Andreas Peters

Department of Aeronautics and Astronautics, Gas Turbine Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02193

Zoltán S. Spakovszky

Department of Aeronautics and Astronautics, Gas Turbine Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02193(zolti@mit.edu)

Certification challenges such as blade containment are acknowledged but not taken into account in the present analysis.

Advance ratio, power coefficient, and thrust coefficient are defined using the average shaft speed $N=(N1+N2)/2$ and average rotor diameter $D=(D1+D2)/2$.

At midspan, fluctuations of up to 20% in pressure coefficient around the mean were found on the front-rotor pressure side compared to 2% on the rear-rotor pressure side.

Because of the weight penalties due structural reinforcements, cabin insulation and increased propulsion system weight, the maximum takeoff weight of the CRP aircraft arrangements increased relative to the datum turbofan powered aircraft. This in turn led to slightly higher Stage 4 noise limits for the CRP powered aircraft configurations.

J. Turbomach 134(1), 011002 (May 24, 2011) (12 pages) doi:10.1115/1.4003223 History: Received July 08, 2010; Revised September 05, 2010; Published May 24, 2011; Online May 24, 2011

## Abstract

Due to their inherent noise challenge and potential for significant reductions in fuel burn, counter-rotating propfans (CRPs) are currently being investigated as potential alternatives to high-bypass turbofan engines. This paper introduces an integrated noise and performance assessment methodology for advanced propfan powered aircraft configurations. The approach is based on first principles and combines a coupled aircraft and propulsion system mission and performance analysis tool with 3D unsteady, full-wheel CRP computational fluid dynamics computations and aeroacoustic simulations. Special emphasis is put on computing CRP noise due to interaction tones. The method is capable of dealing with parametric studies and exploring noise reduction technologies. An aircraft performance, weight and balance, and mission analysis was first conducted on a candidate CRP powered aircraft configuration. Guided by data available in the literature, a detailed aerodynamic design of a pusher CRP was carried out. Full-wheel unsteady 3D Reynolds-averaged Navier-Stokes (RANS) simulations were then used to determine the time varying blade surface pressures and unsteady flow features necessary to define the acoustic source terms. A frequency domain approach based on Goldstein’s formulation of the acoustic analogy for moving media and Hanson’s single rotor noise method was extended to counter-rotating configurations. The far field noise predictions were compared to measured data of a similar CRP configuration and demonstrated good agreement between the computed and measured interaction tones. The underlying noise mechanisms have previously been described in literature but, to the authors’ knowledge, this is the first time that the individual contributions of front-rotor wake interaction, aft-rotor upstream influence, hub-endwall secondary flows, and front-rotor tip-vortices to interaction tone noise are dissected and quantified. Based on this investigation, the CRP was redesigned for reduced noise incorporating a clipped rear-rotor and increased rotor-rotor spacing to reduce upstream influence, tip-vortex, and wake interaction effects. Maintaining the thrust and propulsive efficiency at takeoff conditions, the noise was calculated for both designs. At the interaction tone frequencies, the redesigned CRP demonstrated an average reduction of 7.25 dB in mean sound pressure level computed over the forward and aft polar angle arcs. On the engine/aircraft system level, the redesigned CRP demonstrated a reduction of 9.2 dB in effective perceived noise (EPNdB) and 8.6 EPNdB at the Federal Aviation Regulations (FAR) 36 flyover and sideline observer locations, respectively. The results suggest that advanced open rotor designs can possibly meet Stage 4 noise requirements.

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## Figures

Figure 1

Aerodynamic and acoustic performance assessment framework for counter-rotating propfans

Figure 2

Baseline CRP design

Figure 3

CRP noise estimation methodology

Figure 4

Baseline CRP grid-block topology (left) and close-up of rotor meshes at midspan (right)

Figure 5

Baseline CRP spectrum at 85 deg polar angle from the inlet centerline

Figure 6

Baseline CRP interaction tone noise level at frequency BPF1+BPF2 (left), at 2×BPF1+BPF2 (center), and at BPF1+2×BPF2 (right)

Figure 7

Baseline CRP density distribution at midspan

Figure 8

Baseline CRP density distribution at x/D1=0.12 (top) and blade-tip vortex system (bottom): front-rotor tip-vortices interact with rear rotor

Figure 9

Baseline CRP entropy distribution near hub (at 10% span)

Figure 10

Dissection of CRP noise mechanisms for interaction tones BPF1+BPF2 (left), 2×BPF1+BPF2 (center), and BPF1+2×BPF2 (right), baseline CRP, M=0.25

Figure 11

Baseline CRP noise mechanism contributors to first six interaction tones (percentages based on p′2 averaged over forward and aft polar arcs), M=0.25

Figure 12

Advanced design CRP geometry and near-field density distribution

Figure 13

Comparison of baseline and advanced design CRP directivity at interaction tone frequencies BPF1+BPF2 (left), 2×BPF1+BPF2 (center), and BPF1+2×BPF2 (right), M=0.25

Figure 14

Advanced design CRP noise mechanism contributors to first six interaction tones (percentages based on p′2 averaged over forward and aft polar arcs), M=0.25

Figure 15

Relative change in mean SPL for advanced design CRP compared to baseline CRP

## Errata

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