Research Papers

Investigation of Flutter Mechanisms of a Contra-Rotating Open Rotor

[+] Author and Article Information
Sina C. Stapelfeldt

Vibration University Technology Centre,
Department of Mechanical Engineering,
Imperial College London,
London SW7 2AZ, UK
e-mail: sina.stapelfeldt@imperial.ac.uk

Anthony B. Parry

Fellow ASME
Rolls-Royce plc,
Derby DE24 8BJ, UK
e-mail: anthony.parry@rolls-royce.com

Mehdi Vahdati

Vibration University Technology Centre,
Department of Mechanical Engineering,
Imperial College London,
London SW7 2AZ, UK
e-mail: m.vahdati@imperial.ac.uk

1Corresponding author.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received November 8, 2015; final manuscript received November 22, 2015; published online February 3, 2016. Editor: Kenneth C. Hall.

J. Turbomach 138(5), 051009 (Feb 03, 2016) (10 pages) Paper No: TURBO-15-1251; doi: 10.1115/1.4032186 History: Received November 08, 2015; Revised November 22, 2015

The growing pressure to reduce fuel consumption and cut emissions has triggered renewed interest in contra-rotating open rotor (CROR) technologies. One of their potential issues is self-excited or forced vibration of the unducted, light-weight, highly swept blades. This paper presents a numerical study into the flutter behavior of a CROR rig at take-off conditions. The study presented in this paper aimed to validate the numerical approach and provide insights into the flutter mechanisms of the open rotor under investigation. For the initial validation, pressure profiles and thrust coefficients from steady-state mixing plane calculations were compared against rig measurements. A full domain unsteady analysis predicted front rotor instability at low advance ratios. Flutter occurred in the first torsional mode in 0 and 1 nodal diameter (ND) which agreed with experimental observations. Subsequent unsteady computations focused on the isolated front rotor and first torsional mode. The flow field and aerodynamic damping over a range of advance ratios were studied. It was found that minimum aerodynamic damping occurred at low advance ratios when the flow was highly three-dimensional on the suction side. A correlation between the quasi-steady loading on the blade and aeroelastic stability was made and related to the numerical results. The effects of variations in frequency were then investigated by linking local aerodynamic damping to the unsteady pressure on the blade surface.

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Fig. 2

Computational domain

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Fig. 1

Front rotor response during test campaign (PL-A)

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Fig. 3

Thrust coefficients

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Fig. 4

Comparison of steady-state CFD (lines) and measurements (points)

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Fig. 5

Pitchline A: front rotor streamlines on axial velocity contours (blade planforms not to scale)

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Fig. 6

Pitchline B: front rotor streamlines on axial velocity contours (blade planforms not to scale)

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Fig. 7

Modal displacement history and calculated amplitudes used to evaluate aerodynamic damping (log-dec)

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Fig. 8

Front-rotor eigenmodes with reduced frequency, k, based on chord at 90% radius (normalized axial displacement, planforms not to scale)

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Fig. 10

Aerodynamic damping comparison of three computational setups

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Fig. 11

Modal displacement for Modes 2 and 3 (0ND)

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Fig. 12

Damping variation with advance ratio

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Fig. 9

Modal displacement Mode 3

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Fig. 13 PL-A

radial damping distribution

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Fig. 14 PL-A

log-dec distribution at J = 1.14 (per unit area)

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Fig. 15 PL-A

steady state pressure coefficient at 90% span

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Fig. 16

Lift and moment on aerofoil section

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Fig. 17

Spanwise variation of center of pressure and aerodamping

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Fig. 18

Moment curve at four spanwise positions

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Fig. 19

Spanwise variation of moment curve slope and aerodamping

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Fig. 20

Pitchline A: comparison of unsteady pressure and damping at stable (J = 1.64) and unstable (J = 1.14) operating point

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Fig. 21

Illustration of blade section displacement in torsional mode

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Fig. 22

PL-A J = 1.14: radial damping distribution for two frequencies

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Fig. 23

PL-A J = 1.14: comparison of unsteady pressure and aerodamping for two frequencies



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