Research Papers

Numerical Study on Aeroelastic Instability for a Low-Speed Fan

[+] Author and Article Information
Kuen-Bae Lee

Imperial College London,
Mechanical Engineering Department,
London SW7 2AZ, UK
e-mail: klee2@ic.ac.uk

Mark Wilson

Rolls-Royce plc,
Derby DE24 8BJ, UK
e-mail: mark.wilson@rolls-royce.com

Mehdi Vahdati

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

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received September 27, 2016; final manuscript received November 29, 2016; published online February 23, 2017. Editor: Kenneth Hall.

J. Turbomach 139(7), 071004 (Feb 23, 2017) (8 pages) Paper No: TURBO-16-1262; doi: 10.1115/1.4035569 History: Received September 27, 2016; Revised November 29, 2016

Over recent years, engine designs have moved increasingly toward low specific thrust cycles to deliver significant specific fuel consumption (SFC) improvements. Such fan blades may be more prone to aerodynamic and aeroelastic instabilities than conventional fan blades. The aim of this paper is to analyze the flutter stability of a low-speed/low pressure ratio fan blade. By using a validated computational fluid dynamics (CFD) model (AU3D), three-dimensional unsteady simulations are performed for a modern low-speed fan rig for which extensive measured data are available. The computational domain contains a complete fan assembly with an intake duct and the downstream outlet guide vanes (OGVs), which is a whole low-pressure (LP) domain. Flutter simulations are conducted over a range of speeds to understand flutter characteristics of this blade. Only the first flap (1F) mode is considered in this work. Measured rig data obtained by using the same fan set but with two different lengths of the intake showed a significant difference in the flutter boundary for the two intakes. AU3D computations were performed for both intakes and were used to explain this difference between the two intakes, and showed that intake reflections play an important role in flutter of this blade. This observation indicates that the experiment with the long intake used for the performance test may be misleading for flutter. In the next phase of this work, two possible modifications for increasing the flutter margin of the fan blade were explored: changing the mode shape of the blade and using acoustic liners in the casing. The results show that it is possible to increase the flutter margin of the blade by either decreasing the ratio of the twisting to plunging motion in 1F mode or by introducing deep acoustic liners in the intake. The liners have to be deep enough to attenuate the flutter pressure waves and hence influence the stability. The results indicate the importance of reflection in flutter stability of the fan blade and clearly show that intake duct needs to be included in flutter study of any fan blade.

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Fig. 1

Domain used for flutter computations

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Fig. 2

Pressure ratio versus nondimensional mass flow

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Fig. 3

Instantaneous static pressure (ΔP) contour at mid chord, mref  = 0.92

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Fig. 4

Measured and computed distribution of steady stagnation pressure downstream of rotor: (a) mref  = 0.956 and (b) mref  = 1.053

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Fig. 5

Steady downstream entropy in wake: (a) mref  = 0.956 and (b) mref  = 1.053

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Fig. 6

Configuration of two intakes: (a) short intake with a lined wall and (b) long intake with a hard wall

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Fig. 7

Characteristic map with stability boundaries for the model fan used in this study

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Fig. 8

1F/2ND aero-damping at HWK line

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Fig. 9

Cut-on frequencies upstream and downstream of the blade

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Fig. 10

Time histories of the blade displacement

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Fig. 11

Aerodamping for each component

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Fig. 12

Contours of blade vibration, 1F mode: (a) α = 0.2, (b) α = 0.3, and (c) α = 0.4

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Fig. 13

Time histories of displacement (a) and aerodamping (b) for three mode shapes

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Fig. 15

Geometry of the validation case

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Fig. 16

Unsteady pressure profiles

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Fig. 17

Comparison of CFD results against measured data

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Fig. 18

Insertion loss against liner depth/wavelength

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Fig. 19

1F/2ND aero-damping for hard wall and lined wall

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Fig. 20

1F/3ND aero-damping for a high speed fan

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Fig. 21

Aero-damping for three lined walls




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