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Research Papers

The Effects of Aerodynamic Asymmetric Perturbations on Forced Response of Bladed Disks

[+] Author and Article Information
Tomokazu Miyakozawa, Robert E. Kielb, Kenneth C. Hall

Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708-0300

J. Turbomach. 131(4), 041008 (Jul 02, 2009) (8 pages) doi:10.1115/1.3068319 History: Received August 20, 2008; Revised September 03, 2008; Published July 02, 2009

Most of the existing mistuning research assumes that the aerodynamic forces on each of the blades are identical except for an interblade phase angle shift. In reality, blades also undergo asymmetric steady and unsteady aerodynamic forces due to manufacturing variations, blending, mis-staggered, or in-service wear or damage, which cause aerodynamically asymmetric systems. This paper presents the results of sensitivity studies on forced response due to aerodynamic asymmetry perturbations. The focus is only on the asymmetries due to blade motions. Hence, no asymmetric forcing functions are considered. Aerodynamic coupling due to blade motions in the equation of motion is represented using the single family of modes approach. The unsteady aerodynamic forces are computed using computational fluid dynamics (CFD) methods assuming aerodynamic symmetry. Then, the aerodynamic asymmetry is applied by perturbing the influence coefficient matrix in the physical coordinates such that the matrix is no longer circulant. Therefore, the resulting aerodynamic modal forces in the traveling wave coordinates become a full matrix. These aerodynamic perturbations influence both stiffness and damping while traditional frequency mistuning analysis only perturbs the stiffness. It was found that maximum blade amplitudes are significantly influenced by the perturbation of the imaginary part (damping) of unsteady aerodynamic modal forces. This is contrary to blade frequency mistuning where the stiffness perturbation dominates.

Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Tuned system mode frequencies for stiff and flexible rotors versus nodal diameter

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Figure 2

Unsteady aerodynamic modal forces due to blade motion for first flex mode versus nodal diameter

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Figure 3

Aeroelastic eigenvalues for stiff disk without structural damping for all nodal diameters

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Figure 4

Normalized tuned blade amplitude versus nodal diameter excitations

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Figure 5

FRF for all blades due to single blade force perturbation reduced by 50% that is subjected to a 12ND FTW excitation

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Figure 6

All blade amplifications at resonant frequency due to single blade force perturbation reduced by 50% that is subjected to a 12ND FTW excitation

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Figure 7

Maximum blade amplifications due to single blade force perturbation reduced by 50% versus nodal diameter excitations

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Figure 8

FRF for all blades due to symmetric group force perturbation by ±25% that is subjected to a 9ND FTW excitation

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Figure 9

Maximum blade amplifications due to symmetric group force perturbation by ±25% versus nodal diameter excitations

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Figure 10

FRF for all blades due to random aerodynamic asymmetric force perturbation and frequency mistuning that are subjected to a 3ND BTW

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Figure 11

All blade amplifications at resonant frequency due to random aerodynamic asymmetric force perturbation and frequency mistuning that are subjected to a 3ND BTW

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Figure 12

Top view of perturbed forces of imaginary parts of influence coefficient matrix

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Figure 13

Bottom view of perturbed forces of imaginary parts of influence coefficient matrix

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Figure 14

CFD of maximum blade amplifications due to random aerodynamic asymmetric force perturbation and frequency mistuning that are subjected to a 3ND BTW

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Figure 15

95 percentile of maximum blade amplification factors due to random aerodynamic asymmetric force perturbation versus nodal diameter excitations.

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Figure 16

95 percentile of maximum blade amplification factors due to random aerodynamic asymmetric force perturbation and frequency mistuning versus nodal diameter excitations

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