Effects of Contact Mistuning on Shrouded Blisk Dynamics

Abstract

Mistuning commonly refers to non-cyclically symmetric variations in such an otherwise cyclically-symmetric structure. Mistuning in a blisk due to variations in blade materials and geometry have been studied extensively and are known to have a significant impact on the forced response of blisks. However, mistuning can also arise due to variations at contact interfaces within a blisk with friction damping mechanisms such as under platform dampers or shrouds. Past literature analyzing the effect of this source of contact mistuning is limited, particularly by the use of lumped or single-node contacts at each sector to shorten analysis times. In this paper, we aim to better understand the specific effects of parameter variations across contact interfaces on an otherwise tuned blisk. A blisk with shroud to shroud contacts is considered. Accurate representation of microslip phenomena are incorporated in the analysis by modeling multiple localized node to node contact models at contact surfaces on each blisk sector. Contact stiffnesses which dictate the friction damping dynamics of the shrouds are chosen as the mistuning parameters. The harmonic balance method is used to solve for forced responses. We analyze cases with random patterns of contact stiffnesses in different microslip regimes in the proximity of different modal regions. Probabilistic analysis of nonlinear contact responses are carried out close to a linear region where comparatively high amplification factors are observed. Statistics are also developed for linear cases and compared with the nonlinear case to qualify the dependence of amplification factors of nonlinear forced responses on the level of microslip and on the variance of contact parameters.

Dynamic Modeling and Projection-Based Reduction Methods for Bladed Disks With Nonlinear Frictional and Intermittent Contact Interfaces

“…Mistuning occurs when there is a variability across the sectors of the blisk in a parameter, which affects its dynamics. Variations in different material properties such as mass, stiffness, and damping as well as in geometry across sectors have been studied in literature [4,5,[8][9][10][11][12][13][14]27,[105][106][107]. The consequence is that the sector-level system matrices are nonidentical across sectors.…”

“…If any sector of the blisk is considered to be geometrically and materially identical to any another, the DoFs of the blisk dynamic may be reduced by a factor of the number of blades by the application of suitable constraints. However, it is well known that even small differences, which exist between different sectors in practice due to manufacturing tolerances, collectively called mistuning, can have significant effects on blisk response amplitudes [4][5][6][7][8][9][10][11][12][13][14]. Mistuning may be small when the variability in parameters between the sectors does not significantly affect the modes of the structure, or large when the response of the mistuned blisk in a certain frequency range cannot be captured by a set of nominally cyclic symmetric or tuned modes whose natural frequencies lie in the corresponding range.…”

Vibration analysis of mistuned damped blades with nonlinear friction and contact

“…[1][2][3][4][5][6][7] Blade mistuning is one of the problems that hardly averted in industrial production processes of realistic turbine blades, such as blade manufacturing, installation tolerances, and wear, etc. [8][9][10] Indeed, the vibration amplitude and stress level of blades are highly sensitive to mistuning, especially in frequency steering regions. One evident effect of mistuning is scatters of natural frequencies, mode shapes, and properties in contact interfaces for dampers.…”

“…Kumar and Narayanan 18 presented extensions to a finite difference technique for solution of multidimensional Fokker-Planck equation of mistuned bladed disk assembly subjected to white noise excitation and investigated the effects of stiffness and damping mistuning on the forced response. Mitra et al 9 analyzed random mistuning patterns of contact stiffness on nonlinear contact responses and observed the variation of amplification factors. Joannin et al 19 explained the nonlinear complex modal properties that vary with the vibration amplitude in given cases.…”

A Comparison between Two Reduction Strategies for Shrouded Bladed Disks

“…Although bladed disks are supposed to be made of a set of identical sectors [16], the actual assemblies are never symmetric and small differences, called mistuning in the literature [17], always exist. Furthermore, even in case of a perfectly symmetric system, localization phenomena of the response might occur in the presence of either geometric nonlinearities [18] or scatter of the contact interface parameters [19]. Thus, on one side, for an exhaustive analysis of the dynamic response of bladed disks, it is necessary to study the full bladed disk, while, on the other side, the highly refined meshes typical of industrial bladed disks make the use of full Finite Element (FE) models unfeasible.…”