MISTUNING AND COUPLING EFFECTS IN TURBOMACHINERY BLADINGS

Gerhard Kahl

MTU Aero Engines Munich, Germany Gerhard. Kahl@muc. mtu. de

Abstract A numerical method has been developed to study the effects of structural mis – tuning on the aeroelastic behavior of turbomachinery cascades. The approach used is the combination of a modal reduction technique, where the structural properties of each blade are represented by only a few eigenmodes, with a lin­earized Euler method for the aerodynamic calculations. The method is validated and applied to two test cases, comprising of a high pressure turbine rotor and a transonic compressor rotor. Both are representative of modern turbomachinery designs. The results of the validation confirm the ability of the present method to accurately capture the dominant effects that inflience the aeroelastic behavior of the cascades. Further case studies are performed to assess the influence of alternating and random mistuning on the resonant response amplitudes and on the aeroelastic stability.

1. Introduction

Analysis of the aeroelastic behavior of turbomachinery cascades in many cases still rely on the assumption of a perfectly symmetric structure, where all blades are structurally and aerodynamically identical. In practice small dif­ferences between the individual airfoils of a cascade are unavoidable. The resulting small variations in the structural characteristics of the individual air­foils can cause dramatic effects. For example, strain gauge tests of complete rotors often show a spread in the measured response amplitudes of the differ­ent blades of a factor of more than two. Given the fact that mostly only a few blades are instrumented, this leads to the question of what the true maximum amplitude of all blades might be, since this is what the design of the blade has to take into account. Furthermore, the question arises at to what the effect of the small structural variations on the aeroelastic stability may be, which can often not be measured as readily as the forced response amplitudes.

119

K. C. Hall et al. (eds.),

Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines, 119-131. © 2006 Springer. Printed in the Netherlands.

A wide range of publications exists that assess effects of structural mistun- ing. However, many of these neglect the aerodynamic damping and coupling altogether (e. g. [1], [2], [9], [14] [11]), or use simplified methods, allowing only for two-dimensional rigid motions of the blade sections [7]. In other instances, the aerodynamic interaction between different structural modes is neglected [13]. The present work puts more emphasis on the aerodynamic damping and coupling effects, while using a simple structural model. The ap­proach presented is intended to allow the consideration of multiple modes for each blade and is capable of dealing with sub – and transonic fbw situations. Its main goal is the applicability for design use, meaning that it has to supply results quickly for a large number of configurations.