Convergence-free mapping of non-linear damper-blade performance

Abstract

In the bladed disks of turbomachinery, the problem arises of finding a damping device that reduces as much as possible the amplitude of the alternating stresses produced by the forced vibrations excited by the gas flow. Dry friction solid underplatform dampers are an established solution.The shape and size of the damper, in association with those of the platform, neck and airfoil, determine the non-linear response curves having as parameter the intensity of external excitation, here synthetically represented by a “proof” excitation force. Of greatest importance are the combinations (frequency, excitation force) that realise the maximum amplitude of forced oscillation (measured, for example, at the blade tip) and the maximum value of the amplitude of the variable stress produced by the vibrations at a critical point of the blade. Since the design of the damper-blade coupling is High-Cycle Fatigue driven, this stress amplitude is taken as a reference and related to the value of the excitation force. This can finally take the well-known form of the damper performance curve.Especially in the case of parametric explorations concerning shape, size and contact parameters, the current approach to this non-linear response problem, based on iterative convergence, is numerically prohibitive unless one uses special search techniques, such as the surrogate models of various types that are favoured today.An alternative computational process is presented here, that of integrating PCR (Platform Centred Reduction) with a new approach called Amplitude Layered (excitation) Force Mapping. This process reduces the amount of computation by up to three orders of magnitude compared to standard techniques and is a winning alternative to surrogate models.