Vibrations prediction and measurement of multi-stage bladed disks with non linear behavior due to friction contacts

Abstract

The architecture of current aircraft turbofan engines consists of multi-stage assemblies resulting from the coupling of bladed disks by means of bolted flange joints. The efficiency of such systems in real working condition is strictly related to the dynamic properties of blades and disks constituting them. According to the current design practices, blades and disks are designed so that their weights are reduced and their slenderness increased. Therefore, latest generation turbofan engine thus result much more sensible to mechanical vibrations that may cause failure by high cycle fatigue phenomena. For these reasons robust computational techniques and innovative measurement systems have become necessary tools for the design and validation of multi-stage bladed disks assemblies, in order to preserve their structural integrity while operating in real working conditions. The topics developed in this Ph.D. thesis concern aspects of linear and non-linear dynamics in the turbomachinery field and give a series of important guidelines for the study of multi-stage bladed disks systems from both a numerical and experimental point of view. The research activity has been mainly focused on the following two topics: 1. Development of reduced order model techniques for the prediction of forced response of multi-stage bladed disk assemblies. The main challenge associated with modeling multi-stage assemblies is strictly related to the possible different cyclic symmetry characterizing the coupled stages. In such case a sector representative of the whole multi-stage system does not exist in general and typical dynamic calculations based on cyclic constraints can not be performed as in the case of single bladed disks. Therefore, two novel reduced order model techniques for multi-stage systems have been developed in order to overcome the mentioned drawback while guaranteeing high fidelity in modeling the system dynamics. Furthermore, for the first time the bolted flange joint coupling two bladed disks is considered as a possible source of damping due to friction phenomena. Understanding the effects of such non-linearities in damping blade vibrations could be crucial in design of bolted flange joint. The proposed reduction techniques then also allow the prediction of the forced response of a multi-stage system when friction contacts are present at the flange joint interface while maintaining low computational costs. 2. Validation of the Blade Tip-Timing measurement technique, for the identification of the modal properties of two laboratory dummy disks. In this frame an experimental procedure to validate the Blade Tip-Timing system against the strain gauges measurement has been proposed. Furthermore, a novel methodology for the identification of the operative deflection shape of a vibrating bladed disks in presence of small mistuning has been developed.