title: Structural Modification of Stationary Rotor Systems for Dynamic Performance
Creators: name: Tsai, S
Publisher: name: University of Liverpool Repository
Year: 2020

description: The applications of rotor systems are common in our daily life and industry. Aero engines and power generating equipment are good examples of rotor systems. Due to the recent increasing interest in higher energy efficiency and higher power density, the structural dynamic performance of such a rotor system is becoming more important. A poor structural dynamic design can cause large vibration responses, low power efficiencies, bad user experiences, or possibly a complete failure of the system. For these reasons, the dynamic properties need to be thoroughly considered, designed, and sometimes modified. However, there are cases in which the system of interest is too complex to be accurately modelled, thus making accurate simulations difficult or infeasible. For this problem, the receptance-based inverse structural modification method can potentially be a good solution. In this thesis, a receptance-based inverse structural modification method is studied to improve a rotor system’s dynamic performance. Such a method has a number of merits compared to other methods in the area of structural modification, which include (a) the procedure of the analysis is straightforward in the sense that the modifications to be made are determined by the desired dynamical properties, (b) it does not require the system matrices (M, C, and K) or the modal data to carry out the analysis, (c) the tedious trial-and-error approach can be avoided, (d) experimental data can be directly used in the method, and (e) various dynamic properties can be assigned. As a result, the method continually receives research interest although the idea was proposed slightly more than a decade ago. The relevant work in this area of research is reviewed and the challenges are identified regarding its theoretical developments and practical applications. Some of the challenges are taken as the objectives of this study. First, the receptance-based method is further extended and the assignment problem is cast as an optimization problem to assign various dynamical properties using more than one form of modifications and accommodate structural modifications at more than one location. Several forms of modifications previously reported in the literature can be simultaneously included in the extended method. The applicability of the method is demonstrated by a number of simulations and experiments. It is applied to a laboratory geared rotor-bearing system to achieve natural frequency and antiresonant frequency assignments solely using experimental receptances. Additionally, it is shown by experiments that the locations of the given modifications can be determined without a numerical model so that the highest first bending natural frequency of a rotor system can be achieved. A big unsolved challenge in implementing the receptance-based structural modification method in practice is the lack of high-quality measurement of rotational (in bending) or torsional receptances. A receptance-based indirect measurement technique using a T-block attachment is proposed to address this issue. The numerical model of the system of concern is not required. The proposed technique can take account of the information from a number of excitations and responses, and provides the flexibility in their choices of location. The proposed technique has shown better performance over the torsional receptance estimation technique in the literature and is extended to estimate high-quality rotational receptances. The estimated receptances can now be used in various applications such as modal analysis, model updating, and structural modification. Moreover, the frequency assignment via coupling of subsystems is studied. The subsystems considered are rotor systems which can be rather complex. The theory is developed based on Receptance Coupling technique and formulated as an optimization problem, and only the receptances at the connection ends of the subsystems of interest are required. Both bending natural frequencies and torsional natural frequencies can be assigned using a modifiable joint with multiple DoFs, respectively or simultaneously. The technique is demonstrated by a few simulations and is possible to be implemented in practice through the proposed rotational/torsional receptance estimation technique.