Coupled Mode Flutter of Turbomachinery Blades

With new turbomachinery designs, especially for the fan stage of aero engines, the ratio of blade mass to the surrounding air is significantly reduced. As the aerodynamic forces become relevant in relation to the inertial forces of the structure, aeroelastic coupling cannot be neglected anymore. The classically used decoupled methods for flutter analysis, such as Carta's energy method also known as the work-per-cycle approach, yield a non-conservative statement in predicting the aeroelastic stability boundary. The resulting aeroelastic system of structural dynamics and aerodynamics leads to the aeroelastic stability equation, which itself is a generalized eigenvalue problem depending on an aeroelastic frequency. In fixed-wing analysis, different methods to solve the stability equation were introduced over the decades. The most prominent technique used nowadays is the p-k method as described by Hassig. Within this thesis, the p-k method is adapted for the usage in turbomachinery with respect to the specific numeric setups, such as cyclic symmetry, or the change of mode shapes and natural frequencies over rotor speed and throttling state. Assuming small perturbations in the vicinity of flutter onset, vibrations can be handled by a linearized approach so that aerodynamic responses are independent of the amplitude and allow a superposition. Thus, the unsteady aerodynamic forces are gained from a set of frequency domain forced motion simulations and interpolated at the aeroelastic frequency. The goal of this thesis is to verify and validate the adapted p-k method for coupled-mode flutter in turbomachinery. The results are compared against time-marching fluid/structure-coupled simulations and show good agreement. An intensive investigation of the influencing parameters, i.e. mass ratio, frequency separation and solidity, is performed. Applying the herein established process to a low mass ratio fan blade, it is shown that the flutter-free regime is significantly reduced in comparison to the classical energy method approach

Forced response prediction for industrial gas turbine blades

A highly efficient aeromechanical forced response system is developed for predicting resonant forced vibration of turbomachinery blades with the capabilities of fully 3-D non-linear unsteady aerodynamics, 3-D finite element modal analysis and blade root friction modelling. The complete analysis is performed in the frequency domain using the non linear harmonic method, giving reliable predictions in a fast turnaround time. A robust CFD-FE mesh interface has been produced to cope with differences in mesh geometries, and high mode shape gradients. A new energy method is presented, offering an alternative to the modal equation, providing forced response solutions using arbitrary mode shape scales. The system is demonstrated with detailed a study of the NASA Rotor 67 aero engine fan rotor. Validation of the forced response system is carried out by comparing predicted resonant responses with test data for a 3-stage transonic Siemens industrial compressor. Two fully-coupled forced response methods were developed to simultaneously solve the flow and structural equations within the fluid solver. A novel closed-loop resonance tracking scheme was implemented to overcome the resonant frequency shift in the coupled solutions caused by an added mass effect. An investigation into flow-structure coupling effects shows that the decoupled method can accurately predict resonant vibration with a single solution at the blade natural frequency. Blade root-slot friction damping is predicted using a modal frequency-domain approach by applying linearised contact properties to a finite element model, deriving contact Droperties from an advanced semi-analytical microslip model. An assessment of Coulomb and microslip approaches shows that only the microslip model is suitable for predicting root friction damping

Monitoring For Underdetermined Underground Structures During Excavation Using Limited Sensor Data

A realistic field monitoring application to evaluate close proximity tunneling effects of a new tunnel on an existing tunnel is presented. A blind source separation (BSS)-based monitoring framework was developed using sensor data collected from the existing tunnel while the new tunnel was excavated. The developed monitoring framework is particularly useful to analyze underdetermined systems due to insufficient sensor data for explicit input force-output deformation relations. The analysis results show that the eigen-parameters obtained from the correlation matrix of raw sensor data can be used as excellent indicators to assess the tunnel structural behaviors during the excavation with powerful visualization capability of tunnel lining deformation. Since the presented methodology is data-driven and not limited to a specific sensor type, it can be employed in various proximity excavation monitoring applications

Coriolis effects in bladed discs

New aero-engine architectures are currently being developed to satisfy the increasing demand for fuel-efficiency and lower noise, pushing the boundaries of todays design practice. These unproven designs require additional effort to ensure safety to high-cycle fatigue and flutter, widely acknowledged as a main risk for turbomachinery components. This calls for a new focus on phenomena that have been little investigated in the past due to their minor relevance for traditional designs, like the Coriolis effect. The Coriolis effect can cause an increase in the number of resonance frequencies, and generate global travelling-wave modes that can affect performance and flutter stability. Experimentally validated prediction and analysis methods are essential to ensure the accurate evaluation of the impact of the Coriolis effect on future engine designs. The major finite element (FE) software packages were systematically assessed, and proven to provide reliable simulations of the dynamics of bladed discs when the Coriolis effect is included. Experimental modal tools for the detection and identification of the Coriolis effect are also needed, to provide accurate interpretation of the data for model validation and updating. For this purpose, a dedicated rotating test rig was designed and manufactured. A novel Multiple Input Multiple Output testing framework was developed, based on the use of an array of strain gauges and piezoelectric actuators in combination with a poly-reference identification method, for the extraction of the full set of modal parameters arising in a bladed disc from the Coriolis force. The new technique allowed the successful recovery of Campbell diagrams, damping and strain mode shapes. Left displacement eigenvectors, which appear in the FRF formulation due to the Coriolis effect, could also be extracted and validated for the first time. An accurate comparison was conducted between the measurement data and the FE results, and confirmed the reliability of the new approach.Open Acces

Aeroelastic effects of mistuning and coupling in turbomachinery bladings

A numerical method has been developed to study the effects of structural mistuning on the aeroelastic behaviour of turbomachinery cascades. The specific objectives of the work presented here were the following: A method was to be developed that can assess the major effects of structural mistuning in a cascade in sub- and transonic flow situations. The influence of mechanical and aerodynamic coupling between the blades as well as coupling between multiple modes for each blade was to be included. Accurate representations of the aerodynamic forces, taken from a modern three-dimensional unsteady aerodynamic solver were to be used. The method should be applicable for design use, meaning that it has to supply results quickly for a large number of configurations. The applicability and accuracy of the method was to be demonstrated by comparison of numerical results to available data from recent experiments. The influence of major parameters on the aeroelastic stability and on the resonant amplitude of representative test cases should be assessed. The approach used to achieve these goals is the combination of a linearised Euler method for the aerodynamic calculations with a modal reduction technique, where the structural properties of each blade are represented by only a few eigenmodes. The method is validated and applied to two test cases, comprising of a transonic compressor rotor and a high pressure turbine rotor. Both are representative of modern turbomachinery designs. The final conclusions of this work are: The newly developed method is capable to assess the dominant effects of structural mistuning in turbomachinery cascades, including the mechanical and aerodynamic coupling of adjacent blades and the aerodynamic coupling of multiple modes with arbitrarily complex modeshapes. In this method, the aerodynamic characteristics of the cascade are accurately represented using the generalised unsteady aerodynamic coefficients derived from a modern three-dimensional flow solver, applicable to and validated for both sub- and transonic configurations. The simplifications employed significantly contribute to the computational efficiency of the method, making it applicable for design purposes as well as for the assessment of large parametric variations or for statistical studies of stochastically mistuned configurations. The current method is successfully validated by a comparison of numerical to experimental data. The applicability and accuracy is demonstrated by the favourable agreement between measured and computed results. Based on these validations, the method is applied to study the influence of major parameters on the aeroelastic behaviour of the selected test cases. The results show a wide range of phenomena, dependent on the type and strength of mistuning, frequency, modeshape and interblade phase angle. The results highlight the close inter-dependence of the aeroelastic stability derived from the eigenvalue analysis and the resonant amplitudes derived from the forced response analysis

Dynamic finite element modelling, measurement and updating of cable stayed bridges

SIGLEAvailable from British Library Document Supply Centre- DSC:DXN058072 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Multi-configuration model tuning for precision opto-mechanical systems

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2004.Includes bibliographical references (p. 141-146).This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.It is important for the design of future space-based observatories that simulation models physically represent the designed system and are able to track along configuration changes. This thesis outlines a three-step procedure for model tuning of complex opto-mechanical systems in the presence of measured experimental data. It is the hypothesis of this thesis that this procedure will produce a model that effectively tracks along configuration changes. The first step, engineering insight, applies model heuristics to the simulation model in an effort to produce a simulation model that includes all physical effects in the experiment. The next step, model updating, is an automated procedure whereby an optimization problem is formed in order to set uncertain model parameters. The final step is model tracking across configurations. Configuration changes include, but are not limited to, changes in mass, input/output locations, changes in geometric properties and relative placements. A new metric is provided which helps to gauge the level of experimental/model mismatch in the new configuration (using the updated model) by using the objective function from the optimization in Step 2. Using this metric, one can determine how the model changes with respect to specific configuration changes. Finally, this three-step tuning procedure is compared against traditional model tuning on a testbed at the MIT Space Systems Lab (SSL) in order to gauge its usefulness. The traditional model tuning will be performed by a colleague in the SSL who will use such methods as trial-and-error parameter updating to match the simulation model to the experimental data.(cont.) Using the multi-configuration metric presented in this thesis, it is shown that the model produced using the three step method does track configurations better than the model produced using traditional model tuning.by Deborah Jane Howell.S.M

Experimental modal analysis using ambient and earthquake vibrations : theory, software and applications

This thesis addresses the problem of identifying the modal properties of structures using vibration measurements. Modal identification methodologies are proposed based on vibration measurements induced by artificial, ambient or earthquake loads applied on the structure. A modal model of the structure is identified using a weighted least-squares approach and measured time histories at selected locations of a structure. For artificially induced and ambient vibration measurements, the identification is performed in the frequency domain using respectively frequency response functions and cross power spectral densities. For earthquake induced vibrations, the identification is performed in both time and frequency domains. The modal identification methods presented in this work treat generalized non-classically damped modal models. The identification of the modal parameter (modal frequencies, modal damping ratios, modeshape components and modal participation factors) is accomplished by introducing a computationally very efficient three step approach as follows. In the first step, stabilization diagrams are constructed containing frequency and damping information. The modeshape components and participation factors are estimated in a second least-squares step, based on the user selection of the stabilized poles. The first two steps involve non-iterative procedures and result in solving linear algebraic systems of equations. Finally, in order to improve the estimation of the modal characteristics, especially for the challenging case of closely spaced and overlapping modes, a third step is introduced to solve a fully nonlinear optimization problem using available iterative gradient-based optimization algorithms. In this thesis, theoretical developments as well as software implementation issues are presented. The methodologies and software developed are applied for the identification of the modal characteristics of a small laboratory structure for the case of artificial induced vibration measurements, as well as the identification of the modal characteristics of three bridges, the under construction R/C bridge of Egnatia Odos located at Metsovo (Greece), and two other representative R/C bridges of Egnatia Odos located at Polymylos and Kavala (Greece) for the cases of ambient and earthquake induced vibration measurements. Results provide qualitative and quantitative information on the dynamic behaviour of the systems and their components under different types of excitations. All modal identification methodologies presented in this work are implemented in user-friendly software, termed Modal Identification Tool (MITooL). The software which includes graphical user interface allows the full exploration and analysis of signals that are measured on a structure when it is excited by artificial, ambient or earthquake loads. A user manual is also presented which gives details for the operations and prospects of the MITooL software. Step-by-step examples of modal identification are presented to demonstrate the applicability of the software