bluuebunny/crossref_metadata_2025_split huggingface dataset:
A System Identification Method for Excitation-Frequency Dependent Rotordynamic Coefficients
Authors: Dimond Timothy W. , Younan Amir A. , Allaire Paul 
Published: December 2013

Experimental identification of rotordynamic systems presents unique challenges. Gyroscopics, generally damped systems, and non-self-adjoint systems due to fluid structure interaction forces mean that symmetry cannot be used to reduce the number of parameters to be identified. Rotordynamic system experimental measurements are often noisy, which complicates comparisons with theory. When linearized, the resulting dynamic coefficients are also often a function of excitation frequency, as distinct from operating speed.
In this paper, a frequency domain system identification technique is presented that addresses these issues for rigid-rotor test rigs. The method employs power spectral density functions and forward and backward whirl orbits to obtain the excitation frequency dependent effective stiffness and damping. The method is highly suited for use with experiments that employ active magnetic exciters that can perturb the rotor in the forward and backward whirl directions. Simulation examples are provided for excitation-frequency reduced tilting pad bearing dynamic coefficients. In the simulations, 20 and 50 percent Gaussian output noise was considered. Based on ensemble averages of the coefficient estimates, the 95 percent confidence intervals due to noise effects were within 1.2% of the identified value.
The method is suitable for identification of linear dynamic coefficients for rotordynamic system components referenced to shaft motion. The method can be used to reduce the effect of noise on measurement uncertainty. The statistical framework can also be used to make decisions about experimental run times and acceptable levels of measurement uncertainty. The data obtained from such an experimental design can be used to verify component models and give rotordynamicists greater confidence in the design of turbomachinery.
Analytical Prediction of Fatigue Crack Growth Behavior Under Biaxial Loadings
Authors: Kannusamy Ragupathy , Ramesh K. 
Published: December 2013

Aircraft and pressure vessel components experience stresses that are negative biaxial or multiaxial in nature. Biaxiality is defined as the ratio of stress applied parallel and normal to the crack front. In recent years many experimental studies have been conducted on fatigue crack growth under various biaxial loading conditions. Biaxial loadings affect crack front stresses and strains, fatigue crack growth rate and direction, and crack tip plastic zone size and shape. Many of these studies have focused on positive biaxial loading cases. No conclusive study has been reported out yet that accurately quantifies the influence of negative biaxiality on fatigue crack growth behavior. Lacking validation, implementation on real life problems remains questionable. To ensure safe and optimum designs, it is necessary to better understand and quantify the effect of negative biaxial loading on fatigue crack behavior.
In this paper, attempts were made to quantify the effect of biaxial load cases ranging from B = −0.5 to 1.0 on fatigue crack growth behavior. Also an attempt has been made to establish a simplified approach to incorporate the effect of biaxiality into da/dN curves generated from uniaxial loading using an analytical approach without conducting expensive biaxial crack growth testing. Sensitivity studies were performed with existing test data available for AA2014-T6 aluminum alloy. Detailed elastic–plastic finite element analyses were performed with different stress ranges and stress ratios with various crack sizes and shapes on notched and un-notched geometries. Constant amplitude loads were applied for the current work and comparison studies were made between uniaxial and different biaxial loading cases. It was observed from the study that negative biaxiality has a very pronounced effect on the crack growth rate and direction for AA2014-T6 if the externally applied load exceeds 20% of the yield strength as compared with 40% of externally applied load for alloy of steel quoted in the literature.
Modal Tests and Analysis of a Radial Impeller at Rest: Influence of Surrounding Air on Damping
Authors: Gibert C. , Blanc L. , Almeida P. , Leblanc X. , Ousty J.-P. , Thouverez F. , Laîné J.-P. 
Published: December 2013

HCF risk assessment for turbomachinery blades requires the prediction of vibratory levels, which in turn requires fine damping quantification. This issue is especially sensitive for structures with low structural damping such as monobloc centrifugal compressor disks (blisks). The material composing blisks and aero-dynamic flow both contribute to damping phenomena. A strategy for non-aerodynamic damping characterization is to perform experiments in vacuum.
This paper focuses on the use of modal tests in vacuum to estimate material damping under non-rotating conditions. Experiments are performed on an isolated impeller manufactured from a single piece in a vacuum chamber at different air pressure levels ranging from 10 mbar to 1 bar. Strong dependency of damping ratios on pressure can be found on the first flexural mode, leading to two types of application.
Firstly, measurements enable assessing the validity of extrapolations of non-aerodynamic damping from measurements sometimes performed under less thorough vacuum conditions. Basic fluid-structure interaction models are used to interpret and quantify the evolution of modal quantities when air is progressively removed. Secondly, vacuum measurements can give frequency response functions (FRFs) with much greater separation between resonance peaks. In this study, the damping ratio found in vacuum condition are 3% of these at ambient pressure corresponding to a magnitude 30dB higher at resonance peaks. This contrasts with in-air measurements on cyclic symmetry structures, like blisks, with high modal density that make the direct interpretation of FRFs and their modal analysis more difficult.
The Effect of the Degree of Reaction on the Leakage Loss in Steam Turbines
Authors: Yoon Sungho 
Published: December 2013

The degree of reaction selected in designing steam turbines is of paramount importance. There has been competition between 50% reaction and impulse turbines over a century. It is therefore important to understand the effect of the degree of reaction on aerodynamic performance. In particular, a change in the degree of reaction affects the leakage flow substantially in both the stationary and rotating blades due to a change in the blade loading. The effect of the degree of reaction on the efficiency loss, due to leakage flows, is systematically investigated in this paper using analytical models.
It is shown that the appropriate way to understand the efficiency loss, due to leakage flows, is to estimate the kinetic energy dissipation rather than the leakage mass flow rate, as demonstrated by Yoon et al. [1]. In order to estimate the efficiency loss due to leakage flows, the well-known Denton model [2] is extended by considering the velocity triangles in a repeating turbine stage. The extended model is compared with experimental data, at different degrees of reaction, and shows good agreement with measurements.
It is shown that a reduction in the degree of reaction, at a fixed flow coefficient and a fixed work coefficient, results in an increase in the efficiency loss across the stationary blade, but a decrease in that across the rotating blade. However, the efficiency loss, across the stationary blade hub, is estimated to be smaller than the efficiency loss across the rotating blade tip. A stationary blade can be better sealed than a rotating blade by applying multiple seals and using a leakage path with a low radius.
The efficiency loss, due to the tip leakage flow, is substantially influenced by the choice of the tip configuration. Shrouded blades show several aerodynamic advantages over unshrouded blades in reducing the tip leakage efficiency loss. Employing multiple seals over the shroud decreases the tip leakage mass flow rate significantly. Moreover, as the degree of reaction approaches zero, the tip leakage mass flow rate over the shroud becomes small, since the axial pressure drop across the rotating blade becomes small. In unshrouded blades, a reduction in the degree of reaction is shown to increase the leakage mass flow rate over the tip, because the circumferential pressure difference between the blade pressure side and blade suction side generally increases when the pitch-to-chord ratio remains unchanged.
Case Study: Metallurgical Analysis of Previously Repaired GE Frame 7FA Stage 1 Turbine Buckets
Authors: Nagy Douglas R. , Kuipers Justin H. 
Published: December 2013

A case study is presented in which a destructive metallurgical analysis was performed on three GE Frame 7FA+e stage 1 buckets which had each been repaired using different approaches. The metallurgical condition and extent of the prior repairs are presented for each of the buckets following one additional service interval. The analyses included evaluation of the macroscopic condition, base alloy microstructure, stress rupture properties, internal and external surface coating conditions and the tip integrity.
The first bucket had not been dimensionally restored at the tip during repair (i.e. the tip was still comprised of the original casting). The bucket exhibited moderate oxidation and loss of tip material. Base alloy degradation was observed at the mid airfoil height in the form of coarsening, rounding and agglomeration of the primary gamma prime precipitates. Secondary gamma prime precipitates remained in some of these regions, indicative that the alloy microstructure had not been rejuvenated by full solution treatment during repair. The second bucket had been weld repaired at the tip using a solid solution strengthened weld filler alloy. Following service, the tip exhibited significant oxidation and approximately 6mm of material loss at the mid-chord and trailing edge tip. Within the tip repair material, micro-cracks and creep voids were observed. The base alloy condition appeared similar to the first bucket. The third bucket had been tip weld repaired using a precipitation hardened weld filler alloy and rejuvenated with a full solution treatment. Following service, the bucket exhibited negligible oxidation and material loss at the tip. Overall, microstructural base alloy degradation was found to be minor.
Simulation of 2-Way Fluid Structure Interaction in a 3D Model Combustor
Authors: Shahi Mina , Kok Jim B. W. , Alemela P. R. 
Published: December 2013

The liner of a gas turbine combustor is a very flexible structure that is exposed to the pressure oscillations that occur in the combustor. These pressure oscillations can be of very high amplitude due to thermoacoustic instability, when the fluctuations of the rate of heat release and the acoustic pressure waves amplify each other. The liner structure is a dynamic mechanical system that vibrates at its eigenfrequencies and at the frequencies by which it is forced by the pressure oscillations to which it is exposed. On the other hand the liner vibrations force a displacement of the flue gas near the wall in the combustor. The displacement is very small but this acts like a distributed acoustic source which is proportional to the liner wall acceleration. Hence liner and combustor are a coupled elasto-acoustic system. When this is exposed to a limit cycle oscillation the liner may fail due to fatigue.
In this paper the method and the results will be presented of the partitioned simulation of the coupled acousto-elastic system composed of the liner and the flue gas domain in the combustor. The partitioned simulation uses separate solvers for the flow domain and the structural domain, that operate in a coupled way. In this work 2-way fluid structure interaction is studied for the case of a model combustor for the operating conditions 40–60 kW with equivalence ratio of 0.625. This is done in the framework of the LIMOUSINE project. Computational fluid dynamics analysis is performed to obtain the thermal loading of the combustor liner and finite element analysis renders the temperature, stress distribution and deformation in the liner. The software used is ANSYS workbench V13.0 software, in which the information (pressure and displacement) is also exchanged between fluid and structural domain transiently.
Validation of Linearized Navier-Stokes Based Flutter Prediction Tool: Part 2—Quantification of the Prediction Accuracy on a Turbine Test Case
Authors: Sanz Luengo Antonio , Vogt Damian M. , Schmitt Stefan , Fransson Torsten H. 
Published: December 2013

This is the second part of two papers describing the validation of a tool chain for flutter prediction. The first paper provides an overview of the numerical methods and their verification. The second paper presents the detailed validation of the tool chain on the basis of experimental data obtained from measurements of an annular cascade sector comprising 3D twisted turbine blades.
Aeroelastic test data has been acquired in an isolated blade row consisting of seven free-standing low-pressure (LP) turbine blades. The middle blade has been oscillated in controlled manner in three orthogonal modes and at various frequencies while measuring the unsteady blade surface pressure on several blades. The data has been reduced to aerodynamic influence coefficients and finally recombined to travelling wave mode stability curves. By acquiring data at various spanwise positions, a basis for validating three-dimensional effects has been provided.
The validation of the investigated flutter prediction tool has been performed in a detailed manner and on various levels, started from a critical reduced frequency over stability curves to local work coefficients. At the lowest level of condensation, the unsteady blade surface pressures have been compared. Correlation to test data is shown and discussed rigorously at these various levels giving a detailed assessment of the prediction accuracy of the investigated tool.
Dynamic Characterization of Tilting Pad Journal Bearings From Component and System Level Testing
Authors: Delgado Adolfo , Librashi Mirko , Vannini Giuseppe 
Published: December 2013

The dynamic response of a direct lube, 5-pad, rocker-back pivot tilting pad bearing is characterized in a controlled motion (component level) test rig, and in a spin bunker (full system level) using a dummy rotor mounted on two identical bearings. In the component level test, the force coefficients (stiffness, damping, mass) are identified from pseudorandom excitations using a 2-DOF model. N-DOF system including the pad motions has been shown to yield frequency dependent coefficients that warrant the use of asynchronous coefficients for stability analysis in centrifugal compressors. However, experimental results showed that the real part of the dynamic stiffness is well represented as a constant stiffness and mass coefficients while the imaginary part yields a constant damping coefficient (i.e. frequency independent).
In the system level test, a dedicated dummy rotor (representative of a high speed centrifugal compressor rotor) is excited by a magnetic shaker throughout a frequency range covering the rotor modes of interest while spinning at constant speed. From the rotor harmonic response the damping of each mode is extracted using a curve-fitting method based on a 1-DOF model for a given set of speeds.
The dummy rotor test provides reference values for system logarithmic decrement and further validates the component level test results. The logarithmic decrement prediction using identified bearing force coefficients are in good agreement with the experimental results. In addition, using for prediction identified coefficients in a classical K-C-M or synchronous K-C form yields similar results (within 15%). This indicates that for the given bearing geometry (clearance, offset and size) and operating conditions, synchronously reduced force coefficients are adequate for stability analysis. Comparison of the identified force coefficients with results from commercially available code yielded reasonable agreement on direct coefficients while some discrepancies are highlighted on the cross-coupled coefficients.
The Rolls-Royce MT30 Compact Package Design
Authors: Whatmough Damian , Birtwistle Tim 
Published: December 2013

The new Rolls-Royce MT30 package offers a powerful and lightweight package that is suitable for installation in both commercial and naval vessels. The MT30 can deliver 40MW into either a mechanical water jet or propeller driven application and to a generator in an electrically driven Ship. It can be installed in many sizes of ships capable of operating worldwide.
This paper will discuss the compact MT30 package in the context of all ship applications currently in-use and for future use across the full range of power and propulsion systems. It will highlight the design drivers for the product including power output, operational environment, certification and MT30 and package maintenance.
Identification of Bearing Stiffness and Damping Coefficients Using Phase-Plane Diagrams
Authors: Jáuregui Juan C. , San Andrés Luis , De Santiago Oscar 
Published: December 2013

The reliable identification of dynamic parameters in mechanical systems remains a big challenge, in particular for nonlinear systems. There is not a single mathematical model encompassing the universe of most systems. From a practical point of view, the identification of system parameters depends on the measurement data as well as on the reference model.
This paper presents a novel method for identifying the dynamic parameters of a gas bearing, whose force coefficients are strong functions of frequency. The method is based on the analysis of the phase diagram with the model assuming a mass-damper-spring system with time-dependent force coefficients. The phase diagram could be implemented electronically for on line monitoring and ready fault detection.
Detection of Unsteady Flow in a Kaplan Hydraulic Turbine Using Machine Mechanical Model and Rotor Measured Vibrations
Authors: Pennacchi Paolo , Vania Andrea , Chatterton Steven , Tanzi Ezio 
Published: December 2013

Hydraulic stability is one of the key problems during the design stage of hydraulic turbines. Despite of modern computational tools that help to define dangerous operating conditions and optimize runner design, hydraulic instabilities may fortuitously arise during the turbine life, as a consequence of variable and different operating conditions at which a hydraulic turbine can be subject.
In general, the presence of unsteady flow reveals itself in two different ways: at small flow rate, the swirling flow in the draft tube conical inlet occupies a large portion of the inlet and causes a strong helical vortex rope; at large flow rate conditions the unsteady flow starts midway and causes a breakdownlike vortex bubble, followed by weak helical waves.
In any case, hydraulic instability causes mechanical effects on the runner, on the whole turbine and on the draft tube, which may eventually produce severe damages on the turbine unit and whose most evident symptoms are vibrations. This notwithstanding, condition monitoring systems seldom are installed on this purpose in hydraulic power plants and no examples are reported in literature about the use of model-based methods to detect hydraulic instability onset.
In this paper, by taking the advantage of a testing campaign performed during the commissioning of a 23 MW Kaplan hydraulic turbine unit, a rotordynamic model-based method is proposed. The turbine was equipped by proximity and vibration velocity probes, that allowed measuring lateral and axial vibrations of the shaft-line, under many different operating conditions, including also some off-design ones. The turbine mechanical model, realized by means of finite beam elements and considering lateral and axial degrees of freedom, is used to predict turbine unit response to the unsteady flow. Mechanical system response is then compared to the measured one and the possibility to detect instability onset, especially in real-time, is discussed.
Model Verification and Validation Concepts for a Probabilistic Fracture Assessment Model to Predict Cracking of Knife Edge Seals in the Space Shuttle Main Engine High Pressure Oxidizer
Authors: Pai Shantaram S. , Riha David S. 
Published: December 2013

Physics-based models are routinely used to predict the performance of engineered systems to make decisions such as when to retire system components, how to extend the life of an aging system, or if a new design will be safe or available. Model verification and validation (V&V) is a process to establish credibility in model predictions. Ideally, carefully controlled validation experiments will be designed and performed to validate models or submodels. In reality, time and cost constraints limit experiments and even model development. This paper describes elements of model V&V during the development and application of a probabilistic fracture assessment model to predict cracking in space shuttle main engine high-pressure oxidizer turbopump knife-edge seals.
The objective of this effort was to assess the probability of initiating and growing a crack to a specified failure length in specific flight units for different usage and inspection scenarios. The probabilistic fracture assessment model developed in this investigation combined a series of submodels describing the usage, temperature history, flutter tendencies, tooth stresses and numbers of cycles, fatigue cracking, nondestructive inspection, and finally the probability of failure. The analysis accounted for unit-to-unit variations in temperature, flutter limit state, flutter stress magnitude, and fatigue life properties. The investigation focused on the calculation of relative risk rather than absolute risk between the usage scenarios. Verification predictions were first performed for three units with known usage and cracking histories to establish credibility in the model predictions. Then, numerous predictions were performed for an assortment of operating units that had flown recently or that were projected for future flights. Calculations were performed using two NASA-developed software tools: NESSUS® for the probabilistic analysis, and NASGRO® for the fracture mechanics analysis. The goal of these predictions was to provide additional information to guide decisions on the potential of reusing existing and installed units prior to the new design certification.