bluuebunny/crossref_metadata_2025_split huggingface dataset:
Seven Major Steps to Improve Today’s High-Speed Balancing
Authors: Schneider Hatto , Buschbeck Andreas , Hoffmann Karl-Josef , Bornheimer Albert , Schmidt Joachim 
Published: November 2020

Abstract
There are some statements that today’s high-speed balancing of flexible rotors in a balancing machine may not be very effective. As a consequence often in-situ balancing may have to follow the high speed balancing process in order to achieve smooth running machines in operation.
There are other perceptions that individual rotors cannot be balanced sufficiently at all when coupled with other rotors later on.
If this is true, it may provoke problems with cost and delivery time as well as with the acceptance by the user.
When looking into the development in this field during the last decades it seems that despite (or because of) the use of computers some former experience got lost, and that available software does not fully utilize the latest findings and information on balancing of flexible rotors.
The paper describes possible reasons for this development and outlines seven steps for improving today’s high-speed balancing.
Flexible Rotor Balancing Taking Bearing Non-Linearity Into Account
Authors: Chen Peter Y. P. , Feng Ningsheng , Hahn Eric J. 
Published: November 2020

Abstract
This paper outlines a balancing method for non-linear rotor bearing systems with hydrodynamic bearings which utilises the unbalance response measured at a discrete number of measurement planes at a select number of speeds, as during a single rundown. Numerical experiments confirm that provided the system has been modelled correctly, the unbalance distribution is identified more accurately than by using the influence coefficient approach which assumes system linearity and increasingly so as the response orbits at the bearing increase. The identification accuracy is sensitive to measurement error, and if measurements are limited to field measurement accuracy of two digits, it is necessary to apply constraints, determined with the help of the linear approach, or otherwise reduce the number of unknown unbalances relative to the number of measurements, to extract the true solution.
Twice-Running-Speed Response due to Elliptical Bearing Clearances
Authors: Childs Dara W. 
Published: November 2020

Abstract
Yamamoto [1] examined a vertical Jeffcott rotor model with bearing clearances and showed that a small bearing “dead-band” clearance could have a dramatic influence on synchronous rotor response. Recent test results for a turbopump in a liquid-rocket-engine development program showed a twice-running-speed response (2E) on the order of 8 gs over a wide speed range, while synchronous response levels were only at an 0.5 g level. Childs [2] predicted a sharp 2E response for the Advanced Technology Development, High-Pressure Fuel Turbopump of the Space Shuttle Main Engine due to bearing clearances, when the running speed was nearly one half of a housing-mode natural frequency, i.e., when the 2E frequency was close to the housing mode natural frequency. However, his model did not predict significant 2E response over a broad running-speed range.
Ellipticity of the bearing dead-band clearances was suggested as a possible cause for the observed 2E phenomenon. An extension of Yamamoto’s analysis [1] is presented including bearing ellipticity to examine that proposed explanation. The analysis results show that clearance ellipticity will produce 2E response over a considerable running-speed range during which the bearing clearance is engaged; however, the predicted 2E-response amplitude corresponding to an ellipticity of 0.1 were about 10% of the synchronous levels, and the projected 2E acceleration levels were about 40% of synchronous. The predicted 2E response includes a resonance peak (that can be sharp) at speeds slightly above 25% of the rotor critical speed. The perturbation-analysis results provide an explanation for persistent, lower 2E-response levels observed in many turbopumps, but do not explain the high levels observed with this turbopump.
Theoretical Study on Instability Boundary of Rotor-Hydrodynamic Bearing Systems: Part I — Jeffcott Rotor With External Damping
Authors: Guo Zenglin , Kirk R. Gordon 
Published: November 2020

Abstract
The instability of rotor-hydrodynamic bearing systems has been a major concern in higher-speed rotating machine design. The threshold speed seems to be a barrier that cannot be driven through. However, if some kind of external damping is introduced, the situation may be different. At first, the study is conducted on the Jeffcott rotor with external damping for both vertical and horizontal configurations. For the vertical system, it has been shown by both analytical and numerical study that there is a threshold speed at which the rotor will get into an upper stable region from the previous unstable region. As for the horizontal system, an extended Lund Stability Method is presented, which, and with numerical analysis, has proven that the system may have two threshold speeds, which evolve a stability pattern as one region of instability sandwiched between two regions of stability. Then, the study is expanded to the extended Jeffcott rotor. With the introduction of equivalent journal mass, another threshold speed appears which forms a top and final boundary to the stable operation of the rotor. The results of a numerical study show that with the external damping increased, the first several unstable regions can be reduced or eradicated. The external damping has little effect on the last top threshold which is mainly dependent on the value of journal mass. It has also been shown that a certain value of external damping and small portion of journal mass would be very beneficial to improve the stability characteristics of the system over the entire speed range.
Experimental Investigation of a Platform Damper With Curved Contact Areas
Authors: Jareland Martin H. 
Published: November 2020

Abstract
This paper presents a set of experiments which have been performed with a new friction damper for a high pressure turbine. The main objective of these experiments was to tune a simulation model for the current damper. The simulation model used is valid for both microslip and macroslip. A number of other topics were also investigated, such as influence of glued platforms, repeatability of an experiment, comparison of new and used dampers, and the change in surface structure of the contact areas.
A similar set of experiments has been performed earlier with the current damper. On that occasion, it was found that the experiments and simulations did not agree as desired and therefore a new set of experiments was suggested. This paper presents the new experiments and the new simulations with a more precisely tuned model. The simulation model is tuned by selecting a suitable coefficient of friction and tangential stiffness for the contact interface model. It is found that the simulations agree well with the experimental results when the normal force is held constant and the excitation force is varied. The properties of the contact, contact area and pressure, are unchanged when the normal force is constant.
If a new damper is compared with a damper that has been used in a series of experiments, it is found that the resonance shifts to a lower frequency and that the amplitude increases. This is probably caused by a decrease in coefficient of friction due to wear of the contact surfaces.
The repeatability of an experiment has been investigated by running the same experiment five times in succession. It was found that the repeatability was very good if the position of the damper was not changed. The differences that were found were mainly due to wear on the contact surfaces. A comparison of sweeping forward or backward in frequency was also performed. It showed that the difference was small both for experiments with small motions in the contacts and for experiments with a high amount of macroslip.
Three-Dimensional CFD Rotordynamic Analysis of Gas Labyrinth Seals
Authors: Moore J. Jeffrey 
Published: November 2020

Abstract
Labyrinth seals are utilized inside turbomachinery to provide non-contacting control of internal leakage. These seals can also play an important role in determining the rotordynamic stability of the machine. Traditional labyrinth seal models are based on bulk-flow assumptions where the fluid is assumed to behave as a rigid body affected by shear stress at the interfaces. To model the labyrinth seal cavity, a single, driven vortex is assumed and relationships for the shear stress and divergence angle of the through flow jet are developed. These models, while efficient to compute, typically show poor prediction for seals with small clearances, high running speed, and high pressure (Childs, 1993).
In an effort to improve the prediction of these components, this work utilizes three-dimensional computational fluid dynamics (CFD) to model the labyrinth seal flow path by solving the Reynolds Averaged Navier Stokes equations. Unlike bulk-flow techniques, CFD makes no fundamental assumptions on geometry, shear stress at the walls, as well as internal flow structure. The method allows modeling of any arbitrarily shaped domain including stepped and interlocking labyrinths with straight or angled teeth. When only leakage prediction is required, an axisymmetric model is created. To calculate rotordynamic forces, a full 3D, eccentric model is solved. The results demonstrate improved leakage and rotordynamic prediction over bulk-flow approaches compared to experimental measurements.
A Novel Frequency-Domain Method of Structural Damage Identification
Authors: Lee Usik , Shin Jinho 
Published: November 2020

Abstract
This paper introduces a frequency-domain approach of structural damage identification method (SDIM). The present SDIM is formulated from the exact dynamic stiffness matrix (DSM) equation of motion and then applied to beam structures. The appealing features of the present SDIM are: (1) it needs the DSM only for intact structure, (2) the excitation forces and the measured frequency response functions (FRFs) of damaged structure are only the required input data, and (3) it can locate and quantify many local damages at a time. The feasibility of the present SDIM is verified through some numerically simulated damage identification tests.
Application of Nonlinear Receptance Coupling to Dynamic Stiffness Evaluation for Reconfigurable Machine Tools
Authors: Yigit Ahmet S. , Ulsoy A. Galip 
Published: November 2020

Abstract
A systematic procedure is proposed to evaluate dynamic characteristics of design alternatives, for a Reconfigurable Machine Tool (RMT). The procedure is intended to be used in an automated design environment where various design alternatives are generated by kinematic synthesis based on a given task and specification. The evaluation procedure makes use of a substructuring method called nonlinear receptance coupling. The coupling method includes the effects of weakly nonlinear compliant joints through the use of describing functions for the nonlinearities involved. To demonstrate the utility of the proposed evaluation procedure design alternatives are generated based on a reduced order lumped parameter model of the RMT and examined with respect to the proposed criteria. It is shown that joint nonlinearities may affect dynamic stiffness considerably. It is also shown that the suggested criteria can distinguish various design alternatives with respect to their expected dynamic behavior.
Reduction of Arbitrary Dynamical Systems by Wavelet Bases
Authors: Ghanem Roger , Romeo Francesco 
Published: November 2020

Abstract
A procedure is developed for the identification and classification of nonlinear and time-varying dynamical systems based on measurements of their input and output. The procedure consists of reducing the governing equations with respect to a basis of scaling functions. Given the localizing properties of wavelets, the reduced system is well adapted to predicting local changes in time as well as changes that are localized to particular components of the system. The reduction process relies on traditional Galerkin techniques and recent analytical expressions for evaluating the inner product between scaling functions and their derivatives. Examples from a variety of dynamical systems are used to demonstrate the scope and limitations of the proposed method.
Karhunen-Loeve Decomposition for Reducing the Order and Monitoring the Dynamics of Coupled Oscillators
Authors: Ma Xianghong , Vakakis Alexander F. , Bergman Lawrence A. 
Published: November 2020

Abstract
Karhunen-Loeve - KL modes are used to discretize the dynamics of a four-bay linear truss. This is achieved by defining global KL modal amplitudes and employing the orthogonality relations between KL modes that are inherent in the KL decomposition. It is found that the KL-based low-order models can capture satisfactory the transient dynamics of the truss, even when only a limited number of them is used for the order reduction. A comparison between the exact and low-order dynamics in the frequency domain reveals that the low-order models capture the leading resonances of the truss. A series of experiments with a practical three-bay truss is then performed to validate the theoretical KL decomposition. A comparison between theory and experiment indicates agreement between the predicted and realized dominant KL mode shapes, but less so in the higher order modes. The reasons for this discrepancy between theory and experiment are discussed, and possible applications of the KL-based order reduction to passive and active control of practical large-scale flexible systems are outlined.
Dynamic Instability and Nonlinear Vibration of Doubly-Curved Cross-Ply Shallow Shells
Authors: Nayfeh Ali H. , Alhazza Khaled 
Published: November 2020

Abstract
We consider the dynamic instability and nonlinear vibration of doubly-curved cross-ply laminated shallow shells with simply supported boundary conditions. We investigate their responses and stability to a primary resonance (i.e., Ω ≠ ω11). The governing nonlinear partial-differential equations of motion are based on the von Karman-type geometric nonlinear theory and the first-order shear-deformation theory. We use the Galerkin procedure to reduce the governing nonlinear partial-differential equations to an infinite system of nonlinear coupled ordinary-differential equations. We use a combination of a shooting technique and Floquet theory to calculate the periodic responses of the shell and investigate their bifurcations. We show that for some shell parameters, a single-mode approximation misses some important dynamics, such as period-doubling, bifurcations.
Bayesian Estimation of Cumulative Damage From Seismic Response Records of Buildings
Authors: Heredia-Zavoni Ernesto , Zeballos Antonio , Montes-Iturrizaga Roberto , Esteva Luis 
Published: November 2020

Abstract
This paper discusses the estimation of probability distributions of damage using response records from instrumented buildings subjected to seismic excitations. The objective of the paper is to show how the information on the evolution of the mechanical properties of a system can be used to assess the state of cumulative damage. This implies expressing damage on the structural members in terms of its influence on the residual mechanical properties of the system. The information on the inelastic behavior from response records is used in a bayesian formulation along with a damage function to update prior probability distributions of damage. The damage function models the hysteretic cycles of inelastic response in terms of an initial damage and of the displacement amplitudes of the response cycles. It describes the evolution of the secant stiffness through the cycles of inelastic response as a function of cumulative damage and displacement amplitudes. The updating of probability distributions of damage for single degree of freedom systems is presented first. Extensions to the case of non-linear multi-degree of freedom systems are discussed next. Examples of reinforced concrete frames are given for illustrative purposes.
Analysis of a Class of Coupled Nonlinear Oscillators With an Application to Flow Induced Vibrations
Authors: Haaker T. I. 
Published: November 2020

Abstract
We consider in this paper the following system of coupled nonlinear oscillatorsx..+x-k(y-x)=εf(x,x.),y..+(1+δ)y-k(x-y)=εf(y,y.). In this system we assume ε to be a small parameter, i.e. 0 &lt; ε ≪ 1. A coupling between the two oscillators is established through the terms involving the positive parameter k. The coupling may be interpreted as a mutual force depending on the relative positions of the two oscillators. For both ε and k equal to zero the two oscillators are decoupled and behave as harmonic oscillators with frequencies 1 and 1+δ, respectively. The parameter δ may therefore be viewed as a detuning parameter. Finally, the term ε f represents a small force acting upon each oscillator. Note that this force depends only on the position and velocity of the oscillator upon which the force is acting.
To analyse the system’s dynamic behaviour we use the method of averaging. When k and δ are choosen such that no internal resonance occurs, one typically observes the following behaviour. If the trivial solution is unstable, solutions asymptotically tend to one of the two normal modes or to a mixed mode solution.
For the special case with δ = 0 a system of two identical oscillators is found. If in addition k is O(ε) we obtain a 1 : 1 internal resonant system. The averaged equations may then be reduced to a system of three coupled equations — two for the amplitudes and one for the phase difference. Due to the fact that we consider identical oscillators there is a symmetry in the averaged equations. The normal mode solutions, as found for the non-resonant case, are still present. New mixed mode solutions appear. Moreover, Hopf bifurcations in the averaged system lead to limit cycles that correspond to oscillations in the original system with periodically modulated amplitudes and phases.
We also consider the case with δ = O(ε), i.e. the case with nearly identical oscillators. If k = O(ε) again a 1 : 1 internal resonant system is found. Contrary to the previous cases the normal mode solutions no longer exist. Moreover, different bifurcations are observed due to the disappearance of the symmetry present in the system for s = 0.
We apply some of the results obtained to a model describing aeroelastic oscillations of a structure with two-degrees-of-freedom.
Aeroelastic Oscillations of a Double Seesaw Oscillator
Authors: Lumbantobing H. , Haaker T. I. 
Published: November 2020

Abstract
In this paper we consider the resonan taeroelastic oscillations of a two-degree-of-freedom structure of a double seesaw type. The primary seesaw structure consists of a rigid frame hinged around a main axis. A secondary seesaw structure is hinged around a second axis located at one end of the primary frame. This secondary frame holds at one end a cylinder that interacts with a flow field. As a result, aeroelastic oscillations may occur. Assuming the wind and the damping forces to be small, we can apply the averaging method to analze the asymptotic dynamic behavior.
Here, we consider the case of non-resonance and in particular the cases of 1 : 2 and 1 : 3 resonance. In addition to the stable normal mode solutions as found in the non-resonance case, now we also find stable mixed-mode solutions. In the case of 1 : 3 resonance, we find for certain parameter values a Hopf bifurcation in the averaged equations, resulting in an unstable limit cycle. This limit cycle in the averaged equations corresponds with an oscillation of the original system with periodically modulated amplitudes and phases.
On Asymptotic Approximations of First Integrals for Weakly Nonlinear Oscillators
Authors: Waluya S. B. , van Horssen W. T. 
Published: November 2020

Abstract
In this paper a nonlinear oscillator problem will be studied. It will be shown that the recently developed perturbation method based on integrating vectors can be used to approximate first in tegrals and periodic solutions. The existence, uniqueness, and stability of time-periodic solutions are obtained by using the approximations for the first integrals.
Rain-Wind Induced Vibrations of a Simple Oscillator
Authors: van der Burgh A. H. P. 
Published: November 2020

Abstract
In this paper a relatively simple mechanical oscillator is considered which may be used to study rain-wind induced vibrations of stay cables of cable-stayed bridges. In recent publications mention is made of vibrations of (inclined) stay cables which are excited by a wind-field containing rain drops. The rain drops that hit the cables generate a rivulet on the surface of the cable. The presence of flowing water on the cable changes the cross section of the cable experienced by the wind-field. A symmetric flow pattern around the cable with circular cross section may became asymmetric due to the presence of the rivulet and may consequently induce a lift-force as a mechanism for vibration. During the motion of the cable the position of rivulet (s) may vary as the motion of the cable induces an additional varying aerodynamic force perpendicular to the direction of the wind-field. It seems not to easy to model this phenomenon: several author state that there is no model available y ct.
The idea to model this problem is to consider a horizontal cylinder supported by springs in such a way that only one degree of freedom i.e. vertical vibrations are possible. We consider a ridge on the surface of the cylinder parallel to the axis of the cylinder. Let additionally the cylinder with ridge be able to oscillate, with small amplitude, around the axis such that the oscillationare excited by an external force.
It may be clear that the small amplitude oscillations of the cylinder and hence of the ridge induce a varying lift and drag force. In this approach it is assumed that the motion of the ridge models the dynamics of the rivulet(s) on the cable. By using a quasi-steady approach to model the aerodynamic forces one arrives at a nonlinear second order equation displaying three different kinds of excitation mechanisms: self-excitation, parametric excitation and ordinary forcing. The firstresults of the analysis of the equation of motion show that even in a linear approximation for certain values of the parameters involved stable periodic motions are possible. In the relevant cases where in linear approximation unstable periodic motions are found, results of an analysis of the nonlinear equation are presented.
Parametric Vibration of Viscoelastic Moving Belts Using Standard Linear Solid Model
Authors: Hou Zhichao , Zu Jean W. 
Published: November 2020

Abstract
By using a standard linear solid model to describe the viscoelasticity of the belt material, a vibration analysis of a parametrically excited moving belt is performed. Closed-form solutions at principal resonance and summation resonance are derived at the first order approximation. The existence conditions and stability are discussed for the nontrivial solutions, yielding explicit expressions of the existence and the stability conditions in terms of the detuning parameter. Numerical examples clearly show the effects of tension fluctuations and translating speeds on the amplitudes of dynamic responses, the corresponding existence domains and the stability of the solutions. It is also demonstrated that the stability domains of the nontrivial solutions are different from those corresponding to elastic models.