| 1. |
- Afzal, Mohammad, 1987-
(författare)
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On efficient and adaptive modelling of friction damping in bladed disks
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This work focuses on efficient modelling and adaptive control of friction damping in bladed disks. To efficiently simulate the friction contact, a full-3D time-discrete contact model is reformulated and an analytical expression for the Jacobian matrix is derived that reduces the computation time drastically with respect to the classical finite difference method. The developed numerical solver is applied on bladed disks with shroud contact and the advantage of full-3D contact model compared to a quasi-3D contact model is presented. The developed numerical solver is also applied on bladed disks with strip damper and multiple friction contacts and obtained results are discussed. Furthermore, presence of higher harmonics in the nonlinear contact forces is analyzed and their effect on the excitation of the different nodal diameters of the bladed disk are systematically presented. The main parameters that influence the effectiveness of friction damping in bladed disks are engine excitation order, contact stiffnesses, friction coefficient, relative motion at the friction interface and the normal contact load. Due to variation in these parameters during operation, the obtained friction damping in practice may differ from the optimum value. Therefore, to control the normal load adaptively that will lead to an optimum damping in the system despite these variations, use of magnetostrictive actuator is proposed. The magnetostrictive material that develops an internal strain under the influence of an external magnetic field is employed to increase and decrease the normal contact load. A linearized model of the magnetostrictive actuator is used to characterize the magnetoelastic behavior of the actuator. A nonlinear static contact analysis of the bladed disk reveals that a change of normal load more than 700 N can be achieved using a reasonable size of the actuator. This will give a very good control on friction damping once applied in practice.
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| 2. |
- Liu, Hao, 1986-
(författare)
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Wave Modelling Techniques for Medium and High Frequency Vibroacoustic Analysis Including Porous Materials
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Numerical methods based on wave modelling are explored for the vibroacoustic analysis of wave propagation, sound transmission and interior noise in vehicles and buildings at medium and high frequencies. The presence of sound absorbing porous materials in practical engineering structures is also considered. The wave modelling techniques provide computational efficiency and physical insight, and two such methods having these advantages are developed in this thesis namely: the semi-analytical finite element method and the wave expansion method.The semi-analytical finite element method is applicable to structures which have constant properties in one direction, and it uses a finite element discretization of the cross-section and analytical functions in the third direction. Equations of motion are derived from this method to study wave propagation characteristics, which help understand the vibroacoustic behavior of structures. These characteristics may also be used by high frequency techniques, such as statistical energy analysis. The wave propagation in sandwich panels with a poroelastic core, which is modeled with Biot's theory, is investigated thoroughly.The semi-analytical finite element method retains the flexibility of the finite element method on geometry and also dramatically increases the computational speed thanks to the orthogonality of the analytical functions when used to calculate forced response. The calculated response of partitions is integrated into diffuse field sound transmission loss calculations of, for example, built-up train floor partitions and multilayer panels lined with porous materials. The calculations are computationally efficient and show good agreement with measurements, thus it is interesting for industrial optimizations.The wave expansion method uses a priori defined plane wave solutions to the Helmholtz equation for approximation of the sound field in geometrically complex enclosures. It reduces the requirements regarding the number of degrees of freedom compared to the finite element method, which, furthermore, is polluted by dispersion errors. Therefore, the wave expansion method is particularly appealing for high frequency (or large wavenumber) calculations. Its application in interior sound field predictions is assessed within the automobile context.
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| 3. |
- Gomez, Erik
(författare)
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Centrifugal pendulum vibration absorbers in heavy-duty truck powertrains : Modelling, simulation and experimental investigations
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The heavy-duty vehicle industry is facing big challenges to reduce CO2 emissions. Although electrification is on the rise, the combustion engine will used for some time to come. Unfortunately, CO2 reduction methods, such as downspeeding, down-sizing and increased cylinder gas-pressure, result in increased torsional vibrations and noise which must be addressed. Conventional torsional vibration reduction methods alone, such as the clutch-damper, will not suffice. The centrifugal pendulum vibration absorber (CPVA) is a torsional vibration reduction device that has not conventionally been used in heavy-duty vehicles and is herein investigated.This work presents analytical, simulation and experimental investigations of the CPVA. First, a model of a centrifugal pendulum vibration absorber with a general suspension architecture is derived with Kane's method. The model allows for different pendulum paths and relative rotation of the pendulum body with respect to the rotor. A normal-force friction loss of the pendulum is developed together with a measurement method to determine the friction coefficient. The measurement method may also be used to validate the pendulum model parameters without any special test-apparatus other than standard accelerometers and a data acquisition system. The developed CPVA model is then included in a torsional model of the complete powertrain including gas-pressure based excitation from the engine model. The powertrain model is developed to simulate transient and quasi-steady state conditions in the time-domain. Also, a continuous model of the end-stops of the pendulum are implemented which also facilitates the simulation.It is concluded that it is important to include the system dynamics of the powertrain in the final design of the CPVA. Also, by exploiting the frequency veering property of the CPVA, powertrain resonances can be completely eliminated.
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| 4. |
- Lundberg, Oskar Erik, 1980-
(författare)
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Vibrations induced by surface roughness in nonlinear rolling contacts
- 2014
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- For efficient transportation in either trains, busses or passenger cars, rolling elements such as wheels, tyres, bearings and transmission elements are fundamental. The energy efficiency and the generation of noise and vibrations in rolling contacts depend on the surface roughness of contacting bodies. In order to optimize the surfaces of rolling elements, prediction of its impact on the dynamic response from rolling excitation is required. A computationally efficient method to include surface roughness in the modelling of rolling contacts is presented. More specifically, nonlinear effects on the contact force due to the threedimensional shape and roughness of the contacting surfaces are introduced in a moving point force formulation. As a consequence of the point force approximation follows the assumption that any dynamic wave motion within the contact area is negligible.The rolling contact force is nonlinear due to a varying relative displacement between contacting bodies and is therefore referred to as state-dependent. A study case for the state-dependent method consisting of a steel ball rolling on a steel beam showed good agreement between numerical predictions and measured beam vibrations. Furthermore, an application to the wheel-rail interaction show that roughness-induced contact nonlinearities have a significant impact on the dynamic response caused by rolling excitation.
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| 5. |
- Lundberg, Oskar, 1980-
(författare)
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On the influence of surface roughness on rolling contact forces
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Road vehicle tyres, railway wheels and ball bearings all generate rolling contact forces which are transferred within a finite area of contact between the rolling element and the substrate. Either it is visible or not for the human eye, a certain degree of roughness is always present on the contacting surfaces and it influences the generation of both vertical and lateral contactforces. The purpose of this investigation is to enhance the understanding and modelling of the influence from small-scale surface roughness on the generation of rolling contact forces. To this end, a computationally efficient method to include roughness-induced contact nonlinearities in the dynamic modelling of rolling contacts is proposed. The method is implemented in a time domain model for vertical wheel–track interaction to model rolling-induced rail vibrations, showing good agreement with measurements. Furthermore, a test rig is developed and used for the investigation of tyre–road rolling contact forces. Detailed studies are performed on the influence of substrate roughness on the resulting contact forces for a tyre tread block which is rolling at different operating conditions. The choice of substrate as well as the rolling velocity and the slip ratio is observed to have significant influence on the resulting friction coefficient. For high slip ratios, stick–slip oscillations appear, exhibiting frequency content which is largely dependent on the choice of substrate. The outcomes of this study can potentially be used to improve future tyre–road contacts with respect to wear, traction and noise generation.
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| 6. |
- Zea, Elias, 1989-
(författare)
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Single layer near-field acoustic imaging in space-invariant reflective environments
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The visualization of optical fields, or widely known as photography, is a common practice that can also be applied to acoustic fields. No less than three decades of research in acoustic imaging demonstrate its potential and relevance in plenty applications such as sound source identification and localization, as well as characterization of noise generation mechanisms. In particular, the resolution of the acoustic images can be greatly enhanced by means of measuring the sound in the acoustic near-field. Nevertheless, it is often the case that the field is disturbed by the measurement environment, in the form of noise and reflections, and the source must then be transported to a much quieter place, for instance, an echo-free chamber. Current state-of-the-art methods tackle the aforesaid complications, at the cost, however, of measuring two images instead of one: resulting in twice as many measurement sensors. The contribution of this thesis is the introduction of a novel technique that only requires a single image to be captured, and is specially tailored for acoustic imaging in space-invariant reflective environments, that is, in the presence of reflecting surfaces that are parallel to the measurements.
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| 7. |
- Zhu, Yi, 1984-
(författare)
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Adhesion in the wheel-rail contact under contaminated conditions
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Railway vehicles require a certain level of adhesion between wheel and rail to operate efficiently, reliably, and economically. Different levels of adhesion are needed depending on the vehicle running conditions. In the wheel tread–railhead contact, the dominant problem is low adhesion, as low adhesion on the railhead negatively affects railway operation: on one hand, the vehicle will lose traction resulting in delay when driving on low-adhesion tracks; on the other hand, low adhesion during deceleration will extend the braking distance, which is a safety issue. This thesis examines the influence of several contaminants, i.e., water, oil, and leaves, on the adhesion in the wheel tread–railhead contact. This study will improve our knowledge of the low-adhesion mechanism and of how various contaminants influence adhesion. The thesis consists of a summary overview of the topic and three appended papers (A–C). Papers A and B focus mainly on water and oil contamination examined using two methods, numerical simulation and lab testing. In paper A, real measured wheel and rail surfaces, low- and high-roughness surfaces, along with generated smooth surfaces are used as input to the numerical model for predicting the adhesion coefficient. Water-lubricated, oil-lubricated, and dry contacts are simulated in the model. In the research reported in paper B, scaled testing using a mini traction machine (MTM) was carried out to simulate the wheel–rail contact under lubricated conditions. Two types of disc surfaces of different roughnesses were run at different contact pressures and temperatures. A stylus machine and atomic force microscopy (AFM) were used to measure the surface topography. A study of leaf contamination on the railhead surface, based on field testing, is presented in paper C. Railhead surface samples were cut and the friction coefficient was measured on five occasions over the course of a year. Electron spectroscopy for chemical analysis (ESCA) and glow discharge optical emission spectrometry (GD-OES) were used to detect the chemical composition of the leaf-contamination layer on the railhead surface. The main conclusion of the thesis is that different contaminants reduce the adhesion coefficient in different ways. Oil reduces the adhesion coefficient by carrying the normal force due to its high viscosity. Water can reduce the adhesion coefficient to different degrees depending on the surface topography and water temperature. The mixture of an oxide layer and water contamination may have an essential impact. A leaf-formed blackish layer causes low adhesion by means of a chemical reaction between the leaves and bulk material. The thickness of the friction-reducing oxide layer predicts the friction coefficient and the extent of leaf contamination.
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| 8. |
- Manzari, Luca
(författare)
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High-speed stereo imaging for the characterization of anisotropic viscoelastic media
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis proposes an experimental method for observing and characterizing the viscoelastic properties of anisotropic media using high-speed white light stereo imaging. The method uses short-time video recordings of a specimen undergoing forced harmonic motion.The three-dimensional displacement field of the specimen is then resolved using digital image correlation.Measuring for a short time has multiple advantages: it minimizes the conditioning of the specimen, and gives meaningful results when true stationary conditions are inaccessible (e.g. because of relaxation processes, or changes in the environmental conditions that cannot be accounted for).Moreover, it enables a reduction of the data storage needs and the computational costs associated with the image acquisition and processing.To overcome the intrinsic limitations of a Fourier-based approach for short time records, an optimization algorithm is used to determine the point-wise amplitude, phase and frequency of the full-field harmonic motion.This approach maximizes signal-to-noise ratio, is suitable for the identification of non-linear behaviors and tolerates data records that are non-uniformly spaced in time (e.g. because of momentary data losses and failure of the image matching algorithms).The measurement accuracy is increased by proposing a method to extract the frame of reference of the specimen on a per-frame bases, and express the measured displacement field therein.A cube of melamine foam and a pantographic sheet have been observed using the proposed method, and the measured data compared with the outcome of linear viscoelastic numerical models.The added information obtained about the melamine is believed to improve the accuracy of the characterization of its viscoelastic behavior, and the observation of the pantographic sheet represents and absolute first in the experimental studies of its dynamics.
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| 9. |
- Wang, Bochao
(författare)
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Constitutive models of magneto-sensitive rubber under a continuum mechanics basis and the application in vibration isolation
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Due to its durability, stretchability, relatively low stiffness and high damping, rubber is widely used in engineering anti-vibration fields. However, a major deficiency is that once installed, the mechanical properties of traditional rubber-based devices are fixed where its adaptability to various loading conditions is poor. An alternative to traditional rubber materials is magneto-sensitive (MS) rubber. The main componentsof MS rubber are a rubber matrix and ferromagnetic particles. Under a magnetic field, the modulus of MS rubber can be altered rapidly and reversibly. Therefore, compared with conventional rubber-based devices, the stiffness of MS rubber-based devices can be adapted to various loading conditions and an enhanced vibration reduction effect can be achieved. Measurement results revealed that the mechanical behavior of MS rubber is not simple. To be specific, the dynamic modulus of MS rubber has a magnetic, frequency,amplitude and temperature dependency. In order to promote the applications of MS rubber in the anti-vibration area, models to depict the above properties are needed. The main goal of this thesis is to model the magnetic, frequency, amplitude and temperature dependence of MS rubber under a continuum mechanics basis. The research results regarding the constitutive modeling consist of three papers (Paper A, C and D). The simulation results show a good agreement with the measurement data, which proves the accuracy and feasibility of the developed model. In addition to the constitutive models of MS rubber, an investigation of MS rubber application in the vibration isolation system under harmonic and random loading cases is numerically conducted (Paper B). In order to achieve an enhanced vibration isolation effect, two control algorithms corresponding to the harmonic and random loading are developed. Numerical results verify that the vibration isolation effect ofMS rubber vibration isolator is better than the traditional rubber-based isolator. In this thesis, the model developed for MS rubber deepens the understanding of how magnetic, frequency, amplitude and temperature affect the mechanical performance of MS rubber. Moreover, the research of MS rubber application in vibration isolators and the corresponding control strategies are helpful for the design of MS rubber-based anti-vibration devices.
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