| 1. |
- Afzal, Mohammad, 1987-
(author)
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On efficient and adaptive modelling of friction damping in bladed disks
- 2017
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Doctoral thesis (other academic/artistic)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. |
- Grishenkov, Dmitry, 1983-
(author)
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Polymer-shelled Ultrasound Contrast Agents : Characterization and Application
- 2010
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Doctoral thesis (other academic/artistic)abstract
- Ultrasound-based imaging technique is probably the most used approach for rapid investigationand monitoring of anatomical and physiological conditions of internal organs and tissues.Ultrasound-based techniques do not require the use of ionizing radiation making the tests anexceptionally safe and painless. Operating in the frequency range between 1 to 15 MHz, medicalultrasound provides reliable visual and quantitative information from both superficial structuressuch as muscles and tendons, and also deeper organs such as liver and kidney. From the technicalpoint of view medical ultrasound has a good spatial and temporal resolution. Ultrasound machineis mobile or even portable, which makes it truly bedside modality. And last but not the least,ultrasound investigations are cheaper in comparison to other real time imaging techniques. Ultrasound imaging techniques can be greatly improved by the use of contrast agents to enhancethe signal from the area of interest (e.g. cardiac or liver tissues) relative to the background.Typically ultrasound contrast agent (UCA) is a suspension of the microbubbles consisting of agas core encapsulated within the solid shell. Generally these devices are injected systemically andfunction to passively enhance the ultrasound echo. In recent years, the UCAs have evolved frombeing just a visualization tool to become a new multifunctional and complex device for drug orgene therapy and targeted imaging. The overall objective of the project is to test novel polymer shelled microbubbles (MBs) as apossible new generation of ultrasound contrast agents. During the first year of the project an innovative criterion based on cross-correlation analysis toassess the pressure threshold at which ultrasonic waves fracture the polymer shell of microbubblehas been developed. In addition, acoustic properties of these microbubbles which are relevant totheir use both as contrast agents and drug carriers for localized delivery have been preliminarytested. Furthermore, in order to reconstruct viscoelastic properties of the shell the originalChurch’s model (1995) has been implemented. In collaboration with Karolinska Institutet, imagesof the microbubbles have been acquired with conventional imaging system. These imagesdemonstrate the potential of the novel polymer-shelled microbubbles to be used as contractenhancing agents. The objective of the second year was to describe the acoustic and mechanical properties ofdifferent types of microbubbles synthesised under varied conditions. This task was divided in twointerrelated parts. In the first part acoustic characterization has been completed in low intensityregion with the study of backscattered power, attenuation and phase velocity. In order torecalculate mechanical properties of the shell existing theoretical model has been furtheriimodified to accommodate the frequency dependence of viscoelastic properties andsimultaneously fit the attenuation and phase velocity data. The results concerning acoustic andmechanical properties of the microbubbles have been sent as a feedback to the manufacture inorder to optimize fabrication protocol for effective image acquisition. In the second part acousticcharacterization has been performed in high intensity region under varied parameters ofexperimental set-up. The results that illustrate the dependence of the fracture pressure thresholdon the system parameters allows us to discuss the potential role of polymer-shelled UCAs as drugcarriers and formulate the protocol for save, localized, cavitation-mediated drug delivery. For the third year the major task was to move on from the bulk volume in vitro tests towards themicrocapillary study and even further to incorporate the microcapillary into the tissue mimickingultrasound phantom. The last study has the objective to take into account the wave propagationthrough tissue. And last but not the least, the application of the polymer-shelled microbubblesfor evaluation of perfusion characteristics, i.e. capillary volume and velocity of the flow, has beenperformed. Similar tests are carried out with commercially available phospholipid-shelled UCA.Using destruction/replenishment technique it is suggested that the novel polymer-shelledmicrobubbles have a potential for a more accurate perfusion evaluation compared to that ofcommercially available phospholipid-shelled UCA. In conclusion, proposed polymer-shelled gas-core microbubbles provide a viable system to beused among the next generation of ultrasound contrast agents, which facilitate not only imageenhancement relevant to diagnostics but also localized and specific drug delivery for non-invasivetherapy even in acute conditions.
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| 3. |
- Östberg, Martin, 1982-
(author)
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Modelling tools for quieter vehicles : Waves in poro– and visco–elastic continua
- 2012
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Doctoral thesis (other academic/artistic)abstract
- New modelling tools intended to contribute to the development of components for quieter vehicles are developed. The tools are based on continuum models for wave propagation in poro– and visco–elastic media. By using geometric attributes of the studied components, the computational cost may be radically decreased compared to traditional methods. By assigning known analytical functions for one or two of the spatial directions, the spatial dimension of the remaining numerical problem is reduced. This reduction of spatial dimensions is performed in two di↵erent ways. The first one treats wave propagation in infinitely extended homogeneous and hollowed cylindrical rods, or wave guides, consisting of visco–elastic media. The wave solutions obtained are then used to model rubber vibration isolators of finite length by mode–matching the fields to the radial boundary conditions of interest. The second one is a method for modelling rotationally symmetric multilayered structures consisting of poro–elastic, elastic and fluid domains. By using a harmonic expansion for the azimuthal spatial dependence, the original three–dimensional problem is split up into several, much smaller, two– dimensional ones, radically decreasing the computational load.Moreover, using a mixed measurement/modelling approach, the audible frequency range characteristics of a viscous damper from a truck is studied, illustrating the influence of the rubber bushings by which it is attached to surrounding structures.The modelling approaches presented in this thesis are intended as tools aiding the design process of new vehicles, enabling new technology striving for more sustainable vehicle concepts. More specifically, the tools aim to improve the modelling of sound and vibration properties which are often penalised when seeking new, more sustainable vehicle designs.
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| 4. |
- Afzal, Mohammad, 1987-
(author)
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Numerical modelling and analysis of friction contact for turbine blades
- 2015
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Licentiate thesis (other academic/artistic)abstract
- High cycle fatigue failure of turbine and compressor blades due to resonance in the operating frequency range is one of the main problems in the design of gas turbine engines. To suppress excessive vibrations in the blades and prevent high cycle fatigue, dry friction dampers are used by the engine manufacturers. However, due to the nonlinear nature of friction contact, analysis of such systems becomes complicated.This work focuses on the numerical modelling of friction contact and a 3D friction contact model is developed. To reduce the computation time in the Newton-iteration steps, a method to compute the Jacobian matrix in parallel to the contact forces is proposed. The developed numerical scheme is successfully applied on turbine blades with shroud contact having an arbitrary 3D relative displacement. The equations of motion are formulated in the frequency domain using the multiharmonic balance method to accurately capture the nonlinear contact forces and displacements. Moreover, the equations of motion of the full turbine blade model are reduced to a single sector model by exploiting the concept of the cyclic symmetry boundary condition for a periodic structure.The developed 3D coupled numerical contact model is compared with a 3D contact model having uncoupled tangential motion and drawback of the uncoupled contact model is discussed. Furthermore, presence of higher harmonics in the nonlinear contact forces is analyzed and their effect on the excitation of the different harmonic indices (nodal diameters) of the bladed disk are systematically presented. Moreover, due to the quasi-analytical computation of the Jacobian matrix, the developed scheme is proved to be effective in solving the equations of motion and significant reduction in time is achieved without loss of accuracy.
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| 5. |
- Alberdi-Muniain, Ane, 1983-
(author)
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Magneto-sensitive elastomers in vibration isolation
- 2012
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Doctoral thesis (other academic/artistic)abstract
- Vibration isolators made of rubber are used in numerous engineeringapplications to isolate structures from undesirable effects of vibrations.However, once a vibration isolator is installed in an application, it is not possible to modify its characteristics to adjust to changing conditions. An alternative to obtain more adaptive characteristics is touse magneto-sensitive (MS) elastomers. MS elastomers are a type of smart material consisting of an elastomer matrix, such as natural or synthetic rubber, to which iron particles are added displaying properties that vary rapidly, continuously and reversibly by applying an external magnetic field.The aim of this thesis is to investigate the possibility to use MS natural rubber in vibration isolation.Firstly, dynamic shear properties of MS natural rubber are experimentally studied at various frequencies, dynamic amplitudes and magnetic fields. In addition, the influence on the dynamic properties of adding carbon black and plasticisers to MS rubber is investigated. Carbon black is the most popular reinforcing filler that rubber usually contains in engineering applications to improve mechanical properties where as plasticisers simplify the filler blending process.Furthermore, the effectiveness of MS rubber applied in a vibration isolation system is experimentally investigated by measuring the energy flow into the foundation. The energy flow, including both force and velocity of the foundation, is a suitable measure of the effectiveness of a real vibration isolation system where the foundation is not perfectly rigid. The vibration isolation system in this study consists of a solid aluminium mass excitedby an electro-dynamic shaker and mounted upon four nonlinear frequency,amplitude and magnetic field dependent MS isolators being connected to a relatively stiff foundation. The energy flow through the MS isolators is directly measured by inserting a force transducer below each isolator andan accelerometer on the foundation close to each isolator. MS isolators are shown to be more useful than conventional rubber isolators since the dynamic stiffness varies with the application of an external magnetic field,thus resulting in more effective vibration isolation. In addition, the indirect technique is employed to measure the energy flow while requiring only accelerometers since it is usually difficult to directly measure the force in a real application. The indirect technique is validated by direct measurements.Finally, a model of the energy flow through the nonlinear frequency,amplitude and magnetic field dependent MS isolators is developed for the tested vibration isolation system. Vibration isolators are usually only a small connecting component within a more complex system. Hence, simple discrete models are frequently used to characterise the frequency and dynamic amplitude dependence of rubber. Recently, a model of this type has been modified to include magneto-sensitivity and thus model MS rubber. In this study, this novel MS rubber model is incorporated into the full system to model the MS isolators while the foundation is characterised by its driving-point and transfer inertances at and between the connection points.The energy flow model results are compared to those of measurements,showing good agreement. The developed energy flow model provides a basis to design vibration isolator systems made of MS isolators.
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| 6. |
- Blanco, Blas, 1990-
(author)
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Railway track dynamic modelling
- 2017
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Licentiate thesis (other academic/artistic)abstract
- The railway vehicles are an increasing mean of transportation due to, its reduced impact on environment and high level of comfort provided. These reasons have contributed to settle a positive perception of railway traffic into the European society. In this upward context, the railway industrial sector tackles some important challenges; maintaining low operational costs and controlling the nuisance by-products of trains operation, the most important being railway noise. Track dynamic plays a main role for both issues, since a significant part of the operational costs are associated with the track maintenance tasks and, the noise generated by the track can be dominant in many operational situations. This explains why prediction tools are highly valued by railway companies. The work presented in this licentiate thesis proposes methodologies for accurate and efficient modelling of railway track dynamics. Two core axes have led the development of this task, on one hand, the rail modelling and, on the other hand, the characterisation of the finite length nature of track supports. Firstly, concerning the rail modelling technique, it has evolved under two major premises. On one hand, regarding the frequency domain, it should describe high frequency behaviour of the rail. In order to accomplish with this first premise, a model based on Timoshenko beam theory is used, which can accurately account for the vertical rail behaviour up to 2500 Hz. On the other hand, with respect to the time domain, the response should be smooth and free of discontinuities. This last condition is fulfilled by implementation of the Timoshenko local deformation. Secondly, a model of support that considers its finite length nature is sought. For this purpose, a Timoshenko element over elastic foundation is formulated. Thus, the common model of support, which is based on a concentrated connection, is substituted by a distributed model of support. In this way, several enhancements are achieved; the temporal contact force response is smoothed and a more realistic shape is obtained, the amplitude of the displacement due to the parametric excitation is reduced and the magnitude associated to the ‘pin-pin’ frequency is not overestimated.
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| 7. |
- Gomez, Erik
(author)
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Centrifugal pendulum vibration absorbers in heavy-duty truck powertrains : Modelling, simulation and experimental investigations
- 2021
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Doctoral thesis (other academic/artistic)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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| 8. |
- Lundberg, Oskar, 1980-
(author)
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On the influence of surface roughness on rolling contact forces
- 2016
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Doctoral thesis (other academic/artistic)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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| 9. |
- Sun, Jia
(author)
-
Rotating Structure Modeling and Damping Measurements
- 2011
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Licentiate thesis (other academic/artistic)abstract
- The structural damping is of importance to suppress the vibration amplitude of compressor blades rotating at high angular velocity under a high cycle impact. To avoid the appearance of the high cycle fatigue (HCF), damping materials may be applied to the compressor blades. To quantify the effect while using damping materials, a numerical tool needs to be developed for the damping prediction of a dynamic rotating blade. This thesis is divided into two parts: Paper A develops a dynamic model of a rotating blade and Paper B a damping structure model including measurements.In Paper A, a dynamic rotating blade model is developed by using a plate model at an arbitrary stagger angle. Hamilton’s principle is applied to derive a system of equations of motion and the corresponding boundary conditions. Numerical simulation is implemented to perform eigenfrequency analysis by the Extended Galerkin method. In addition, parametric analysis is performed with respect to rotation speed and stagger angle, respectively. Results show a good agreement with those of the finite element method. Finally, forced response analysis is determined for two cases; a point force and a distribution force, using a proportional damping model.In Paper B, unconstrained and constrained damping techniques are applied to increase the structural damping of the blades, including measurement and modeling results. Two specimens, titanium and stainless steel, are treated by aluminum oxide and epoxy coating material. Measurement results show that both treatments give damping increase, where aluminum oxide is more effective for damping improvement than the corresponding epoxy treatment. The unconstrained damping layer model is used to predict the total material damping of the combined structure as well as the material damping of coating layer. Furthermore, the constrained-layer model is used to optimize the damping configuration. Two compressor blades in titanium and stainless steel are tested in air and vacuum. One reason is being that the radiation loss factor increases the total damping comparing with that under vacuum condition. The calculation of radiation loss factor is performed to match the measurement data. Finally, increased material damping decreases peak stress and therefore increases the life time of the compressor blades.
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| 10. |
- Sun, Jia, 1981-
(author)
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Vibration Characteristics and Structural Damping of Rotating Compressor Blades
- 2012
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Doctoral thesis (other academic/artistic)abstract
- Nowadays, the increasing demand on the high efficiency energy, low fuel consumption and environment friendly leads the turbomachinery to be operating under a critical high rotation speed at high temperature and pressure. This severe operation condition will definitely increase the probability of the occurrence of the high cycle fatigue. To reduce the risk of appearing high cycle fatigue, the structural damping of turbomachinery components has to be increased. Since the structural damping is always positive while the aerodynamic damping can be negative at some situation, increasing structural damping is nevertheless an interesting field in turbomachinery research. One efficient way of increasing damping is to treat damping material over the blade surface. Traditional damping materials, such as rubber, are not applicable in the severe operation environment. Therefore, hard coating material is applied due to its high stiffness and good sustainability in rough environments.Numerical tools are developed to predict the structural damping of a dynamic rotating blade while varying several important designing parameters. Two types of rotating blades are modeled using the Hamilton’s principle: the straight blade by plate theory and pretwisted blade by shell theory. The extended Galerkin method and Chebyshev collocation method are applied for the numerical simulation, such as modal analysis and frequency response analysis. The parametric analysis is performed with respect to rotation speed, stagger angle, pretwisted angle, aspect ratio, etc. Proportional damping isused in all dynamic models to investigate the damping characteristics of the blades.Alternatively, a multilayer rotating blade is modeled by a high order layerwise theory, where the validated results reveal the modal damping exchanges between modes dueto frequency loci veering and the influence of the damping configurations on the total damping of the multilayered structure. Finally, a commercial finite element software isused to predict the damping of a real compressor blade treated by the hard coating while varying the coating thickness and distributions.
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