"uuid","repository link","title","author","contributor","publication year","abstract","subject topic","language","publication type","publisher","isbn","issn","patent","patent status","bibliographic note","access restriction","embargo date","faculty","department","research group","programme","project","coordinates"
"uuid:158da0dc-e2a8-4c59-8af8-5b50b2b96c94","http://resolver.tudelft.nl/uuid:158da0dc-e2a8-4c59-8af8-5b50b2b96c94","Interplay of Structural and Light-induced Carrier Dynamics in Metal Halide Perovskites","Zhao, J. (TU Delft ChemE/Opto-electronic Materials)","Savenije, T.J. (promotor); Houtepen, A.J. (copromotor); Delft University of Technology (degree granting institution)","2024","As one of the fastest-growing renewable energy technologies, photovoltaics play an increasingly important role in the global energy transition. Over the past decade, metal halide perovskite solar cells (PSCs) have emerged as the most promising candidates for next-generation solar cells, with a certified power conversion efficiency of 26.1% for single-junction cells. Despite these significant advances in this performance, understanding the fundamental optoelectronic properties of various compositions is crucial to improve the efficiency and stability of the development of single-junction and multi-junction solar cells, including perovskite/silicon and all-perovskite tandem solar cells. In this thesis, we have investigated the generation, recombination, and extraction of photo-generated carriers in various metal halide perovskites (MHPs) in combination with selective transport layers (TLs) mainly using the time-resolved microwave conductivity (TRMC) technique. Moreover, structural properties were revealed using various techniques including XRD, XPS, and SEM. In addition, different deposition methods of perovskite thin films are studied with the aim of providing insights into the relationship between structure and optoelectronic properties.....","","en","doctoral thesis","","","","","","","","2024-03-19","","","ChemE/Opto-electronic Materials","","",""
"uuid:d848c617-2eae-491d-aae1-d524495e9e65","http://resolver.tudelft.nl/uuid:d848c617-2eae-491d-aae1-d524495e9e65","Hydrogenated nanocrystalline silicon-based layers for silicon heterojunction and perovskite/c-Si tandem solar cells","Zhao, Y. (TU Delft Photovoltaic Materials and Devices)","Weeber, A.W. (promotor); Isabella, O. (promotor); Zeman, M. (promotor); Delft University of Technology (degree granting institution)","2023","Large-scale deployment of photovoltaic (PV) technology is imperative for realizing a future sustainable and electrified energy system. Over the past decades, technological advancements that enhance the efficiency of PV technologies have been one of the crucial aspects for significantly reducing the cost of PV-generated electricity. Among various crystalline silicon (c-Si) PV technologies, silicon heterojunction (SHJ) solar cells, which have achieved the highest efficiency of single-junction c-Si solar cells, hold great promise for advancing the energy transition facilitated by PV technologies even further. Moreover, notable efficiency enhancements, which are well beyond the theoretical efficiency limit of single-junction c-Si solar cells, have been experimentally demonstrated by combining SHJ solar cells with semi-transparent perovskite solar cells in tandem configurations. This thesis focuses on addressing the challenges of efficient deployments of doped hydrogenated nanocrystalline silicon-based (nc-Si:H-based) layers for high-efficiency front/back-contacted (FBC) SHJ solar cells and applications of FBC-SHJ bottom-cells in two-terminal (2T) and four-terminal (4T) tandem devices with perovskite top-cells, supported by advanced opto-electrical simulations.","","en","doctoral thesis","","978-94-6473-313-6","","","","","","","","","Photovoltaic Materials and Devices","","",""
"uuid:2aff1a7e-45eb-4d10-9944-8e06ef12b9fa","http://resolver.tudelft.nl/uuid:2aff1a7e-45eb-4d10-9944-8e06ef12b9fa","Learning Analytics Technology to Understand Learner Behavioral Engagement in MOOCs","Zhao, Y. (TU Delft Web Information Systems)","Houben, G.J.P.M. (promotor); Hauff, C. (copromotor); Lofi, C. (copromotor); Delft University of Technology (degree granting institution)","2019","As one of the most prominent examples of technology-enhanced learning, massive open online courses (MOOCs) have attracted extensive attention of learners, educators, and researchers since 2012. However, a low completion rate is a ubiquitous and severe problem in MOOCs, which means that only a small portion of learners got scores higher than or equal to the course requirements in MOOCs. Learner engagement is commonly presumed to be highly related to the completion rates of MOOCs...","","en","doctoral thesis","","978-94-028-1462-0","","","","","","","","","Web Information Systems","","",""
"uuid:b4f772b3-c9ea-4760-b68e-a8c85fd099b6","http://resolver.tudelft.nl/uuid:b4f772b3-c9ea-4760-b68e-a8c85fd099b6","Sequential ultrasonic spot welding of thermoplastic composites: An experimental study on the welding process and the mechanical behaviour of (multi-)spot welded joints","Zhao, T. (TU Delft Structural Integrity & Composites)","Benedictus, R. (promotor); Villegas, I.F. (copromotor); Delft University of Technology (degree granting institution)","2018","The popularity of thermoplastic composites (TPCs) has been growing steadily in the last decades in the aircraft industry. This is not only because of their excellent material properties, but also owing to their fast and cost-effective manufacturing process. Fusion bonding, or welding, is a typical joining method for TPCs due to the intrinsic properties of thermoplastic polymers. Among different welding technologies, ultrasonic welding has been regarded as one of the most promising techniques for the assembly of TPC components. Ultrasonic welding is by nature a spot welding technique. As it is known that a series of problems result from using mechanical fasteners for joining composite structures, e.g. breaking fibres during drilling and extensive labour work, ultrasonic spot welding can be considered as a promising alternative from the perspective of fast manufacturing cycle. However, fundamental understanding is still lacking to achieve application of ultrasonic spot welding in composite structures to be achieved:","Thermoplastic composites; Ultrasonic spot welding; Mechanical behaviour; Fractographic analysis","en","doctoral thesis","","978-94-6295-916-3","","","","","","","","","Structural Integrity & Composites","","",""
"uuid:48218fb4-1b37-4b83-81ee-a0cadc16d8e9","http://resolver.tudelft.nl/uuid:48218fb4-1b37-4b83-81ee-a0cadc16d8e9","Unified correspondence and canonicity","Zhao, Z. (TU Delft Ethics & Philosophy of Technology)","Palmigiano, A. (promotor); van de Poel, I.R. (promotor); Delft University of Technology (degree granting institution)","2018","Correspondence theory originally arises as the study of the relation between modal formulas and first-order formulas interpreted over Kripke frames. We say that a modal formula and a first-order formula correspond to each other if they are valid on the same class of Kripke frames. Canonicity theory is closely related to correspondence theory. We say that a modal formula is canonical if it is valid on its canonical frame, or equivalently,if its validity is preserved from a modal algebra to its canonical extension, or from a descriptive general frame to its underlying Kripke frame. Canonicity is closely related to completeness. If a modal formula is canonical, then the normal modal logic axiomatized by this modal formula is complete with respect to the class of Kripke frames defined by it.
In the development of correspondence theory, the algorithmic aspect receives increasing attention. The Sahlqvist-van Benthem theorem provides an algorithm to transform a class of modal formulas, which are later called Sahlqvist formulas, into their corresponding first-order formulas. The algorithm SQEMA provides a modal language-based algorithm to transform a modal formula into a pure modal formula in an expanded language, and then translate the pure modal formula into the first-order language. SQEMA succeeds on a strictly larger class of modal formulas, which are called inductive formulas.
In recent years, unified correspondence theory is developed based on duality-theoretic and order-algebraic insights. In this approach, a very general syntactic definition of Sahlqvist and inductive formulas is given, which applies uniformly to each logical signature and is given purely in terms of the order-theoretic properties of the algebraic interpretations of the logical connectives. In addition, the Ackermann lemma based algorithm ALBA, which is a generalization of SQEMA based on order-theoretic and algebraic insights, is given, which effectively computes first-order correspondents of input formulas/inequalities, and is guaranteed to succeed on the Sahlqvist and inductive classes of formulas/inequalities.
This dissertation belong to the line of research of unified correspondence theory.
Chapter 3 applies the unified correspondence methodology to possibility semantics, and gives alternative proofs of Sahlqvist-type correspondence results to the ones in [196], and extends these results from Sahlqvist formulas to the strictly larger class of inductive formulas, and from the full possibility frames to filter-descriptive possibility frames. Chapter 4 applies the unified correspondence methodology to modal compact Hausdorff spaces, and gives alternative proofs of canonicity-type preservation results to the ones in [14]. Chapter 5 examines the power and limits of the translation method in obtaining correspondence and canonicity results. Chapter 6 is about an application of unified correspondence theory to the proof theory of strict implication logics, showing the usefulness of unified correspondence theory in the design of analytic Gentzen sequent calculi, especially when it comes to computing the corresponding analytic rules of a given sequent.","","en","doctoral thesis","","","","","","","","","","","Ethics & Philosophy of Technology","","",""
"uuid:e74bcb28-5409-4ba2-ac5c-40dc201313aa","http://resolver.tudelft.nl/uuid:e74bcb28-5409-4ba2-ac5c-40dc201313aa","Aircraft Life Cycle Cost Analysis and Design Integration: A Knowledge Based Engineering Approach","Zhao, X. (TU Delft Air Transport & Operations)","Curran, R. (promotor); Verhagen, W.J.C. (copromotor); Delft University of Technology (degree granting institution)","2016","","","en","doctoral thesis","","978-94-6186-712-4","","","","","","2017-06-07","","","Air Transport & Operations","","",""
"uuid:ee435ef7-f71c-493c-812b-68cf1e39aa33","http://resolver.tudelft.nl/uuid:ee435ef7-f71c-493c-812b-68cf1e39aa33","Breach growth in cohesive embankments due to overtopping","Zhao, G. (TU Delft Rivers, Ports, Waterways and Dredging Engineering)","Vrijling, J.K. (promotor); Visser, P.J. (copromotor); Delft University of Technology (degree granting institution)","2016","Breaching is the most frequent form of embankment failure in the world. Due to overtopping, an embankment starts to breach when part of the embankment actually breaks away, leaving an opening for water to flood the land protected by the embankment. A breach can be a sudden or gradual failure that is caused by surface erosion and/or headcut erosion in the embankment. The magnitude and extent of the losses depend highly on the rate of breaching of the embankment, which determines the discharge through the breach and the speed and rate of inundation of the valley or polder. Therefore, modelling of the breach evolution in embankments is of significant interest for damage assessment and risk analysis. It is also important for the development of early warning systems for dike and dam failures and of evacuation plans for people at risk.
The breach flow plays an important role in the embankment breaching process, coupling the hydraulic process and the sediment transport process. During the breaching process, the flow overtops the entire embankment crest and generates the breach channel in the initial phase of the breach development. As the breach further develops, the breach flow goes only through the breach channel due to the decrease of the upstream water level. The breach flow can thus be classified as compound weir flow and weir flow, each having own special characteristics. In a breach, the helicoidal flow accelerates the sediment undermining at the toe of the breach slopes and widens the breach in the lateral direction of the embankment. A triangular hydraulic jump happens when the breach flow changes from supercritical flow into subcritical flow, with a triangular critical area at the toe of the breach. The triangular hydraulic jump works as a driving force to the headcut erosion in the breaching process and the scour hole development at the toe of embankment. According to the hydraulic energy loss in the breach, the discharge coefficients are deducted for both weir flow condition and compound weir flow condition. The resulting discharge coefficients can be used in the calculation of the breach discharge in a breach model.
In the present study, five runs of breach experiments were conducted in a relative large laboratory flume. The experimental results clearly expose the hydrodynamic process and the erosion process in the breaching of the cohesive embankment. The breaching starts with the initial erosion of the embankment surface washing away the embankment surface. Due to the surface erosion at the toe of the embankment, the headcut erosion is stimulated on the embankment slope. While headcut migration stimulates the breach to develop in longitudinal direction, the lateral erosion triggers the breach to widen in lateral direction. Three types of erosion (surface erosion, headcut erosion and lateral erosion) contribute to the breach erosion process in the embankment, however, the breach flow is the driving force for the erosion. Sediment deposition in the breaching process, generally ignored in the embankment breaching studies, is also of importance.
A mathematical model has been developed that couples weir flow and erosion (surface erosion, headcut erosion and lateral erosion). The breaching process is simplified into initial development, deepening development and widening development, corresponding with surface erosion, headcut erosion and lateral erosion, respectively. As the link between flow and embankment material, erosion plays a key role in the embankment breach model. Mathematical descriptions of the headcut migration and the lateral migration rate have been developed to simulate the breaching process in cohesive embankments. The headcut erosion and the lateral erosion are considered to occur in the form of clay blocks instead of in the form of individual clay particles.","","en","doctoral thesis","","97890-6562-3935","","","","","","","","","Rivers, Ports, Waterways and Dredging Engineering","","",""
"uuid:c227a75a-20b7-4d88-a0b4-7175b7e956b8","http://resolver.tudelft.nl/uuid:c227a75a-20b7-4d88-a0b4-7175b7e956b8","Fast solvers for concentrated elastic contact problems","Zhao, J.","Oosterlee, C.W. (promotor); Vollebregt, E.A.H. (promotor)","2015","Rail transportation plays an important role in our everyday life, and there is fast development and modernization in the railway industry to meet the growing demand for swifter, safer and more comfortable trains. At the same time, the security of train operation and the maintenance of rails have to be considered. A lot of research on these issues has been carried out, among which the study of the contact between a train's wheel and the rail is particularly significant. The contact problem considers two elastic bodies. When they are pressed together, a contact area is formed where the two body surfaces coincide with each other. Moreover, an elastic field of stress, strain and displacement arises in each body. These stresses consist of normal stress (pressure) and frictional stress (traction) acting in the tangential direction. When solving the so-called {\it normal contact problem}, we search for the contact area and the pressure on it. The {\it tangential contact problem} is studied when the two bodies are brought into relative motion. If the relative velocity of the two surfaces is small, a creeping motion may be observed which is largely caused by the elastic deformation at the contact region. In those parts of the contact area where the tangential stress is small, the surfaces of the two bodies stick to each other. Otherwise, local relative sliding may occur. The research question is to find the adhesion and slip areas, and the tangential tractions. The solution methods for contact problems have been studied from the late nineteenth century, resulting in a variety of analytic and numerical approaches, w.r.t. their own specific applications. Motivated by the requirement of fast computation for involved applications such as the simulation of railway wheel-rail dynamics, we aim at developing fast numerical solvers for concentrated elastic contact problems in this thesis. Our work focuses on the contact between bodies of linear homogeneous elastic material. Moreover, it is a concentrated contact, i.e. the contact area is small compared to the dimensions of the contacting bodies. The models in use are provided by a variational formulation, which is based on a boundary element method (BEM). It gives rise to a convex optimization problem with linear or nonlinear constraints. The corresponding Karush-Kuhn-Tucker conditions provide the governing equations and contact conditions, that are numerically solved. The most time-consuming part attributes to solving a Fredholm integral of the first kind, resulting from the BEM. The corresponding Green's function expresses the relation between tractions and deformation, using a half-space approach. This integral yields linear systems with coefficient matrices that are dense, symmetric and positive definite. Moreover, they are Toeplitz in two-dimensional (2D) problems and block Toeplitz with Toeplitz blocks in three-dimensional (3D) problems. Fast computing techniques such as the fast Fourier transform (FFT) are explored. We start our work by solving the normal contact problem in Chapter 2. It is modeled by a linear complementarity problem, for which a full multigrid method (FMG) is presented. This method combines a multigrid (MG) method, an active set strategy and a nested iteration technique. It is applied to a Hertzian smooth contact and a rough surface contact. The results show the efficiency and robustness of the FMG method. Tangential contact is considered in Chapter 3 and Chapter 4. A 2D no-slip tangential problem is first studied in Chapter 3, where we mainly solve the surface integral. A fast MG method is proposed with an FFT smoother, where a Toeplitz preconditioner is constructed to approximate the inverse of the coefficient matrix. This smoother reduces many error components but enlarges some smooth error modes. Techniques such as subdomain deflation and row sum modification (RSM) are incorporated. Numerical experiments indicate rapid convergence and mesh-independence of MG with the FFT+RSM smoother. Moreover, FFT+RSM as a stand-alone solver also shows its efficiency. The complexity of these two methods is $\mathcal{O}(n\log (n))$, with $n$ the number of unknowns. We work on the 3D tangential contact in Chapter 4, where a nonlinear constrained optimization problem arises. A fast solver, called TangCG, is proposed. It combines an active set strategy and a nonlinear conjugate gradient method. The most pronounced component of this method is that it employs two types of variables in the adhesion and slip areas. Techniques including the FFT and diagonal preconditioning are also incorporated. The TangCG method is tested for Cattaneo shift problems, with different amounts of slip. It dramatically reduces the computational time, compared to the state-of-art ConvexGS method. The numerical methods presented above are based on the influence coefficients (ICs) that give the relation between tractions and deformation. In Chapter 5, we investigate ICs by computing them numerically. Based on a concentrated contact setting, an elastic model is built for this purpose and a finite element method (FEM) is employed. Suggestions about the FEM meshing and element types are given, considering the accuracy and computational cost. The effects of employing the numerical ICs on contact solutions are examined. The work in this chapter provides a guidance for developing fast solvers for conformal contact problems, which typically are governed by a larger and curved contact region. With the research presented in the present PhD thesis on efficient numerical solution techniques, the numerical solution of full-scale train-rail contact problems may have come one step closer. With the research presented in present PhD thesis, and with the resulting improved numerical solution techniques, it becomes one step closer to incorporate detailed contact models in the numerical simulation of rail vehicle dynamics and in the simulation of rail and wheel wear and track deterioration.","computational contact mechanics; Numerical algorithms; optimization problems; boundary element method; finite element method; multigrid method","en","doctoral thesis","","","","","","","","2015-06-23","Electrical Engineering, Mathematics and Computer Science","Applied mathematics","","","","52.00667, 4.35556"
"uuid:d6d0698c-8961-414e-9818-220ee08647e6","http://resolver.tudelft.nl/uuid:d6d0698c-8961-414e-9818-220ee08647e6","Solid-State Camera System for Fluorescence Lifetime Microscopy","Zhao, Q.","Young, I.T. (promotor)","2014","Fluorescence microscopy is a well-established platform for biology and biomedical research (Chapter 2). Based on this platform, fluorescence lifetime imaging microscopy (FLIM) has been developed to measure fluorescence lifetimes, which are independent of fluorophore concentration and excitation intensity and offer more information about the physical and chemical environment of the fluorophore (Chapter 3). The frequency domain FLIM technique offers fast acquisition times required for dynamic processes at the sub-cellular level. A conventional frequency-domain FLIM system employs a CCD camera and an image intensifier, the gain of which is modulated at the same frequency as the light source with a controlled phase shift (time delay). At the moment these systems, based on modulated image intensifiers, have disadvantages such as high cost, low image quality (distortions, low resolution), low quantum efficiency, prone to damage by overexposure, and require high voltage sources and RF amplifiers. These disadvantages complicate the visualization of small sub-cellular organelles that could provide valuable fundamental information concerning several human diseases (Chapter 3 and 4). In order to characterize the constraints involved in current fluorescent microscope systems that are used for lifetime as well as intensity measurements and to design and fabricate new systems, we have constructed a mathematical model to analyze the photon efficiency of frequency-domain fluorescence lifetime imaging microscopy (FLIM) (Chapter 5). The power of the light source needed for illumination in a FLIM system and the signalto-noise ratio (SNR) of the detector have led us to a photon “budget”. A light source of only a few milliWatts is sufficient for a FLIM system using fluorescein as an example. For every 100 photons emitted, around one photon will be converted to a photoelectron, leading to an estimate for the ideal SNR for one fluorescein molecule in an image as 5 (14 dB). We have performed experiments to validate the parameters and assumptions used in the mathematical model. The transmission efficiencies of the lenses, filters, and mirrors in the optical chain can be treated as constant parameters. The Beer-Lambert law is applicable to obtain the absorption factor in the mathematical model. The Poisson distribution assumption used in deducing the SNR is also valid. We have built compact FLIM systems based on new designs of CCD image sensors that can be modulated at the pixel level. Two different designs: the horizontal toggled MEM-FLIM1 camera and vertical toggled MEM-FLIM2 camera are introduced (Chapter 6). By using the camera evaluation techniques described in Chapter 7, these two versions of the MEM-FLIM systems are extensively studied and compared to the conventional image intensifier based FLIM system (Chapter 8). The low vertical charge transport efficiency limited the MEM-FLIM1 camera to perform lifetime experiments, however, the MEM-FLIM2 camera is a success. The MEM-FLIM2 camera not only gives comparable lifetime results with the reference intensifier based camera, but also shows a much better image quality and reveals more detailed structures in the biological samples. The novel MEM-FLIM systems are able to shorten the acquisition time since they allows recording of two phase images at once. The MEM-FLIM2 camera is, however, not perfect. It can only be modulated at a single frequency (25 MHz) and requires that the light source be switched off during readout due to an aluminum mask that had a smaller area than intended. A redesign of the architecture based on the vertical toggling concept leads to the MEM-FLIM3 camera (Chapter 9). Several improvements have been made in the sensor design for the MEMFLIM3 camera, such as higher fill factor, greater number of pixels etc. The MEM-FLIM3 camera is able to operate at higher frequencies (40, 60 and 80 MHz) and has an option for electron multiplication. Evaluations of this updated MEM-FLIM system are presented (Chapter 10). The images obtained from the MEM-FLIM3 camera at 20 and 40 MHz can be used directly for the lifetime calculation and the obtained lifetimes are comparable with the ones from the reference camera. There are, however, differences in the even and odd columns (20 MHz) and four image sections (40 MHz) for the intensity and lifetime images. For higher frequencies (60 and 80 MHz) calibrations are needed before calculating lifetimes. The lifetimes derived from the modulation depth after the calibrations are in a reasonable range while the lifetime derived from the phase cannot be used. At 60 and 80 MHz we can use one phase register from the MEM-FLIM3 camera for the lifetime calculation, the same way the reference camera operates. The lifetimes obtained by this method from the MEM-FLIM3 at 60 and 80 MHz are comparable with the ones from the reference camera. The MEM-FLIM3 camera also has an electron multiplication feature for low-light experimental condition. We could get approximately 500 times multiplication. Lifetime measurement using the EM function, however, has not been tested due to the limitation of the project time.","fluorescence lifetime imaging microscopy; camera; modulated; frequency-domain","en","doctoral thesis","","","","","","","","","Applied Sciences","Imaging Science & Technology","","","",""
"uuid:5db5ad07-21fa-4054-aba1-a56daf5a2043","http://resolver.tudelft.nl/uuid:5db5ad07-21fa-4054-aba1-a56daf5a2043","High-Performance mm-Wave and Wideband Large-Signal Amplifiers","Zhao, Y.","Long, J.R. (promotor)","2013","","passive-aided RF and mm-wave circuit design; power amplifier; power combiner; optical modulator driver; distributed amplifier; transformer; transmission line","en","doctoral thesis","","","","","","","","","Electrical Engineering, Mathematics and Computer Science","Microelectronics","","","",""
"uuid:e959451d-857d-47f6-9e1f-58e65bcf3948","http://resolver.tudelft.nl/uuid:e959451d-857d-47f6-9e1f-58e65bcf3948","Dynamic Wheel/Rail Rolling Contact at Singular Defects with Application to Squats","Zhao, X.","Molenaar, A.A.A. (promotor); Dollevoet, R.P.B.J. (promotor); Li, Z. (promotor)","2012","Squats, as a kind of short wavelength rail surface defects, have become one of the main rolling contact fatigue problems in railways worldwide. The purpose of this work is to better understand the squatting phenomenon, contribute to reduction and even prevention of squat occurrence, and thereby reduce the related costs. To such an end, a new modeling approach has been developed in this dissertation, i.e. a three-dimensional finite element (FE) model of the vehicle–track interaction. Both wheel set and rail are simulated as three-dimensional continua. A detailed surface-to-surface contact algorithm is integrated within the FE model in order to solve the frictional rolling contact between the wheel and rail. Different traction/braking efforts are simulated. Detailed modeling of the wheel and rail ensures the consideration of important eigen-modes related to squats, mainly in the high frequency range. Other structures of the vehicle–track system are also modeled to appropriate extents. An estimate of contact stresses with sufficient accuracy is the basis of further dynamic, stress, and fatigue analyses of squats. The FE model has been validated for both normal and tangential contact solutions by comparing it to the widely accepted Hertz theory and Kalker’s CONTACT program. Due to the fact that Hertz theory and CONTACT are only applicable to static contact problems, the steady-state rolling contact between smooth wheels and smooth rails, with the contact occurring in the middle of the rail top, is simulated by the FE model for the purpose of validation. The results show that the FE model is reliable for the solution of frictional rolling contact. On the other hand, the FE model can also take into account actual contact geometry, material non-linearity, and transient effects, which are required for more complicated cases like the wheel–rail rolling contact at a squat. Therefore, the newly developed modeling approach provides a valid and promising tool to solve the problem of rolling contact in the presence of friction. With the validated FE model, the influence of plastic deformation on the solution of frictional wheel–rail rolling contact is further investigated. A bi-linear elasto-plastic material model is employed. It is found that the contact geometry change caused by plastic deformation can significantly modify both the normal and the tangential solutions. When squat type defects are added to the rail top, the calculated dynamic contact forces show a good agreement in wavelength with observed squats in the field. This means that vibrations related to squats are captured by the FE model, proving the applicability of the FE modeling in treating the high frequency dynamics of a system containing rolling contact. Furthermore, based on the simulations and field observations, a growth process of squats from light to mature state has been postulated. This postulation has been validated by track monitoring conducted in the Netherlands. Further analyses of the FE simulations show that squats mainly excite the vehicle–track system at two frequencies. The vibration component with the lower frequency can transfer down to the ballast layer, especially at the support close to the squat. The high frequency vibration component has similar magnitude at several fastenings near the squat and is negligible at the ballast layer. For the investigated rolling speed range between 40 and 140 km/h, both two vibration components increase in magnitude and wavelength with the rolling speed. The vibration component at the higher frequency can be absent when the rolling speed is sufficiently low, e.g. at 40 km/h for the simulated system. By evaluating the stress under rolling contact and comparing it with material strength, it is derived that an initial rail surface defect such as an indentation can only grow into a mature squat when it is over a critical size of 6–8 mm under the typical Dutch railway condition. This critical size has also been verified by monitoring tests. The work of this dissertation formed the basis for a ‘Guideline to Best Practice of Squat Treatment’, written upon invitation by the International Union of Railways.","Railway; Rolling contact fatigue; Vhicle-track interaction; Wheel-rail rolling contact; Frictional rolling contact; Finite element method; Transient contact; Squats","en","doctoral thesis","Wöhrmann Print Service, Zutphen, the Netherlands","","","","","","","2013-07-01","Civil Engineering and Geosciences","Structural Engineering","","","",""
"uuid:9eb29aca-c5d1-466a-b734-e6bc1997685f","http://resolver.tudelft.nl/uuid:9eb29aca-c5d1-466a-b734-e6bc1997685f","Fire-Induced Spalling Modeling of High-Performance Concrete","Zhao, J.","Van Breugel, K. (promotor)","2012","","","en","doctoral thesis","Haveka BV","","","","","","","","Civil Engineering and Geosciences","Structural Engineering","","","",""
"uuid:6cbe28fb-302e-4f65-9e2a-988d24dbf00f","http://resolver.tudelft.nl/uuid:6cbe28fb-302e-4f65-9e2a-988d24dbf00f","13C labeling studies in the central metabolism of Penicillium Chrysogenum","Zhao, Z.","Heijnen, J.J. (promotor); Wahl, S.A. (promotor)","2011","","","en","doctoral thesis","","","","","","","","","Applied Sciences","BT/Biotechnology","","","",""
"uuid:f5c638c3-00cf-410c-b628-04aabfbf0fd9","http://resolver.tudelft.nl/uuid:f5c638c3-00cf-410c-b628-04aabfbf0fd9","Measurements of fluid flow in weld pools","Zhao, C.","Richardson, I.M. (promotor)","2011","Understanding the fluid flow in weld pools contributes significantly toward controlling the heat distribution in the base material and the mass distribution of molten base and additive materials. Currently, most investigations focus primarily on numerical models, due to the experimental difficulties associated with the challenging environment in and around the weld pool. Numerical simulations based on computational fluid dynamics are currently addressing the dynamic behaviour of weld pools, such as melting, solidification, temperature and velocity fields; these results provide information about conditions inside the weld pool that is impossible to gain experimentally. However, there is still a shortage of experimental validation to support these models. In this thesis, experimental methods are applied to study the fluid flow of the weld pool in both arc and laser generated weld pools. The general conclusions from the experimental study provides some suggestions for understanding the fluid flow in the weld pool, such as flow asymmetry, rotational flow and interface instabilities. For example, the interface instabilities can affect the energy and mass distribution. Describing these instabilities numerically requires three dimensional flow models. Moreover, the current experimental results question the validity of works where the surfactant concentration is assumed to be constant in the weld pool without consideration of dynamic diffusion and convection. This work has focused on experimental measurement of the fluid dynamics of weld pools, and current results provide a number of validation cases for further numerical simulations.","Weld pool; experiment; visualization; fluid flow","en","doctoral thesis","","","","","","","","","Mechanical, Maritime and Materials Engineering","Materials Science & Engineering","","","",""
"uuid:b5df2455-90a8-42f4-bc75-069bfd193897","http://resolver.tudelft.nl/uuid:b5df2455-90a8-42f4-bc75-069bfd193897","Curvature Lines for Lesion Detection and Visualization in CT Colonography","Zhao, L.","Jansen, F.W. (promotor); Botha, C.P. (promotor)","2011","In this thesis, we present an automatic polyp detection approach that integrates knowledge from flow visualization techniques. Our primary goal was to compute additional characteristic polyp features that improve the CAD performance of existing polyp detection approaches by eliminating found false-positive polyp candidates. We found that surface principal curvature directions presented discriminating patterns on surface areas that belong to colonic polyps. These could be visualized using lines of curvatures. We developed novel approaches for generating lines of curvature on the colon wall. In order to provide sufficient shape information, lines of curvature were distributed using curvature-adaptive streamline seeding and spacing strategies. Geometric features of lines of curvature were used to differentiate between true polyp detections and false positives. The visualization of the colon wall was also enhanced by visualizing lines of curvature.","Medical Visualization; CT Colonography; Computer-aided Diagnosis; Lines of Curvature","en","doctoral thesis","","","","","","","","2011-03-10","Electrical Engineering, Mathematics and Computer Science","Mediamatics","","","",""
"uuid:5d5632be-f9a3-4b89-b49f-ce7044bc8844","http://resolver.tudelft.nl/uuid:5d5632be-f9a3-4b89-b49f-ce7044bc8844","Hierarchical EMC Design for Inverters in Motor Drive Systems","Zhao, D.","Ferreira, J.A. (promotor); Leferink, F.B.J. (promotor)","2009","Power electronics applications are usually accompanied by high voltage and current amplitudes, and steep voltage and current transients. The EMI (Electromagnetic Interference) issue is regarded as the main side effect of power electronics applications. Noise level prediction is a vital task for filter design, but many difficulties are encountered. For instance, a large amount of experience is needed to build equivalent circuits, the use of simplified models often requires sacrificing details in the high frequency range, individual approaches must be developed for various topologies, etc.. A new method for EMI prediction of power electronics applications is thus desirable. The universal method should overcome the following common characteristics of power electronics circuits: (a) the large difference between the time constants, and (b) the long time required to reach steady state. The time domain approach is very time-consuming. The assumptions used in the frequency domain approach, namely, periodic noise sources and a fixed propagation path, are also not valid. For example, the slopes of voltage and current transients depend on the operating points, and the assumption of periodic noise sources does not apply. Because the junction capacitors of power rectifiers and switches change with the reverse voltage, the values of the components in the noise propagation path also change. In this thesis, a hierarchical approach is proposed for the EMC design of inverters in motor drive systems. It combines the advantage of the time domain and frequency domain approach to achieve a fast, universal and accurate result. The approach is validated by observations in the time domain and the frequency domain. The proposed approach has three steps. In the first step (functional level), a simple model of switches in the system is developed. The operating points of each switching transient and all time intervals are derived and the narrowband signals of the EMI noise can be derived. The second step (transient level) results in detailed transient waveforms which take the variation of the nonlinear switching transient into account. In the third step (propagation level), the noise propagation through the system is described by the transfer ratio, and the EMC performance is evaluated. The approach is described in detail, and then a PWM voltage source inverter feeding an induction motor is analyzed using this approach. This approach is also applied to a resonant inverter that operates under ZVS conditions. The experimental results are compared to calculated results. Two approaches are proposed for EMI suppression in this thesis. The first approach is by adding a passive filter on the dc-bus. It can be concluded that the same noise suppression performance can be achieved using a dc-bus filter as the conventional ac side filter. The advantage of this approach is that the connections can be made very short which can significantly improve the suppression. A new active filter called the ""fourth leg compensator"" is proposed in this thesis. The fourth leg inverter generates a signal to compensate the transients of other three legs. It can suppress the fast transients of common mode voltages while reducing the amplitude of the common mode voltage by 50%. A method to determine the values of the additional components is described. It is shown that the CM voltage can be compensated, even with nonideal coupling in the CM transformer and when leakage inductance is present.","EMC; EMI; Power electronics; Inverter; Motor Drive System","en","doctoral thesis","","","","","","","","","Electrical Engineering, Mathematics and Computer Science","Electrica Sustainable Energy","","","",""
"uuid:df6af2ed-a2b2-4aad-b6ea-8ccdcc08ebce","http://resolver.tudelft.nl/uuid:df6af2ed-a2b2-4aad-b6ea-8ccdcc08ebce","Integral pavement/soil-wall structures: A numerical study","Zhao, Y.","Blaauwendraad, J. (promotor); Scarpas, A. (promotor)","2009","","","en","doctoral thesis","","","","","","","","","Civil Engineering and Geosciences","","","","",""