287 research outputs found
Photon-detecting superconducting resonators
No full textOne of the greatest challenges in astronomy is observing star and planetary formation, redshifted distant galaxies and molecular spectral ‘fingerprints’ in the far-infrared spectrum of light, using highly sensitive and large cameras. In this thesis we investigate superconducting resonators for photon detection. In superconductors the electrons are paired. The incoming light then breaks these pairs into unpaired electrons, so-called quasiparticles, influencing the superconductor’s inductance. Consequently, the resonance frequency shifts. These resonators are extremely sensitive, since they are operated at temperatures where less than a billionth of the electrons are unpaired. By giving each resonator (or pixel) a slightly different length, like the pipes in an organ, many can be read out simultaneously. This allows for the construction of large cameras. These cameras work best when pairing is slow – the quasiparticles eventually recombine and the signal is lost – and the noise is low. In this thesis we focus on two main topics: the quasiparticle recombination process as well as the frequency noise of these resonators. At low temperatures we find relaxation times as long as milliseconds for Al and several tens of microseconds for Ta. The relaxation times clearly saturate at low temperatures in both materials, indicating an additional recombination channel in the superconducting films. The low temperature relaxation is made faster by the implantation of magnetic as well as nonmagnetic atoms, indicating that it arises from an enhancement of disorder. In addition, we show that the frequency noise mainly arises at interfaces, whereas deviations in the temperature dependence of the resonance frequency arise from dipole defects in the volume of dielectrics. Finally, we significantly decrease the noise by widening the geometry of the resonator waveguide.Kavli Institute of NanoScienceApplied Science
The influence of anisotropy on the consolidation behaviour of peat
No full textSoft soil is often described as an anisotropic heterogeneous material. Standard soil investigation mainly involves vertically retrieved samples. Information on the parameters working in the horizontal direction remains scarce. Principally each soil property like permeability, stiffness or strength might show anisotropic behaviour. This thesis focuses on anisotropy in stiffness and the possibility to use conventional laboratory measurement techniques to determine the level of anisotropy in stiffness.Civil Engineering and Geoscience
Stochastic characterization of geological heterogeneity and its impact on groundwater contaminant transport
No full textAbstract not availableCivil Engineering and Geoscience
Adaptive Multiscale Finite Element Method for Subsurface Flow Simulation
No full textNatural geological formations generally show multiscale structural and functional heterogeneity evolving over many orders of magnitude in space and time. In subsurface hydrological simulations the geological model focuses on the structural hierarchy of physical sub units and the flow model addresses the functional hierarchy of the flow process. Flow quantities like pressure, flux and dissipation relate to each other by constitutive relations and structural sub-unit parameters like porosity and hydraulic permeability. Hydraulic permeability includes the (steady state) intrinsic permeability of the solid phase and the (time dependent) relative permeability. The permeability is the dominant parameter and a highly heterogeneous parameter affecting the groundwater flow at field scale. Laboratory experiments provide measurements of the sub-unit parameters on a fine scale. If laboratory measurements are treated stochastically within the geological model, then the structural model of the subsurface should be built on the same scale as these indicating measurements. Fully resolved flow simulations on the field scale are however intractable and a new adaptive multiscale technique has therefore been developed. Though constitutive relations may change at different scales, Darcy’s law is supposed to remain valid on both laboratory scale and field scale. Nowadays upscaling methods are applied, which aim to propagate information over this hierarchy of scales both functionally and structurally from the fine scale to the coarse scale and not vise versa, by computing effective or equivalent material behavior. Permeability is not an additive parameter so it is not possible to calculate an equivalent coarse scale permeability as a simple average of fine scale measurements. A flow criterion or a criterion about energy dissipation often defines an equivalent permeability. Only if the scale of variation is much smaller than the coarse observation scale then the equivalent permeability matches the effective permeability. This effective permeability is a constant second order tensor variable on the coarse scale, whereas the equivalent permeability is non-unique and depends on the boundary conditions of the sample domain. The effective permeability holds for discrete hierarchical systems where scales can be decomposed. The newly developed adaptive multiscale technique extends the original two-level multiscale finite element method to a hierarchy of scales. Multiscale finite element methods capture the fine scale behavior on a coarse mesh by multiscale basis functions. The weights of the multiscale basis functions follow from local flow simulation. The procedure removes fine scale nodes from the subdomain, but introduces errors at the subdomain boundaries. The two-level method forms a class of subdomain decomposition techniques. It can be shown that a sequential implementation of the method is not faster than an optimal solver like the full multigrid solver, however the method is suitable for parallel implementation. The proposed method is based on a conformal nodal finite element formulation over simplex elements. The conformal finite element method obtains mass conservation on a nodal basis, and does not preserve continuity of flux over the inter-element boundaries. A mesh refinement criterion detects zones in which large errors occur over the element edges, and an adaptive refinement procedure enriches the mesh locally to correct the error in the velocity field. Multiscale basis functions follow from solving local flow problems over patches of simplex elements. Linear boundary conditions close oversampled subdomains and reduce the effect of the imposed boundary condition on the patch. This procedure obtains more accurate coarse scale behavior then a procedure that operates on the patch directly. However, over-sampled local flow problems introduce discontinuities in the basis functions and introduce new nodal connectivities on the coarser scale. For this reason closure of the local flow problems by dimensionally reduced flow problems is preferred. A second refinement criterion compares oversampled and non-oversampled function values. Pressure-dissipation averaging approximates the multiscale coarse grid operator and supports a functional adaptive formulation. The multiscale averaging procedure computes equivalent permeability tensor components, and reproduces the sparse matrix structure on the coarse scale. The loading cases for the local problems follow from a summation of multiscale basis functions. The multiscale basis functions extrapolate the coarse scale solution to the fine scale. On this scale discontinuities in the velocity field are detected and compared to the refinement criterion. The computed equivalent permeability is used in the framework of a geometric multigrid solver to compute the coarse scale operators. The hierarchy of multiscale basis functions, which relate the pressure on each coarse level to the next fine level, generates the intergrid transfer operators. The proposed approach provides a robust and efficient algorithm, based on the concept of the multiscale finite element method, for simulating partly saturated subsurface flow and fully coupled solute transport and heat transport through hierarchical heterogeneous formations. The multiscale finite element formulation produces numerically homogenized discrete flow equations, and upscales the permeability. The adaptive formulation obtains locally refined velocity fields, which support accurate transport computations. The performance of the method is illustrated by a set of realistic case studies.Geo-EngineeringGeotechnologyCivil Engineering and Geoscience
Localization in Dutch dune sand and organic clay. New insight into localization mechanisms
No full textOne of the challenging research puzzles in soil mechanics is the subject of strain localization. The investigation of the physico-chemical mechanisms and the conditions under which the strain will localize has been going on for more than a century. The focus of these studies was continuously adapted as new engineering problems occurred or new technologies became available. The objective of the present study is to collect new laboratory evidence of strain localization in Dutch dune sand and organic clay with the emphasis on the application of novel laboratory techniques. In addition to mathematical aspects at the macro level the interdisciplinary knowledge of micro-geology and bio-geochemistry was incorporated in the micro-mechanical approach. The first part of this thesis describes specific microscopic laboratory findings and their macroscopic effects of a typical Dutch dune sand. Chapter 2 reports on triaxial elemental tests, which were performed to revisit the interpretation of dilatancy of sand and its implication on failure mechanisms. Specific attention was given to pre-peak volume change characteristics. The corresponding characteristic stress state (CSS) was measured with respect to the evolution of deformations. A special mathematical approach in line with Desai yield surfaces was developed to model the observed volume change of sand by incorporating characteristic stress states with an additional state parameter. A hitherto unrecognised intermixing deformation mode, i.e. cone-radial bands of varying density, has been identified. The characteristic stress state is found to be a precursor of the formation of such a deformation mode. Once such an intermixing deformation mode occurs the measured strength, either peak or post-peak strength, is difficult to establish. Chapter 3 formulates the intermixing deformation mode using the technique of limit analysis. Two failure mechanisms were involved, i.e. a single shear rupture plane and double cones-radial loosening bands. Both associated and non-associated kinematical fields were considered. The Mohr-Coulomb yield criterion was applied. The classical mechanism of energy dissipation in a shear rupture has been adopted. Upper bound solutions were obtained and the orientation of shear-rupture surfaces (cones)-radial loosening bands in a cylindrical sample was formulated in terms of apparent friction angle, apparent dilatancy angle, confining pressure, slenderness of the sample and tensile strength. This finding is important for the interpretation of triaxial compression strength when measured in the laboratory. Dutch organic clay is the second important subject in this thesis, in recognition of its importance to geotechnical engineering practice in the Netherlands. This material has a distinct mechanical behaviour different from non-organic clays. The reason for its high effective strength index and low coefficient of lateral stress remains as yet unknown. Fundamental aspects of the behaviour of Dutch organic clays, including the effects of strain localization (Chapter 4), the fabric and micro-deformation of organic and clay mineral contents (Chapter 5 and Chapter 6) are reported in this thesis. Based on observed laboratory behaviour the strain rate dependent characteristics of soft clays was demonstrated and examined in terms of the instability of the material (Chapter 4). The results suggest that the observed phenomena of rate-dependent deformation mechanisms, creep rupture and pre-failure softening are related to this rate-dependent instability. This viscosity-induced instability can lead to strain localization of saturated soft clay, which is different from the critical state. The combined effect of viscosity and drainage was also addressed by demonstrating its correlation with the type of failure of soft clay. The commonly applied undrained conditions adopted for the usual theoretical analysis and the standard engineering interpretation for the strength of soft clay do not occur in the laboratory or practice. Local drainage has to be taken into account and awareness is growing that the introduction of localization into the study of soft clays is relevant, both scientifically and practically. A high quality electron microscope system (ESEM and EDAX) combined with a mini-loading module was made available. Chapter 5 includes the laboratory investigation of fabric and related micro-deformation of Dutch organic clay with this modern sophisticated experimental technology. Four types of clay states were studied: natural, remoulded, artificial inorganic and fractured. The results suggest that Dutch organic clay has an unusual fabric where microstructures of organics and microfossils play a central role. Emphasis was put on the identification of the complicated microstructures and on live measurement of related micro-deformations in a controlled environment (temperature and humidity). Eventually, at a scale of tens of micrometers, the clay fabric and its micro-deformation characteristics were analysed. Applying the technique of distinct element modelling (DEM) a sound relation between fabric and common geotechnical properties of clay could be formulated on basis of micro-scale observations (Chapter 6), including an explanation accounting for the fabric matrix suction with regard to the unusual mechanical properties of Dutch organic clay. Micro-scale approach is promising and inevitable for the enhancement of the fundamental understanding of the fabric of organic clays and its geotechnical implications. Moreover, the study of mechanical behaviour of modified or contaminated clay could benefit from it as well.Civil Engineering and Geoscience
Dynamics of railway transition zones in soft soils
No full textGeo-EngineeringGeotechnologyCivil Engineering and Geoscience
Technisch rapport waterspanningen bij dijken Waterkeringen (TAW)
No full textBundelt alle aspecten over waterspanningen, zoals die in de laatste decennia zijn ontwikkeld en in vigerende leidraden en voorschriften voor dijkbeheer en \u96ontwerp zijn genoemd. Voor verschillende typen dijkprofiel zijn concrete situaties in detail uitgewerkt. In dit rapport is in het bijzonder ingegaan op de praktische toepassing bij toetsen en ontwerpen. Een waterkering wordt echter mede gekarakteriseerd door discontinuïteiten, zoals scherpe variatie in de geologie, constructieovergangen en kruisingen (leidingen e.d.).TAW/EN
Innovative quay structures and developments of the port Eemshaven, the Netherlands
No full textThe Eemshaven, located in the north eastern part of the Netherlands, is developing rapidly since 2004 introducing new port infrastructure for various clients such as AG Ems (Borkum ferry line), Wijnne & Barends stevedoors, Bio Value, Essent/Vopak/Gasunie (LNG plant) and the Energy Park comprising amongst others new power plants for Nuon and RWE
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