1,893 research outputs found

    Experimental and computational investigation of flow of pebbles in a pebble bed nuclear reactor

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    The Pebble Bed Reactor (PBR) is a 4th generation nuclear reactor which is conceptually similar to moving bed reactors used in the chemical and petrochemical industries. In a PBR core, nuclear fuel in the form of pebbles moves slowly under the influence of gravity. Due to the dynamic nature of the core, a thorough understanding about slow and dense granular flow of pebbles is required from both a reactor safety and performance evaluation point of view. In this dissertation, a new integrated experimental and computational study of granular flow in a PBR has been performed. Continuous pebble re-circulation experimental set-up, mimicking flow of pebbles in a PBR, is designed and developed. Experimental investigation of the flow of pebbles in a mimicked test reactor was carried out for the first time using non-invasive radioactive particle tracking (RPT) and residence time distribution (RTD) techniques to measure the pebble trajectory, velocity, overall/zonal residence times, flow patterns etc. The tracer trajectory length and overall/zonal residence time is found to increase with change in pebble\u27s initial seeding position from the center towards the wall of the test reactor. Overall and zonal average velocities of pebbles are found to decrease from the center towards the wall. Discrete element method (DEM) based simulations of test reactor geometry were also carried out using commercial code EDEM and simulation results were validated using the obtained benchmark experimental data. In addition, EDEM based parametric sensitivity study of interaction properties was carried out which suggests that static friction characteristics play an important role from a packed/pebble beds structural characterization point of view. To make the RPT technique viable for practical applications and to enhance its accuracy, a novel and dynamic technique for RPT calibration was designed and developed. Preliminary feasibility results suggest that it can be implemented as a non-invasive and dynamic calibration methodology for RPT technique which will enable its industrial applications. --Abstract, page iii

    Three-dimensional simulation of aggregate and asphalt mixture using parameterized shape and size gradation

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    Aggregate occupies at least three-quarters of the volume of asphalt mixture and can significantly affect the performance of pavement. The geometrical morphology influences the slippage and interlock among aggregates for resisting and distributing applied loads. In recent years, the discrete-element method (DEM) has been employed for simulation of asphalt mixture structure. This paper introduces an approach for simulation of aggregate and asphalt mixtures using parameterized shape and size gradation. Both the plane geometry factor (PGF) and the section aspect ratio (SAR) were employed to describe the three-dimensional (3D) geometric characteristics of aggregates. A numerical technique of aggregate models was implemented with probabilistic parameters depending on statistical results of PGFs and SARs. The 3D numerical model of asphalt mixtures was assembled with three different components, and was validated by uniaxial compression tests via comparison with the laboratory result. It was found that the PGF and SAR are appropriate to describe the three-dimensional features of aggregate shapes, because a simplified space object can be described by a two-dimensional (2D) graphical projection and a vector scalar corresponding to the space vector. Probability distribution curves of PGFs and SARs between coarse aggregates were in concordance with the Gauss-type function, because their correlation coefficients were all greater than 95%. It was verified that the developed clumping algorithm of aggregates was reasonable in terms of the shape and size gradation. Based on the parallel-bond model and Burger's model, the results of virtual tests were in good agreement with those of laboratory uniaxial tests. The angularity (PGF) of aggregates has a beneficial effect on the strength and stability of asphalt mixtures, whereas the flat-elongated feature (SAR) has a negative effect on the strength and stability of asphalt mixtures

    Prehistoric Cypriot mud buildings and their impact on the formation of archaeological sites

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    The current research grew out of work on the Erimi Culture sites in the west of Cyprus at Lemba Lakkous, Kissonerga Mosphilia and Kissonerga Mylouthkia dating from c3500-2500/2300 B.C. The need to understand the nature of the archaeological deposits and the buildings which form the largest element of those deposits channelled the research along four main paths or aims: 1) the characterisation and classification of all Chalcolithic building elements setting them within a proposed scheme of building types, 2) the establishment of these within the framework of Middle Eastern building traditions with an indication of any cultural links, 3) the identification and characterisation of prehistoric building deposits and materials and, 4) the contribution of some thoughts to the understanding of site formation processes. Three avenues of study were followed. The first involved an investigation of the behaviour of the key elements of soil, clay and lime in order to be able to characterise building materials. A study of the archaeological evidence was then undertaken and a scheme for classifying building elements proposed. This was tested in the second avenue of study through experimental reconstruction at Lemba in which comparative modern materials and building types were examined. Further comparative material was obtained from the study and excavation of the recently abandoned village of Souskiou, this being the third avenue of study. This has also been used to identify ways in which buildings collapse or decay and become part of the archaeological record. From these studies a reference collection of comparative building materials and deposits is being assembled and used to explain prehistoric remains.In the final discussion, eight Chalcolithic building types were identified and described. The development of house form is also discussed and the emergence of two very characteristic building types, the courtyard house and the temple /shrine is proposed. A tradition stretching back into the Neolithic of Cyprus and the southern Levant is suggested with key architectural traits being highlighted. The significance of the lime plaster industry is also discussed in its Middle Eastern context. Finally, the key characteristics for identifying common building materials are described. This can be achieved using a hand-held, calibrated 10x magnification lens and is easily accessible to all field archaeologists. The understanding of site formation processes is a much longer term aim but initial results from the experimental work at Lemba and from the studies at Souskiou are presented

    Hyper-Velocity Impacts on Rubble Pile Asteroids

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    Most asteroids in the size range of approximately 100m to 100km are rubble piles, aggregates of rocky material held together mainly by gravitational forces, and only weak cohesion. They contain high macroporosities, indicating a large amount of void space in their interiors. How these voids are distributed is not yet known, as in-situ measurements are still outstanding. In this work, a model to create rubble pile asteroid simulants for use in SPH impact simulations is presented. Rubble pile asteroids are modelled as gravitational re-aggregating remnant fragments of a catastrophically disrupted parent body, which are represented by spherical pebbles. It is shown that this approach allows to explicitly follow the internal restructuring of rubble pile asteroids during impact events, while preserving the expected properties of the bulk asteroid as known from observations and experiments. The bulk behaviour of asteroid simulants, as characterized by the stability against disruption and fragment size distribution, follows the expected behaviour and is not sensitive to the exact distribution of voids in the interior structure, but rather to the void fraction as the amount of consolidated void space in between the constituent fragment pebbles. No exact a priory knowledge of the fragment size distribution inside the body is therefore needed to use this model in impact simulations. Modelling the behaviour of the large-scale rubble pile constituents during impact events is used as a tool to infer the internal structure of asteroids by linking surface features like hills or pits to the creation of sub-catastrophic craters. In this work, the small rubble pile asteroid (2867) S?teins is analysed. The flyby of the Rosetta spacecraft at S?teins has revealed several interesting features: the large crater Diamond close to the southern pole, a hill like feature almost opposite to the crater, and a catena of crater pits extending radially from the rim of the crater. A possible link between these two structures and the cratering event is investigated in a series of impact simulations varying the interior of a plausible shape of S?teins prior to the event that formed crater Diamond. A connection between the cratering event and the hill is shown to be highly unlikely. Therefore, the hill is most likely a remnant of the formation of S?teins. Its size therefore helps to infer the initial size distribution of fragments forming the asteroid. The formation of a fracture radially from the crater can be observed for rubble pile simulants with highly collimated voids. This fracture could plausibly form the catena of pits observed on S?teins. This can therefore serve as a link between observable surface features and S?teins internal structure. The interior of S?teins is most likely an aggregate of fragments that themselves are only lightly fractured, and large void spaces might be found inside the asteroid. As S?teins seems to be a good example of a YORPoid, an asteroid that has been evolved to a top-like shape by radiative forces due to the YORP effect, this gives first insights in the distribution of voids in the interior of this class of rubble pile asteroids

    Numerical upscaling of parametric microstructures in a possibilistic uncertainty framework with tensor trains

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    A fuzzy arithmetic framework for the efficient possibilistic propagation of shape uncertainties based on a novel fuzzy edge detection method is introduced. The shape uncertainties stem from a blurred image that encodes the distribution of two phases in a composite material. The proposed framework employs computational homogenisation to upscale the shape uncertainty to a effective material with fuzzy material properties. For this, many samples of a linear elasticity problem have to be computed, which is significantly sped up by a highly accurate low-rank tensor surrogate. To ensure the continuity of the underlying mapping from shape parametrisation to the upscaled material behaviour, a diffeomorphism is constructed by generating an appropriate family of meshes via transformation of a reference mesh. The shape uncertainty is then propagated to measure the distance of the upscaled material to the isotropic and orthotropic material class. Finally, the fuzzy effective material is used to compute bounds for the average displacement of a non-homogenized material with uncertain star-shaped inclusion shapes

    GEOLOGY, TAPHONOMY, AND PALEOECOLOGY OF A UNIQUE UPPER CRETACEOUS BONEBED NEAR THE CRETACEOUS-TERTIARY BOUNDARY IN SOUTH DAKOTA

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    This research examines a Konzentrat-Lagerstätte, the Bone Butte site, in the Upper Cretaceous Hell Creek Formation of South Dakota. Field data from a diverse multispecific bonebed in Harding County, South Dakota, including sedimentologic, stratigraphic, taphonomic, floral, and faunal observations, are used to reconstruct the depositional history, depositional environment, and paleoecology of the site, while comparing it to other sites within the Hell Creek Formation. Taphonomic analyses of bone completeness, abrasion, and sorting reveal the presence of autochthonous and allochthonous faunas, and demonstrate the heterogeneity of the Hell Creek dinosaurian fauna. Particularly noteworthy components of the Bone Butte ecology are undescribed insect and ornithurine bird faunas. Also, a rare instance of dinosaur predation is evidenced by theropod tooth marks and an embedded tooth found in two fused proximal hadrosaur vertebrae. The precise temporal resolution of the site, less than 7 years as indicated by sedimentologic, paleobotanical, and vertebrate ontogenetic data, provides the basis of a reconstruction of the local climate and ecology. Finely preserved clay-pebble laminae show evidence of annual rainfall magnitude, and pyrofusain-enriched laminae may indicate a chronology of significant local forest fires. The optimum temperature requirements for plants at Bone Butte, for example the gymnosperm Taxodium, give a rough estimation of annual temperature variation. Thousands of collected vertebrate specimens provide a means of tracking ontogenetic ratios from hatchlings to adulthood in the local stratigraphic column. In some of the dinosaur specimens, the presence of medullary bone allowed estimation of their seasonal nesting habits. Laboratory experiments and examination of modern fluvial analogs augmented the analysis of data from the Bone Butte site. A reconstruction of the Bone Butte river system and site-forming events are presented in this work. In particular, the presence of marine organisms at Bone Butte indicates the river system's close proximity to the paleoshoreline

    Modeling asteroid collisions and impact processes

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    As a complement to experimental and theoretical approaches, numerical modeling has become an important component to study asteroid collisions and impact processes. In the last decade, there have been significant advances in both computational resources and numerical methods. We discuss the present state-of-the-art numerical methods and material models used in "shock physics codes" to simulate impacts and collisions and give some examples of those codes. Finally, recent modeling studies are presented, focussing on the effects of various material properties and target structures on the outcome of a collision.Comment: Chapter to appear in the Space Science Series Book: Asteroids IV. Includes minor correction

    Some Fundamental Concepts of Discrete Tomography

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    The word tomography refers to a variety of imaging methods where a penetrating wave is used to collect data about an unknown object of interest. The waves usually need to be sent through the object from a large number of different angles in order to have enough data for a successful reconstruction. The problems can be expressed in a form where the measured data is known to be equal to the unknown object (expressed as a function) multiplied by a known operator. Reconstructing either a two-, three-, or in case of dynamic tomography, four-dimensional image based on data is not a simple matter of inverting said operator. The measurement noise, which is always a factor in imaging situations, can be amplified greatly in the reconstruction, making the inverse problem in question ill-posed. To avoid this, some regularization method in which a stable, unique problem close to the original, ill-posed one, needs to be applied. A method called Tikhonov regularization is one of the most commonly used ones. Discrete tomography differs from general tomography by limiting the objects or images being reconstructed to ones consisting of only a small set of different densities or colours. This a priori knowledge of the object makes it possible to make successful reconstructions based on a much smaller amount of data. Traditionally discrete tomography has only focused on making reconstructions of binary images but more recently algorithms have been developed that allow the number of different colours or densities to be as large as five. There are some very promising new algorithms in the field of discrete tomography but due to the requirements set by new applications, an ever-increasing number of researchers are working on new ones. In this thesis a small, simulated example of tomographic reconstruction is made using both Tikhonov regularization and DART (discrete algebraic reconstruction technique), which is an algorithm of discrete tomography. Both methods give reasonably good results in all of the situations that were studied. It is found, however, that for an image fulfilling the requirements for using DART (small enough number of different colours), DART performs significantly better when the number of projection angles is decreased.Sanalla tomografia viitataan erilaisiin kuvantamismenetelmiin, joissa tuntemattomasta kappaleesta kerätään tietoa käyttäen läpäisevää aaltoa. Useimmissa tapauksissa aalto täytyy lähettää kappaleen lävitse monesta eri kulmasta, jotta saadaan kerättyä riittävästi tietoa hyvän rekonstruktion aikaansaamiseksi. Tämäntyyppiset ongelmat voidaan ilmaista muodossa, jossa mitattu data tiedetään yhtä suureksi kuin (funktion muodossa esitetty) tuntematon kappale kerrottuna tunnetulla operaattorilla. Joko kaksi-, kolmi- tai dynaamisen tomografian tapauksessa neliulotteisen rekonstruktion rakentaminen perustuen dataan ei onnistu yksinkertaisesti kääntämällä kyseinen operaattori. Mittaustilanteissa aina vaikuttavana tekijänä oleva kohina saattaa vahvistua rekonstruktiossa merkittävästi, tehden kyseessä olevasta inversio-ongelmasta huonostiasetetun. Jotta tämä pystytään välttämään, on käytettävä jotakin regularisaatiomenetelmää, jossa tutkitaan ongelmaa, joka on lähellä alkuperäistä, huonosti asetettua ongelmaa, mutta yksikäsitteinen ja stabiili. Tikhonov-regularisaationa tunnettu menetelmä on yksi näiden joukossa laajimmin käytetyistä. Diskreetti tomografia poikkeaa yleisestä tomografiasta siten, että kappaleet tai kuvat joita yritetään rekonstruoida koostuvat siinä ainoastaan muutamista eri tiheyksistä tai väreistä. Tämä a priori-tieto mahdollistaa sen, että hyviä rekonstruktioita voidaan saada aikaan paljon pienemmän datamäärän perusteella. Alun perin diskreetti tomografia keskittyi ainoastaan binääristen kuvien rekonstruoimiseen, mutta viime vuosina on kehitetty uusia algoritmeja, jotka sallivat eri tiheyksien tai värien määrän olla jopa viisi. Jotkut uusista algoritmeista ovat hyvin lupaavia, mutta uusien sovellusten asettamien vaatimuksien takia yhä kasvava joukko tutkijoita työskentelee uusien algoritmien kehittämisen parissa. Tässä tutkielmassa pieni, simuloitu esimerkki tomografisesta rekonstruktiosta on toteutettu käyttäen sekä Tikhonov regularisaatioa että DART-algoritmia, joka on eräs diskreetin tomografian algoritmi. Molemmat menetelmät toimivat hyvin kaikissa tutkielmassa käsitellyissä tilanteissa. Huomataan kuitenkin, että kuvalle, jossa eri värien määrä on riittävän pieni, DART antaa selkeästi parempia rekonstruktioita kun projektiokulmien määrää vähennetään

    The origin of massive sandstone facies in an ancient braided river deposits

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    Lateral profiling techniques have been utilised to define the three-dimensional fluvial architecture of the Fell Sandstone Group (Arundian-Holkerian) of the Northumberland Basin, UK; the Lee-type sandstones (Morrowan-Atokan) of the central Appalachian Basin, USA; the Mansfield and Brazil Formations (Morrowan-Atokan) of the Illinois Basin, USA; and the Anisian Hawkesbury Sandstone of the Sydney Basin, Australia. These strata are characterised by sandstones of braided fluvial origin. Individual fluvial channels are dominated by downstream accreting mesoforms and macroforms, interpreted to represent mid-channel and bank attached bars and dunes. Palaeocurrents are unimodal and of low variance. Evidence of low stage reworking is rare, indicating that the fluvial systems were perennial. Cross-stratified sandstones are interbedded with structureless sand bodies, which display three distinct geometric forms: Sms, Smc and Sme. The texture and composition of facies Sms, Smc and Sme are distinct from associated structured facies. Facies Sms forms erosively based sandsheets 250 m parallel and transverse to the flow. The upper surface is planar. Facies Smc forms elongate channels trending at high angles to the palaeoflow of fluvial channels. The sandbodies preserve a symmetrical cross-section with margins dipping 6 m thick, and may be traced >200 m parallel and transverse to flow direction. Amalgamation of the facies results in sandsheets >20m thick. Scours, elongate both parallel and oblique to fluvial flow are preserved along the basal surface. A classification scheme of massive sandstone facies has been developed. The facies are interpreted in terms of deposition from highly concentrated, laminar sediment/water flows. Sediment-laden currents were generated through primary and secondary mechanisms related to flooding and mass flow
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