This project is to analyse the energy spectrum of a spin-molecular tunnelling junction which is composed of molecules confined between two conducting metallic leads. By letting a continuous stream of electrons flow across the junction the molecules can interact with each other with an indirect force called exchange interaction, and those exchange interactions which are of interest in this project are described by models called the Heisenberg, the Ising and the Dzyaloshinski-Moriya models. The molecules may also interact with themselves anisotropically and if there is an external magnetic field there will be yet another kind of interaction. The goal of this project is to see the contribution of all these spin interactions and how they affect the resulting energy spectrum under the variation of the junction's chemical potential and the voltage bias between the leads. This project is of a theoretical nature where the models are analytically adapted for a restricted scenario and is later on numerically calculated to be graphed and analysed. The models are restricted to only consider molecules of same spin and approximated to only consider interactions between closest neighbouring molecules. The results are composed of both analytically derived energy values and numerical computed values which show that there exists certain critical values of the variation parameters which naturally splits the ground state of the system and that the self-interaction may further split the degenerate ground state. A possible outcome of these result could be the possibility to control the magnetic order of the molecules to either be locked in an anti-ferromagnetic configuration or be easily mixed by manipulating the chemical potential or the voltage bias. / Detta projekt handlar om att analysera energispektrumet från en spinn-molekyl-tunnelkor-sning som består av molekyler instängda mellan två ledande metaller. När en kontinuerlig elektronström korsar tunnelkorsningen så kan molekylerna växelverka med varandra via en indirekt kraft kallad utbytesinteraktion, och de utbytesinteraktioner som är relevanta i denna uppställning beskrivs av de så kallade Heisenberg-, Ising- och Dzyaloshinski-Moriya-modellerna. Molekylerna kan också växelverka med sig själva anisotropt och om det finns ett externt magnetfält så tillkommer ytterligare en interaktionsterm. Målet för detta projekt är att se hur alla dessa spinnbidrag påverkar det slutliga energispektrumet under variation av korsningens kemiska potential och spänningen mellan metalledarna.       Projektet är teoretiskt lagt på så sätt att modellerna är analytiskt anpassade för ett begränsat scenario samt att de är numeriskt beräknade så att energispektrumet kan plottas i grafer och analyseras. Modellerna är begränsade för molekyler av samma spinn och är approximerade så att endast närmsta-granne-interaktioner är beaktade.       Resultaten är uppdelade i både analytiskt framtagna energivärden samt numeriskt beräknande energinivåer och båda visar att det finns kritiska värden på variationsparametrarna som automatiskt delar grundtillståndet för systemet samt att självinteraktionerna ytterligare kan dela det degenererade grundtillståndet. Ett möjligt utfall av dessa resultat är att de kan användas till att kontrollera systemets magnetiska ordning på så sätt att det antingen är låst i en antiferromagnetisk konfiguration eller med enkelhet kan mixas genom att ändra den kemiska potentialen eller spänningen mellan metalledarna.

This thesis is a study of the celestial gamma-ray flux. It reviews several of the proposed mechanisms for producing high energy gamma rays and describes several of the attempts to detect their presence. Also included is a short historical review of the spark chamber, along with a qualitative description of its operation.

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Traditionally, Retarding Potential Analyzers (RPAs) operate exclusively on large satellites due to the size, power, and mass constraints posed by nano-satellites like CubeSats. These sensors take in-situ measurements of Earth's atmospheric ion current during a range of time-varied ``retarding" voltage steps. Curve-fitting the retarding voltage versus collected current data provides derived measurements of ion density, ram velocity, and temperature. In order to successfully miniaturize these instruments and validate their performance prior to launch, thorough calibration and comprehensive end-to-end testing must be performed. This paper discusses the difficulties of performing complete system validation in ground-based vacuum chamber testing for RPAs. A procedure for RPA instrument calibration will be presented along with the calibration results for the Lower Atmosphere/Ionosphere Coupling Experiment (LAICE) CubeSat RPA. This paper presents a user-friendly and robust software control suite developed to read, parse, and interpret the data from the LAICE RPA. Electronics noise testing and analysis defines the performance boundaries of the instrument electronics. End-to-end testing of the LAICE RPA with a hot-filament ion source simulating the space plasma verifies the function of the LAICE RPA sensor and electronics, as well as the software control, thus qualifying the instrument for on-orbit use. / Master of Science

In the deep burn research of Very High Temperature Reactor (VHTR), it is desired to make an accurate estimation of absorption cross sections and absorption rates in burnable poison (BP) pins. However, in traditional methods, multi-group cross sections are generated from single bundle calculations with specular reflection boundary condition, in which the energy spectral effect in the core environment is not taken into account. This approximation introduces errors to the absorption cross sections especially for BPs neighboring reflectors and control rods. In order to correct the BP absorption cross sections in whole core diffusion calculations, energy spectrum reconstruction (ESR) methods have been developed to reconstruct the fine group spectrum (and in-core continuous energy spectrum). Then, using the reconstructed spectrum as boundary condition, a BP pin cell local transport calculation serves an imbedded module within the whole core diffusion code to iteratively correct the BP absorption cross sections for improved results. The ESR methods were tested in a 2D prismatic High Temperature Reactor (HTR) problem. The reconstructed fine-group spectra have shown good agreement with the reference spectra. Comparing with the cross sections calculated by single block calculation with specular reflection boundary conditions, the BP absorption cross sections are effectively improved by ESR methods. A preliminary study was also performed to extend the ESR methods to a 2D Pebble Bed Reactor (PBR) problem. The results demonstrate that the ESR can reproduce the energy spectra on the fuel-outer reflector interface accurately.

This thesis concerns the occurrence of the large-scale bed and plan forms known as alternate bars and meandering, and the internal structures of the flow associated with their formation. The work is to be viewed as an extension of previous work by da Silva (1991), Yalin (1992), and Yalin and da Silva (2001). As a first step in this work, the criteria for occurrence of alternate bars and meandering of Yalin and da Silva (2001) is re-considered in view of additional field and laboratory data from the recent literature and data resulting from two series of experimental runs carried out in two sediment transport flumes. This leads to a number of modifications of the boundary-lines in the related existence-region diagram of Yalin and da Silva. The size of the largest horizontal coherent structures (HCS’s) of an alternate bar inducing flow was then investigated experimentally on the basis of three series of flow velocity measurements. These were carried out in a 21m-long, 1m-wide straight channel, conveying a 4cm-deep flow. The bed consisted of a silica sand having a grain size of 2mm; its surface was flat. The measurements were carried out using a Sontek 2D Micro ADV. The horizontal burst length was found to be between five and seven times the flow width. The effect of the HCS’s on the mean flow was also investigated. A slight internal meandering of the flow caused by the superimposition of burst-sequences on the mean flow was clearly detectable. Finally, with the aid of three new series of measurements in the same channel, an attempt was made to penetrate the dynamics and life-cycle of the HCS’s. For this purpose, quadrant analysis was used; the cross-sectional distribution of relevant statistical turbulence-related parameters was investigated; and cross-correlations of flow velocity along the flow depth and across the channel were performed. The analysis indicates that the HCS’s originate near the channel banks, with the location of ejections and sweeps being anti-symmetrically arranged with regard to the channel centreline, and then evolve so as to occupy the entire depth of the water and the entire width of the channel. / Thesis (Ph.D, Civil Engineering) -- Queen's University, 2010-03-09 10:20:53.596

Yes / A set of shallow-water equations (SWEs) based on a k-epsilon Reynold stress model is established to simulate the turbulent flows over a complex roughness bed. The fundamental equations are discretized by the second-order finite-difference method (FDM), in which spatial and temporal discretization are conducted by staggered-grid and leap-frog schemes, respectively. The turbulent model in this study stems from the standard k-epsilon model, but is enhanced by replacing the conventional vertical production with a more rigorous and precise generation derived from the energy spectrum and turbulence scales. To verify its effectiveness, the model is applied to compute the turbulence in complex flow surroundings (including a rough bed) in an abrupt bend and in a natural waterway. The comparison of the model results against experimental data and other numerical results shows the robustness and accuracy of the present model in describing hydrodynamic characteristics, especially turbulence features on the complex roughness bottom. / National Key Research and Development Program of China (Grant No: 2016YFE0122500, 2013CB036401 and 2013CB036402), China Postdoctoral Science Foundation (Grant No: 2016M591184) and Programme of Introducing Talents of Discipline to Universities (Grant No: BC2018038) / Research Development Fund Publication Prize Award winner, June 2019.

<p>Recent developments in signal processing called Compressive Sampling (CS) show that the measurement and reconstruction of sparse signals often requires fewer samples than is estimated by the sampling theorem. CS is a combination of a linear sampling scheme and a reconstruction method and, typically, the signal is assumed to be sparse, compressible, or having a prior distribution, with the reconstruction error measured in the \ell^2 norm. This thesis investigates the application of CS to turbulence signals, particularly for estimating some statistics or nonlinear functions of the signals. The main original research result of the thesis is a proposed method, Spectrum Estimation by Sparse Optimization (SpESO), which uses a priori information about isotropic homogeneous turbulent flows and the multilevel structure of wavelet transforms to reconstruct energy spectra from compressive measurements, with errors measured on a logarithmic scale. The method is tested numerically on a variety of 1D and 2D turbulence signals, and is able to approximate energy spectra with an order of magnitude fewer samples than with traditional fixed rate sampling. The results demonstrate that SpESO performs much better than Lumped Orthogonal Matching Pursuit (LOMP), and as well or better than wavelet-based best M-term methods in many cases, even though these methods require complete sampling of the signal before compression.</p> / Master of Science (MSc)

The reliable prediction of turbulent non-equilibrium flows is of high academic and industrial interest in several engineering fields. Most turbulent flows are often predicted using single-time-scale Reynolds-Averaged-Navier-Stokes (RANS) turbulence models which assume the flows can be modelled through a single time or length scale which is an admittedly incorrect assumption. Therefore they are not expected to capture the lag in the response of the turbulence in non-equilibrium flows. In attempts to improve prediction of these flows, by taking into consideration some features of the turbulent kinetic energy spectrum, the multiple-time-scale models arose. A number of two-scale models have been proposed, but so far their use has been rather limited.This work thus focusses on the development of two-time-scale approaches. Two two-time-scale linear-eddy-viscosity models, referred to as NT1 and NT2 models, have been developed and the initial stages of the development of two-time-scale non-linear-eddy-viscosity models are also reported. The models' coefficients have been determined through asymptotic analysis of decaying grid turbulence, homogeneous shear flows and the flow in a boundary layer in local equilibrium. Three other important features of these models are that there is consistent partition of the large and the small scales for all above limiting cases, model sensitivity to the partition and production rate ratios and sensitivity of the eddy viscosity sensitive to the mean strain rates.The models developed have been tested through computations of a wide range of flows such as homogeneous shear and normally strained flows, fully developed channel flows, zero-pressure-gradient, adverse-pressure-gradient, favourable-pressure-gradient and oscillatory boundary layer flows, fully developed oscillatory and ramp up pipe flows and steady and pulsated backward-facing-step flows.The proposed NT1 and NT2 two-scale models have been shown to perform well in all test cases, being, among the benchmarked models tested, the models which best performed in the wide range of dimensionless shear values of homogeneous shear flows, the only linear-eddy-viscosity models which predicted well the turbulent kinetic energy in the normally strained cases and the only models which showed satisfactory sensitivity in predicting correctly the reattachment point in the unsteady backward facing step cases with different forcing frequencies. Although the development of the two-time-scale non-linear-eddy-viscosity models is still in progress, the interim versions proposed here have resulted in predictions of the Reynolds normal stresses similar to those of much more complex models in all test cases studied and in predictions of the turbulent kinetic energy in normally strained flows which are better than those of the other models tested in this study.

In this thesis, three improved analysis techniques for scatterometer wind estimation are presented. These techniques build upon previous methods that help validate scatterometer data. This thesis examines the theory connecting the 1D and 2D kinetic energy spectra and uses QuikSCAT data to measure the 2D kinetic energy spectrum of ocean winds. The measured 2D kinetic energy spectrum is compared to the traditional 1D kinetic energy spectrum. The relationship between the 2D kinetic energy spectra and the 1D kinetic energy spectra confirms findings from previous studies that ocean winds modeled in 2D are isotropic and nondivergent. The 1D and 2D kinetic energy spectra also confirm the known conclusion that the zonal and meridional components of ocean winds are uncorrelated. Through simulation, the wind response function (WRF) is calculated for three different QuikSCAT processing algorithms. The WRF quantifies the contribution that the wind at each point of the surface makes to a given wind estimate. The spatial resolution of the different processing algorithms is estimated by their WRFs. The WRFs imply that the spatial resolution of ultrahigh resolution (UHR) processing is finer than the spatial resolution of conventional drop-in-the-bucket (DIB) processing; the spatial resolution of UHR processing is ~5-10 km while the spatial resolution of DIB slice processing is ~12-15 km and the spatial resolution of coarse resolution DIB egg processing is ~30 km. Simulation is used to analyze the effectiveness of various wind retrieval and ambiguity selection algorithms. To assist in the simulation, synthetic wind fields are created through extrapolating the 2D Fourier transform of a numerical weather prediction wind field. These synthetic wind fields are sufficiently realistic to evaluate ambiguity selection algorithms. The simulation employs the synthetic wind fields to compare wind estimation with and without direction interval retrieval (DIR) applied. Both UHR and DIB wind estimation processes are performed in the simulation and UHR winds are shown to resolve finer resolution wind features than DIB winds at the cost of being slightly noisier. DIR added to standard QuikSCAT UHR wind estimation drops the wind direction root-mean-squared error by ~10° to ~24.74° in the swath sweet spot.