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This thesis studies the implementation of an Explicit Algebraic Reynolds-Stress Model(EARSM) for Atmospheric Boundary Layer (ABL) in an open source ComputationalFluid Dynamics (CFD) software, OpenFOAM, following the guidance provided by thewind company ENERCON that aims to make use of this novel model to improvesites’ wind-field predictions. After carefully implementing the model in OpenFOAM,the EARSM implementation is verified and validated by testing it with a stratifiedCouette flow case. The former was done by feeding mean flow properties, takenfrom OpenFOAM, in a python tool containing the full EARSM system of equationsand constants, and comparing the resulting flux profiles with the ones extracted bythe OpenFOAM simulations. Subsequently, the latter was done by comparing theprofiles of the two universal functions used by Monin-Obukhov Similarity Theory(MOST) for mean velocity and temperature to the results obtained by Želi et al. intheir study of the EARSM applied to a single column ABL, in “Modelling of stably-stratified, convective and transitional atmospheric boundary layers using the explicitalgebraic Reynolds-stress model” (2021). The verification of the model showed minordifferences between the flux profiles from the python tool and OpenFOAM thus, themodel’s implementation was deemed verified, while the validation step showed nodifference in the unstable and neutral stratification cases, but a significant discrepancyfor stably stratified flow. Nonetheless, the reason behind the inconsistency is believedto be related to the choice of boundary conditions thus, the model’s implementationitself is considered validated. Finally, the comparison between the EARSM and the k − ε model showed thatthe former is able to capture the physics of the flow properties where the latter failsto. In particular, the diagonal momentum fluxes resulting from the EARSM reflectthe observed behaviour of being different from each other, becoming isotropic withaltitude in the case of unstable stratification, and having magnitude u′u′ > v′v′ > w′w′ for stably stratified flows. On the other hand, the eddy viscosity assumption used bythe k − ε model computes the diagonal momentum fluxes as being equal to each other.Moreover, the EARSM captures more than one non-zero heat flux component in theCouette flow case, which has been observed to be the case in literature, while the eddydiffusivity assumption used by the k − ε model only accounts for one non-zero heat fluxcomponent.

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Thanks to high Tc of 40 K, high Jc of > 10^7 A.cm^-2, and no weak link behavior across the grain boundary in MgB2 material. This highest Tc among all conventional BCS superconductors, and better material properties of MgB2 compared to high Tc cuprate superconductors makes this material attractive for many applications including, but not limited to, power cables, Josephson junction based electronic devices, SRF cavities, THz sensors and single photon counters. Ultrathin superconducting films are a key element in various detectors utilized in remote sensing over a large part of the entire electromagnetic spectrum. The superconducting hot electron bolometer (HEB) mixer is a crucial detector for high-resolution spectroscopy at THz frequencies. The state-of-the-art NbN phonon-cooled HEB mixers have a relatively narrow (IF) bandwidth ~ 3- 4 GHz as a direct result of the poor acoustic transparency of the film-substrate interface and low sound velocity in NbN reducing the phonon escape time in the film. Alternatively, MgB2 displays a very short τe-ph ~ ps. The phonon escape time is also short due to the high sound velocity in the material (~ 7 Km.s^-2) thus giving rise to a broader IF bandwidth. Also, smaller magnetic penetration depth (λ ≈ 40 nm) of MgB2 makes material of choices for single photon detector application. The response time of an SNSPD is proportional to the square of its magnetic penetration depth λ. Therefore, MgB2 may potentially operate 10-fold faster than the NbN (λ =200 nm) based SNSPD. In this work, I present my effort to fabricate high quality ultrathin superconducting MgB2 films on 6H-SiC (0001) substrates, and study their superconducting and electronic properties. C- epitaxial 10 nm showed Tc of above 36 K, while residual resistivity of up to 26 μΩ.cm was achieved. Critical currents of more than 6 × 10^6 A · cm^−2 at 20 K have been measured for the films with thicknesses iv ranging from 10 to 100 nm. Fishtail structures have been observed in the magnetic field dependence of the critical current density for the thinnest of these films, indicating the presence of defects, which act as vortex pinning centers. From the magnetic field dependence, an average distance between adjacent pinning centers of 35 nm has been obtained for the thinnest films. Ultrathin film as thin as 1.8 nm (6 unit cells) can be achieved by Hybrid Physical-Chemical Vapor Deposition (HPCVD) followed by low angle Ar ion milling. These post processed films exhibit better superconducting properties compared to directly grown films. The 1.8 nm, showed Tc > 28 K and Jc > 10^6 A/cm^2 4 K. The surface roughness of the films was significantly improved and the suppression of Tc from the bulk value is much slower in milled films than in as-grown films. These results show the great potential of these ultrathin films for superconducting devices and present a possibility to explore superconductivity in MgB2 at the 2D limit. Finally, I measured the upper critical field of MgB2 films of various thickness and extracted their thickness dependent in-plane intraband diffusivities by using Gurevich model developed for two-band MgB2 superconductor in dirty limit. Results showed that π band diffusivity (Dπ) decreases rapidly from 71.12 cm^2/s for 100 nm film to 4.6 cm^2/s for 5 nm film where as �� band diffusivity (����) decreases much slower from 2.8 cm^2/s for 100 nm film to 0.8 cm^2/s for 5 nm film. This larger Dπ than ���� indicates the cleaner π band. / Physics

After a few remarks about the history of diffusion in solids we remind the reader to some basics of diffusion such as tracer diffusion, interdiffusion, high- diffusivity paths, and basic diffusion mechanism in solids. We then summarize self-diffusion in cubic, hexagonal metals and metals with phase transformations. Then we summarize diffusion of substitutional impurities (solutes) in metals and remind the reader to the phenomena of slow solute diffusion in aluminium and of fast solute diffusion in polyvalent metals. We finish the part on solute diffusion with some remarks on interstitial impurities We start the part on intermetallic alloys by reminding the reader to some of the more frequent structures. We consider examples of the influence of order-disorder transformation on diffusion. We then discuss diffusion in cubic B2-structured phases and in B2-, L12 - and DO 3- intermetallics, We then discuss uniaxial L1 0 -intermetallics and C11 b-structured molybdenum disilicide. We finish with some remarks diffusion in the cubic Laves phase Co2 Nb.:1. Introduction – some historical remarks 2. Some basics 2.1 Tracer diffusion 2.2 Interdiffusion 2.2.2 The ‘random alloy’ approximation for interdiffusion 2.2.1 Boltzmann-Matano method 2.3 High-diffusivity paths in solids 2.4 Diffusion mechanisms in solids 3. Self-diffusion in metals 3.1 Cubic metals 3.2 Uniaxial metals 3.3 Metals with phase transitions 4, Impurity diffusion in metals 4.1 Diffusion of substitutional impurities 4.2 Slow impurity diffusion in aluminium 4.3 Fast impurity diffusion in polyvalent metals 4.4 Diffusion of fast diffusing interstitial impurities and of hydrogen 5. Diffusion in Intermetallics 5.1 Some structures of intermetallics 5.2 Influence of order-disorder transitions on diffusionThis work is licensed under a Creative Commons Attribution 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/1 5.3 Diffusion in B2 phases and B2-intermetallics 5.4 Diffusion in L12-intermetallics 5.5 Diffusion in DO3-intermetallics 5.5 Diffusion in L10-intermetallics 5.6 Diffusion in C11b-structured molybdenum disilicide 5.6 Diffusion in the Laves phase Co 2Nb

Release of gas fission products in nuclear fuels is a practically important phenomenon based on the bubble diffusivity in crystal lattice. The bubble diffusion coefficient can be expressed through the volume and surface self-diffusion coefficients Dvol and Dsurf.

Yes / Based on the Lagrangian approach, this study proposes a random displacement model (RDM) to predict the concentration of suspended sediment in vegetated steady open channel flow. Validation of the method was conducted by comparing the simulated results by using the RDM with available experimental measurements for uniform open-channel flows. The method is further validated with the classical Rouse formula. To simulate the important vertical dispersion caused by vegetation in the sediment-laden open channel flow, a new integrated sediment diffusion coefficient is introduced in this study, which is equal to a coefficient multiplying the turbulent diffusion coefficient. As such, the RDM approach for sandy flow with vegetation was established for predicting the suspended sediment concentration in low-sediment-concentration flow with both the emergent and submerged vegetation. The study shows that the value of for submerged vegetation flow is larger than that for emergent vegetation flow. The simulated result using the RDM is in good agreement with the available experimental data, indicating that the proposed sediment diffusion coefficient model can be accurately used to investigate the sediment concentration in vegetated steady open channel flow. / National Natural Science Foundation (No. 51439007, 11672213, and 11872285); Open Funding of State Key Laboratory of Water Resources and Hydropower Engineering Science (WRHES), Wuhan University (Project No: 2018HLG01)