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The possibility of guiding light in air has fascinated optical scientists and engineers since the dawn of optical fiber technology. In the last few years, hollow core optical fibers have been attracting the attention of an expanding worldwide research community, furthering the design, fabrication and device implementation of specialty optical fibers. Hollow core optical fibers are entering almost any specific application field of optics from medicine to security; from telecommunication to industrial processing; from instrumentation to biology. In parallel to the increased number of applications, major advances are still being made on the optimization of hollow core fiber designs and on the study of its underlying guiding properties, as well as in the use of different materials and fabrication techniques, which, in turn, are providing even more ways of exploitation of this technology and new technical challenges. This Special Issue of Fibers rides the wave of this increasing interest in the field of hollow core optical fibers by providing an overview of the recent progress in this field as well as an updated and indicative sample of current research activities worldwide.

hollow optical fiber --- carbon dioxide laser --- endoscopic laser applications --- hollow-core fibers --- Raman lasers --- negative curvature fibers --- microstructured optical fibers --- hollow-core antiresonant fiber --- numerical modeling --- modal fiber properties --- terahertz --- THz --- 3D printing --- addictive manufacturing --- waveguide --- optical fiber --- fiber optics --- fiber design and fabrication --- microstructured fibers --- anti-resonant fibers --- CO2 lasers --- negative curvature fibers --- chalcogenide glass --- fiber loss --- mid-IR --- microstructured optical fiber splicing --- optical fiber sensors --- hollow core fibers --- photonic crystal fibers --- multiphoton fluorescence spectroscopy --- hollow core fiber --- beam delivery --- ultrafast lasers --- fiber filters --- hollow core fibers --- anti-resonant --- photonic crystal fibers --- fabrication --- hollow-core photonic crystal fiber --- gas photonics --- n/a

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This Special Issue presents the work of 30 scientists from 11 countries. It confirms that the impacts of global change, resulting from both climate change and increasing anthropogenic pressure, are huge on worldwide coastal areas (and critically so on some islands in the Pacific Ocean), with highly negative effects on coastal groundwater resources, which are widely affected by seawater intrusion. Some improved research methods are proposed in the contributions: using innovative hydrogeological, geophysical, and geochemical monitoring; assessing impacts of the changing environment on the coastal groundwater resources in terms of quantity and quality; and using modelling, especially to improve management approaches. The scientific research needed to face these challenges must continue to be deployed by different approaches based on the monitoring, modelling and management of groundwater resources. Novel and more efficient methods must be developed to keep up with the accelerating pace of global change.

artificial neural network --- support vector machine --- time series model --- freshwater-saltwater interface --- direct prediction --- recursive prediction --- geophysics --- groundwater --- hydrogeology --- aquifer --- atoll --- freshwater lens --- sea-level rise --- flooding --- groundwater --- tide --- submarine groundwater discharge --- atoll island --- groundwater storage --- freshwater resilience --- MODFLOW/SEAWAT --- saltwater intrusion --- water resources management --- seawater intrusion --- coastal aquifer --- arid and semi-arid regions --- cation exchange --- Tripoli --- Libya --- seawater intrusion --- multi-layered coastal aquifer --- offshore geophysics --- numerical model --- tidal signal --- sea–aquifer relations --- fish ponds --- sharp interface numerical modeling --- freshwater lens --- saltwater intrusion --- well salinization --- small islands --- Tongatapu --- saltwater intrusion --- sea level rise --- Nile Delta aquifer --- fresh groundwater volume --- extraction --- Nile Delta governorates --- SGD --- SGD model --- Radon --- coastal aquifer --- nutrient discharge --- Gaza Strip --- saltwater intrusion --- groundwater resources --- coastal aquifer --- climate change --- modelling --- monitoring --- salinization --- water resources management

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Fluid flow and heat transfer processes play an important role in many areas of science and engineering, from the planetary scale (e.g., influencing weather and climate) to the microscopic scales of enhancing heat transfer by the use of nanofluids; understood in the broadest possible sense, they also underpin the performance of many energy systems. This topical Special Issue of Energies is dedicated to the recent advances in this very broad field. This book will be of interest to readers not only in the fields of mechanical, aerospace, chemical, process and petroleum, energy, earth, civil ,and flow instrumentation engineering but, equally, biological and medical sciences, as well as physics and mathematics; that is, anywhere that “fluid flow and heat transfer” phenomena may play an important role or be a subject of worthy research pursuits.

performance characteristics --- Positive Temperature Coefficient (PTC) elements --- heat transfer --- thermal performance --- Computational Fluid Dynamics (CFD) simulation --- air heater --- impingement heat transfer enhancement --- orthogonal jet --- turbulence --- flat plate --- Colebrook equation --- Colebrook-White --- flow friction --- iterative procedure --- logarithms --- Padé polynomials --- hydraulic resistances --- turbulent flow --- pipes --- computational burden --- thermodynamic --- numerical simulation --- thermal effect --- axial piston pumps --- microbubble pump --- bubble generation --- pump efficiency --- bubble size --- concentration --- particle counter --- flow-induced motion --- sharp sections --- T-section prism --- load resistances --- section aspect ratios --- energy conversion --- thermosyphon --- phase change --- two-phase flow --- visualization --- superheated steam --- triaxial stress --- thermogravimetry --- X-ray microtomography --- thermal cracking --- microbubbles --- fluidics --- flow oscillation --- oscillators --- energetics --- pressure loss --- pressure drop --- friction factor --- multiphase flow --- flow rate --- flow regime --- POD --- entropy generation --- boundary layer --- laminar separation bubble --- two-phase flow --- pump performance --- computational fluid dynamics --- centrifugal pump --- flow behavior --- magnetic field --- ferrofluid --- porous cavity --- heat transfer --- mass transfer --- numerical modeling --- numerical modeling --- surrogate model --- correlation --- fin-tube --- spiral fin-tube --- CFD --- ( A g ? F e 3 O 4 / H 2 O ) hybrid nanofluid --- nonlinear thermal radiation --- heat transfer --- chemical reaction --- mass transfer --- method of moment --- numerical results --- transient analysis --- pumps --- moment of inertia --- water hammer --- pipe flow --- wind turbine --- downwind --- tower shadow --- load --- tower --- BEM --- actuator disc --- CANDU-6 --- PHWR --- moderator --- turbulence --- OpenFOAM --- printed circuit heat exchanger --- supercritical LNG --- zigzag type --- heat transfer performance --- gas turbine engine --- particle deposition --- capture efficiency --- multiphase flow --- tip leakage flow --- detached-eddy simulation --- vortex breakdown --- transonic compressor --- POD --- tip leakage flow --- decomposition region --- decomposition dimensionalities --- vortex identification --- SPIV --- fire-spreading characteristics --- real vehicle experiments --- toxic gases --- temperature distributions --- unsteady heat release rate --- thermal energy recovery --- flue gas --- dew point temperature --- condensation --- Aspen® --- thermoacoustic electricity generator --- multi-stage --- traveling-wave heat engine --- push-pull --- inertance-compliance --- acoustic streaming --- n/a

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This book is a collection of recent publications from researchers all over the globe in the broad area of high-voltage engineering. The presented research papers cover both experimental and simulation studies, with a focus on topics related to insulation monitoring using state-of-the-art sensors and advanced machine learning algorithms. Special attention was given in the Special Issue to partial discharge monitoring as one of the most important techniques in insulation condition assessment. Moreover, this Special Issue contains several articles which focus on different modeling techniques that help researchers to better evaluate the condition of insulation systems. Different power system assets are addressed in this book, including transformers, outdoor insulators, underground cables, and gas-insulated substations.

space/interface charge --- electrical field strength --- temperature --- oil-paper insulation --- simulation --- bipolar charge transport model --- earthing systems --- electrode’s geometry --- fast-impulses --- high-magnitude currents and impulse polarity --- grounding --- grounding electrodes --- high impulse conditions --- seasonal --- soil resistivity --- partial discharges (PD) --- partial discharge --- calibrator --- Tettex 9520 --- DDX 8003 --- DDX 9121b --- secondary arc --- short-circuit discharge --- numerical modeling --- plasma discharge --- post insulator --- non-uniform pollution between windward and leeward sides --- residual resistance formulation --- flashover dynamic model --- artificial flashover tests --- flashover characteristics --- feature selection --- insulation health index --- machine learning --- oil/paper insulation --- transformer asset management --- thermal effect --- cable --- XLPE --- laying modes --- Comsol Multiphysics --- thermal parameters --- cable ampacity --- partial discharge --- surface discharge --- UHF sensor --- FDTD simulation --- cavity discharge --- partial discharge --- denoising --- RF signal --- wavelet transform --- artificial neural network --- curve fitting --- saline mechanism --- shoreline --- wind speed --- outdoor insulators --- dry band arcing --- flashover --- tracking --- ageing --- hydrophobicity --- leakage current --- dry band arcing --- degradation --- polymeric insulation --- tracking test setup --- composite insulator --- dry band formation --- heat transfer model --- generalized finite difference time domain --- corona discharge --- electric field analysis --- ion flow field --- space charge density --- UFVM --- partial discharge --- optical-UHF integrated detection --- photoelectric fusion pattern --- GIL --- NSCT --- cable joint --- charge simulation method --- electrical tree --- random walk theory --- finite element analysis --- partial discharge modeling --- high-frequency --- fast-rise square wave voltages --- silicone gel --- wide bandgap power modules --- surface discharge --- flashover --- gas --- modelling --- pressure --- thermal properties