Low-loss criterion and effective area considerations for photonic crystal fibers

We study the class of endlessly single-mode all-silica photonic crystal fibers with a triangular air-hole cladding. We consider the sensibility to longitudinal nonuniformities and the consequences and limitations for realizing low-loss large-mode area photonic crystal fibers. We also discuss the dominating scattering mechanism and experimentally we confirm that both macro and micro-bending can be the limiting factor.Comment: Accepted for Journal of Optics A - Pure and Applied Optic

All-optical switching and strong coupling using tunable whispering-gallery-mode microresonators

We review our recent work on tunable, ultrahigh quality factor whispering-gallery-mode bottle microresonators and highlight their applications in nonlinear optics and in quantum optics experiments. Our resonators combine ultra-high quality factors of up to Q = 3.6 \times 10^8, a small mode volume, and near-lossless fiber coupling, with a simple and customizable mode structure enabling full tunability. We study, theoretically and experimentally, nonlinear all-optical switching via the Kerr effect when the resonator is operated in an add-drop configuration. This allows us to optically route a single-wavelength cw optical signal between two fiber ports with high efficiency. Finally, we report on progress towards strong coupling of single rubidium atoms to an ultra-high Q mode of an actively stabilized bottle microresonator.Comment: 20 pages, 24 figures. Accepted for publication in Applied Physics B. Changes according to referee suggestions: minor corrections to some figures and captions, clarification of some points in the text, added references, added new paragraph with results on atom-resonator interactio

Natural convection in a square cavity with uniformly heated and/or insulated walls using marker-and-cell method

In this study, a numerical investigation has been performed using the computational Harlow-Welch MAC (Marker and Cell) finite difference method to analyse the unsteady state two-dimensional natural convection in lid-driven square cavity with left wall maintained at constant heat flux and remaining walls kept thermally insulated. The significant parameters in the present study are Reynolds number (Re), thermal Grashof number (Gr) and Prandtl number (Pr) and Peclét number (Pe =PrRe). The structure of thermal convection patterns is analysed via streamline, vorticity, pressure and temperature contour plots. The influence of the thermophysical parameters on these distributions is described in detail. Validation of solutions with earlier studies is included. Mesh independence is also conducted. It is observed that an increase in Prandtl number intensifies the primary circulation whereas it reduces the heat transfer rate. Increasing thermal Grashof number also decreases heat transfer rates. Furthermore the isotherms are significantly compressed towards the left (constant flux) wall with a variation in Grashof number while Peclét number is fixed. The study is relevant to solar collector heat transfer simulations and also crystal growth technologies

Natural convection of liquid metal in a vertical annulus with lateral and volumetric heating in the presence of a horizontal magnetic field

MHD free convection of a liquid metal is studied in a closed vertical annulus in which the upper and bottom walls are adiabatic while the cylindrical walls are kept at different temperatures. The flow is driven by two mechanisms: the temperature difference between the two cylindrical walls and the volumetric heating. A constant horizontal magnetic field is also imposed resisting the fluid motion. The laminar and turbulent regimes of the flow are assessed by performing three-dimensional direct numerical simulations. The results show that in the absence of the magnetic field, turbulent flow is developed in most of the cases, while as the magnetic field increases the flow becomes laminar. The highest temperature is found in the upper-central part of the annular cavity when the fluid is heated volumetrically, resulting in the creation of two convection currents as the hot fluid ascends in the central part and descends close to both colder walls. The Hartmann and Roberts layers developing near the walls normal and parallel to the magnetic field, respectively, are found to be responsible for the loss of axisymmetry of the present flow. (C) 2011 Elsevier Ltd. All rights reserved

MAGNETOHYDRODYNAMIC NATURAL CONVECTION OF LIQUID METAL BETWEEN COAXIAL ISOTHERMAL CYLINDERS DUE TO INTERNAL HEATING

Direct numerical simulation results are presented of magnetohydrodynamic liquid metal flow between two vertical coaxial cylinders under the effect of internal heating and a horizontal magnetic field. The end walls are thermally insulated, while the cylindrical walls are at the same temperature. The flow is driven by the heat sources under the stabilizing effect of the external magnetic field. The problem is characterized by the internal Rayleigh number, expressing the internal heating, and the Hartmann number corresponding to the magnetic field. Depending on their value laminar, transitional or turbulent flow may occur. The heat sources create bi-cellular flow patterns as the maximum temperature in inside the fluid bulk. Te flow is azimuthally asymmetric due to the Hartmann and Robert layers formed on the walls normal and parallel to the magnetic field, respectively

MHD natural convection in a laterally and volumetrically heated square cavity

A numerical study is presented of unsteady two-dimensional natural convection of an electrically conducting fluid in a laterally and volumetrically heated square cavity under the influence of a magnetic field. The flow is characterized by the external Rayleigh number, Ra-E, determined from the temperature difference of the side walls, the internal Rayleigh number, Raj, determined from the volumetric heat rate, and the Hartmann number, Ha, deter-mined from the strength of the imposed magnetic field. Starting from given values of Ra-E and Ha, for which the flow has a steady unicellular pattern, and gradually increasing the ratio S = Ra-1/Ra-E, oscillatory convective flow may occur. The initial steady unicellular flow for S = 0 may undergo transition to steady or unsteady multicellular flow up to a threshold value, Ra-1,Ra-cr, Of the internal Rayleigh number depending on Ha. Oscillatory multicellular flow fields were observed for S values up to 100 for the range 10(5)-10(6) of Ra-E studied. The increase of the ratio S results usually in a transition from steady to unsteady flow but there have also been cases where the increase of S results in an inverse transition from unsteady to steady flow. Moreover, the usual damping effect of increasing Hartmann number is not found to be straightforward connected with the resulting flow patterns in the present flow configuration. (c) 2005 Elsevier Ltd. All rights reserved

Numerical simulations of the IPPE target geometry flows

A high speed water and liquid lithium (Li) flow is computed over the IPPE geometry to evaluate the performance of different turbulence models in 2D and 3D simulations. Results reported are the thickness of the liquid jet, irregularities in the surface, transient phenomena at the wall which can affect fluid surface and effect of the variation in bulk velocity on these quantities. All models show good near wall resolution of the boundary layer and expected profiles for the free surface flow. Predicted turbulent kinetic energy compare well with published data. Fluctuations of the flow surface at the control location (center of the curved section) and elsewhere are well within 1 mm for all models. However it was observed that the predictions are strongly dependent on the model used. Overall, the predictions of RANS models are close to each other whereas predictions of laminar simulations are close to those obtained with LES models. (C) 2013 Elsevier B.V. All rights reserved

Magnetohydrodynamic natural convection in a vertical cylindrical cavity with sinusoidal upper wall temperature

A series of numerical simulations were performed in order to study liquid metal MHD natural convection in a vertical cylindrical container with a sinusoidal temperature distribution at the upper wall and the other surfaces being adiabatic. Starting from the basic hydrodynamic case, the effect of vertical (axial) and horizontal magnetic fields is assessed. Depending on the magnitude of the Rayleigh and Hartmann numbers, both turbulent and laminar (azimuthally symmetric or not) flows are observed. The results show that the increase of Rayleigh number promotes heat transfer by convection while the increase of Hartmann number favors heat conduction. The vertical magnetic field reduces the Nusselt number more than the horizontal. The circulation patterns for the most convective cases are confined close to the top corner of the container with the simultaneous formation of a secondary flow pattern at the bottom corner, while for the more conductive cases only one circulation pattern exists covering the entire domain. (c) 2008 Elsevier Ltd. All rights reserved

Investigation of various nozzles configurations with respect to IFMIF and liquid walls concepts

The study of liquid-metal free surface flows is of great interest in the fusion research, for example in the IFMIF and liquid walls concepts. In the IFMIF project, the main goal is to test candidate metallic materials in irradiation conditions similar to those present in a fusion reactor. More specifically, an intense neutron source will be produced by bombarding a high-speed liquid lithium target jet with two deuterium beams of 40 MeV. The source will then be used to test samples of the candidate materials. In the so called "Liquid walls" project, the use of liquid film free surface flows as plasma facing components (PFCs) is studied as an alternative to metallic plasma facing materials. The free surface PFCs could result in important advantages over solid walls, such as the minimization of corrosion defects and faster maintenance. In both concepts the feeding of the liquid film will be achieved by the use of a nozzle. The main scope of this work is to focus on the optimization of the flow uniformity that comes out from the nozzle. According to the literature, the use of nozzles based on the Shima profile formulation has been favoured to improve the film stability. Based on the above, a number of flows springing from several variations of "Shima" type nozzles are numerically investigated here with main goal to define the most optimum geometry in terms of minimizing turbulent defects and flow deformations. © 2015 Elsevier B.V. All rights reserved

Analytical and numerical study of MHD natural convection in a horizontal shallow cavity with heat generation

A study is presented of two-dimensional MHD natural convection flow in an internally heated horizontal shallow cavity in the presence of an external uniform vertical magnetic field. The main feature of the present flow is a symmetric double-cell Hadley circulation with the fluid ascending in the center of the cavity and descending near the vertical cold walls. All walls are electrically insulated, with the horizontal being adiabatic and the vertical isothermal. The method of the matched asymptotic expansions is used to obtain solutions of the flow and heat transfer problem. This analysis which is valid for large cavity aspect ratios and for any magnetic field strength is particularly helpful for the inexpensive determination of the flow and heat transfer characteristics. In addition, numerical simulations are performed for a range of Hartmann, Prandtl and Rayleigh numbers in order to verify the accuracy and validity of the analytical results, to calculate the constants arising by the analytical approach and to obtain details of the flow in the vicinity of the vertical side walls. The comparison of the analytical and numerical results is very good indicating the correctness of the present analysis and its applicability. (C) 2014 Elsevier Ltd. All rights reserved