5,234 research outputs found

    Prediction of airfoil stall using Navier-Stokes equations in streamline coordinates

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    A Navier-Stokes procedure to calculate the flow about an airfoil at incidence was developed. The parabolized equations are solved in the streamline coordinates generated for an arbitrary airfoil shape using conformal mapping. A modified k-epsilon turbulence model is applied in the entire domain, but the eddy viscosity in the laminar region is suppressed artificially to simulate the region correctly. The procedure was applied to airfoils at various angles of attack, and the results are quite satisfactory for both laminar and turbulent flows. It is shown that the present choice of the coordinate system reduces the error due to numerical diffusion, and that the lift is accurately predicted for a wide range of incidence

    Giant Shapiro Resonances in a Flux Driven Josephson Junction Necklace

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    We present a detailed study of the dynamic response of a ring of NN equally spaced Josephson junctions to a time-periodic external flux, including screening current effects. The dynamics are described by the resistively shunted Josephson junction model, appropriate for proximity effect junctions, and we include Faraday's law for the flux. We find that the time-averaged I−VI-V characteristics show novel {\em subharmonic giant Shapiro voltage resonances}, which strongly depend on having phase slips or not, on NN, on the inductance and on the external drive frequency. We include an estimate of the possible experimental parameters needed to observe these quantized voltage spikes.Comment: 8 pages RevTeX, 3 figures available upon reques

    Phase and Charge reentrant phase transitions in two capacitively coupled Josephson arrays with ultra-small junction

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    We have studied the phase diagram of two capacitively coupled Josephson junction arrays with charging energy, EcE_c, and Josephson coupling energy, EJE_J. Our results are obtained using a path integral Quantum Monte Carlo algorithm. The parameter that quantifies the quantum fluctuations in the i-th array is defined by αi≡EciEJi\alpha_i\equiv \frac{E_{{c}_i}}{E_{J_i}}. Depending on the value of αi\alpha_i, each independent array may be in the semiclassical or in the quantum regime: We find that thermal fluctuations are important when α≲1.5\alpha \lesssim 1.5 and the quantum fluctuations dominate when 2.0≲α2.0 \lesssim \alpha . We have extensively studied the interplay between vortex and charge dominated individual array phases. The two arrays are coupled via the capacitance CinterC_{{\rm inter}} at each site of the lattices. We find a {\it reentrant transition} in Υ(T,α)\Upsilon(T,\alpha), at low temperatures, when one of the arrays is in the semiclassical limit (i.e. α1=0.5\alpha_{1}=0.5 ) and the quantum array has 2.0≤α2≤2.52.0 \leq\alpha_{2} \leq 2.5, for the values considered for the interlayer capacitance. In addition, when 3.0≤α2<4.03.0 \leq \alpha_{2} < 4.0, and for all the inter-layer couplings considered above, a {\it novel} reentrant phase transition occurs in the charge degrees of freedom, i.e. there is a reentrant insulating-conducting transition at low temperatures. We obtain the corresponding phase diagrams and found some features that resemble those seen in experiments with 2D JJA.Comment: 25 Latex pages including 8 encapsulated poscript figures. Accepted for publication in Phys. Rev B (Nov. 2004 Issue

    Damage and repair classification in reinforced concrete beams using frequency domain data

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    This research aims at developing a new vibration-based damage classification technique that can efficiently be applied to a real-time large data. Statistical pattern recognition paradigm is relevant to perform a reliable site-location damage diagnosis system. By adopting such paradigm, the finite element and other inverse models with their intensive computations, corrections and inherent inaccuracies can be avoided. In this research, a two-stage combination between principal component analysis and Karhunen-Loéve transformation (also known as canonical correlation analysis) was proposed as a statistical-based damage classification technique. Vibration measurements from frequency domain were tested as possible damage-sensitive features. The performance of the proposed system was tested and verified on real vibration measurements collected from five laboratory-scale reinforced concrete beams modelled with various ranges of defects. The results of the system helped in distinguishing between normal and damaged patterns in structural vibration data. Most importantly, the system further dissected reasonably each main damage group into subgroups according to their severity of damage. Its efficiency was conclusively proved on data from both frequency response functions and response-only functions. The outcomes of this two-stage system showed a realistic detection and classification and outperform results from the principal component analysis-only. The success of this classification model is substantially tenable because the observed clusters come from well-controlled and known state conditions

    Optimum Tower Crane Selection and Supporting Design Management

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    To optimize tower crane selection and supporting design, lifting requirements (as well as stability) should be examined, followed by a review of economic feasibility. However, construction engineers establish plans based on data provided by equipment suppliers since there are no tools with which to thoroughly examine a support design’s suitability for various crane types, and such plans lack the necessary supporting data. In such cases it is impossible to optimize a tower crane selection to satisfy lifting requirements in terms of cost, and to perform lateral support and foundation design. Thus, this study is intended to develop an optimum tower crane selection and supporting design management method based on stability. All cases that are capable of generating an optimization of approximately 3,000 ~ 15,000 times are calculated to identify the candidate cranes with minimized cost, which are examined. The optimization method developed in the study is expected to support engineers in determining the optimum lifting equipment management

    The Extended Shapes of Galactic Satellites

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    We are exploring the extended stellar distributions of Galactic satellite galaxies and globular clusters. For seven objects studied thus far, the observed profile departs from a King function at large r, revealing a ``break population'' of stars. In our sample, the relative density of the ``break'' correlates to the inferred M/L of these objects. We discuss opposing hypotheses for this trend: (1) Higher M/L objects harbor more extended dark matter halos that support secondary, bound, stellar ``halos''. (2) The extended populations around dwarf spheroidals (and some clusters) consist of unbound, extratidal debris from their parent objects, which are undergoing various degrees of tidal disruption. In this scenario, higher M/L ratios reflect higher degrees of virial non-equilibrium in the parent objects, thus invalidating a precept underlying the use of core radial velocities to obtain masses.Comment: 8 pages, including 2 figures Yale Cosmology Workshop: The Shapes of Galaxies and Their Halo

    Orientational pinning and transverse voltage: Simulations and experiments in square Josephson junction arrays

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    We study the dependence of the transport properties of square Josephson Junctions arrays with the direction of the applied dc current, both experimentally and numerically. We present computational simulations of current-voltage curves at finite temperatures for a single vortex in the array (Ha2/Φ0=f=1/L2Ha^2/\Phi_0=f=1/L^2), and experimental measurements in 100×1000100\times1000 arrays under a low magnetic field corresponding to f≈0.02f\approx0.02. We find that the transverse voltage vanishes only in the directions of maximum symmetry of the square lattice: the [10] and [01] direction (parallel bias) and the [11] direction (diagonal bias). For orientations different than the symmetry directions, we find a finite transverse voltage which depends strongly on the angle ϕ\phi of the current. We find that vortex motion is pinned in the [10] direction (ϕ=0\phi=0), meaning that the voltage response is insensitive to small changes in the orientation of the current near ϕ=0\phi=0. We call this phenomenon orientational pinning. This leads to a finite transverse critical current for a bias at ϕ=0\phi=0 and to a transverse voltage for a bias at ϕ≠0\phi\not=0. On the other hand, for diagonal bias in the [11] direction the behavior is highly unstable against small variations of ϕ\phi, leading to a rapid change from zero transverse voltage to a large transverse voltage within a few degrees. This last behavior is in good agreement with our measurements in arrays with a quasi-diagonal current drive.Comment: 9 pages, 9 figure
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