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Caltech Authors (161.377 recursos)

Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 18.603

  1. Optimization principles and the figure of merit for triboelectric generators

    Peng, Jun; Kang, Stephen Dongmin; Snyder, G. Jeffrey
    Energy harvesting with triboelectric nanogenerators is a burgeoning field, with a growing portfolio of creative application schemes attracting much interest. Although power generation capabilities and its optimization are one of the most important subjects, a satisfactory elemental model that illustrates the basic principles and sets the optimization guideline remains elusive. We use a simple model to clarify how the energy generation mechanism is electrostatic induction but with a time-varying character that makes the optimal matching for power generation more restrictive. By combining multiple parameters into dimensionless variables, we pinpoint the optimum condition with only two independent parameters, leading to predictions...

  2. Search for pair production of vector-like T and B quarks in single-lepton final states using boosted jet substructure in proton-proton collisions at √s = 13 TeV

    Sirunyan, A. M.; Anderson, D.; Bendavid, J.; Bornheim, A.; Lawhorn, J. M.; Newman, H. B.; Nguyen, T.; Pena, C.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhang, Z.; Zhu, R. Y.; Dubinin, M.
    A search for pair production of massive vector-like T and B quarks in proton-proton collisions at √s = 13 TeV is presented. The data set was collected in 2015 by the CMS experiment at the LHC and corresponds to an integrated luminosity of up to 2.6 fb^(−1). The T and B quarks are assumed to decay through three possible channels into a heavy boson (either a W, Z or Higgs boson) and a third generation quark. This search is performed in final states with one charged lepton and several jets, exploiting techniques to identify W or Higgs bosons decaying hadronically...

  3. Multiwavelength follow-up of a rare IceCube neutrino multiplet

    Aartsen, M. G.; Bellm, E.
    On February 17, 2016, the IceCube real-time neutrino search identified, for the first time, three muon neutrino candidates arriving within 100 s of one another, consistent with coming from the same point in the sky. Such a triplet is expected once every 13.7 years as a random coincidence of background events. However, considering the lifetime of the follow-up program the probability of detecting at least one triplet from atmospheric background is 32%. Follow-up observatories were notified in order to search for an electromagnetic counterpart. Observations were obtained by Swift’s X-ray telescope, by ASAS-SN, LCO and MASTER at optical wavelengths, and by VERITAS...

  4. Recoupling Coefficients and Quantum Entropies

    Christandl, Matthias; Sahinoğlu, M. Burak; Walter, Michael
    We prove that the asymptotic behavior of the recoupling coefficients of the symmetric group S_k is characterized by a quantum marginal problem: they decay polynomially in k if there exists a quantum state of three particles with given eigenvalues for their reduced density operators and exponentially otherwise. As an application, we deduce solely from symmetry considerations of the coefficients the strong subadditivity property of the von Neumann entropy, first proved by Lieb and Ruskai (J Math Phys 14:1938–1941, 1973). Our work may be seen as a non-commutative generalization of the representation-theoretic aspect of the recently found connection between the quantum...

  5. Measurement of Circumstellar Disk Sizes in the Upper Scorpius OB Association with ALMA

    Barenfeld, Scott A.; Carpenter, John M.; Sargent, Anneila I.; Isella, Andrea; Ricci, Luca
    We present detailed modeling of the spatial distributions of gas and dust in 57 circumstellar disks in the Upper Scorpius OB Association observed with ALMA at submillimeter wavelengths. We fit power-law models to the dust surface density and CO J = 3–2 surface brightness to measure the radial extent of dust and gas in these disks. We found that these disks are extremely compact: the 25 highest signal-to-noise disks have a median dust outer radius of 21 au, assuming an R^(-1) dust surface density profile. Our lack of CO detections in the majority of our sample is consistent with these...

  6. Characterizing Dust Attenuation in Local Star-forming Galaxies: Inclination Effects and the 2175 Å Feature

    Battisti, A. J.; Calzetti, D.; Chary, R.-R.
    We characterize the influence that inclination has on the shape and normalization in average dust attenuation curves derived from a sample of ~10,000 local star-forming galaxies. To do this, we utilize aperture-matched multiwavelength data from the Galaxy Evolution Explorer, the Sloan Digital Sky Survey, the United Kingdom Infrared Telescope, and the Two Micron All-sky Survey. We separate our sample into groups according to axial ratio (b/a) and obtain an estimate of their average total-to-selective attenuation k(λ). The attenuation curves are found to be shallower at UV wavelengths with increasing inclination, whereas the shape at longer wavelengths remains unchanged. The highest...

  7. Dynamics of many-body localization in the presence of particle loss

    van Nieuwenburg, Evert; Yago Malo, Jorge; Daley, Andrew J.; Fischer, Mark Hannes
    At long times, residual couplings to the environment become relevant even in the most isolated experiments, a crucial difficulty for the study of fundamental aspects of many-body dynamics. A particular example is many-body localization in a cold-atom setting, where incoherent photon scattering introduces both dephasing and particle loss. Whereas dephasing has been studied in detail and is known to destroy localization already on the level of non-interacting particles, the effect of particle loss is less well understood. A difficulty arises due to the 'non-local' nature of the loss process, complicating standard numerical tools using matrix product decomposition. Utilizing symmetries of...

  8. Computational modelling of impact damage in brittle materials

    Camacho, G. T.; Ortiz, M.
    A Lagrangian finite element method of fracture and fragmentation in brittle materials is developed. A cohesive-law fracture model is used to propagate multiple cracks along arbitrary paths. In axisymmetric calculations, radial cracking is accounted for through a continuum damage model. An explicit contact/friction algorithm is used to treat the multi-body dynamics which inevitably ensues after fragmentation. Rate-dependent plasticity, heat conduction and thermal coupling are also accounted for in calculations. The properties and predictive ability of the model are exhibited in two case studies: spall tests and dynamic crack propagation in a double cantilever beam specimen. As an example of application...

  9. Ductile fracture by vacancy condensation in f.c.c. single crystals

    Cuitiño, A. M.; Ortiz, M.
    We explore the feasibility of vacancy condensation as the void-nucleating mechanism underlying ductile fracture by void growth and coalescence in single crystals at room temperature. Vacancies are presumed to be primarily generated by the dragging of intersection jogs. The equations governing the rate of growth of voids by vacancy condensation are derived. These equations are used to follow the evolution of vacancy concentrations and void sizes in the Wang and Anderson [Acta metall. 39, 779 (1991)] [1] Σ9 test. We find that, when pipe diffusions are taken into account, the time required for the nucleation of a macroscopic void in...

  10. Three-dimensional crack-tip fields in four-point-bending copper single-crystal specimens

    Cuitiño, A. M.; Ortiz, M.
    The three-dimensional near-tip fields in copper single crystals loaded in four-point bending are characterized numerically. For comparison purposes, the corresponding plane-strain fields are also computed numerically and their asymptotic behavior determined semi-analytically. On the basis of these analyses, we investigate: (i) the dependence of the fields on the hardening law; (ii) the degree of correlation between surface and interior fields in finite specimens; and (iii) the degree of correlation between plane-strain and three-dimensional fields. In particular, we endeavor to ascertain the extent to which surface observations of near-tip fields in single crystals, such as those obtained by Moire interferometry, are...

  11. The Two-Dimensional Structure of Dynamic Boundary Layers and Shear Bands in Thermoviscoplastic Solids

    Gioia, G.; Ortiz, M.
    A general boundary layer theory for thermoviscoplastic solids which accounts for inertia, rate sensitivity, hardening, thermal coupling, heat convection and conduction, and thermal softening is developed. In many applications of interest, the boundary layer equations can be considerably simplified by recourse to similarity methods, which facilitates the determination of steady-state and transient fully non-linear two-dimensional solutions. A simple analysis of the asymptotic behavior of the steady-state solutions leads to a classification of stable and unstable regimes. Under adiabatic conditions, the resulting material stability criterion coincides with that previously derived by Molinari and Clifton [(1987) Analytical characterization of shear localization in...

  12. Finite‐element modeling of dry sliding wear in metals

    Molinari, J. F.; Ortiz, M.; Radovitzky, R.; Repetto, E. A.
    This paper is concerned with the calibration and validation of a finite‐element model of dry sliding wear in metals. The model is formulated within a Lagrangian framework capable of accounting for large plastic deformations and history‐dependent material behavior. We resort to continuous adaptive meshing as a means of eliminating deformation‐induced element distortion, and of resolving fine features of the wear process such as contact boundary layers. Particular attention is devoted to a generalization of Archard’s law in which the hardness of the soft material is allowed to be a function of temperature. This dependence of hardness on temperature provides a...

  13. A theory of subgrain dislocation structures

    Ortiz, M.; Repetto, E. A.; Stainier, L.
    We develop a micromechanical theory of dislocation structures and finite deformation single crystal plasticity based on the direct generation of deformation microstructures and the computation of the attendant effective behavior. Specifically, we aim at describing the lamellar dislocation structures which develop at large strains under monotonic loading. These microstructures are regarded as instances of sequential lamination and treated accordingly. The present approach is based on the explicit construction of microstructures by recursive lamination and their subsequent equilibration in order to relax the incremental constitutive description of the material. The microstructures are permitted to evolve in complexity and fineness with increasing...

  14. Large-eddy simulation of flow over a grooved cylinder up to transcritical Reynolds numbers

    Cheng, W.; Pullin, D. I.; Samtaney, R.
    We report wall-resolved large-eddy simulation (LES) of flow over a grooved cylinder up to the transcritical regime. The stretched-vortex subgrid-scale model is embedded in a general fourth-order finite-difference code discretization on a curvilinear mesh. In the present study grooves are equally distributed around the circumference of the cylinder, each of sinusoidal shape with height ε, invariant in the spanwise direction. Based on the two parameters, ε/D and the Reynolds number Re_D = U_∞D/ν where U_∞ is the free-stream velocity, D the diameter of the cylinder and ν the kinematic viscosity, two main sets of simulations are described. The first set...

  15. Large-eddy simulation of flow over a grooved cylinder up to transcritical Reynolds numbers

    Cheng, W.; Pullin, D. I.; Samtaney, R.
    We report wall-resolved large-eddy simulation (LES) of flow over a grooved cylinder up to the transcritical regime. The stretched-vortex subgrid-scale model is embedded in a general fourth-order finite-difference code discretization on a curvilinear mesh. In the present study grooves are equally distributed around the circumference of the cylinder, each of sinusoidal shape with height ε, invariant in the spanwise direction. Based on the two parameters, ε/D and the Reynolds number Re_D = U_∞D/ν where U_∞ is the free-stream velocity, D the diameter of the cylinder and ν the kinematic viscosity, two main sets of simulations are described. The first set...

  16. Regularized integral formulation of mixed Dirichlet-Neumann problems

    Akhmetgaliyev, Eldar; Bruno, Oscar P.
    This paper presents a theoretical discussion as well as novel solution algorithms for problems of scattering on smooth two-dimensional domains under Zaremba boundary conditions, for which Dirichlet and Neumann conditions are specified on various portions of the domain boundary. The theoretical basis of the proposed numerical methods, which is provided for the first time in the present contribution, concerns detailed information about the singularity structure of solutions of the Helmholtz operator under boundary conditions of Zaremba type. The new numerical method is based on the use of Green functions and integral equations, and it relies on the Fourier continuation method...

  17. Regularized integral formulation of mixed Dirichlet-Neumann problems

    Akhmetgaliyev, Eldar; Bruno, Oscar P.
    This paper presents a theoretical discussion as well as novel solution algorithms for problems of scattering on smooth two-dimensional domains under Zaremba boundary conditions, for which Dirichlet and Neumann conditions are specified on various portions of the domain boundary. The theoretical basis of the proposed numerical methods, which is provided for the first time in the present contribution, concerns detailed information about the singularity structure of solutions of the Helmholtz operator under boundary conditions of Zaremba type. The new numerical method is based on the use of Green functions and integral equations, and it relies on the Fourier continuation method...

  18. Variants of the Entropy Power Inequality

    Bobkov, Sergey G.; Marsiglietti, Arnaud
    An extension of the entropy power inequality to the form N_r^α (X + Y) ≥ N_r^α (X) + N_r^α (Y) with arbitrary independent summands X and Y in R^n is obtained for the Rényi entropy and powers α ≥ (r + 1)/2.

  19. Variants of the Entropy Power Inequality

    Bobkov, Sergey G.; Marsiglietti, Arnaud
    An extension of the entropy power inequality to the form N_r^α (X + Y) ≥ N_r^α (X) + N_r^α (Y) with arbitrary independent summands X and Y in R^n is obtained for the Rényi entropy and powers α ≥ (r + 1)/2.

  20. Three-dimensional quasi-periodic shifted Green function throughout the spectrum, including Wood anomalies

    Bruno, Oscar P.; Shipman, Stephen P.; Turc, Catalin; Venakides, Stephanos
    This work, part II in a series, presents an efficient method for evaluation of wave scattering by doubly periodic diffraction gratings at or near what are commonly called ‘Wood anomaly frequencies’. At these frequencies, there is a grazing Rayleigh wave, and the quasi-periodic Green function ceases to exist. We present a modification of the Green function by adding two types of terms to its lattice sum. The first type are transversely shifted Green functions with coefficients that annihilate the growth in the original lattice sum and yield algebraic convergence. The second type are quasi-periodic plane wave solutions of the Helmholtz...

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