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Numerical investigation of nanoporous evaporation using direct simulation Monte Carlo

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Physical Review Fluids 2019年第11期4卷 113401-113401页

作者： Benzi John Ryan Enright James E. Sprittles Livio Gibelli David R. Emerson Duncan A. Lockerby Scientific Computing Department STFC Daresbury Laboratory Warrington WA4 4AD United Kingdom Efficient Energy Transfer Department (ηET) Nokia Bell Labs Dublin D15 Y6NT Ireland Mathematics Institute University of Warwick Warwick CV4 7AL United Kingdom School of Engineering University of Edinburgh Edinburgh EH9 3FB United Kingdom School of Engineering University of Warwick Warwick CV4 7AL United Kingdom

Evaporation is an effective cooling mechanism widely exploited in the thermal management of modern electronic devices, with a growing interest in the evaporation process in thin-film-based nanoporous membrane technologies. At such scales, classical approaches fail and one requires solutions of the Boltzmann equation; these are obtained here using the direct simulation Monte Carlo method. In particular, the evaporation from representative nanoporous meniscus shapes, corresponding to different operating conditions, has been investigated. Evaporation rates for the different conditions have been characterized as a function of a wide range of Knudsen numbers and free-stream Mach numbers. Additionally, the influence of porosity and evaporation coefficient on the nanoporous evaporation rates has been assessed. Investigations have also been carried out to consider cases where the meniscus has sunk within the pore, and cooling efficacy compared with cases where the meniscus is pinned to the top of the pore. This work demonstrates that the net evaporative mass flux is ultimately determined by the interplay between various physical effects, whose dominance is quantified by the Knudsen number, porosity, evaporation coefficient, and the meniscus shape. This work thus provides useful information for the design of nanoporous membrane-based cooling devices.

关键词： Evaporation Interfacial flows Nanofluidics Rarefied flows

Computation of avoidance regions for driver assistance systems by using a Hamilton-Jacobi approach

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arXiv 2019年

作者： Xausa, Ilaria Baier, Robert Bokanowski, Olivier Gerdts, Matthias Institut Fuer Mathematik Und Rechneranwendung FakulTaet Fuer Luft- Und Raumfahrttechnik Universitaet Der Bundeswehr Muenchen Neubiberg/MuenchenD-85577 Germany Chair of Applied Mathematics University of Bayreuth Bayreuth95440 Germany F-75005 Paris France Sorbonne Université Ljll ParisF-75005 France Institute of Applied Mathematics and Scientific ComputIng Department of Aerospace Engineering Bundeswehr UniVersity Munich Werner-Heisenberg-Weg 39 Neubiberg85577 Germany

We consider the problem of computing safety regions, modeled as nonconvex backward reachable sets, for a nonlinear car collision avoidance model with time-dependent obstacles. The Hamilton-Jacobi-Bellman framework is used. A new formulation of level set functions for obstacle avoidance is given and sufficient conditions for granting the obstacle avoidance on the whole time interval are obtained, even though the conditions are checked only at discrete times. Different scenarios including various road configurations, different geometry of vehicle and obstacles, as well as fixed or moving obstacles, are then studied and computed. Computations involve solving nonlinear partial differential equations of up to five space dimensions plus time with nonsmooth obstacle representations, and an efficient solver is used to this end. A comparison with a direct optimal control approach is also done for one of the examples. Copyright © 2019, The Authors. All rights reserved.

关键词： Collision avoidance

Memristor Crossbar based Implementation of a Multilayer Perceptron

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Memristor Crossbar based Implementation of a Multilayer Perc...

IEEE National Aerospace and Electronics Conference (NAECON)

作者： Yakopcic, Chris Taha, Tarek M. BioTechMed-Graz Graz Austria Institute for Mathematics and Scientific Computing University of Graz 36 Graz Austria Institute of Medical Engineering Graz University of Technology Graz Austria

ISBN: (纸本)9781538632000

This paper describes a memristor-based neuromorphic system that can be used for ex-situ training of various multi-layer neural network algorithms. This system is based on an analog neuron circuit that is capable of performing an accurate dot product calculation. The presented ex-situ programming technique can be used to map many key neural algorithms directly onto the grid of resistances in a memristor crossbar. Using this weight-to-crossbar mapping approach along with the dot product calculation circuit, complex neural algorithms can be easily implemented using this system. To show the effectiveness and versatility of this circuit, a Multilayer Perceptron (MLP) is trained to perform Sobel edge detection. Following these simulations, an analysis was presented that shows how both memristor accuracy and neuron circuit gain relates to output error. Additionally, this paper discusses how circuit noise and neural network layout contribute to testing accuracy.

关键词： Memristors Training Biological neural networks Neurons Neuromorphics Image edge detection Resistance

Nonstandard finite element de Rham complexes on cubical meshes

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arXiv 2018年

作者： Gillette, Andrew Hu, Kaibo Zhang, Shuo Department of Mathematics University of Arizona 617 N. Santa Rita Ave. TucsonAZ United States Department of Mathematics University of Oslo PO Box 1053 Blindern OsloNO 0316 Norway LSEC Institute of Computational Mathematics and Scientific/Engineering Computing Academy of Mathematics and System Sciences National Centre for Mathematics and Interdisciplinary Sciences Chinese Academy of Sciences Beijing100190 China

MSC Codes 65N30We propose two general operations on finite element differential complexes on cubical meshes that can be used to construct and analyze sequences of "nonstandard" convergent methods. The first operation, called DoF-transfer, moves edge degrees of freedom to vertices in a way that reduces global degrees of freedom while increasing continuity order at vertices. The second operation, called serendipity, eliminates interior bubble functions and degrees of freedom locally on each element without affecting edge degrees of freedom. These operations can be used independently or in tandem to create nonstandard complexes that incorporate Hermite, Adini and "trimmed-Adini" elements. We show that the resulting elements provide convergent, non-conforming methods for problems requiring stronger regularity and satisfy a discrete Korn inequality. We discuss potential benefits of applying these elements to Stokes, biharmonic and elasticity problems. Copyright © 2018, The Authors. All rights reserved.

关键词： Finite element method

Search for an isotropic gravitational-wave background with the Parkes Pulsar Timing Array

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arXiv 2023年

作者： Reardon, Daniel J. Zic, Andrew Shannon, Ryan M. Hobbs, George B. Bailes, Matthew Di Marco, Valentina Kapur, Agastya Rogers, Axl F. Thrane, Eric Askew, Jacob Ramesh Bhat, N.D. Cameron, Andrew Curylo, Malgorzata Coles, William A. Dai, Shi Goncharov, Boris Kerr, Matthew Kulkarni, Atharva Levin, Yuri Lower, Marcus E. Manchester, Richard N. Mandow, Rami Miles, Matthew T. Nathan, Rowina S. Oslowski, Stefan Russell, Christopher J. Spiewak, Renée Zhang, Songbo Zhu, Xing-Jiang Centre for Astrophysics and Supercomputing Swinburne University of Technology P.O. Box 218 HawthornVIC3122 Australia OzGrav: The Australian Research Council Centre of Excellence for Gravitational Wave Discovery HawthornVIC3122 Australia Australia Telescope National Facility CSIRO Space and Astronomy P.O. Box 76 EppingNSW1710 Australia Department of Physics and Astronomy MQ Research Centre in Astronomy Astrophysics and Astrophotonics Macquarie University NSW2109 Australia School of Physics and Astronomy Monash University VIC3800 Australia OzGrav: The Australian Research Council Centre of Excellence for Gravitational Wave Discovery ClaytonVIC3800 Australia Institute for Radio Astronomy & Space Research Auckland University of Technology Private Bag 92006 Auckland1142 New Zealand International Centre for Radio Astronomy Research Curtin University BentleyWA6102 Australia Astronomical Observatory University of Warsaw Aleje Ujazdowskie 4 Warsaw00-478 Poland Electrical and Computer Engineering University of California La Jolla San DiegoCA United States School of Science Western Sydney University Locked Bag 1797 Penrith South DCNSW2751 Australia L'AquilaI-67100 Italy INFN Laboratori Nazionali del Gran Sasso AssergiI-67100 Italy Space Science Division US Naval Research Laboratory 4555 Overlook Ave SW WashingtonDC20375 United States Physics Department Columbia Astrophysics Laboratory Columbia University 538 West 120th Street New YorkNY10027 United States Center for Computational Astrophysics Flatiron Institute 162 5th Ave NY10011 United States Manly Astrophysics 15/41-42 East Esplanade ManlyNSW2095 Australia CSIRO Scientific Computing Australian Technology Park Locked Bag 9013 AlexandriaNSW1435 Australia Jodrell Bank Centre for Astrophysics Department of Physics and Astronomy University of Manchester ManchesterM13 9PL United Kingdom Purple Mountain Observatory Chinese Academy of Sciences Nanjing210008 China Advanced Instit

Pulsar timing arrays aim to detect nanohertz-frequency gravitational waves (GWs). A background of GWs modulates pulsar arrival times and manifests as a stochastic process, common to all pulsars, with a signature spatial correlation. Here we describe a search for an isotropic stochastic gravitational-wave background (GWB) using observations of 30 millisecond pulsars from the third data release of the Parkes Pulsar Timing Array (PPTA), which spans 18 years. Using current Bayesian inference techniques we recover and characterize a common-spectrum noise process. Represented as a strain spectrum hc = A(f/1yr−1)α, we measure (Equation presented) and α = −0.45 ± 0.20 respectively (median and 68% credible interval). For a spectral index of α = −2/3, corresponding to an isotropic background of GWs radiated by inspiraling supermassive black hole binaries, we recover an amplitude of (Equation presented). However, we demonstrate that the apparent signal strength is time-dependent, as the first half of our data set can be used to place an upper limit on A that is in tension with the inferred common-spectrum amplitude using the complete data set. We search for spatial correlations in the observations by hierarchically analyzing individual pulsar pairs, which also allows for significance validation through randomizing pulsar positions on the sky. For a process with α = −2/3, we measure spatial correlations consistent with a GWB, with an estimated false-alarm probability of p. 0.02 (approx. 2σ). The long timing baselines of the PPTA and the access to southern pulsars will continue to play an important role in the International Pulsar Timing Array. Copyright © 2023, The Authors. All rights reserved.

关键词： Pulsars

Using machine learning to understand physics graduate school admissions

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arXiv 2019年

作者： Young, Nicholas T. Caballero, Marcos D. Department of Physics and Astronomy Michigan State University East LansingMI48824 United States Department of Computational Mathematics Science and Engineering Michigan State University East LansingMI48824 United States Center for Computing in Science Education Department of Physics University of Oslo OsloN-0316 Norway CREATE for STEM Institute Michigan State University East LansingMI48824 United States

Among all of the first-year graduate students enrolled in doctoral-granting physics departments, the percentage of female and racial minority students has remained unchanged for the past 20 years. The current graduate program admissions process can create challenges for achieving diversity goals in physics. In this paper, we will investigate how the various aspects of a prospective student’s application to a physics doctoral program affect the likelihood the applicant will be admitted. Admissions data was collected from a large, Midwestern public research university that has a decentralized admissions process and included applicants’ undergraduate GPAs and institutions, research interests, and GRE scores. Because the collected data varied in scale, we used supervised machine learning algorithms to create models that predict who was admitted into the PhD program. We find that using only the applicant’s undergraduate GPA and physics GRE score, we are able to predict with 75% accuracy who will be admitted to the program. © 2019, CC BY.

关键词： Students

Tangible reduction in learning sample complexity with large classical samples and small quantum system

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arXiv 2019年

作者： Song, Wooyeong Wieśniak, Marcin Liu, Nana Pawlowski, Marcin Lee, Jinhyoung Kim, Jaewan Bang, Jeongho Department of Physics Hanyang University Seoul04763 Korea Republic of Seoul02792 Korea Republic of Institute of Theoretical Physics and Astrophysics Faculty of Mathematics Physics and Informatics University of Gdańsk Gdańsk80-308 Poland International Centre for Theory of Quantum Technologies University of Gdańsk Gdańsk80-308 Poland Institute of Natural Science Shanghai Jiao Tong University Shanghai200240 China Ministry of Education Key Laboratory in Scientific and Engineering Computing Shanghai Jiao Tong University Shanghai200240 China University of Michigan-Shanghai Jiao Tong University Joint Institute Shanghai200240 China School of Computational Sciences Korea Institute for Advanced Study Seoul02455 Korea Republic of Electronics and Telecommunications Research Institute Daejeon34129 Korea Republic of

Quantum computation requires large classical datasets to be embedded into quantum states in order to exploit quantum parallelism. However, this embedding requires considerable resources in general. It would therefore be desirable to avoid it, if possible, for noisy intermediate-scale quantum (NISQ) implementation. Accordingly, we consider a classical-quantum hybrid architecture, which allows large classical input data, with a relatively small-scale quantum system. This hybrid architecture is used to implement a sampling oracle. It is shown that in the presence of noise in the hybrid oracle, the effects of internal noise can cancel each other out and thereby improve the query success rate. It is also shown that such an immunity of the hybrid oracle to noise directly and tangibly reduces the sample complexity in the framework of computational learning theory. This NISQ-compatible learning advantage is attributed to the oracle’s ability to handle large input features. Copyright © 2019, The Authors. All rights reserved.

关键词： Quantum optics

A New Fully Polynomial Time Approximation Scheme for the Interval Subset Sum Problem

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arXiv 2017年

作者： Diao, Rui Liu, Ya-Feng Dai, Yu-Hong State Key Laboratory of Scientific and Engineering Computing Institute of Computational Mathematics and Scientific/Engineering Computing Academy of Mathematics and Systems Science Chinese Academy of Sciences Beijing100190 China

The interval subset sum problem (ISSP) is a generalization of the well-known subset sum problem. Given a set of intervals {[ai,1, ai,2]}ni=1 and a target integer T, the ISSP is to find a set of integers, at most one from each interval, such that their sum best approximates the target T but cannot exceed it. In this paper, we first study the computational complexity of the ISSP. We show that the ISSP is relatively easy to solve compared to the 0-1 Knapsack problem (KP). We also identify several subclasses of the ISSP which are polynomial time solvable (with high probability), albeit the problem is generally NP-hard. Then, we propose a new fully polynomial time approximation scheme (FPTAS) for solving the general ISSP problem. The time and space complexities of the proposed scheme are O (n max {1/ǫ, log n}) and O (n + 1/ǫ), respectively, where ǫ is the relative approximation error. To the best of our knowledge, the proposed scheme has almost the same time complexity but a significantly lower space complexity compared to the best known scheme. Both the correctness and efficiency of the proposed scheme are validated by numerical simulations. In particular, the proposed scheme successfully solves ISSP instances with n = 100, 000 and ǫ = 0.1% within one second. Copyright © 2017, The Authors. All rights reserved.

关键词： computational complexity

Sparse and switching infinite horizon optimal control with nonconvex penalizations

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arXiv 2018年

作者： Kalise, Dante Kunisch, Karl Rao, Zhiping Department of Mathematics Imperial College London South Kensington Campus LondonSW7 2AZ United Kingdom Institute for Mathematics and Scientific Computing University of Graz Heinrichstraße 36 GrazA-8010 Austria Radon Institute of Computational and Applied Mathematics Austrian Academy of Sciences School of Mathematics and Statistics Wuhan University Wuhan Hubei430072 China

A class of infinite horizon optimal control problems involving mixed quasi-norms of Lp-type cost functionals for the controls is discussed. These functionals enhance sparsity and switching properties of the optimal controls. The existence of optimal controls and their structural properties are analyzed on the basis of first order optimality conditions. A dynamic programming approach is used for numerical realization.93D15, 93B52, 93C05, 93C20 Copyright © 2018, The Authors. All rights reserved.

关键词： Dynamic programming