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Evaluation of reynolds-averaged Navier-Stokes turbulence models in open channel flow over salmon redds

Journal of Hydrodynamics 2024年第4期36卷 741-756页

作者： Bishal Bhattarai Brandon Hilliard Daniele Tonina William J.reeder ralph Budwig Benjamin T.martin Tao Xing Department of Mechanical Engineering University of IdahoMoscowUSA Center for Ecohydraulics research University of IdahoBoiseUSA Institute for Biodiversity and Ecosystem Dynamics University of AmsterdamAmsterdamThe Netherlands Hamm School of Engineering University of MaryBismarckUSA

This study evaluates computational fluid dynamics(CFD)turbulence closures for reynolds-averaged Navier-Stokes(rANS)equations against experimental data to model complex open channel flows,like those occurring over dune-shaped salmon spawning nests called"redds".Open channel flow complexity,characterized by near-bed turbulence,adverse pressure,and free surfaces,requires suitable turbulence closure capable of capturing the flow structure between streambed and water *** evaluated three rANS models:Standard k-,shear-stress transport(SST)k-@and realizable k-,along with four wall treatments for the realizable k-:Standard,and scalable wall functions,enhanced wall treatment,and an unconventional closure combining standard wall function with near-wall mesh resolving the viscous *** all models generally capturing the bulk flow characteristics,considerable discrepancies were evident in their ability to predict specific flow features,such as flow *** realizable k-model,with standard wall function and mesh resolving viscous sublayer,outperformed other closures in predicting near-wall flow separations,velocity fields,and free surface *** realizable k-model with a log-layer resolved mesh predicted the free surface elevation equally well but lacked precision for near-wall *** SST k-@model outperformed in predicting turbulent kinetic energy and provided better predictions of the near-boundary velocity distributions than realizable k-closure with any of the conventional wall treatments but overestimated the separation vortex *** standard k-o model also overestimated near-wall *** study highlights the variability in accuracy among turbulence models,underlining the need for careful model selection based on specific prediction regions.

关键词： Salmon redd numerical simulation turbulence models volume of fluid(VOF)

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Ballistic tests on hot-rolled Ti-6Al-4V plates:Experiments and numerical approaches

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Defence Technology（防务技术） 2023年第8期26卷 39-53页

作者： Alexander Janda Benjamin James ralph Yael Demarty Marcel Sorger Stefan Ebenbauer Aude Prestl Ingo Siller.martin Stockinger Helmut Clemens Department of Materials Science Montanuniversitat LeobenFranz Josef-Straße 188700 LeobenAustria Department Product Engineering Montanuniversitat LeobenFranz Josef-Straße 188700 LeobenAustria French-German research Institute of Saint-Louis 5 Rue du General Cassagnou68300 Saint-LouisFrance voestalpine BOEHLEr Bleche GmbH&Co KG Boehler-Gasse 18680MürzzuschlagAustria

Superior ballistic performance and the lightweight character of Ti alloys are considered as main reasons for their use in armour applications against a broad spectrum of ballistic threats,***,fragment or blast *** dynamic loading caused by typical penetrators is characterized by high strain rates,only specific test methods allow a closer investigation of the respective material *** the present study,quasi-static and dynamic compression tests as well as ballistic tests were conducted on a twophase a+βalloy Ti-6Al-4V(in m%)manufactured by ***-deformation heat treatments,influencing microstructure and mechanical properties were applied in order to compare three different microstructural configurations:as-rolled,mill-annealed and ***,on the one hand,ballistic tests were employed for the determination of the ballistic limit velocity v_(50),compression tests,on the other hand,delivered essential input parameters for the application of the Johnson-Cook constitutive model in a finite element simulation of the impact *** comparison of experimental results to simulation results was supplemented by means of microstructural characterization of tested samples with the focus set on the prevalently observed deformation and damage mechanisms,as for example adiabatic shearing.

关键词： Ti-6Al-4V Ballistic performance Split Hopkinson pressure bar FE simulation Adiabatic shear bands Intermetallic phase

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Painting patches:reducing flicker in painterly re-rendering of video

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Science China(Information Sciences) 2011年第12期 2612-2621页

作者： martin ralph School of Computer Science and Informatics Cardiff University Cardiff Wales CF24 3AA UK

This paper presents a novel method for re-rendering video in a stroke-based painterly style. Previous methods typically place and adjust strokes on a frame by frame basis, guided by an analysis of motion vectors. Our method constructs painting patches which last for multiple frames, and paints them just once, compositing them after placing and clipping each one in each output frame. Painting patches are constructed by clustering pixels with similar motions, representing moving objects. This is done using a multi-frame window, to take account of objects which are present in consecutive frames, and which may occur a few frames apart with occlusion. The appearance of a given cluster across a sequence of frames is warped to a common reference to produce the painting patch; a global optimization of the warp is used to minimize distortion in the painting strokes. This approach outperforms prior algorithms in problem areas of the image, where flickering typically occurs, while producing comparable results elsewhere. In particular, stable strokes are produced at occlusion boundaries where objects emerge, and at image borders exposed by camera panning. A further advantage is consistent rendering of regions before and after brief occlusion, enhancing temporal stability of the output of discontiguous frames.

关键词： non-photorealistic rendering painterly rendering stroke placement region clustering video ren- dering flicker

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Modulation of ice ages via precession and dust-albedo feedbacks

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Geoscience Frontiers 2016年第6期7卷 891-909页

作者： ralph Ellis Michael Palmer 105-6817 route Pommier St Martin Bellevue 74370France University of Waterloo Department of ChemistryWaterlooOntarioCanada

We present here a simple and novel proposal for the modulation and rhythm of ice-ages and interglacials during the late Pleistocene. While the standard Milankovitch-precession theory fails to explain the long intervals between interglacials, these can be accounted for by a novel forcing and feedback system involving CO2, dust and albedo. During the glacial period, the high albedo of the northern ice sheets drives down global temperatures and CO2 concentrations, despite subsequent precessional forcing maxima. Over the following millennia more CO2 is sequestered in the oceans and atmospheric concentrations eventually reach a critical minima of about 200 ppm, which combined with arid conditions, causes a die-back of temperate and boreal forests and grasslands, especially at high altitude. The ensuing soil erosion generates dust storms, resulting in increased dust deposition and lower albedo on the northern ice sheets. As northern hemisphere insolation increases during the next Milankovitch cycle, the dust-laden ice-sheets absorb considerably more insolation and undergo rapid melting, which forces the climate into an interglacial period. The proposed mechanism is simple, robust, and comprehensive in its scope, and its key elements are well supported by empirical evidence.

关键词： Paleoclimatology Ice-age Precession CO2 Albedo Dust

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Video Structure Analysis

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Tsinghua Science and Technology 2007年第6期12卷 714-718页

作者：张松海张一飞陈韬 Peter M. Hall ralph martin Department of Computer Science and Technology Tsinghua University Department of Computer Science University of Bath Bath BA2 7AY United Kingdom School of Computer Science Cardiff University Wales CF24 3AA United Kingdom

Video structure analysis is a basic requirement for most content-based video editing and processing systems. This paper presents a fast video structure analysis method based on image segmentation in each frame, with region matching between frames. The structure analysis decomposes the video into several moving objects, including information about their colors, positions, shapes, movements, and lifetimes. The method also supports user interactions to improve the results. The result shows that this method is fast and stable and can complete video analyzinq interactivelv.

关键词： video structure analysis video segmentation image segmentation region matching

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