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Applications of Vortex Gas Models to Tornadogenesis and Maintenance

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Open Journal of Fluid Dynamics 2017年第4期7卷 596-622页

作者： pavel bělík Douglas P. Dokken Corey k. Potvin kurt Scholz Mikhail M. Shvartsman Department of Mathematics Statistics and Computer Science Augsburg University Minneapolis MN USA Department of Mathematics University of St. Thomas St. Paul MN USA Cooperative Institute for Mesoscale Meteorological Studies School of Meteorology University of Oklahoma NOAA/OAR/National Severe Storms Laboratory Norman OK USA

Processes related to the production of vorticity in the forward and rear flank downdrafts and their interaction with the boundary layer are thought to play a role in tornadogenesis. We argue that an inverse energy cascade is a plausible mechanism for tornadogenesis and tornado maintenance and provides supporting evidence which is both numerical and observational. We apply a three-dimensional vortex gas model to supercritical vortices produced at the surface boundary layer possibly due to interactions of vortices brought to the surface by the rear flank downdraft and also to those related to the forward flank downdraft. Two-dimensional and three-dimensional vortex gas models are discussed, and the three-dimensional vortex gas model of Chorin, developed further by Flandoli and Gubinelli, is proposed as a model for intense small-scale subvortices found in tornadoes and in recent numerical studies by Orf et al. In this paper, the smaller scales are represented by intense, supercritical vortices, which transfer energy to the larger-scale tornadic flows (inverse energy cascade). We address the formation of these vortices as a result of the interaction of the flow with the surface and a boundary layer.

关键词： Vortex Gas Negative Temperature Supercritical Vortices Inverse Energy Cascade Tornadogenesis Tornado Maintenance

Equilibrium Energy and Entropy of Vortex Filaments in the Context of Tornadogenesis and Tornadic Flows

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Open Journal of Fluid Dynamics 2023年第3期13卷 144-176页

作者： pavel bělík Douglas P. Dokken Mikhail M. Shvartsman Eric bibelnieks Robert laskowski Alek lukanen Department of Mathematics Statistics and Computer Science Augsburg University Minneapolis USA Department of Mathematics University of St. Thomas St. Paul USA

In this work, we study approximations of supercritical or suction vortices in tornadic flows and their contribution to tornadogenesis and tornado maintenance using self-avoiding walks on a cubic lattice. We extend the previous work on turbulence by A. Chorin and collaborators to approximate the statistical equilibrium quantities of vortex filaments on a cubic lattice when both an energy and a statistical temperature are involved. Our results confirm that supercritical (smooth, “straight”) vortices have the highest average energy and correspond to negative temperatures in this model. The lowest-energy configurations are folded up and “balled up” to a great extent. The results support A. Chorin’s findings that, in the context of supercritical vortices in a tornadic flow, when such high-energy vortices stretch, they need to fold and transfer energy to the surrounding flow, contributing to tornado maintenance or leading to its genesis. The computations are performed using a Markov Chain Monte Carlo approach with a simple sampling algorithm using local transformations that allow the results to be reliable over a wide range of statistical temperatures, unlike the originally used pivot algorithm that only performs well near infinite temperatures. Efficient ways to compute entropy are discussed and show that a system with supercritical vortices will increase entropy by having these vortices fold and transfer their energy to the surrounding flow.

关键词： Tornadogenesis Supercritical Vortices Vortex Filaments Negative Temperature kinetic Energy Entropy Statistical Mechanics Equilibrium Statistics Self-Avoiding Walks Cubic lattice Monte-Carlo Techniques Pivot Algorithm

Time-headway distribution for random-sequential-update TASEP with periodic and open boundaries

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Journal of Traffic and Transportation Engineering(English Edition) 2020年第1期7卷 30-41页

作者： pavel Hrabák Faculty of Information Technology Czech Technical University in PraguePrague 16000Czechia

The temporal-headway distribution for Totally Asymmetric Simple Exclusion Process(TASEP) with random-sequential update is *** the stationary/steady state of the process,exact formula for the step-headway distribution is derived for conditions when the stationary measure is bernoulli,le.,for periodic boundaries and for open boundaries with entering boundary rate α and leaving boundary rate 0 satisfying α+β=*** step-headway formula for general values of boundary rates is calculated numerically by means of the matrix product *** formula is applicable mainly for the model defined on finite small lattice representing short segment of complex *** this case the dependency of the motion of individual particles is noticeable and cannot be *** finite lattice results are compared to continuous time distribution obtained by mans of the large l limit It can be observed that the scaled distribution converges quite fast to continuous time ***,in the case of rather small lattice the distribution significantly differs from the limiting ***,in the case of bernoulli stationary measure,the distribution is not dependent on the position of the reference site on the *** general values of boundary parameters,the shape of the distribution is influenced by the density profile of the process near *** influence vanishes with increasing lattice size.

关键词： Transportation TASEP Time-headway distribution Random-sequential update Periodic and open boundaries

Patterning Nanoroads and Quantum Dots on Fluorinated Graphene

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Nano Research 2011年第1期4卷 143-152页

作者： Morgana A. Ribas Abhishek k. Singh pavel b. Sorokin boris I. Yakobson Department of Mechanical Engineering and Materials Science and Department of Chemistry Rice University Houston Texas 77005 USA Materials Research Centre Indian Institlzte of Science Bangalore 560012 India

Using ab initio methods we have investigated the fluorination of graphene and find that different stoichiometric phases can be formed without a nucleation barrier, with the complete ＂2D-Teflon＂ CF phase being thermody- namically most stable. The fluorinated graphene is an insulator and turns out to be a perfect matrix-host for patterning nanoroads and quantum dots of pristine graphene. The electronic and magnetic properties of the nanoroads can be tuned by varying the edge orientation and width. The energy gaps between the highest occupied and lowest unoccupied molecular orbitals （HOMO-lUMO） of quantum dots are size-dependent and show a confinement typical of Dirac fermions. Furthermore, we study the effect of different basic coverage of F on graphene （with stoichiometries CF and C4F） on the band gaps, and show the suitability of these materials to host quantum dots of graphene with unique electronic properties.

关键词： Graphene fluorinated graphene fluorographene nanoroads quantum dots

Defect aggregates in the Sr_2MgSi_2O_7 persistent luminescence material

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Journal of Rare Earths 2011年第12期29卷 1130-1136页

作者： Jorma Hls Taneli laamanen Mika lastusaari pavel Novák Department of Chemistry University of Turku Turku University Centre for Materials and Surfaces(MatSurf) Graduate School of Materials Research(GSMR) Institute of Physics Academy of Sciences of the Czech Republic

The crystal and electronic structure of the Eu^2＋ doped and defect containing Sr2MgSi2O7 persistent luminescence material were studied using the density functional theory （DFT）. The defects may act as energy storage or even luminescence quenching centres in these materials, however their role is very difficult to confirm experimentally. The probability of vacancy formation was studied using the total energy of the defect containing host. Significant structural modifications in the environment of the isolated defects, especially the strontium vacancy, as well as defect aggregates were found. The experimental band gap energy of Sr2MgSi2O7 was well reproduced by the calculations. The defect induced electron traps close to the host＇s conduction band were found to act as energy storage sites contributing to its efficient persistent luminescence. The interactions between the defects were found to modify both the Eu^2＋ 4f^7 ground state energy as well as the trap structure. The effect of charge compensation induced by the rare earth co-doping on the defect structure and energy storage properties of the persistent luminescence materials was discussed.

关键词： strontium magnesium disilicate vacancy defect aggregate persistent luminescence density functional theory calculations rare earths

Interaction of water droplets with pyrolyzing coal particles and tablets

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Defence Technology（防务技术） 2023年第12期30卷 1-12页

作者： Anastasia Islamova pavel Tkachenko pavel Strizhak National Research Tomsk Polytechnic University Heat and Mass Transfer Laboratory30 Lenin AvenueTomsk634050Russia

The paper presents the experimental research findings for the patterns of collisions of water droplets with pressed tableted samples used as substrates and with small particles of a pyrolyzing solid *** coal samples were ***-substrate interactions were studied when varying the droplet diameter in the range from 1 to 4 mm and velocity from 0.5 to 4 m/*** corresponded to the Weber number range of *** coal tablet surface temperature was varied from 20 to 700℃.In the interactions of water droplets(0.7-1.5 mm in diameter,pre-collision velocity from 1 to 3 m/s)with coal particles(with a size of 0.2-1 mm,pre-collision velocity 0.7-2 m/s),the temperature of the latter was varied in the range of 330-480℃.The following regimes of the interaction of droplets with solid particles during chemical reactions and phase transformations were distinguished:spreading/agglomeration and break-up/*** in the characteristics of the interaction of water droplets with coal particles at varying temperatures were ***-particle interaction regime maps for b(We),We(Oh)and We(Ca)were *** collision regime boundaries were described using fitted curves that can be utilized to develop the existing mathematical models of droplet-particle collisions in *** was established that the gaseous volatile production in coal pyrolysis has a modest effect on the regimes and characteristics of the droplet destruction in the temperature range under consideration(20-700℃).

关键词： Droplet Particle Solid substrate Collision Agglomeration Separation

Crystal field and magnetism with Wannier functions: rare-earth doped aluminum garnets

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Journal of Rare Earths 2015年第12期33卷 1316-1323页

作者： Eva Mihóková pavel Novák Valentin V.laguta Institute of Physics Czech Academy of Sciences

Using the recently developed method we calculated the crystal field parameters m yttrium anct lutetium aluminum garnets doped with seven trivalent krarners rare-earth ions. We then inserted calculated parameters into the atomic-like Hamiltonian taking into account the electron-electron, spin-orbit and Zeeman interactions and determined the multiplet splitting by the crystal field as well as magnetic tensors. We compared calculated results with available experimental data. Very good agreement with the spectroscopic data and qualitative agreement with experimental tensors was found.

关键词： crystal field ab initio calculations gamets rare earths

Influence of stress path change on the resistance to plastic deformation of cold rolled sheets

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Journal of University of Science and Technology beijing 2005年第6期12卷 521-526页

作者： pavel Huml Metal Forming Department of Materials Science and Engineering The Royal Institute of Technology 10044 Stockholm Sweden

Flat workpieces have been tested in order to investigate the influence of stress path change （loading mode） while keeping strain path unchanged. These investigations are pertinent to the testing of cold rolled strips and to subsequent forming. The workpieces which first compressed by plane strain compression in thickness direction were then tested in perpendicular direction in order to measure the influence of strain and stress path. The tension workpieces came from flat die compression test at different deformation histories. Two different materials were investigated： 18/8 Ti stainless steel and AW-1050 aluminium. The results show that the plastic flow by tension in lengthwise direction after pre-strain by compression in thickness direction will begin at an appreciably lower stress than that of the workpieces unloaded after pre-compression. Comparing with two materials, it can be seen that both 18/8 Ti stainless steel and AW-1050 aluminium behave similarly. The drop in yield stress is lower for AW-1050 aluminium than that for 18/8 Ti stainless steel. However, reloading in different directions than in the precious step results in significantly higher strain hardening.

关键词： cold working plastic deformation strain path stress path loading mode flow stress yield stress strain hardening

Evaluation of the seismic hazard for 20 cities in Romania using Monte Carlo based simulations

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Earthquake Engineering and Engineering Vibration 2017年第3期16卷 513-523页

作者： Florin pavel Radu Vacareanu Seismic Risk Assessment Research Center Technical University of Civil Engineering Bucharest Romania

This work focuses on the evaluation of the seismic hazard for Romania using earthquake catalogues generated by a Monte Carlo approach. The seismicity of Romania can be attributed to the Vrancea intermediate-depth seismic source and to 13 other crustal seismic sources. The recurrence times of large magnitude seismic events（both crustal and subcrustal）, as well as the moment release rates are computed using simulated earthquake catalogues. The results show that the largest contribution to the overall moment release for the crustal seismic sources is from the seismic regions in bulgaria, while the seismic regions in Romania contribute less than 5% of the overall moment release. In addition, the computations show that the moment release rate for the Vrancea subcrustal seismic source is about ten times larger than that of all the crustal seismic sources. Finally, the Monte Carlo approach is used to evaluate the seismic hazard for 20 cities in Romania with populations larger than 100,000 inhabitants. The results show some differences between the seismic hazard values obtained through Monte-Carlo simulation and those in the Romanian seismic design code P100-1/2013, notably for cities situated in the western part of Romania that are influenced by local crustal seismic sources.

关键词： seismic sources uncertainty seismic moment GMPE percentile