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direct numerical simulation of flow over a slotted cylinder at low Reynolds number

APPLIED OCEAN RESEARCH 2019年 87卷 9-25页

作者： Zhu, Hongjun Zhao, Honglei Zhou, Tongming Southwest Petr Univ State Key Lab Oil & Gas Reservoir Geol & Exploita Chengdu 610500 Sichuan Peoples R China Univ Western Australia Dept Civil Environm & Min Engn Crawley WA 6009 Australia

direct numerical simulation was conducted to investigate the flow past a slotted cylinder at low Reynolds number (Re) of 100. The slotting of cylinder affects the boundary layer separation, vortex formation position, recirculation region length and wake width, which are determined by the type of slit. The streamwise slit (SS1), T-shaped slit (SS3) and Y-shaped slit (SS4) act as passive jets, while the transverse slit (SS2) achieves an alternate self-organized boundary layer suction and blowing. The flow rate in slits fluctuates over time due to the alternate vortex shedding and fluctuating pressure distribution around the cylinder surface. One fluctuation cycle of flow rate is caused by a pair of vortices shedding for SS2, SS3 and SS4, while it is created by each vortex shedding for SS1. The wall shear stress and flow impact on the slit wall partly contribute to the hydrodynamic forces acting on the slotted cylinder. Taking into account the internal wall of slit, the transverse slit plays the best role in suppressing the fluid forces with drag reduction of 1.7% and lift reduction of 17%.

关键词： direct numerical simulation Slotted cylinder Vortex suppression Lift reduction

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direct numerical simulation of Turbulence Collapse and Rebirth in Stably Stratified Ekman Flow

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BOUNDARY-LAYER METEOROLOGY 2017年第3期162卷 401-426页

作者： Gohari, S. M. Iman Sarkar, Sutanu Univ Calif San Diego Mech & Aerosp Engn 9500 Gilman Dr La Jolla CA 92093 USA

direct numerical simulations of an Ekman layer are performed to study flow evolution during the response of an initially neutral boundary layer to stable stratification. The Obukhov length, L, is varied among cases by imposing a range of stable buoyancy fluxes at the surface to mimic ground cooling. The imposition of constant surface buoyancy flux , i.e. constant-flux stability, leads to a buoyancy difference between the ground and background that tends to increase with time, unlike the constant-temperature stability case where a constant surface temperature is imposed. The initial collapse of turbulence in the surface layer owing to surface cooling that occurs over a time scale proportional to , where is the friction velocity, is followed by turbulence recovery. The flow accelerates, and a "low-level jet" (LLJ) with inertial oscillations forms during the turbulence collapse. Turbulence statistics and budgets are examined to understand the recovery of turbulence. Vertical turbulence exchange, primarily by pressure transport, is found to initiate fluctuations in the surface layer and there is rebirth of turbulence through enhanced turbulence production as the LLJ shear increases. The turbulence recovery is not monotonic and exhibits temporal intermittency with several collapse/rebirth episodes. The boundary layer adjusts to an increase in the surface buoyancy flux by increased super-geostrophic velocity and surface stress such that the Obukhov length becomes similar among the cases and sufficiently large to allow fluctuations with sustained momentum and heat fluxes. The eventual state of fluctuations, achieved after about two inertial periods (), corresponds to global intermittency with turbulent patches in an otherwise quiescent background. Our simplified configuration is sufficient to identify turbulence collapse and rebirth, global and temporal intermittency, as well as formation of low-level jets, as in observations of the stratified atmospheric boundary layer.

关键词： direct numerical simulation Ekman flow Intermittency Stratified boundary layer Turbulence collapse

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direct numerical simulation of bi-disperse particle-laden gravity currents in the channel configuration

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APPLIED MATHEMATICAL MODELLING 2017年 49卷 739-752页

作者： Francisco, Ezequiel P. Espath, L. F. R. Silvestrini, J. H. Pontificia Univ Catolica Rio Grande do Sul Fac Engn Lab Simulacao Escoamentos Turbulentos Av Ipiranga 6681 Porto Alegre RS Brazil

We present a numerical investigation of bi-disperse particle-laden gravity currents in the lock-exchange configuration. Previous results, based on numerical simulation and laboratory experiments, are used to establish comparisons. Our discussion focuses on explaining how the presence of more than one particle diameter influences the main features of the flow, such as deposit profile, the evolution of the front location and suspended mass. We develop the complete energy budget equation for bi-disperse flows. A set of two and three-dimensional direct numerical simulations (DNS), with different initial compositions of coarse and fine particles, are carried out for Reynolds number equal to 4000. Such simulations show that the energy terms are strongly affected by varying the initial particle fractions. The addition of a small amount of fine particles into a current predominantly composed of coarse particles increases its run-out distance. In particular, it is shown that higher amounts of coarse particles have a dumping effect on the current development. Comparisons show that the two-dimensional simulation does not reproduce the intense turbulence generated in 3D cases accurately, which results in a significant difference in the suspended mass, front position as well as the dissipation term due to the advective motion. (C) 2017 Elsevier Inc. All rights reserved.

关键词： Bi-disperse flows Particle-laden gravity currents direct numerical simulation Energy budget

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direct numerical simulation of turbulence-radiation interactions in a statistically one-dimensional nonpremixed system

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JOURNAL OF QUANTITATIVE SPECTROSCOPY & RADIATIVE TRANSFER 2008年第14期109卷 2391-2400页

作者： Deshmukh, K. V. Modest, M. F. Haworth, D. C. Penn State Univ Dept Mech & Nucl Engn University Pk PA 16802 USA

An important issue in chemically reacting turbulent flows is the interaction between turbulence and radiation (TRI), which arises from highly nonlinear coupling between fluctuations in temperature and species composition of the now field with the fluctuations of radiative intensity. Here direct numerical simulation (DNS) has been employed to investigate TRI in canonical nonpremixed systems in three-dimensional geometries. A photon Monte Carlo method has been used to solve the radiative transfer equation (RTE), which has been coupled with the flow solver. Radiation properties employed here correspond to a nonscattering fictitious gray gas with a Planck-mean absorption coefficient, which mimics that of typical hydrocarbon-air combustion products. Individual contributions of emission and absorption TRI have been isolated and quantified. The temperature self-correlation, the absorption coefficient-Planck function correlation, and the absorption coefficient-intensity correlation have been examined for intermediate-to-large values of the optical thickness, and contributions from all three correlations were found significant but the relative importance of their contribution varies with optical thickness. (C) 2008 Elsevier Ltd. All rights reserved.

关键词： turbulence-radiation interactions direct numerical simulation photon Monte Carlo method

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direct numerical simulation of the effect of compression on the flow, temperature and composition under engine-like conditions

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PROCEEDINGS OF THE COMBUSTION INSTITUTE 2015年第3期35卷 3069-3077页

作者： Schmitt, Martin Frouzakis, Christos E. Tomboulides, Ananias G. Wright, Yuri M. Boulouchos, Konstantinos Swiss Fed Inst Technol Aerothermochem & Combust Syst Lab Zurich Switzerland Univ Western Macedonia Dept Mech Engn Kozani Greece

The effect of compression on the flow, temperature and composition inside a cylinder is investigated using direct numerical simulation (DNS). The initial conditions are obtained by a separate DNS of the intake stroke in an open-valve setup which includes thermal and species mixing. The results show significant changes of the turbulence and temperature fields during compression: The decrease of kinematic viscosity resulting from the increasing pressure results in smaller turbulent length scales and higher dissipation rates. Temperature fluctuations away from the walls decrease slightly during the first half but increase strongly during the second half of the compression stroke towards the Top Dead Center (TDC) due to heat transfer to and from the walls and turbulent transport. At TDC the turbulent flow field is anisotropic, and the axial fluctuation velocity is approximately 30% smaller than the fluctuation velocities in the radial and azimuthal directions. The integral length scale of temperature is approximately 25% higher than the integral length scale of turbulent kinetic energy. The stratification in the species concentration is found to be practically negligible. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

关键词： direct numerical simulation Internal combustion engine HCCI Compression Thermal stratification

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direct numerical simulation of flame/spontaneous ignition interaction fueled with hydrogen under SACI engine conditions

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INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 2017年第6期42卷 3842-3852页

作者： Zhang, F. Yu, R. Bai, X. S. Yao, M. Peng, Z. Tianjin Univ State Key Lab Engines Tianjin Peoples R China Lund Univ Div Fluid Mech Lund Sweden Hertfortshire Univ Sch Engn & Technol Hatfield Hants England

In spark assisted homogeneous charge compression ignition (SACI) engines a premixed flame is first established;the propagating flame results in compression heating of the end gas and finally auto-ignition of the gas. Two dimensional direct numerical simulation (2D DNS) and one dimensional (1D) detailed simulations of flame/spontaneous ignition interaction in a lean hydrogen/air mixture under a constant volume enclosure relevant to SACI engine conditions are performed using detailed chemistry and detailed transport properties. In a 2D outward propagating spherical flame, thermal-diffusive instability is observed and once auto-ignition starts a low temperature region in the preheat zone of the flame is formed. Subsequently, the ignition in the preheat zone is suppressed. Then 1D flame/ignition interactions in H-2/air, syngas/air and methane/air mixtures are studied using detailed numerical simulations. The results reveal that due to preferential diffusion of hydrogen, heat and mass transfer in the preheat zone inhibits the ignition process, yielding a slower reaction rate hence an even lower temperature in the region. It is shown further that the low temperature region is not affected by the domain size and one step chemistry but it will disappear at very low initial temperature conditions. This is due to the absence of auto-ignition under low temperature conditions. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

关键词： direct numerical simulation Flame/ignition interaction Hydrogen Thermal-diffusive instability SACI engines

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direct numerical simulation of Three-Dimensional Richtmyer-Meshkov Instability

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Chinese Physics Letters 2008年第1期25卷 188-190页

作者：傅德薰马延文李新亮 State Key Laboratory of Nonlinear Mechanics Institute of Mechanics Chinese Academy of Sciences Beijing 100080

direct numerical simulation （DNS） is used to study flow characteristics after interaction of a planar shock with a spherical media interface in each side of which the density is different. This interracial instability is known as the Richtmyer-Meshkov （R-M） instability. The compressible Navier-Stoke equations are discretized with group velocity control （GVC） modified fourth order accurate compact difference scheme. Three-dimensional numerical simulations are performed for R-M instability installed passing a shock through a spherical interface. Based on numerical results the characteristics of 3D R-M instability are analysed. The evaluation for distortion of the interface, the deformation of the incident shock wave and effects of refraction, reflection and diffraction are presented. The effects of the interracial instability on produced vorticity and mixing is discussed.

关键词： direct numerical simulation Three-Dimensional Richtmyer-Meshkov Instability group velocity control (GVC)

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direct numerical simulation of water-ethanol flows in a T-mixer

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CHEMICAL ENGINEERING JOURNAL 2017年 324卷 168-181页

作者： Schikarski, Tobias Peukert, Wolfgang Avila, Marc Friedrich Alexander Univ Erlangen Nurnberg Inst Fluid Mech Dept Chem & Biol Engn D-91058 Erlangen Germany Friedrich Alexander Univ Erlangen Nurnberg Inst Particle Technol Dept Chem & Biol Engn D-91058 Erlangen Germany Univ Bremen Ctr Appl Space Technol & Micrograv D-28359 Bremen Germany

The efficient mixing of fluids is key in many applications, such as chemical reactions and nanoparticle precipitation. Detailed experimental measurements of the mixing dynamics are however difficult to obtain, and so predictive numerical tools are helpful in designing and optimizing many processes. If two different fluids are considered, the viscosity and density of the mixture depend often nonlinearly on the composition, which makes the modeling of the mixing process particularly challenging. Hence water-water mixtures in simple geometries such as T-mixers have been intensively investigated, but little is known about the dynamics of more complex mixtures, especially in the turbulent regime. We here present a numerical method allowing the accurate simulation of two-fluid mixtures. Using a high-performance implementation of this method we perform direct numerical simulations resolving the spatial and temporal dynamics of water-ethanol flows for Reynolds numbers from 100 to 2000. The flows states encountered during turbulence transition and their mixing properties are discussed in detail and compared to water-water mixtures. (C) 2017 Elsevier B.V. All rights reserved.

关键词： direct numerical simulation Miscible fluids Water-ethanol Turbulent flows T-mixer Mixing Transition to turbulence

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direct numerical simulation of turbulent wake behind a surface-mounted square cylinder

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JOURNAL OF FLUIDS AND STRUCTURES 2014年 51卷 20-39页

作者： Saeedi, Mohammad LePoudre, Philip P. Wang, Bing-Chen Univ Manitoba Dept Mech Engn Winnipeg MB R3T 5V6 Canada Univ Saskatchewan Dept Mech Engn Saskatoon SK S7N 5A9 Canada

In this research, direct numerical simulation has been performed to study the turbulent wake behind a wall-mounted square cylinder with aspect ratio 4 and Reynolds number 12 000 (based on the free-stream velocity and obstacle side length) in a developing boundary layer. Owing to the relatively high Reynolds number and high aspect ratio of the cylinder tested, the wake is wide spread behind the cylinder and exhibits complex and energetic vortex motions. The lateral and tip vortex shedding patterns at different frequencies, coherent structures downstream of the obstacle, the production rate and distribution of turbulent kinetic energy, and the instantaneous pressure distribution in the wake region have been thoroughly investigated. In order to validate the numerical results, the first-and second-order flow statistics obtained from the simulations have been carefully compared against available wind-tunnel measurement data. (C) 2014 Elsevier Ltd. All rights reserved.

关键词： Turbulence Bluff body flow Vortex shedding direct numerical simulation

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direct numerical simulation of a differentially heated cavity of aspect ratio 4 with Rayleigh numbers up to 10¹¹ - Part II: Heat transfer and flow dynamics

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INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER 2010年第4期53卷 674-683页

作者： Trias, F. X. Gorobets, A. Soria, M. Oliva, A. Tech Univ Catalonia UPC CTTC ETSEIAT Terrassa 08222 Spain

This is the second of a two-part paper on five direct numerical simulations of a differentially heated cavity of aspect ratio 4 with adiabatic horizontal walls (Rayleigh numbers based on the cavity height Ra = 6.4 x 10(8) 2 x 10(9), 10(10), 3 x 10(10) and 10(11), Pr = 0.71). The numerical methods and the time-averaged flow results were presented in the Part I. The heat transfer and the flow dynamics, including the turbulent statistics, the global kinetic energy balances and the internal waves motion phenomenon, are herewith described and discussed. The power-law scalings of the total kinetic dissipation rate and the Nusselt number suggest that a state of transition to a new scaling regime has been reached for the highest Ra. (C) 2009 Elsevier Ltd. All rights reserved.

关键词： direct numerical simulation Differentially heated cavity Natural convection Turbulence

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