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Zaichik, L. I., N. I. Drobyshevsky, A. S. Filippov, R. V. Mukin et V. F. Strizhov. « A diffusion-inertia model for predicting dispersion and deposition of low-inertia particles in turbulent flows ». International Journal of Heat and Mass Transfer 53, no 1-3 (janvier 2010) : 154–62. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2009.09.044.
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Demenkov, A. G., B. B. Ilyushin, D. Ph Sikovsky, V. F. Strizhov et L. I. Zaichik. « Development of the diffusion-inertia model of particle deposition in turbulent flows ». Journal of Engineering Thermophysics 18, no 1 (mars 2009) : 39–48. http://dx.doi.org/10.1134/s1810232809010056.
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Maloth, Raj Kumar Nayak, Roger E. Khayat et Christopher T. DeGroot. « Bubble Growth in Supersaturated Liquids ». Fluids 7, no 12 (25 novembre 2022) : 365. http://dx.doi.org/10.3390/fluids7120365.
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Bubble formation and dissolution have a wide range of industrial applications, from the production of beverages to foam manufacturing processes. The rate at which the bubble expands or contracts has a significant effect on these processes. In the current work, the hydrodynamics of an isolated bubble expanding due to mass transfer in a pool of supersaturated gas–liquid solution is investigated. The complete scalar transportation equation (advection–diffusion) is solved numerically. It is observed that the present model accurately predicted bubble growth when compared with existing approximated models and experiments. The effect of gas–liquid solution parameters such as inertia, viscosity, surface tension, diffusion coefficient, system pressure, and solubility of the gas has been investigated. It is found that the surface tension and inertia have a very minimal effect during the bubble expansion. However, it is observed that the viscosity, system pressure, diffusion, and solubility have a considerable effect on bubble growth.
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ARABSHAHI, H., REZAEE ROKN-ABADI et S. GOLAFROZ. « COMPARISON OF TWO-VALLEY HYDRODYNAMIC MODEL IN BULK SiC AND ZnO MATERIALS ». Modern Physics Letters B 23, no 23 (10 septembre 2009) : 2807–18. http://dx.doi.org/10.1142/s0217984909020916.
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This report reviews the feasibility of two-dimensional hydrodynamic models in bulk SiC and ZnO semiconductor materials. Although the single-gas hydrodynamic model is superior to the drift-diffusion or energy balance model, it is desirable to direct the efforts of future research in the direction of multi-valley hydrodynamic models. The hydrodynamic model is able to describe inertia effects which play an increasing role in different fields of micro and optoelectronics where simplified charge transport models like the drift-diffusion model and the energy balance model are no longer applicable. Results of extensive numerical simulations are shown for SiC and ZnO materials, which are in fair agreement with other theoretical or experimental methods.
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Meng, Meng, Stefan Z. Miska, Mengjiao Yu et Evren M. Ozbayoglu. « Fully Coupled Modeling of Dynamic Loading of the Wellbore ». SPE Journal 25, no 03 (14 novembre 2019) : 1462–88. http://dx.doi.org/10.2118/198914-pa.
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Summary Loadings acting on a wellbore are more realistically regarded as dynamic rather than static, and the wellbore response under dynamic loading can be different from that under static loading. Under dynamic loading, the inertia term should be considered and the changing rate of loading could induce a change in the mechanical properties of the wellbore, which might compromise wellbore stability and integrity. In this paper, a fully coupled poroelastodynamic model is proposed to study wellbore behavior. This model not only considers fully coupled deformation/diffusion effects, but also includes both solid and fluid inertia terms. The implicit finite-difference method was applied to solve the governing equations, which allows this model to handle all kinds of dynamic loading conditions. After modifying the existing code only slightly, our numerical solution can neglect inertia terms. The numerical results were validated by comparing them to the analytical solution with a simulated sinusoidal boundary condition. To understand this model better, a sensitivity analysis was performed, and the influence of inertia terms was investigated. After that, the model was applied to analyze wellbore stability under tripping operations. The results show that the inertial effect is insignificant for tripping and a fully coupled, quasistatic model is recommended for wellbore stability under tripping operations. The fully coupled poroelastodynamic model should be used for rapid dynamic loading conditions, such as earthquakes and perforations.
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Kuznetsov, Yu I., et A. G. Rzhanov. « A model of injection laser with allowance for the inertia of currier diffusion processes ». Physics of Wave Phenomena 21, no 4 (octobre 2013) : 283–86. http://dx.doi.org/10.3103/s1541308x13040080.
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Drobyshevsky, N. I., L. I. Zaichik, R. V. Mukin, V. F. Strizhov et A. S. Filippov. « Development and application of a diffusion-inertia model for simulating gas-dispersed turbulent flows ». Thermophysics and Aeromechanics 16, no 4 (décembre 2009) : 521–38. http://dx.doi.org/10.1134/s0869864309040039.
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RUYER-QUIL, C., P. TREVELEYAN, F. GIORGIUTTI-DAUPHINÉ, C. DUPRAT et S. KALLIADASIS. « Modelling film flows down a fibre ». Journal of Fluid Mechanics 603 (30 avril 2008) : 431–62. http://dx.doi.org/10.1017/s0022112008001225.
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Consider the gravity-driven flow of a thin liquid film down a vertical fibre. A model of two coupled evolution equations for the local film thickness h and the local flow rate q is formulated within the framework of the long-wave and boundary-layer approximations. The model accounts for inertia and streamwise viscous diffusion. Evolution equations obtained by previous authors are recovered in the appropriate limit. Comparisons to experimental results show good agreement in both linear and nonlinear regimes. Viscous diffusion effects are found to have a stabilizing dispersive effect on the linear waves. Time-dependent computations of the spatial evolution of the film reveal a strong influence of streamwise viscous diffusion on the dynamics of the flow and the wave selection process.
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YOUNG, JOHN, et ANGUS LEEMING. « A theory of particle deposition in turbulent pipe flow ». Journal of Fluid Mechanics 340 (10 juin 1997) : 129–59. http://dx.doi.org/10.1017/s0022112097005284.
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The paper describes a theory of particle deposition based formally on the conservation equations of particle mass and momentum. These equations are formulated in an Eulerian coordinate system and are then Reynolds averaged, a procedure which generates a number of turbulence correlations, two of which are of prime importance. One represents ‘turbulent diffusion’ and the other ‘turbophoresis’, a convective drift of particles down gradients of mean-square fluctuating velocity. Turbophoresis is not a small correction; it dominates the particle dynamic behaviour in the diffusion-impaction and inertia-moderated regimes.Adopting a simple model for the turbophoretic force, the theory is used to calculate deposition from fully developed turbulent pipe flow. Agreement with experimental measurements is good. It is found that the Saffman lift force plays an important role in the inertia-moderated regime but that the effect of gravity on deposition from vertical flows is negligible. The model also predicts an increase in particle concentration close to the wall in the diffusion-impaction regime, a result which is partially corroborated by an independent ‘direct numerical simulation’ study.The new deposition theory represents a considerable advance in physical understanding over previous free-flight theories. It also offers many avenues for future development, particularly in the simultaneous calculation of laminar (pure inertial) and turbulent particle transport in more complex two- and three-dimensional geometries.
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Zaichik, L. I., A. P. Skibin et S. L. Solov'ev. « Simulation of the Distribution of Bubbles in a Turbulent Liquid Using a Diffusion-Inertia Model ». High Temperature 42, no 1 (janvier 2004) : 111–18. http://dx.doi.org/10.1023/b:hite.0000020098.97475.9c.
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Drobyshevsky, N. I., L. I. Zaichik, R. V. Mukin et A. S. Filippov. « Development and application of a diffusion-inertia model for calculating aerosol particle deposition from turbulent flows ». Journal of Engineering Thermophysics 18, no 4 (14 novembre 2009) : 271–78. http://dx.doi.org/10.1134/s181023280904002x.
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Alkuwaiti, Hamda, Hadi Belhaj, Mohammed Aldhuhoori, Bisweswar Ghosh et Ryan Fernandes. « An Extensive Study on Desorption Models Generated Based on Langmuir and Knudsen Diffusion ». Energies 14, no 19 (8 octobre 2021) : 6435. http://dx.doi.org/10.3390/en14196435.
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Although gas desorption is a known phenomenon, modeling fluid flow in tight gas reservoirs often ignores the governing desorption effect, assuming that viscous transport is the predominant controller, resulting in an erroneous prediction of mass transport and fluid flow calculations. Thus, developing a new model accommodating all the major contributing forces in such a medium is essential. This work introduces a new comprehensive flow model suitable for tight unconventional reservoirs, including viscous, inertia, diffusion, and sorption forces, to account for fluid transport. Based on Langmuir law and Knudsen diffusion effect, three models were generated and compared with different known models using synthetic data. The model was solved and analyzed for different scenario cases, and parametric studies were conducted to evaluate the desorption effect on different reservoir types using MATLAB. Results show that the contribution of the sorption mechanism to the flow increases with the reducing permeability of the medium and lower viscosity of the flowing fluid and an additional pressure drop up to 10 psi was quantified.
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Ghiaasiaan, S. M., et G. F. Yao. « A Theoretical Model for Deposition of Aerosols in Rising Spherical Bubbles due to Diffusion, Convection, and Inertia ». Aerosol Science and Technology 26, no 2 (janvier 1997) : 141–53. http://dx.doi.org/10.1080/02786829708965420.
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ROKN-ABADI, M. REZAEE, H. ARABSHAHI et M. R. BENAM. « DISCRETIZATION METHOD OF HYDRODYNAMIC EQUATIONS FOR SIMULATION OF GaN MESFETs ». Modern Physics Letters B 22, no 16 (30 juin 2008) : 1599–608. http://dx.doi.org/10.1142/s0217984908016339.
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A finite discretization method in two dimensions has been developed and used to model electron transport in wurtzite phase GaN MESFETs. The model is based on the solutions of the highly-coupled nonlinear hydrodynamic partial differential equations. These solutions allow us to calculate the electron drift velocity and other device parameters as a function of the applied electric field. This model is able to describe inertia effects which play an increasing role in different fields of micro and optoelectronics where simplified charge transport models like drift-diffusion model and energy balance model are no longer applicable. Results of numerical simulations are shown for a two-dimensional GaN MESFET device which are in fair agreement with other theoretical or experimental methods.
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Zaichik, L. I., R. V. Mukin, L. S. Mukina, V. F. Strizhov et A. S. Filippov. « Development of a diffusion-inertia model for calculating bubble turbulent flows : Isothermal monodispersed flow in a vertical pipe ». High Temperature 50, no 1 (février 2012) : 70–77. http://dx.doi.org/10.1134/s0018151x12010191.
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Zaichik, L. I., R. V. Mukin, L. S. Mukina et V. F. Strizhov. « Development of a diffusion-inertia model for calculating bubble turbulent flows : Isothermal polydispersed flow in a vertical pipe ». High Temperature 50, no 5 (septembre 2012) : 621–30. http://dx.doi.org/10.1134/s0018151x12040220.
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ARABSHAHI, H., et M. R. BENAM. « A SHOCK-CAPTURING UPWIND DISCRETIZATION METHOD FOR CHARACTERIZATION OF SiC MESFETs ». International Journal of Computational Methods 05, no 02 (juin 2008) : 341–49. http://dx.doi.org/10.1142/s0219876208001509.
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A finite difference shock-capturing upwind discretization method in two dimensions is presented in detail for simulation of homogeneous and nonhomogeneous devices. The model is based on the solutions to the highly coupled nonlinear partial differential equations of the full hydrodynamic model. These solutions allow one to calculate the electron drift velocity and other device parameters as a function of the applied electric field. The hydrodynamic model is able to describe inertia effects which play an increasing role in different fields of micro- and optoelectronics where simplified charge transport models like the drift-diffusion model and the energy balance model are no longer applicable. Results of numerical simulations are shown for a two-dimensional SiC MESFET device, and are in fair agreement with other theoretical or experimental methods.
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Tellier, L. N. « From the Weber Problem to a ‘Topodynamic’ Approach to Locational Systems ». Environment and Planning A : Economy and Space 24, no 6 (juin 1992) : 793–806. http://dx.doi.org/10.1068/a240793.
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In any given space, a sequence of interdependent Weber problems of a certain type leads to a pattern of locations which can be mathematically characterized. Conversely, the observed evolution of a given locational system corresponds to certain characteristics of an analogous Weberian locational system. Determining such characteristics leads to the simulating and forecasting of the evolution of the observed locational system. A model corresponding to such a ‘topodynamic’ approach is presented and an application is made. Three different effects are integrated into the model: an interdependency effect which determines the polarization level; an ‘attraction — repulsion’ effect which determines the center — periphery equilibrium; and a distance deterrence effect which determines the diffusion process and the inertia level.
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Gungor, Arif Can, Stefan M. Koepfli, Michael Baumann, Hande Ibili, Jasmin Smajic et Juerg Leuthold. « Modeling Hydrodynamic Charge Transport in Graphene ». Materials 15, no 12 (10 juin 2022) : 4141. http://dx.doi.org/10.3390/ma15124141.
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Graphene has exceptional electronic properties, such as zero band gap, massless carriers, and high mobility. These exotic carrier properties enable the design and development of unique graphene devices. However, traditional semiconductor solvers based on drift-diffusion equations are not capable of modeling and simulating the charge distribution and transport in graphene, accurately, to its full extent. The effects of charge inertia, viscosity, collective charge movement, contact doping, etc., cannot be accounted for by the conventional Poisson-drift-diffusion models, due to the underlying assumptions and simplifications. Therefore, this article proposes two mathematical models to analyze and simulate graphene-based devices. The first model is based on a modified nonlinear Poisson’s equation, which solves for the Fermi level and charge distribution electrostatically on graphene, by considering gating and contact doping. The second proposed solver focuses on the transport of the carriers by solving a hydrodynamic model. Furthermore, this model is applied to a Tesla-valve structure, where the viscosity and collective motion of the carriers play an important role, giving rise to rectification. These two models allow us to model unique electronic properties of graphene that could be paramount for the design of future graphene devices.
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KASMUDIN et A. SULAKSONO. « IMPACTS OF PARAMETERS ADJUSTMENT OF RELATIVISTIC MEAN FIELD MODEL ON NEUTRON STAR PROPERTIES ». International Journal of Modern Physics E 20, no 05 (mai 2011) : 1271–85. http://dx.doi.org/10.1142/s0218301311018320.
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Analysis of the parameters adjustment effects in isovector as well as in isoscalar sectors of effective field based relativistic mean field (E-RMF) model in the symmetric nuclear matter and neutron-rich matter properties has been performed. The impacts of the adjustment on slowly rotating neutron star are systematically investigated. It is found that the mass–radius relation obtained from adjusted parameter set G2** is compatible not only with neutron stars masses from 4U 0614+09 and 4U 1636-536, but also with the ones from thermal radiation measurement in RX J1856 and with the radius range of canonical neutron star of X7 in 47 Tuc, respectively. It is also found that the moment inertia of PSR J073-3039A and the strain amplitude of gravitational wave at the Earth's vicinity of PSR J0437-4715 as predicted by the E-RMF parameter sets used are in reasonable agreement with the extracted constraints of these observations from isospin diffusion data.
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Abdou, M. M. M. « Effect of radiation with temperature dependent viscosity and thermal conductivity on unsteady a stretching sheet through porous media ». Nonlinear Analysis : Modelling and Control 15, no 3 (25 juillet 2010) : 257–70. http://dx.doi.org/10.15388/na.15.3.14322.
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A numerical model is developed to study the effect of thermal radiation on unsteady boundary layer flow with temperature dependent viscosity and thermal conductivity due to a stretching sheet in porous media. The Rosseland diffusion approximation is used to describe the radiative heat flux in the energy equation. The governing equations reduced to similarity boundary layer equations using suitable transformations and then solved using the Runge–Kutta numerical integration, procedure in conjunction with shooting technique. A parametric study illustrating the influence of the radiation R, variable viscosity ε, Darcy number Da, porous media inertia coefficient γ, thermal conductivity κ and unsteady A parameters on skin friction and Nusselt number.
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Moser, Raphael, Chun Xia-Bauer, Johannes Thema et Florin Vondung. « Solar Prosumers in the German Energy Transition : A Multi-Level Perspective Analysis of the German ‘Mieterstrom’ Model ». Energies 14, no 4 (23 février 2021) : 1188. http://dx.doi.org/10.3390/en14041188.
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The expansion of photovoltaics in German cities has so far fallen short of expectations. The concept of ‘tenant electricity’ (‘Mieterstrom’ in German), in which tenants of a building are supplied with solar power produced on site, offers great potential here. A study on behalf of the German Federal Ministry for Economic Affairs and Energy estimated the number of tenant households with good conditions for solar tenant electricity at 3.8 million. At the same time, the federal tenant electricity promotion scheme has been in place since 2017, but only about 1% of the annual budget has been claimed. The aim of this study is to identify the barriers for and drivers of diffusion of the tenant electricity model. To this end, a qualitative document analysis and a range of semi-structured expert interviews have been conducted. The theoretical framework used to guide the analysis is the multi-level perspective. The main barrier found for tenant electricity diffusion is the legal framework on the regime level, which also leads to high transaction costs of implementing tenant electricity. A social barrier is the inertia of some residents to actively concern themselves with their electricity supply and switch to a tenant electricity contract. Among its drivers are long-term trends such as the increasing electricity demand in urban areas, technical developments like blockchain technology and the increasing deployment of smart meters, and the EU Renewable Energy Directive. As long as the restrictive legal framework prevails, the further diffusion of tenant electricity will remain limited.
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Guimond, Stephen R., Jon M. Reisner, Simone Marras et Francis X. Giraldo. « The Impacts of Dry Dynamic Cores on Asymmetric Hurricane Intensification ». Journal of the Atmospheric Sciences 73, no 12 (9 novembre 2016) : 4661–84. http://dx.doi.org/10.1175/jas-d-16-0055.1.
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Abstract The fundamental pathways for tropical cyclone (TC) intensification are explored by considering axisymmetric and asymmetric impulsive thermal perturbations to balanced, TC-like vortices using the dynamic cores of three different nonlinear numerical models. Attempts at reproducing the results of previous work, which used the community WRF Model, revealed a discrepancy with the impacts of purely asymmetric thermal forcing. The current study finds that thermal asymmetries can have an important, largely positive role on the vortex intensification, whereas other studies find that asymmetric impacts are negligible. Analysis of the spectral energetics of each numerical model indicates that the vortex response to asymmetric thermal perturbations is significantly damped in WRF relative to the other models. Spectral kinetic energy budgets show that this anomalous damping is primarily due to the increased removal of kinetic energy from the vertical divergence of the vertical pressure flux, which is related to the flux of inertia–gravity wave energy. The increased kinetic energy in the other two models is shown to originate around the scales of the heating and propagate upscale with time from nonlinear effects. For very large thermal amplitudes (50 K), the anomalous removal of kinetic energy due to inertia–gravity wave activity is much smaller, resulting in good agreement between models. The results of this paper indicate that the numerical treatment of small-scale processes that project strongly onto inertia–gravity wave energy can lead to significant differences in asymmetric TC intensification. Sensitivity tests with different time integration schemes suggest that diffusion entering into the implicit solution procedure is partly responsible for the anomalous damping of energy.
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Yu, Huijun, Wu Zhou, Bei Peng, Xiaoping He, Xiaohong Hao et Zhi Zeng. « Modeling the Boron-Doping Silicon Beam by a Multilayer Model ». Mathematical Problems in Engineering 2014 (2014) : 1–6. http://dx.doi.org/10.1155/2014/894286.
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The boron-doping silicon beam commonly used in microdevices exhibits a nonuniform material property along its thickness or width because of the gradient of boron concentration induced by diffusion process. The constant of rigidity, one of the most important parameters of microbeam, needs to be accurately calculated and designed in the development of high precise sensors and actuators. Current design methods, mainly depending on the analytical solutions derived under the assumption of a uniform material property or some commercial software for a varied property, are not adequate and time consuming to calculate the constant of rigidity of boron-doping silicon beam. A multilayer model is proposed in this paper to replace the continuous solid model by dividing the beam into separated layers glued together. The finite element lamination method is utilized to acquire the equivalent Young modulus and moment of inertia of cross section of multilayer model. The equivalent values are calculated from double-layer structures to multilayer ones based on the small deformation theory and the material mechanics theory. The proposed method provides an effective method to design the stiffness or frequency of microdevice and its results are validated by COMSOL simulation.
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DATTA, S. « THERMODYNAMIC PROPERTIES OF A TRAPPED BOSE GAS : A DIFFUSION MONTE CARLO STUDY ». International Journal of Modern Physics B 22, no 24 (30 septembre 2008) : 4261–73. http://dx.doi.org/10.1142/s021797920804870x.
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We investigate the thermodynamic properties of a trapped Bose gas of Rb atoms interacting through a repulsive potential at low but finite temperature (kBT < μ < Tc) by Quantum Monte Carlo method based upon the generalization of Feynman-Kac method1-3 applicable to many-body systems at T=0 to finite temperatures. In this paper, we report temperature variation of condensation fraction, chemical potential, density profile, total energy of the system, release energy, frequency shifts and moment of inertia within the realistic potential model (Morse type) for the first time by diffusion Monte Carlo technique. The most remarkable success was in achieving the same trend in the temperature variation of frequency shifts as was observed in JILA4 for both m=2 and m=0 modes. For other things, we agree with the work of Giorgini et al.,5 Pitaevskii et al.6 and Krauth.7
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Huang, Jian-Sheng. « Numerical Study of Thermophoresis on Mass Transfer from Natural Convection Flow over a Vertical Porous Medium with Variable Wall Heat Fluxes ». Applied Sciences 11, no 21 (5 novembre 2021) : 10418. http://dx.doi.org/10.3390/app112110418.
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This study investigates heat and mass transfer under natural convection flow along a vertical permeable surface with variable wall heat fluxes through a porous medium. The non-Darcian model is employed for the medium. The effects of suction/blowing, inertia, buoyancy ratio, exponent of heat flux, position parameter, Schmidt number, and thermophoresis are considered. The governing equations of continuity, momentum, energy, and concentration are solved by adopting similarity transformation and Runge–Kutta integration with a shooting technique. Results of interest, such as velocity, temperature, and concentration profiles related to local Nusselt and Sherwood numbers, are obtained for the selected buoyancy ratio at different magnitudes of the thermophoretic effect. The numerical solutions help us to realize the gas diffusion phenomena and control the transport technology.
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Dietze, Georg F., et Christian Ruyer-Quil. « Films in narrow tubes ». Journal of Fluid Mechanics 762 (27 novembre 2014) : 68–109. http://dx.doi.org/10.1017/jfm.2014.648.
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AbstractWe consider the axisymmetric arrangement of an annular liquid film, coating the inner surface of a narrow cylindrical tube, in interaction with an active core fluid. We introduce a low-dimensional model based on the two-phase weighted residual integral boundary layer (WRIBL) formalism (Dietze & Ruyer-Quil, J. Fluid Mech., vol. 722, 2013, pp. 348–393) which is able to capture the long-wave instabilities characterizing such flows. Our model improves upon existing works by fully representing interfacial coupling and accounting for inertia as well as streamwise viscous diffusion in both phases. We apply this model to gravity-free liquid-film/core-fluid arrangements in narrow capillaries with specific attention to the dynamics leading to flooding, i.e. when the liquid film drains into large-amplitude collars that occlude the tube cross-section. We do this against the background of linear stability calculations and nonlinear two-phase direct numerical simulations (DNS). Due to the improvements of our model, we have found a number of novel/salient physical features of these flows. First, we show that it is essential to account for inertia and full interphase coupling to capture the temporal evolution of flooding for fluid combinations that are not dominated by viscosity, e.g. water/air and water/silicone oil. Second, we elucidate a viscous-blocking mechanism which drastically delays flooding in thin films that are too thick to form unduloids. This mechanism involves buckling of the residual film between two liquid collars, generating two very pronounced film troughs where viscous dissipation is drastically increased and growth effectively arrested. Only at very long times does breaking of symmetry in this region (due to small perturbations) initiate a sliding motion of the liquid film similar to observations by Lister et al. (J. Fluid Mech., vol. 552, 2006, pp. 311–343) in thin non-flooding films. This kickstarts the growth of liquid collars anew and ultimately leads to flooding. We show that streamwise viscous diffusion is essential to this mechanism. Low-frequency core-flow oscillations, such as occur in human pulmonary capillaries, are found to set off this sliding-induced flooding mechanism much earlier.
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Elbaz, S. B., et A. D. Gat. « Dynamics of viscous liquid within a closed elastic cylinder subject to external forces with application to soft robotics ». Journal of Fluid Mechanics 758 (7 octobre 2014) : 221–37. http://dx.doi.org/10.1017/jfm.2014.527.
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AbstractViscous flows in contact with elastic structures apply both pressure and shear stress at the solid–liquid interface and thus create internal stress and deformation fields within the solid structure. We study the interaction between the deformation of elastic structures, subject to external forces, and an internal viscous liquid. We neglect inertia in the liquid and solid and focus on viscous flow through a thin-walled slender elastic cylindrical shell as a basic model of a soft robot. Our analysis yields an inhomogeneous linear diffusion equation governing the coupled viscous–elastic system. Solutions for the flow and deformation fields are obtained in closed analytical form. The functionality of the viscous–elastic diffusion process is explored within the context of soft-robotic applications, through analysis of selected solutions to the governing equation. Shell material compressibility is shown to have a unique effect in inducing different flow and deformation regimes. This research may prove valuable to applications such as micro-swimmers, micro-autonomous systems and soft robotics by allowing for the design and control of complex time-varying deformation fields.
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Liu, Rengguang, Shidong Ding et Guoshuai Ju. « Numerical Study of Leakage and Diffusion of Underwater Oil Spill by Using Volume-of-Fluid (VOF) Technique and Remediation Strategies for Clean-Up ». Processes 10, no 11 (9 novembre 2022) : 2338. http://dx.doi.org/10.3390/pr10112338.
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An oil spill accident will cause serious harm to marine ecology and the environment. Rapid response and effective prevention methods are required to minimize the damage of oil spill accidents. The critical problems that marine emergency rescue teams face are when the spilled oil reaches the sea surface, the extent of the spilled oil, and how far they are from the drilling platform. However, there is no reliable model to predict the diffusion distance of spilled oil. Accurately predicting the diffusion characteristics of underwater spilled oil can provide timely and accurate information for the treatment of oil spill accidents and guide the correct implementation of emergency treatment. In this paper, the computational fluid dynamics (CFD) method was used to establish a two-phase flow model for the diffusion of a submarine oil spill. The volume-of-fluid (VOF) technique was implemented to track the interface between oil–water phases. The effects of different parameters on leakage and diffusion characteristics were investigated by adjusting spilled oil velocity, ocean current velocity, crude oil density, and crude oil viscosity. The logarithmic velocity profile was adopted for ocean currents to conform to the actual flow near the sea surface. A user-defined function (UDF) was developed and applied for CFD modeling. The focus was on analyzing the diffusion range (rising height Hp and lateral migration distance Wp) from full-field data. The results indicate that the oil spill velocity, ocean current velocity, crude oil density, and crude oil viscosity impact the viscous shear force, the oil spill’s inertia force, and the current shear effect. The formula for calculating the lateral migration distance of spilled oil under different working conditions was obtained by fitting. The results of this study can provide a scientific basis for formulating an emergency treatment plan for offshore oil spill accidents and minimizing the harm to marine ecology and the environment.
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Tan, C. A., et C. D. Mote. « Analysis of a Hydrodynamic Bearing Under Transverse Vibration of an Axially Moving Band ». Journal of Tribology 112, no 3 (1 juillet 1990) : 514–23. http://dx.doi.org/10.1115/1.2920288.
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This paper presents a mathematical model of the flow and pressure developed in a hydrodynamic guide bearing film under transverse vibration of a translating band. The guide bearing is commonly used in band and circular sawing systems. The perturbed pressure is derived in the frequency domain based on a linearized, incompressible fluid film model. Unsteady fluid inertia, caused by the high frequency vibration modes of the flexible band, is included in the model. The perturbed pressure is generated by two mechanisms, one caused by the band convection, and the other by the band transverse vibration. Moreover, the pressure generations are governed by fluid impedance functions, which characterize the viscous diffusion across the film. Results for both the transient and steady state responses of the film are presented and discussed. It is shown that the pressure component caused by the band convection, which has not been considered in the literature, is important at low frequency. The pressure component caused by the band transverse vibration is dominant at high frequency.
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Rani, Sarma L., Rohit Dhariwal et Donald L. Koch. « A stochastic model for the relative motion of high Stokes number particles in isotropic turbulence ». Journal of Fluid Mechanics 756 (5 septembre 2014) : 870–902. http://dx.doi.org/10.1017/jfm.2014.461.
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AbstractThe probability density function (PDF) kinetic equation describing the relative motion of inertial particle pairs in a turbulent flow requires closure of the phase-space diffusion current. A novel analytical closure for the diffusion current is presented that is applicable to high-inertia particle pairs with Stokes numbers $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}{\mathit{St}}_r \gg 1$. Here ${\mathit{St}}_r$ is a Stokes number based on the time scale $\tau _r$ of eddies whose size scales with pair separation $r$. In the asymptotic limit of ${\mathit{St}}_r \gg 1$, the pair PDF kinetic equation reduces to an equation of Fokker–Planck form. The diffusion tensor characterizing the diffusion current in the Fokker–Planck equation is equal to $1/\tau _v^2$ multiplied by the time integral of the Lagrangian correlation of fluid relative velocities along particle-pair trajectories. Here, $\tau _v$ is the particle viscous relaxation time. Closure of the diffusion tensor is achieved by converting the Lagrangian correlations of fluid relative velocities ‘seen’ by pairs into Eulerian fluid-velocity correlations at pair separations that remain essentially constant during time scales of $O(\tau _r)$; the pair centre of mass, however, is not stationary and responds to eddies with time scales comparable to or smaller than $\tau _v$. For isotropic turbulence, Eulerian fluid-velocity correlations may be expressed as Fourier transforms of the velocity spectrum tensor, enabling us to derive a closed-form expression for the diffusion tensor. A salient feature of this closure is that it has a single, unique form for pair separations spanning the entire spectrum of turbulence scales, unlike previous closures that involve velocity structure functions with different forms for the integral, inertial subrange, and Kolmogorov-scale separations. Using this closure, Langevin equations, which are statistically equivalent to the Fokker–Planck equation, were solved to evolve particle-pair relative velocities and separations in stationary isotropic turbulence. The Langevin equation approach enables the simulation of the full PDF of pair relative motion, instead of only the first few moments of the PDF as is the case in a moments-based approach. Accordingly, PDFs $\varOmega (U|r)$ and $\varOmega (U_r|r)$ are computed and presented for various separations $r$, where the former is the PDF of relative velocity $U$ and the latter is the PDF of the radial component of relative velocity $U_r$, both conditioned upon the separation $r$. Consistent with the direct numerical simulation (DNS) study of Sundaram & Collins (J. Fluid Mech., vol. 335, 1997, pp. 75–109), the Langevin simulations capture the transition of $\varOmega (U|r)$ from being Gaussian at integral-scale separations to an exponential PDF at Kolmogorov-scale separations. The radial distribution functions (RDFs) computed from these simulations also show reasonable quantitative agreement with those from the DNS study of Février, Simonin & Legendre (Proceedings of the Fourth International Conference on Multiphase Flow, New Orleans, 2001).
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Kazmierczak, M., et D. Poulikakos. « Transient Double Diffusion in a Fluid Layer Extending Over a Permeable Substrate ». Journal of Heat Transfer 113, no 1 (1 février 1991) : 148–57. http://dx.doi.org/10.1115/1.2910519.
Texte intégralRésumé :
The problem of transient double diffusion in a composite layer is studied numerically. The composite layer consists of a fluid region extending over a fluid-saturated permeable substrate. Initially, the fluid in the system is motionless, isothermal, and stably stratified with a linear salt distribution. A constant uniform heat flux is then suddenly applied to the bottom wall of the system. The resulting coupled flow, temperature, and concentration fields as they evolve in time are obtained numerically. The flow in the fluid region was determined by solving the complete form of the two-dimensional laminar governing equations subjected to the usual Boussinesq approximations. The flow in the porous region was modeled using the general flow model, which includes both the effects of macroscopic shear (Brinkman effect) and flow inertia (Forchheimer effect). Interesting results were obtained and are presented in a systematic manner so as to document the effect of changing the important system parameters, which include the height of the permeable substrate, its permeability, and the ratio of the thermal to the solutal Rayleigh number. It was found that these parameters had a major impact on the system behavior and their effects are thoroughly discussed.
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Aubert, Julien, et Nicolas Gillet. « The interplay of fast waves and slow convection in geodynamo simulations nearing Earth’s core conditions ». Geophysical Journal International 225, no 3 (10 février 2021) : 1854–73. http://dx.doi.org/10.1093/gji/ggab054.
Texte intégralRésumé :
SUMMARY Ground observatory and satellite-based determinations of temporal variations in the geomagnetic field probe a decadal to annual timescale range where Earth’s core slow, inertialess convective motions and rapidly propagating, inertia-bearing hydromagnetic waves are in interplay. Here we numerically model and jointly investigate these two important features with the help of a geodynamo simulation that (to date) is the closest to the dynamical regime of Earth’s core. This model also considerably enlarges the scope of a previous asymptotic scaling analysis, which in turn strengthens the relevance of the approach to describe Earth’s core dynamics. Three classes of hydrodynamic and hydromagnetic waves are identified in the model output, all with propagation velocity largely exceeding that of convective advection: axisymmetric, geostrophic Alfvén torsional waves, and non-axisymmetric, quasi-geostrophic Alfvén and Rossby waves. The contribution of these waves to the geomagnetic acceleration amounts to an enrichment and flattening of its energy density spectral profile at decadal timescales, thereby providing a constraint on the extent of the $f^{-4}$ range observed in the geomagnetic frequency power spectrum. As the model approaches Earth’s core conditions, this spectral broadening arises because the decreasing inertia allows for waves at increasing frequencies. Through non-linear energy transfers with convection underlain by Lorentz stresses, these waves also extract an increasing amount of energy from the underlying convection as their key timescale decreases towards a realistic value. The flow and magnetic acceleration energies carried by waves both linearly increase with the ratio of the magnetic diffusion timescale to the Alfvén timescale, highlighting the dominance of Alfvén waves in the signal and the stabilizing control of magnetic dissipation at non-axisymmetric scales. Extrapolation of the results to Earth’s core conditions supports the detectability of Alfvén waves in geomagnetic observations, either as axisymmetric torsional oscillations or through the geomagnetic jerks caused by non-axisymmetric waves. In contrast, Rossby waves appear to be too fast and carry too little magnetic energy to be detectable in geomagnetic acceleration signals of limited spatio-temporal resolution.
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de la Fuente, Rebeca, Gábor Drótos, Emilio Hernández-García, Cristóbal López et Erik van Sebille. « Sinking microplastics in the water column : simulations in the Mediterranean Sea ». Ocean Science 17, no 2 (9 mars 2021) : 431–53. http://dx.doi.org/10.5194/os-17-431-2021.
Texte intégralRésumé :
Abstract. We study the vertical dispersion and distribution of negatively buoyant rigid microplastics within a realistic circulation model of the Mediterranean sea. We first propose an equation describing their idealized dynamics. In that framework, we evaluate the importance of some relevant physical effects (inertia, Coriolis force, small-scale turbulence and variable seawater density), and we bound the relative error of simplifying the dynamics to a constant sinking velocity added to a large-scale velocity field. We then calculate the amount and vertical distribution of microplastic particles on the water column of the open ocean if their release from the sea surface is continuous at rates compatible with observations in the Mediterranean. The vertical distribution is found to be almost uniform with depth for the majority of our parameter range. Transient distributions from flash releases reveal a non-Gaussian character of the dispersion and various diffusion laws, both normal and anomalous. The origin of these behaviors is explored in terms of horizontal and vertical flow organization.
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Achatz, Ulrich, et Gerhard Schmitz. « Shear and Static Instability of Inertia–Gravity Wave Packets : Short-Term Modal and Nonmodal Growth ». Journal of the Atmospheric Sciences 63, no 2 (1 février 2006) : 397–413. http://dx.doi.org/10.1175/jas3636.1.
Texte intégralRésumé :
Abstract The problem of nonmodal instabilities of inertia–gravity waves (IGW) in the middle atmosphere is addressed, within the framework of a Boussinesq model with realistic molecular viscosity and thermal diffusion, by singular-vector analysis of horizontally homogeneous vertical profiles of wind and buoyancy obtained from IGW packets at their statically least stable or most unstable horizontal location. Nonmodal growth is always found to be significantly stronger than that of normal modes, most notably at wave amplitudes below the static instability limit where normal-mode instability is very weak, whereas the energy gain between the optimal perturbation and singular vector after one Brunt–Väisälä period can be as large as two orders of magnitude. Among a multitude of rapidly growing singular vectors for this optimization time, small-scale (wavelengths of a few 100 m) perturbations propagating in the horizontal parallel to the IGW are most prominent. These parallel optimal perturbations are amplified by a roll mechanism, while transverse perturbations (with horizontal scales of a few kilometers) are to a large part subject to an Orr mechanism, both controlled by the transverse wind shear in the IGW at its statically least stable altitude, but further enhanced by reduced static stability. The elliptic polarization of the IGW leaves its traces in an additional impact of the roll mechanism via the parallel wind shear on the leading transverse optimal perturbation.
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Ghaddar, Nesreen, Kamel Ghali et Jihad Harathani. « Modulated Air Layer Heat and Moisture Transport by Ventilation and Diffusion From Clothing With Open Aperture ». Journal of Heat Transfer 127, no 3 (1 mars 2005) : 287–97. http://dx.doi.org/10.1115/1.1857949.
Texte intégralRésumé :
A two-dimensional model is developed for the modulated internal airflow, due to walking, in the gap between clothing and skin surface in the presence of clothing apertures. The normal airflow renewing the air layer through the fabric is modeled using the Ghali et al. three-node fabric ventilation model with corrected heat and moisture transport coefficients within the fabric voids to include the diffusion-dominated transport processes in the fabric at low normal flow rates that occur near the open aperture. The parallel flow is induced by a periodic pressure difference between environmental pressure at the aperture of the clothing system and trapped air layer pressure. The parallel flow in the trapped air layer is assumed to be locally governed by the Womersley solution of time-periodic laminar flow in a plane channel. The two-dimensional (2D) model that uses, in the parallel direction, the Womersley flow of the trapped air layer has predicted significantly lower flow rates than a model based on an inertia-free quasi-steady Poisueille flow model (valid only at low ventilation frequencies). In addition, the model predicted lower sensible and latent heat losses from the sweating skin in the presence of open apertures in the clothing system. The percentage drop in total heat loss due to open aperture is 7.52%, and 2.63%, at ventilation frequencies of 25, and 35 revolution per minute, respectively. The reported results showed that under walking conditions, a permeable clothing system with an open aperture reduced heat loss from the skin when compared to a normal ventilation model (closed aperture). These results were consistent with previously published empirical data of Lotens and Danielsson on air layer resistance for open and closed apertures in high air permeable fabrics.
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Shih, Dong-Sin, et Gour-Tsyh Yeh. « Studying Inertia Effects in Open Channel Flow Using Saint-Venant Equations ». Water 10, no 11 (14 novembre 2018) : 1652. http://dx.doi.org/10.3390/w10111652.
Texte intégralRésumé :
One-dimensional (1D) Saint-Venant equations, which originated from the Navier–Stokes equations, are usually applied to express the transient stream flow. The governing equation is based on the mass continuity and momentum equivalence. Its momentum equation, partially comprising the inertia, pressure, gravity, and friction-induced momentum loss terms, can be expressed as kinematic wave (KIW), diffusion wave (DIW), and fully dynamic wave (DYW) flow. In this study, the method of characteristics (MOCs) is used for solving the diagonalized Saint-Venant equations. A computer model, CAMP1DF, including KIW, DIW, and DYW approximations, is developed. Benchmark problems from MacDonald et al. (1997) are examined to study the accuracy of the CAMP1DF model. The simulations revealed that CAMP1DF can simulate almost identical results that are valid for various fluvial conditions. The proposed scheme that not only allows a large time step size but also solves half of the simultaneous algebraic equations. Simulations of accuracy and efficiency are both improved. Based on the physical relevance, the simulations clearly showed that the DYW approximation has the best performance, whereas the KIW approximation results in the largest errors. Moreover, the field non-prismatic case of the Zhuoshui River in central Taiwan is studied. The simulations indicate that the DYW approach does not ensure achievement of a better simulation result than the other two approximations. The investigated cross-sectional geometries play an important role in stream routing. Because of the consideration of the acceleration terms, the simulated hydrograph of a DYW reveals more physical characteristics, particularly regarding the raising and recession of limbs. Note that the KIW does not require assignment of a downstream boundary condition, making it more convenient for field application.
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di Schio, Eugenia Rossi, Abderrahim Mokhefi, Andrea Natale Impiombato et Cesare Biserni. « Numerical Analysis of the Unsteady Mixed Convection of a Nanofluid in a Concentric Tube Heat Exchanger ». Defect and Diffusion Forum 429 (12 décembre 2023) : 13–32. http://dx.doi.org/10.4028/p-1ezhc5.
Texte intégralRésumé :
In the present work, a numerical investigation of the unsteady mixed convection and entropy generation of a nanofluid in an annular cylindrical space is presented using the Buongiorno’s two-phase flow model. It deals with a concentric tube heat exchanger where the inner cylinder rotates with a constant frequency and is maintained at hot temperature, while the outer cylinder is cold. The aim of the present investigation is to highlight the effects of some parameters on the hydrodynamic, thermal and mass behavior of the considered nanofluid as well as on the system irreversibility, namely: the inertia (1 ⩽ Re ⩽ 20), the buoyancy (0 ⩽ Ri ⩽ 5), the mass diffusion (0.1 ⩽ Le ⩽ 10) and the vertical positions of the inner cylinder (-0.4 ⩽ H ⩽ 0.4). Moreover, at specific parameters, an optimal position in terms of heat transfer has been determined. The flow of the nanofluid is two-dimensional and governed by the equations of continuity, momentum, energy as well as volume fraction conservation. After performing a finite element method mesh test and validation with the literature, the Nusselt number and the entropy generation are discussed. The results show that the heat transfer rate and entropy generation increase with increasing values of Richardson and Reynolds number, especially when positioning the inner cylinder in the lower part. On the other hand, the nanoparticles migration under the thermophoretic diffusion decrease with the increase of the Lewis number, which consequent decrease of the heat transfer rate.
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Ireland, Peter J., et Lance R. Collins. « Direct numerical simulation of inertial particle entrainment in a shearless mixing layer ». Journal of Fluid Mechanics 704 (2 juillet 2012) : 301–32. http://dx.doi.org/10.1017/jfm.2012.241.
Texte intégralRésumé :
AbstractWe present the first computational study of the dynamics of inertial particles in a shearless turbulence mixing layer. We parametrize our direct numerical simulations to isolate the effects of turbulence, Reynolds number, particle inertia, and gravity on the entrainment process. By analysing particle concentrations, particle and fluid velocities, particle size distributions, and higher-order velocity moments, we explore the impact of particle inertia and gravity on the mechanism of turbulent mixing. We neglect thermodynamic processes, including phase changes between the drops and surrounding air, which is equivalent to assuming the air is saturated (i.e. 100 % humidity). Entrainment is found to be governed by the large scales of the flow and is relatively insensitive to the Reynolds number over the range considered. Our results show that both fluid and particle velocities exhibit intermittency and that gravity and turbulent diffusion interact in unexpected ways to dictate particle dynamics. An analysis of the temporal evolution of fluid and particle statistics suggests that particle concentration profiles and velocities are self-similar under certain circumstances. We also observe large fluctuations in particle concentrations resulting from entrainment and introduce a model to estimate the impact these fluctuations have on the radial distribution function, a statistic that is often used to quantify inertial particle clustering. Our study is both a computational counterpart to and an extension of the wind tunnel experiments by Gerashchenko, Good & Warhaft (J. Fluid Mech., vol. 668, 2011, pp. 293–303) and Good, Gerashchenko, & Warhaft (J. Fluid Mech., vol. 694, 2012, pp. 371–398). We find good agreement between these experimental studies and our computational results. We anticipate that a better understanding of the role of gravity and turbulence in inertial particle entrainment will lead to improved cloud evolution predictions.
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Roy, Nepal Chandra, et Sadia Siddiqa. « Effect of Nanofluid on Heat Transfer Enhancement for Mixed Convection Flow Over a Corrugated Surface ». Journal of Non-Equilibrium Thermodynamics 45, no 4 (25 octobre 2020) : 373–83. http://dx.doi.org/10.1515/jnet-2020-0008.
Texte intégralRésumé :
AbstractA mathematical model for mixed convection flow of a nanofluid along a vertical wavy surface has been studied. Numerical results reveal the effects of the volume fraction of nanoparticles, the axial distribution, the Richardson number, and the amplitude/wavelength ratio on the heat transfer of Al2O3-water nanofluid. By increasing the volume fraction of nanoparticles, the local Nusselt number and the thermal boundary layer increases significantly. In case of \mathrm{Ri}=1.0, the inclusion of 2 % and 5 % nanoparticles in the pure fluid augments the local Nusselt number, measured at the axial position 6.0, by 6.6 % and 16.3 % for a flat plate and by 5.9 % and 14.5 %, and 5.4 % and 13.3 % for the wavy surfaces with an amplitude/wavelength ratio of 0.1 and 0.2, respectively. However, when the Richardson number is increased, the local Nusselt number is found to increase but the thermal boundary layer decreases. For small values of the amplitude/wavelength ratio, the two harmonics pattern of the energy field cannot be detected by the local Nusselt number curve, however the isotherms clearly demonstrate this characteristic. The pressure leads to the first harmonic, and the buoyancy, diffusion, and inertia forces produce the second harmonic.
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Ramos-Pérez, David. « LA VIVIENDA VACACIONAL EN LA PALMA (CANARIAS) : EVOLUCIÓN Y DISTRIBUCIÓN ESPACIAL (2015-2020) ». Cuadernos de Turismo, no 50 (28 novembre 2022) : 143–81. http://dx.doi.org/10.6018/turismo.541901.
Texte intégralRésumé :
Studies focusing on the diffusion of dwellings for tourist short-term rental outside large cities are almost non-existent in the academic literature. As a result, it is unknown whether the impacts in these territories replicate those observed in urban areas. By exploiting a database of these dwellings from different sources, the article answers some of these questions for the case of the island of La Palma (Canary Islands). The results show the strong inertia that settlement and the traditional distribution model of tourist accommodation have on the location of holiday homes, while the real disruption appears to come from their growth in the island's small urban centres. Los trabajos centrados en la difusión del alquiler de viviendas para uso turístico fuera de las grandes ciudades son casi inexistentes en la literatura académica. De ahí que se desconozca si los impactos en estos territorios replican los observados en los ámbitos urbanos. Explotando una base de datos de estas viviendas elaborada a partir de diferentes fuentes, el artículo responde a algunos de esos interrogantes para el caso de la isla de La Palma (Canarias). Los resultados muestran la fuerte inercia que el poblamiento y el tradicional modelo de distribución del alojamiento turístico ejercen sobre la localización de la vivienda vacacional, mientras la verdadera disrupción aparece vinculada a su eclosión en los pequeños centros urbanos de la isla.
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Mariani, Valerio, Gian Marco Bianchi, Giulio Cazzoli et Stefania Falfari. « A one-dimensional model for the motor oil-fuel dilution under gasoline engine boundary conditions ». E3S Web of Conferences 197 (2020) : 06004. http://dx.doi.org/10.1051/e3sconf/202019706004.
Texte intégralRésumé :
Nowadays the climate change caused by the green-house effect increasing is a world-wide issue with which scientists have to face. Being the passenger vehicles filled with fossil fuels, the emission of the carbon dioxide green-house gas is unavoidable. In order to slowdown both the global warming and the fossil fuels wasting, lower fuel consumptions, thus high efficiency engines, are required. Currently, the coupled use of downsizing and direct injection in spark ignited engines meets both high efficiency and power-demand requirements. Being downsized engines more compact, the distance between the injector and the engine walls are shorter. Thus, depending on the engine operating condition, the fuel spray wall impingement may occur leading to the formation of a liquid fuel film. If the wall impingement occurs against the cylinder liner wall, which is wetted by a thin oil layer, the motor oil and the landed fuel dilute until the piston arrival. At this time, the mass transport by diffusion have promoted the creation of an oil-fuel mixture. Thus, while thicker part of the mixture layer is scraped into the piston top land crevice, the thinner, whose properties are degraded by the fuel contamination, remains on the cylinder liner. The mixture accumulated inside the crevice may be scattered due to the piston inertia leading to the oil detachment and transport in the combustion chamber. The latter may cause both abnormal combustions named preignitions at high loads and increased particulate emissions at low loads, cold states, and cat-heating. This paper reports the implementation of a onedimensional numerical model for the dilution between fuel and motor oil in a stroke of a gasoline engine. The model aims to give the composition of the oil-fuel mixture both on the cylinder liner after the piston arrival and in the piston top land crevice.
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Cirrone, Donatella, Dmitriy Makarov et Vladimir Molkov. « Spontaneous Ignition of Cryo-Compressed Hydrogen in a T-Shaped Channel System ». Hydrogen 3, no 3 (20 août 2022) : 348–60. http://dx.doi.org/10.3390/hydrogen3030021.
Texte intégralRésumé :
Sudden releases of pressurised hydrogen may spontaneously ignite by the so-called “diffusion ignition” mechanism. Several experimental and numerical studies have been performed on spontaneous ignition for compressed hydrogen at ambient temperature. However, there is no knowledge of the phenomenon for compressed hydrogen at cryogenic temperatures. The study aims to close this knowledge gap by performing numerical experiments using a computational fluid dynamics model, validated previously against experiments at atmospheric temperatures, to assess the effect of temperature decrease from ambient 300 K to cryogenic 80 K. The ignition dynamics is analysed for a T-shaped channel system. The cryo-compressed hydrogen is initially separated from the air in the T-shaped channel system by a burst disk (diaphragm). The inertia of the burst disk is accounted for in the simulations. The numerical experiments were carried out to determine the hydrogen storage pressure limit leading to spontaneous ignition in the configuration under investigation. It is found that the pressure limit for spontaneous ignition of the cryo-compressed hydrogen at temperature 80 K is 9.4 MPa. This is more than 3 times larger than pressure limit for spontaneous ignition of 2.9 MPa in the same setup at ambient temperature of 300 K.
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Huang, Jia-Wei, Ming-Liang Zhao, Yu-Ru Zhang, Fei Gao et You-Nian Wang. « Investigation of stochastic heating and its influence on plasma radial uniformity in biased inductively coupled Ar discharges by hybrid simulation ». Physics of Plasmas 30, no 4 (avril 2023) : 043508. http://dx.doi.org/10.1063/5.0142345.
Texte intégralRésumé :
A bias power is usually applied in inductively coupled plasmas (ICP) to realize the separate control of the plasma density and the ion energy. In this research, a two-dimensional fluid/electron Monte Carlo hybrid model is developed to self-consistently investigate the bias effect on the stochastic heating and on the radial homogeneity in a biased argon ICP operated at low pressure (3 mTorr). The results show that the temporal evolution of the stochastic heating exhibits a plateau and a peak when the sheath collapses at high bias voltages, due to the limited sheath heating and the electron inertia. In addition, the plasma density in the diffusion chamber increases with bias voltage and bias frequency, because of the more pronounced stochastic heating both at the substrate and at the grounded wall. In the main discharge chamber, the plasma density decreases with bias voltage, due to the compression of the bulk plasma region, and this trend becomes less obvious at high bias frequency, because of the enhanced power absorption caused by the stochastic heating. Therefore, it is concluded that by tuning the bias voltage and bias frequency, the plasma radial uniformity could be modulated efficiently, which is very important for improving plasma processing.
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Wang, Kun, Wenjin Chen, Shifang Xiao, Jun Chen et Wangyu Hu. « Pattern Formation under Deep Supercooling by Classical Density Functional-Based Approach ». Entropy 25, no 5 (24 avril 2023) : 708. http://dx.doi.org/10.3390/e25050708.
Texte intégralRésumé :
Solidification patterns during nonequilibrium crystallization are among the most important microstructures in the natural and technical realms. In this work, we investigate the crystal growth in deeply supercooled liquid using the classical density functional-based approaches. Our result shows that the complex amplitude expanded phase-field crystal (APFC) model containing the vacancy nonequilibrium effects proposed by us could naturally reproduce the growth front nucleation (GFN) and various nonequilibrium patterns, including the faceted growth, spherulite, symmetric and nonsymmetric dendrites among others, at the atom level. Moreover, an extraordinary microscopic columnar-to-equiaxed transition is uncovered, which is found to depend on the seed spacing and distribution. Such a phenomenon could be attributed to the combined effects of the long-wave and short-wave elastic interactions. Particularly, the columnar growth could also be predicted by an APFC model containing inertia effects, but the lattice defect type in the growing crystal is different due to the different types of short-wave interactions. Two stages are identified during the crystal growth under different undercooling, corresponding to diffusion-controlled growth and GFN-dominated growth, respectively. However, compared with the second stage, the first stage becomes too short to be noticed under the high undercooling. The distinct feature of the second stage is the dramatic increments of lattice defects, which explains the amorphous nucleation precursor in the supercooled liquid. The transition time between the two stages at different undercooling is investigated. Crystal growth of BCC structure further confirms our conclusions.
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Baskharone, E. A. « Finite-Element Analysis of Turbulent Flow in Annular Exhaust Diffusers of Gas Turbine Engines ». Journal of Fluids Engineering 113, no 1 (1 mars 1991) : 104–10. http://dx.doi.org/10.1115/1.2926479.
Texte intégralRésumé :
A finite-element model of the turbulent flow field in the annular exhaust diffuser of a gas turbine engine is developed. The analysis is based on a modified version of the Petrov-Galerkin weighted residual method, coupled with a highly accurate biquadratic finite element of the Lagrangian type. The elemental weight functions in the finite-element formulation are so defined to ensure upwinding of the convection terms in the flow-governing equations while reverting to the conventional Galerkin’s definition for all other terms. This approach is equivalent to altering the integration algorithm as the convection terms in the element equations are derived, with the exception that the latter technique is tailored for low-order elements of the linear and bilinear types. Numerical results of the current analysis indicate that spurious pressure modes associated with this type of inertia-dominated flow are alleviated while the false numerical diffusion in the finite-element equations is simultaneously minimized. Turbulence of the flow field is modeled using the two-layer algebraic turbulence closure of Baldwin and Lomax, and the eddy viscosity calculations are performed at variably spaced points which are different from those in the finite-element discretization model. This enhances the accuracy in computing the wall shear stress and the inner/outer layer interface location. The computational model is verified using a set of experimental data at design and off-design operation modes of the exhaust diffuser in a commercial gas turbine engine. Assessment of the results in this case is favorable and, as such, provides evidence of the model capability as an accurate predictive tool in the diffuser detailed design phase.
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MIKHAILOV, Dmitri, Kevin H. MAYO, Azra PERVIN et Robert J. LINHARDT. « 13C-NMR relation study of heparin—disaccharide interactions with tripeptides GRG and GKG ». Biochemical Journal 315, no 2 (15 avril 1996) : 447–54. http://dx.doi.org/10.1042/bj3150447.
Texte intégralRésumé :
Heparin is a polydisperse sulphated copolymer consisting mostly of 1 → 4 linked glucosamine and uronic acid residues, i.e. 2-deoxy-2-sulphamido-D-glucopyranose 6-sulphate and L-idopyranosyluronic acid 2-sulphate. 13C NMR has been used to study the interactions of heparinase-derived and purified heparin disaccharide with N- and C-terminally-blocked tripeptides GRG and GKG. Titration of the disaccharide with peptide indicates that GRG binds the disaccharide more strongly than does GKG, with interactions in either case being stronger at uronate ring positions. In the presence of GRG, a carboxylate pKa depression suggests electrostatic interactions between the arginine guanidinium group and the uronate carboxylate group. 13C relaxation data have been acquired for all disaccharide and peptide carbons in the presence and absence of GRG and GKG. 13C relaxation rates for the disaccharide are significantly faster in the presence of peptide, especially with GRG. Analysis of these relaxation data has been done in terms of molecular diffusion constants, D⊥ and D‖, and an angle α between D‖ and a molecular frame defined by the moment of inertia tensor calculated for an internally rigid disaccharide. Disaccharide conformational space in these calculations has been sampled for both uronate half-chair forms (2H1 and 1H2) and over a range of glycosidic bond angles defined by motional order parameters and inter-residue nuclear Overhauser effects (±30° from the average). In the absence of peptide, the ratio D⊥/D‖ falls between 0.4 and 0.7; therefore molecular diffusion occurs preferentially about D‖, which runs through both disaccharide rings. In the presence of peptide, D⊥/D‖ is decreased, indicating that GRG is oriented along D‖ and proximal to the uronic acid ring. A model for this is shown.
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Buongiorno, J. « Convective Transport in Nanofluids ». Journal of Heat Transfer 128, no 3 (15 août 2005) : 240–50. http://dx.doi.org/10.1115/1.2150834.
Texte intégralRésumé :
Nanofluids are engineered colloids made of a base fluid and nanoparticles (1-100nm). Nanofluids have higher thermal conductivity and single-phase heat transfer coefficients than their base fluids. In particular, the heat transfer coefficient increases appear to go beyond the mere thermal-conductivity effect, and cannot be predicted by traditional pure-fluid correlations such as Dittus-Boelter’s. In the nanofluid literature this behavior is generally attributed to thermal dispersion and intensified turbulence, brought about by nanoparticle motion. To test the validity of this assumption, we have considered seven slip mechanisms that can produce a relative velocity between the nanoparticles and the base fluid. These are inertia, Brownian diffusion, thermophoresis, diffusiophoresis, Magnus effect, fluid drainage, and gravity. We concluded that, of these seven, only Brownian diffusion and thermophoresis are important slip mechanisms in nanofluids. Based on this finding, we developed a two-component four-equation nonhomogeneous equilibrium model for mass, momentum, and heat transport in nanofluids. A nondimensional analysis of the equations suggests that energy transfer by nanoparticle dispersion is negligible, and thus cannot explain the abnormal heat transfer coefficient increases. Furthermore, a comparison of the nanoparticle and turbulent eddy time and length scales clearly indicates that the nanoparticles move homogeneously with the fluid in the presence of turbulent eddies, so an effect on turbulence intensity is also doubtful. Thus, we propose an alternative explanation for the abnormal heat transfer coefficient increases: the nanofluid properties may vary significantly within the boundary layer because of the effect of the temperature gradient and thermophoresis. For a heated fluid, these effects can result in a significant decrease of viscosity within the boundary layer, thus leading to heat transfer enhancement. A correlation structure that captures these effects is proposed.
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Rehder, Zoé, Thomas Kleinen, Lars Kutzbach, Victor Stepanenko, Moritz Langer et Victor Brovkin. « Simulated methane emissions from Arctic ponds are highly sensitive to warming ». Biogeosciences 20, no 14 (17 juillet 2023) : 2837–55. http://dx.doi.org/10.5194/bg-20-2837-2023.
Texte intégralRésumé :
Abstract. The Arctic is warming at an above-average rate, and small, shallow waterbodies such as ponds are vulnerable to this warming due to their low thermal inertia compared to larger lakes. While ponds are a relevant landscape-scale source of methane under the current climate, the response of pond methane emissions to warming is uncertain. We employ a new, process-based model for methane emissions from ponds (MeEP) to investigate the methane emission response of polygonal-tundra ponds in northeastern Siberia to warming. MeEP is the first dedicated model of pond methane emissions which differentiates between the three main pond types of the polygonal-tundra, ice-wedge, polygonal-center, and merged polygonal ponds and resolves the three main pathways of methane emissions – diffusion, ebullition, and plant-mediated transport. We perform idealized warming experiments, with increases in the mean annual temperature of 2.5, 5, and 7.5 ∘C on top of a historical simulation. The simulations reveal an approximately linear increase in emissions from ponds of 1.33 g CH4 yr−1 ∘C−1 m−2 in this temperature range. Under annual temperatures 5 ∘C above present temperatures, pond methane emissions are more than 3 times higher than now. Most of this emission increase is due to the additional substrate provided by the increased net productivity of the vascular plants. Furthermore, plant-mediated transport is the dominating pathway of methane emissions in all simulations. We conclude that vascular plants as a substrate source and efficient methane pathway should be included in future pan-Arctic assessments of pond methane emissions.
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Ormel, Chris W., et Beibei Liu. « Catching drifting pebbles ». Astronomy & ; Astrophysics 615 (juillet 2018) : A178. http://dx.doi.org/10.1051/0004-6361/201732562.
Texte intégralRésumé :
Turbulence plays a key role in the transport of pebble-sized particles. It also affects the ability of pebbles to be accreted by protoplanets because it stirs pebbles out of the disk midplane. In addition, turbulence suppresses pebble accretion once the relative velocities become too high for the settling mechanism to be viable. Following Paper I, we aim to quantify these effects by calculating the pebble accretion efficiency ε using three-body simulations. To model the effect of turbulence on the pebbles, we derive a stochastic equation of motion (SEOM) applicable to stratified disk configurations. In the strong coupling limit (ignoring particle inertia) the limiting form of this equation agrees with previous works. We conduct a parameter study and calculate ε in 3D, varying pebble and gas turbulence properties and accounting for the planet inclination. We find that strong turbulence suppresses pebble accretion through turbulent diffusion, agreeing closely with previous works. Another reduction of ε occurs when the turbulent rms motions are high and the settling mechanism fails. In terms of efficiency, the outer disk regions are more affected by turbulence than the inner regions. At the location of the H2O iceline, planets around low-mass stars achieve much higher efficiencies. Including the results from Paper I, we present a framework to obtain ε under general circumstances.