59,475 research outputs found
Lattice Boltzmann Magnetohydrodynamics
Lattice gas and lattice Boltzmann methods are recently developed numerical
schemes for simulating a variety of physical systems. In this paper a new
lattice Boltzmann model for modeling two-dimensional incompressible
magnetohydrodynamics (MHD) is presented. The current model fully utilizes the
flexibility of the lattice Boltzmann method in comparison with previous lattice
gas and lattice Boltzmann
MHD models, reducing the number of moving directions from in other
models to only. To increase computational efficiency, a simple single time
relaxation rule is used for collisions, which directly controls the transport
coefficients.
The bi-directional streaming process of the particle distribution function in
this paper is similar to the original model [ H. Chen and W. H. Matthaeus,
Phys. Rev. Lett., {\bf 58}, 1845(1987), S.Chen, H.Chen, D.Mart\'{\i}nez and
W.H.Matthaeus, Phys. Rev. Lett. {\bf 67},3776 (1991)], but has been greatly
simplified, affording simpler implementation of boundary conditions and
increasing the feasibility of extension into a workable three-dimensional
model. Analytical expressions for the transport coefficients are presented.
Also, as example cases, numerical calculation for the Hartmann flow is
performed, showing a good agreement between the theoreticalComment: 45 pages, to appear in Physics of Plasma
The lattice Boltzmann modeling of two-phase electroosmotic flow in microchannels
Technial Session - 2C Multi-phase Flows(1): no. S2C5In this paper, a numerical framework based on the lattice Boltzmann method is presented for modeling two-phase electroosmotic flow within microchannels. In the model, lattice Boltzmann schemes are designed for all the governing equations involved such as Navier-Stokes equations for momentum transport, Nernst-Planck equations for ion transport, the Cahn-Hilliard equation for the immiscible fluid interface motion, and Poisson equation for the electric potential referring the model proposed in Shao’s work [6]. Related boundary schemes are also proposed to modeling the slip effect on the microchannel surfaces. The theoretical analysis shows that the model has second order accuracy.published_or_final_versio
A Unified Gas-kinetic Scheme for Continuum and Rarefied Flows IV: full Boltzmann and Model Equations
Fluid dynamic equations are valid in their respective modeling scales. With a
variation of the modeling scales, theoretically there should have a continuous
spectrum of fluid dynamic equations. In order to study multiscale flow
evolution efficiently, the dynamics in the computational fluid has to be
changed with the scales. A direct modeling of flow physics with a changeable
scale may become an appropriate approach. The unified gas-kinetic scheme (UGKS)
is a direct modeling method in the mesh size scale, and its underlying flow
physics depends on the resolution of the cell size relative to the particle
mean free path. The cell size of UGKS is not limited by the particle mean free
path. With the variation of the ratio between the numerical cell size and local
particle mean free path, the UGKS recovers the flow dynamics from the particle
transport and collision in the kinetic scale to the wave propagation in the
hydrodynamic scale.
The previous UGKS is mostly constructed from the evolution solution of
kinetic model equations. This work is about the further development of the UGKS
with the implementation of the full Boltzmann collision term in the region
where it is needed. The central ingredient of the UGKS is the coupled treatment
of particle transport and collision in the flux evaluation across a cell
interface, where a continuous flow dynamics from kinetic to hydrodynamic scales
is modeled. The newly developed UGKS has the asymptotic preserving (AP)
property of recovering the NS solutions in the continuum flow regime, and the
full Boltzmann solution in the rarefied regime. In the mostly unexplored
transition regime, the UGKS itself provides a valuable tool for the flow study
in this regime. The mathematical properties of the scheme, such as stability,
accuracy, and the asymptotic preserving, will be analyzed in this paper as
well
Analytical model of nanowire FETs in a partially ballistic or dissipative transport regime
The intermediate transport regime in nanoscale transistors between the fully
ballistic case and the quasi equilibrium case described by the drift-diffusion
model is still an open modeling issue. Analytical approaches to the problem
have been proposed, based on the introduction of a backscattering coefficient,
or numerical approaches consisting in the MonteCarlo solution of the Boltzmann
transport equation or in the introduction of dissipation in quantum transport
descriptions. In this paper we propose a very simple analytical model to
seamlessly cover the whole range of transport regimes in generic quasi-one
dimensional field-effect transistors, and apply it to silicon nanowire
transistors. The model is based on describing a generic transistor as a chain
of ballistic nanowire transistors in series, or as the series of a ballistic
transistor and a drift-diffusion transistor operating in the triode region. As
an additional result, we find a relation between the mobility and the mean free
path, that has deep consequences on the understanding of transport in nanoscale
devices
Electron Monte Carlo Simulations of Nanoporous Si Thin Films -- The Influence of Pore-Edge Charges
Electron transport within nanostructures can be important to varied
engineering applications, such as thermoelectrics and nanoelectronics. In
theoretical studies, electron Monte Carlo simulations are widely used as an
alternative approach to solving the electron Boltzmann transport equation,
where the energy-dependent electron scattering, exact structure shape, and
detailed electric field distribution can be fully incorporated. In this work,
such electron Monte Carlo simulations are employed to predict the electrical
conductivity of periodic nanoporous Si films that have been widely studied for
thermoelectric applications. The focus is on the influence of pore-edge charges
on the electron transport. The results are further compared to our previous
modeling [Hao et al., J. Appl. Phys. 121, 094308 (2017)], where the pore-edge
electric field has its own scattering rate to be added to the scattering rates
of other mechanisms
Core-Collapse Supernovae: Modeling between Pragmatism and Perfectionism
We briefly summarize recent efforts in Garching for modeling stellar core
collapse and post-bounce evolution in one and two dimensions. The transport of
neutrinos of all flavors is treated by iteratively solving the coupled system
of frequency-dependent moment equations together with a model Boltzmann
equation which provides the closure. A variety of progenitor stars, different
nuclear equations of state, stellar rotation, and global asymmetries due to
large-mode hydrodynamic instabilities have been investigated to ascertain the
road to finally successful, convectively supported neutrino-driven explosions.Comment: 8 pages, contribution to Procs. 12th Workshop on Nuclear
Astrophysics, Ringberg Castle, March 22-27, 200
Impact Ionization and Hot-Electron Injection Derived Consistently from Boltzmann Transport
We develop a quantitative model of the impact-ionizationand hot-electron–injection processes in MOS devices from first principles. We begin by modeling hot-electron transport in the drain-to-channel depletion region using the spatially varying Boltzmann transport equation, and we analytically find a self consistent distribution function in a two step process. From the electron distribution function, we calculate the probabilities of impact ionization and hot-electron injection as functions of channel current, drain voltage, and floating-gate voltage. We compare our analytical model results to measurements in long-channel devices. The model simultaneously fits both the hot-electron- injection and impact-ionization data. These analytical results yield an energydependent impact-ionization collision rate that is consistent with numerically calculated collision rates reported in the literature
Implication of the Mott-limit violation in high-Tc cuprates
The Fermi arc is a striking manifestation of the strong-correlation physics
in high-T_c cuprates. In this paper, implications of the metallic transport in
the lightly hole-doped regime of the cuprates, where the Fermi arcs are found,
are examined in conjunction with competing interpretations of the Fermi arcs in
terms of small hole pockets or a large underlying Fermi surface. It is
discussed that the latter picture provides a more natural understanding of the
metallic transport in view of the Mott-limit argument. Furthermore, it is shown
that a suitable modeling of the Fermi arcs in the framework of the Boltzmann
theory allows us to intuitively understand why the transport properties are
apparently determined by a "small" carrier density even when the underlying
Fermi surface, and hence k_F, is large.Comment: 5 pages, 5 figures; manuscript for the Proceedings of SNS2007 to be
published as a Special Issue of the Journal of Physics and Chemistry of
Solid
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