5,405 research outputs found
Variance Reduction For A Discrete Velocity Gas
We extend a variance reduction technique developed by Baker and Hadjiconstantinou [1] to a discrete velocity gas. In our previous work, the collision integral was evaluated by importance sampling of collision partners [2]. Significant computational effort may be wasted by evaluating the collision integral in regions where the flow is in equilibrium. In the current approach, substantial computational savings are obtained by only solving for the deviations from equilibrium. In the near continuum regime, the deviations from equilibrium are small and low noise evaluation of the collision integral can be achieved with very coarse statistical sampling. Spatially homogenous relaxation of the Bobylev-Krook-Wu distribution [3,4], was used as a test case to verify that the method predicts the correct evolution of a highly non-equilibrium distribution to equilibrium. When variance reduction is not used, the noise causes the entropy to undershoot, but the method with variance reduction matches the analytic curve for the same number of collisions. We then extend the work to travelling shock waves and compare the accuracy and computational savings of the variance reduction method to DSMC over Mach numbers ranging from 1.2 to 10.Aerospace Engineering and Engineering Mechanic
Anomalous Defects and Their Quantized Transverse Conductivities
Using a description of defects in solids in terms of three-dimensional
gravity, we study the propagation of electrons in the background of
disclinations and screw dislocations. We study the situations where there are
bound states that are effectively localized on the defect and hence can be
described in terms of an effective 1+1 dimensional field theory for the low
energy excitations. In the case of screw dislocations, we find that these
excitations are chiral and can be described by an effective field theory of
chiral fermions. Fermions of both chirality occur even for a given direction of
the magnetic field. The ``net'' chirality of the system however is not always
the same for a given direction of the magnetic field, but changes from one sign
of the chirality through zero to the other sign as the Fermi momentum or the
magnitude of the magnetic flux is varied. On coupling to an external
electromagnetic field, the latter becomes anomalous, and predicts novel
conduction properties for these materials.Comment: New material added. ReVTeX , 31 pgs., 4 figs.(uses epsf
a study of a megalitic complex in Central Kerala, south India
This thesis is a study of the Iron Age / megalithic burial sites of central Kerala, south
India. The thesis organizes the scattered information we have on the Iron Age burials of
central Kerala alongside the theoretical studies on the same, so as to identify the general
trends and the major gaps. Specifically it takes up the problematic of spatial
organization, a largely ignored theme in the studies on the Iron Age of Kerala. The base
assumption of the work is that space is a dynamic concept that is experientially
constituted and can be restructured. Spaces, especially symbolic architectural spaces
like megalithic monuments, may represent power in terms of visual dimensions of
domination, through visibility, by the division of space, by the privilege of inclusion, or
by exclusion from the knowledge repre W
N K
are examined in order to see how space was organized by the builders of the monuments
at inter regional, inter-site and intra site levels. The thesis is a pilot study that initiates
an effort to bring the concepts of spatial organisation and landscape relations to the
centre of the discussion on the Iron Age of Kerala, and offers certain practical
guidelines to generate data that facilitates such a discussionInstituto Politécnico de Tomar. UTA
Far Field Deposition Of Scoured Regolith Resulting From Lunar Landings
As a lunar lander approaches a dusty surface, the plume from the descent engine impinges on the ground, entraining loose regolith into a high velocity dust spray. Without the inhibition of a background atmosphere, the entrained regolith can travel many kilometers from the landing site. In this work, we simulate the flow field from the throat of the descent engine nozzle to where the dust grains impact the surface many kilometers away. The near field is either continuum or marginally rarefied and is simulated via a loosely coupled hybrid DSMC - Navier Stokes (DPLR) solver. Regions of two-phase and polydisperse granular flows are solved via DSMC. The far field deposition is obtained by using a staged calculation, where the first stages are in the near field where the flow is quasi-steady and the outer stages are unsteady. A realistic landing trajectory is approximated by a set of discrete hovering altitudes which range from 20m to 3m. The dust and gas motions are fully coupled using an interaction model that conserves mass, momentum, and energy statistically and inelastic collisions between dust particles are also accounted for. Simulations of a 4 engine configuration are also examined, and the erosion rates as well as near field particle fluxes are discussed.Astronom
Transition to the ultimate regime in two-dimensional Rayleigh-B\'enard convection
The possible transition to the so-called ultimate regime, wherein both the
bulk and the boundary layers are turbulent, has been an outstanding issue in
thermal convection, since the seminal work by Kraichnan [Phys. Fluids 5, 1374
(1962)]. Yet, when this transition takes place and how the local flow induces
it is not fully understood. Here, by performing two-dimensional simulations of
Rayleigh-B\'enard turbulence covering six decades in Rayleigh number Ra up to
for Prandtl number Pr , for the first time in numerical
simulations we find the transition to the ultimate regime, namely at
. We reveal how the emission of thermal plumes enhances
the global heat transport, leading to a steeper increase of the Nusselt number
than the classical Malkus scaling [Proc.
R. Soc. London A 225, 196 (1954)]. Beyond the transition, the mean velocity
profiles are logarithmic throughout, indicating turbulent boundary layers. In
contrast, the temperature profiles are only locally logarithmic, namely within
the regions where plumes are emitted, and where the local Nusselt number has an
effective scaling , corresponding to the
effective scaling in the ultimate regime.Comment: 6 pages, 4figure
Model based methodology development for energy recovery in ash heat exchange systems
Flash tank evaporation combined with a condensing heat exchanger can be used when heat exchange is required between two streams and where at least one of these streams is difficult to handle (in terms of solid particles content, viscosity, pH, consistency etc.). To increase the efficiency of heat exchange, a cascade of these units in series can be used. Heat transfer relationships in such a cascade are very complex due to their interconnectivity, thus the impact of any changes proposed is difficult to predict. In this report, a mathematical model of a single unit ash tank evaporator combined with a condensing heat exchanger unit is proposed. This model is then developed for a chain of the units. The purpose of this model is to allow an accurate evaluation of the effect and result of an alteration to the system. The resulting model
is applied to the RUSAL Aughinish Alumina digester area
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