17,905 research outputs found
Adjoint operator approach to shape design for internal incompressible flows
The problem of determining the profile of a channel or duct that provides the maximum static pressure rise is solved. Incompressible, laminar flow governed by the steady state Navier-Stokes equations is assumed. Recent advances in computational resources and algorithms have made it possible to solve the direct problem of determining such a flow through a body of known geometry. It is possible to obtain a set of adjoint equations, the solution to which permits the calculation of the direction and relative magnitude of change in the diffuser profile that leads to a higher pressure rise. The solution to the adjoint problem can be shown to represent an artificially constructed flow. This interpretation provides a means to construct numerical solutions to the adjoint equations that do not compromise the fully viscous nature of the problem. The algorithmic and computational aspects of solving the adjoint equations are addressed. The form of these set of equations is similar but not identical to the Navier-Stokes equations. In particular some issues related to boundary conditions and stability are discussed
Internal Stresses and Formation of Switchable Nanowires at Thin Silica Film Edge
At vertical edges, thin films of silicon oxide (SiO_{2-x}) contain
semiconductive c-Si layered nanocrystals (Si NC) embedded in and supported by
an insulating g-SiO2 matrix. Tour et al. have shown that a trenched thin film
geometry enables the NC to form switchable nanowires (SNW) when trained by an
applied field. The field required to form SNW decreases rapidly within a few
cycles, or by annealing at 600 C in even fewer cycles, and is stable to 700C.
Here we describe the intrinsic evolution of Si NC and SNW in terms of the
competition between internal stresses and electro-osmosis. The analysis relies
heavily on experimental data from a wide range of thin film studies, and it
explains why a vertical edge across the planar Si-SiOx interface is necessary
to form SNW. The discussion also shows that the formation mechanisms of Si NC
and Si/SiO_{2-x} SNW are intrinsic and result from optimization of nanowire
conductivity in the presence of residual host misfit stresses
Rich variety of defects in ZnO via an attractive interaction between O-vacancies and Zn-interstitials
As the concentration of intrinsic defects becomes sufficiently high in
O-deficient ZnO, interactions between defects lead to a significant reduction
in their formation energies. We show that the formation of both O-vacancies and
Zn-interstitials becomes significantly enhanced by a strong attractive
interaction between them, making these defects an important source of n-type
conductivity in ZnO.Comment: 12 pages, 4 figure
Stochastic Reinforcement Learning
In reinforcement learning episodes, the rewards and punishments are often
non-deterministic, and there are invariably stochastic elements governing the
underlying situation. Such stochastic elements are often numerous and cannot be
known in advance, and they have a tendency to obscure the underlying rewards
and punishments patterns. Indeed, if stochastic elements were absent, the same
outcome would occur every time and the learning problems involved could be
greatly simplified. In addition, in most practical situations, the cost of an
observation to receive either a reward or punishment can be significant, and
one would wish to arrive at the correct learning conclusion by incurring
minimum cost. In this paper, we present a stochastic approach to reinforcement
learning which explicitly models the variability present in the learning
environment and the cost of observation. Criteria and rules for learning
success are quantitatively analyzed, and probabilities of exceeding the
observation cost bounds are also obtained.Comment: AIKE 201
Singlet fermionic dark matter
We propose a renormalizable model of a fermionic dark matter by introducing a
gauge singlet Dirac fermion and a real singlet scalar. The bridges between the
singlet sector and the standard model sector are only the singlet scalar
interaction terms with the standard model Higgs field. The singlet fermion
couples to the standard model particles through the mixing between the standard
model Higgs and singlet scalar and is naturally a weakly interacting massive
particle (WIMP). The measured relic abundance can be explained by the singlet
fermionic dark matter as the WIMP within this model. Collider implication of
the singlet fermionic dark matter is also discussed. Predicted is the elastic
scattering cross section of the singlet fermion into target nuclei for a direct
detection of the dark matter. Search of the direct detection of the dark matter
provides severe constraints on the parameters of our model.Comment: 12 pages, 7 figure
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