18,003 research outputs found
Unsteady aerodynamic effects in small-amplitude pitch oscillations of an airfoil
High-fidelity wall-resolved large-eddy simulations (LES) are utilized to
investigate the flow-physics of small-amplitude pitch oscillations of an
airfoil at Re = 100,000. The investigation of the unsteady phenomenon is done
in the context of natural laminar flow airfoils, which can display sensitive
dependence of the aerodynamic forces on the angle of attack in certain
"off-design" conditions. The dynamic range of the pitch oscillations is chosen
to be in this sensitive region. Large variations of the transition point on the
suction-side of the airfoil are observed throughout the pitch cycle resulting
in a dynamically rich flow response. Changes in the stability characteristics
of a leading-edge laminar separation bubble has a dominating influence on the
boundary layer dynamics and causes an abrupt change in the transition location
over the airfoil. The LES procedure is based on a relaxation-term which models
the dissipation of the smallest unresolved scales. The validation of the
procedure is provided for channel flows and for a stationary wing at Re =
400,000.Comment: 37 pages. 19 figure
On self-sustained oscillations in two-dimensional compressible flow over rectangular cavities
Numerical simulations are used to investigate the resonant instabilities in two-dimensional flow past an open cavity. The compressible Navier–Stokes equations are solved directly (no turbulence model) for cavities with laminar boundary layers upstream. The computational domain is large enough to directly resolve a portion of the radiated acoustic field, which is shown to be in good visual agreement with schlieren photographs from experiments at several different Mach numbers. The results show a transition from a shear-layer mode, primarily for shorter cavities and lower Mach numbers, to a wake mode for longer cavities and higher Mach numbers. The shear-layer mode is characterized well by the acoustic feedback process described by Rossiter (1964), and disturbances in the shear layer compare well with predictions based on linear stability analysis of the Kelvin–Helmholtz mode. The wake mode is characterized instead by a large-scale vortex shedding with Strouhal number independent of Mach number. The wake mode oscillation is similar in many ways to that reported by Gharib & Roshko (1987) for incompressible flow with a laminar upstream boundary layer. Transition to wake mode occurs as the length and/or depth of the cavity becomes large compared to the upstream boundary-layer thickness, or as the Mach and/or Reynolds numbers are raised. Under these conditions, it is shown that the Kelvin–Helmholtz instability grows to sufficient strength that a strong recirculating flow is induced in the cavity. The resulting mean flow is similar to wake profiles that are absolutely unstable, and absolute instability may provide an explanation of the hydrodynamic feedback mechanism that leads to wake mode. Predictive criteria for the onset of shear-layer oscillations (from steady flow) and for the transition to wake mode are developed based on linear theory for amplification rates in the shear layer, and a simple model for the acoustic efficiency of edge scattering
A Conversation with Shayle R. Searle
Born in New Zealand, Shayle Robert Searle earned a bachelor's degree (1949)
and a master's degree (1950) from Victoria University, Wellington, New Zealand.
After working for an actuary, Searle went to Cambridge University where he
earned a Diploma in mathematical statistics in 1953. Searle won a Fulbright
travel award to Cornell University, where he earned a doctorate in animal
breeding, with a strong minor in statistics in 1959, studying under Professor
Charles Henderson. In 1962, Cornell invited Searle to work in the university's
computing center, and he soon joined the faculty as an assistant professor of
biological statistics. He was promoted to associate professor in 1965, and
became a professor of biological statistics in 1970. Searle has also been a
visiting professor at Texas A&M University, Florida State University,
Universit\"{a}t Augsburg and the University of Auckland. He has published
several statistics textbooks and has authored more than 165 papers. Searle is a
Fellow of the American Statistical Association, the Royal Statistical Society,
and he is an elected member of the International Statistical Institute. He also
has received the prestigious Alexander von Humboldt U.S. Senior Scientist
Award, is an Honorary Fellow of the Royal Society of New Zealand and was
recently awarded the D.Sc. Honoris Causa by his alma mater, Victoria University
of Wellington, New Zealand.Comment: Published in at http://dx.doi.org/10.1214/08-STS259 the Statistical
Science (http://www.imstat.org/sts/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Chapter 10: Algebraic Algorithms
Our Chapter in the upcoming Volume I: Computer Science and Software
Engineering of Computing Handbook (Third edition), Allen Tucker, Teo Gonzales
and Jorge L. Diaz-Herrera, editors, covers Algebraic Algorithms, both symbolic
and numerical, for matrix computations and root-finding for polynomials and
systems of polynomials equations. We cover part of these large subjects and
include basic bibliography for further study. To meet space limitation we cite
books, surveys, and comprehensive articles with pointers to further references,
rather than including all the original technical papers.Comment: 41.1 page
Quantum Computation and Spin Electronics
In this chapter we explore the connection between mesoscopic physics and
quantum computing. After giving a bibliography providing a general introduction
to the subject of quantum information processing, we review the various
approaches that are being considered for the experimental implementation of
quantum computing and quantum communication in atomic physics, quantum optics,
nuclear magnetic resonance, superconductivity, and, especially, normal-electron
solid state physics. We discuss five criteria for the realization of a quantum
computer and consider the implications that these criteria have for quantum
computation using the spin states of single-electron quantum dots. Finally, we
consider the transport of quantum information via the motion of individual
electrons in mesoscopic structures; specific transport and noise measurements
in coupled quantum dot geometries for detecting and characterizing
electron-state entanglement are analyzed.Comment: 28 pages RevTeX, 4 figures. To be published in "Quantum Mesoscopic
Phenomena and Mesoscopic Devices in Microelectronics," eds. I. O. Kulik and
R. Ellialtioglu (NATO Advanced Study Institute, Turkey, June 13-25, 1999
Absolute linear instability in laminar and turbulent gas/liquid two-layer channel flow
We study two-phase stratified flow where the bottom layer is a thin laminar
liquid and the upper layer is a fully-developed gas flow. The gas flow can be
laminar or turbulent. To determine the boundary between convective and absolute
instability, we use Orr--Sommerfeld stability theory, and a combination of
linear modal analysis and ray analysis. For turbulent gas flow, and for the
density ratio r=1000, we find large regions of parameter space that produce
absolute instability. These parameter regimes involve viscosity ratios of
direct relevance to oil/gas flows. If, instead, the gas layer is laminar,
absolute instability persists for the density ratio r=1000, although the
convective/absolute stability boundary occurs at a viscosity ratio that is an
order of magnitude smaller than in the turbulent case. Two further unstable
temporal modes exist in both the laminar and the turbulent cases, one of which
can exclude absolute instability. We compare our results with an
experimentally-determined flow-regime map, and discuss the potential
application of the present method to non-linear analyses.Comment: 33 pages, 20 figure
Knudsen diffusivity in random billiards: spectrum, geometry, and computation
We develop an analytical framework and numerical approach to obtain the
coefficient of self-diffusivity for the transport of a rarefied gas in channels
in the limit of large Knudsen number. This framework provides a method for
determining the influence of channel surface microstructure on the value of
diffusivity that is particularly effective when the microstructure exhibits
relatively low roughness. This method is based on the observation that the
Markov transition (scattering) operator determined by the microstructure, under
the condition of weak surface scattering, has a universal form given, up to a
multiplicative constant, by the classical Legendre differential operator. We
also show how characteristic numbers of the system -- namely geometric
parameters of the microstructure, the spectral gap of a Markov operator, and
the tangential momentum accommodation coefficient of a commonly used model of
surface scattering -- are all related. Examples of microstructures are
investigated to illustrate the relation of these quantities numerically and
analytically.Comment: 29 pages, 12 figure
Absolute and convective instabilities of an inviscid compressible mixing layer: Theory and applications
This study aims to examine the effect of compressibility on unbounded and parallel shear flow linear instabilities. This analysis is of interest for industrial, geophysical, and astrophysical flows. We focus on the stability of a wavepacket as opposed to previous single-mode stability studies. We consider the notions of absolute and convective instabilities first used to describe plasma instabilities. The compressible-flow modal theory predicts instability whatever the Mach number. Spatial and temporal growth rates and Reynolds stresses nevertheless become strongly reduced at high Mach numbers. The evolution of disturbances is driven by Kelvin -Helmholtz instability that weakens in supersonic flows. We wish to examine the occurrence of absolute instability, necessary for the appearance of turbulent motions in an inviscid and compressible two-dimensional mixing layer at an arbitrary Mach number subject to a three-dimensional disturbance. The mixing layer is defined by a parametric family of mean-velocity and temperature profiles. The eigenvalue problem is solved with the help of a spectral method. We ascertain the effects of the distribution of temperature and velocity in the mixing layer on the transition between convective and absolute instabilities. It appears that, in most cases, absolute instability is always possible at high Mach numbers provided that the ratio of slow-stream temperature over fast-stream temperature may be less than a critical maximal value but the temporal growth rate present in the absolutely unstable zone remains small and tends to zero at high Mach numbers. The transition toward a supersonic turbulent regime is therefore unlikely to be possible in the linear theory. Absolute instability can be also present among low-Mach-number coflowing mixing layers provided that this same temperature ratio may be small, but nevertheless, higher than a critical minimal value. Temperature distribution within the mixing layer also has an effect on the growth rate, this diminishes when the slow stream is heated. These results are applied to the dynamics of mixing layers in the interstellar medium and to the dynamics of the heliopause, frontier between the interstellar medium, and the solar wind. (C) 2009 American Institute of Physics
Unexpected distribution phenomenon resulting from Cantor series expansions
We explore in depth the number theoretic and statistical properties of
certain sets of numbers arising from their Cantor series expansions. As a
direct consequence of our main theorem we deduce numerous new results as well
as strengthen known ones.Comment: 32 page
Inverse Optimization with Noisy Data
Inverse optimization refers to the inference of unknown parameters of an
optimization problem based on knowledge of its optimal solutions. This paper
considers inverse optimization in the setting where measurements of the optimal
solutions of a convex optimization problem are corrupted by noise. We first
provide a formulation for inverse optimization and prove it to be NP-hard. In
contrast to existing methods, we show that the parameter estimates produced by
our formulation are statistically consistent. Our approach involves combining a
new duality-based reformulation for bilevel programs with a regularization
scheme that smooths discontinuities in the formulation. Using epi-convergence
theory, we show the regularization parameter can be adjusted to approximate the
original inverse optimization problem to arbitrary accuracy, which we use to
prove our consistency results. Next, we propose two solution algorithms based
on our duality-based formulation. The first is an enumeration algorithm that is
applicable to settings where the dimensionality of the parameter space is
modest, and the second is a semiparametric approach that combines nonparametric
statistics with a modified version of our formulation. These numerical
algorithms are shown to maintain the statistical consistency of the underlying
formulation. Lastly, using both synthetic and real data, we demonstrate that
our approach performs competitively when compared with existing heuristics
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