569 research outputs found
Aeorodynamic characteristics of an air-exchanger system for the 40- by 80-foot wind tunnel at Ames Research Center
A 1/50-scale model of the 40- by 80-Foot Wind Tunnel at Ames Research Center was used to study various air-exchange configurations. System components were tested throughout a range of parameters, and approximate analytical relationships were derived to explain the observed characteristics. It is found that the efficiency of the air exchanger could be increased (1) by adding a shaped wall to smoothly turn the incoming air downstream, (2) by changing to a contoured door at the inlet to control the flow rate, and (3) by increasing the size of the exhaust opening. The static pressures inside the circuit then remain within the design limits at the higher tunnel speeds if the air-exchange rate is about 5% or more. Since the model is much smaller than the full-scale facility, it is not possible to completely duplicate the tunnel, and it will be necessary to measure such characteristics as flow rate and tunnel pressures during implementation of the remodeled facility. The aerodynamic loads estimated for the inlet door and for nearby walls are also presented
Invariant, super and quasi-martingale functions of a Markov process
We identify the linear space spanned by the real-valued excessive functions
of a Markov process with the set of those functions which are quasimartingales
when we compose them with the process. Applications to semi-Dirichlet forms are
given. We provide a unifying result which clarifies the relations between
harmonic, co-harmonic, invariant, co-invariant, martingale and co-martingale
functions, showing that in the conservative case they are all the same.
Finally, using the co-excessive functions, we present a two-step approach to
the existence of invariant probability measures
Fisher information and asymptotic normality in system identification for quantum Markov chains
This paper deals with the problem of estimating the coupling constant
of a mixing quantum Markov chain. For a repeated measurement on the
chain's output we show that the outcomes' time average has an asymptotically
normal (Gaussian) distribution, and we give the explicit expressions of its
mean and variance. In particular we obtain a simple estimator of whose
classical Fisher information can be optimized over different choices of
measured observables. We then show that the quantum state of the output
together with the system, is itself asymptotically Gaussian and compute its
quantum Fisher information which sets an absolute bound to the estimation
error. The classical and quantum Fisher informations are compared in a simple
example. In the vicinity of we find that the quantum Fisher
information has a quadratic rather than linear scaling in output size, and
asymptotically the Fisher information is localised in the system, while the
output is independent of the parameter.Comment: 10 pages, 2 figures. final versio
F/A-18 1/9th scale model tail buffet measurements
Wind tunnel tests were carried out on a 1/9th scale model of the F/A-18 at high angles of attack to investigate the characteristics of tail buffet due to bursting of the wing leading edge extension (LEX) vortices. The tests were carried out at the Aeronautical Research Laboratory low-speed wind tunnel facility and form part of a collaborative activity with NASA Ames Research Center, organized by The Technical Cooperative Program (TTCP). Information from the program will be used in the planning of similar collaborative tests, to be carried out at NASA Ames, on a full-scale aircraft. The program covered the measurement of unsteady pressures and fin vibration for cases with and without the wing LEX fences fitted. Fourier transform methods were used to analyze the unsteady data, and information on the spatial and temporal content of the vortex burst pressure field was obtained. Flow visualization of the vortex behavior was carried out using smoke and a laser light sheet technique
Effects of Long-Term Use of Nonoxynol-9 on Vaginal Flora
OBJECTIVE—Products containing nonoxynol-9 have been used as spermicidal contraceptives for many years, but limited data have been published describing the long-term effects of nonoxynol-9 use on the vaginal microbial ecosystem. This longitudinal study was conducted to examine the effects of nonoxynol-9 on the vaginal ecology.
METHODS—Vaginal swabs were obtained from 235 women enrolled in a randomized clinical trial before initiation of use of 1 of 5 different formulations of nonoxynol-9 for contraception, and up to 3 more samples were gathered over 7 months of use. The swab samples were evaluated in a single laboratory. The prevalence of several constituents of the normal vaginal flora was evaluated. The associations between nonoxynol-9 dosage, formulation, average product use per week, and number of sex acts per week were calculated.
RESULTS—The changes in prevalence of vaginal microbes after nonoxynol-9 use were minimal for each of the different nonoxynol-9 formulations. However, when both nonoxynol-9 concentration and number of product uses are taken into account, nonoxynol-9 did have dose-dependant effects on the increased prevalence of anaerobic gram-negative rods (odds ratio [OR] 2.4, 95% confidence interval [CI] 1.1–5.3), H2O2-negative lactobacilli (OR 2.0, 95% CI 1.0–4.1), and bacterial vaginosis (OR 2.3, 95% CI 1.1–4.7).
CONCLUSION—This study demonstrated that most nonoxynol-9 users experienced minimal disruptions in their vaginal ecology. There were no differences between the different formulations evaluated with respect to changes in vaginal microflora. However, independent of the nonoxynol-9 formulation, there was a dose-dependent effect with increased exposure to nonoxynol-9 on the risk of bacterial vaginosis and its associated flora
Random billiards with wall temperature and associated Markov chains
By a random billiard we mean a billiard system in which the standard specular
reflection rule is replaced with a Markov transition probabilities operator P
that, at each collision of the billiard particle with the boundary of the
billiard domain, gives the probability distribution of the post-collision
velocity for a given pre-collision velocity. A random billiard with
microstructure (RBM) is a random billiard for which P is derived from a choice
of geometric/mechanical structure on the boundary of the billiard domain. RBMs
provide simple and explicit mechanical models of particle-surface interaction
that can incorporate thermal effects and permit a detailed study of
thermostatic action from the perspective of the standard theory of Markov
chains on general state spaces.
We focus on the operator P itself and how it relates to the
mechanical/geometric features of the microstructure, such as mass ratios,
curvatures, and potentials. The main results are as follows: (1) we
characterize the stationary probabilities (equilibrium states) of P and show
how standard equilibrium distributions studied in classical statistical
mechanics, such as the Maxwell-Boltzmann distribution and the Knudsen cosine
law, arise naturally as generalized invariant billiard measures; (2) we obtain
some basic functional theoretic properties of P. Under very general conditions,
we show that P is a self-adjoint operator of norm 1 on an appropriate Hilbert
space. In a simple but illustrative example, we show that P is a compact
(Hilbert-Schmidt) operator. This leads to the issue of relating the spectrum of
eigenvalues of P to the features of the microstructure;(3) we explore the
latter issue both analytically and numerically in a few representative
examples;(4) we present a general algorithm for simulating these Markov chains
based on a geometric description of the invariant volumes of classical
statistical mechanics
Convergence to equilibrium for many particle systems
The goal of this paper is to give a short review of recent results of the
authors concerning classical Hamiltonian many particle systems. We hope that
these results support the new possible formulation of Boltzmann's ergodicity
hypothesis which sounds as follows. For almost all potentials, the minimal
contact with external world, through only one particle of , is sufficient
for ergodicity. But only if this contact has no memory. Also new results for
quantum case are presented
On the flow-level stability of data networks without congestion control: the case of linear networks and upstream trees
In this paper, flow models of networks without congestion control are
considered. Users generate data transfers according to some Poisson processes
and transmit corresponding packet at a fixed rate equal to their access rate
until the entire document is received at the destination; some erasure codes
are used to make the transmission robust to packet losses. We study the
stability of the stochastic process representing the number of active flows in
two particular cases: linear networks and upstream trees. For the case of
linear networks, we notably use fluid limits and an interesting phenomenon of
"time scale separation" occurs. Bounds on the stability region of linear
networks are given. For the case of upstream trees, underlying monotonic
properties are used. Finally, the asymptotic stability of those processes is
analyzed when the access rate of the users decreases to 0. An appropriate
scaling is introduced and used to prove that the stability region of those
networks is asymptotically maximized
CLTs and asymptotic variance of time-sampled Markov chains
For a Markov transition kernel P and a probability distribution
μ on nonnegative integers, a time-sampled Markov chain evolves according
to the transition kernel Pμ = Σkμ(k)Pk. In this note we obtain CLT
conditions for time-sampled Markov chains and derive a spectral formula
for the asymptotic variance. Using these results we compare efficiency of
Barker's and Metropolis algorithms in terms of asymptotic variance
Noisy Monte Carlo: Convergence of Markov chains with approximate transition kernels
Monte Carlo algorithms often aim to draw from a distribution by
simulating a Markov chain with transition kernel such that is
invariant under . However, there are many situations for which it is
impractical or impossible to draw from the transition kernel . For instance,
this is the case with massive datasets, where is it prohibitively expensive to
calculate the likelihood and is also the case for intractable likelihood models
arising from, for example, Gibbs random fields, such as those found in spatial
statistics and network analysis. A natural approach in these cases is to
replace by an approximation . Using theory from the stability of
Markov chains we explore a variety of situations where it is possible to
quantify how 'close' the chain given by the transition kernel is to
the chain given by . We apply these results to several examples from spatial
statistics and network analysis.Comment: This version: results extended to non-uniformly ergodic Markov chain
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