4,091 research outputs found
Interactional aerodynamics and acoustics of a propeller-augmented compound coaxial helicopter
The aerodynamic and acoustic characteristics of a generic hingeless coaxial helicopter with a tail-mounted propulsor and stabiliser have been simulated using Brown's Vorticity Transport Model. This has been done to investigate the ability of models of this type to capture the aerodynamic interactions that are generated between the various components of realistic, complex helicopter configurations. Simulations reveal the aerodynamic environment of the coaxial main rotor of the configuration to be dominated by internal interactions that lead to high vibration and noise. The wake of the main rotor is predicted to interact strongly with the tailplane, particularly at low forward speed, to produce a strong nose-up pitching moment that must be countered by significant longitudinal cyclic input to the main rotor. The wake from the main rotor is ingested directly into the tail propulsor over a broad range of forward speeds, where it produces significant vibratory excitation of the system as well as broadband noise. The numerical calculations also suggest the possibility that poor scheduling of the partition of the propulsive force between the main rotor and propulsor as a function of forward speed may yield a situation where the propulsor produces little thrust but high vibration as a result of this interaction. Although many of the predicted effects might be ameliorated or eliminated entirely by more careful or considered design, the model captures many of the aerodynamic interactions, and the resultant effects on the loading on the system, that might be expected to characterise the dynamics of such a vehicle. It is suggested that the use of such numerical techniques might eventually allow the various aeromechanical problems that often beset new designs to be circumvented - hopefully well before they manifest on the prototype or production aircraft
Hydrodynamic interactions of spherical particles in suspensions confined between two planar walls
Hydrodynamic interactions in a suspension of spherical particles confined
between two parallel planar walls are studied under creeping-flow conditions.
The many-particle friction matrix in this system is evaluated using our novel
numerical algorithm based on transformations between Cartesian and spherical
representations of Stokes flow. The Cartesian representation is used to
describe the interaction of the fluid with the walls and the spherical
representation is used to describe the interaction with the particles. The
transformations between these two representations are given in a closed form,
which allows us to evaluate the coefficients in linear equations for the
induced-force multipoles on particle surfaces. The friction matrix is obtained
from these equations, supplemented with the superposition lubrication
corrections. We have used our algorithm to evaluate the friction matrix for a
single sphere, a pair of spheres, and for linear chains of spheres. The
friction matrix exhibits a crossover from a quasi-two-dimensional behavior (for
systems with small wall separation H) to the three-dimensional behavior (when
the distance H is much larger than the interparticle distance L). The crossover
is especially pronounced for a long chain moving in the direction normal to its
orientation and parallel to the walls. In this configuration, a large pressure
buildup occurs in front of the chain for small values of the gapwidth H, which
results in a large hydrodynamic friction force. A standard wall superposition
approximation does not capture this behavior
Non-Gaussian dynamic Bayesian modelling for panel data
A first order autoregressive non-Gaussian model for analysing panel data is proposed. The main feature is that the model is able to accommodate fat tails and also skewness, thus allowing for outliers and asymmetries. The modelling approach is to gain sufficient flexibility, without sacrificing interpretability and computational ease. The model incorporates individual effects and we pay specific attention to the elicitation of the prior. As the prior structure chosen is not proper, we derive conditions for the existence of the posterior. By considering a model with individual dynamic parameters we are also able to formally test whether the dynamic behaviour is common to all units in the panel. The methodology is illustrated with two applications involving earnings data and one on growth of countries.autoregressive modelling; growth convergence; individual effects; labour earnings; prior elicitation; posterior existence; skewed distributions
Patterns of neutral diversity under general models of selective sweeps
Two major sources of stochasticity in the dynamics of neutral alleles result
from resampling of finite populations (genetic drift) and the random genetic
background of nearby selected alleles on which the neutral alleles are found
(linked selection). There is now good evidence that linked selection plays an
important role in shaping polymorphism levels in a number of species. One of
the best investigated models of linked selection is the recurrent full sweep
model, in which newly arisen selected alleles fix rapidly. However, the bulk of
selected alleles that sweep into the population may not be destined for rapid
fixation. Here we develop a general model of recurrent selective sweeps in a
coalescent framework, one that generalizes the recurrent full sweep model to
the case where selected alleles do not sweep to fixation. We show that in a
large population, only the initial rapid increase of a selected allele affects
the genealogy at partially linked sites, which under fairly general assumptions
are unaffected by the subsequent fate of the selected allele. We also apply the
theory to a simple model to investigate the impact of recurrent partial sweeps
on levels of neutral diversity, and find that for a given reduction in
diversity, the impact of recurrent partial sweeps on the frequency spectrum at
neutral sites is determined primarily by the frequencies achieved by the
selected alleles. Consequently, recurrent sweeps of selected alleles to low
frequencies can have a profound effect on levels of diversity but can leave the
frequency spectrum relatively unperturbed. In fact, the limiting coalescent
model under a high rate of sweeps to low frequency is identical to the standard
neutral model. The general model of selective sweeps we describe goes some way
towards providing a more flexible framework to describe genomic patterns of
diversity than is currently available.Comment: 44 pages. 5 figure
Single-Event Upset Analysis and Protection in High Speed Circuits
The effect of single-event transients (SETs) (at a combinational node of a design) on the system reliability is becoming a big concern for ICs manufactured using advanced technologies. An SET at a node of combinational part may cause a transient pulse at the input of a flip-flop and consequently is latched in the flip-flop and generates a soft-error. When an SET conjoined with a transition at a node along a critical path of the combinational part of a design, a transient delay fault may occur at the input of a flip-flop. On the other hand, increasing pipeline depth and using low power techniques such as multi-level power supply, and multi-threshold transistor convert almost all paths in a circuit to critical ones. Thus, studying the behavior of the SET in these kinds of circuits needs special attention. This paper studies the dynamic behavior of a circuit with massive critical paths in the presence of an SET. We also propose a novel flip-flop architecture to mitigate the effects of such SETs in combinational circuits. Furthermore, the proposed architecture can tolerant a single event upset (SEU) caused by particle strike on the internal nodes of a flip-flo
Obervational Model for Microarcsecond Astrometry with the Space Interferometry Mission
The Space Interferometry Mission (SIM) is a space-based long-baseline optical
interferometer for precision astrometry. One of the primary objectives of the
SIM instrument is to accurately determine the directions to a grid of stars,
together with their proper motions and parallaxes, improving a priori knowledge
by nearly three orders of magnitude. The basic astrometric observable of the
instrument is the pathlength delay, a measurement made by a combination of
internal metrology measurements that determine the distance the starlight
travels through the two arms of the interferometer and a measurement of the
white light stellar fringe to find the point of equal pathlength. Because this
operation requires a non--negligible integration time to accurately measure the
stellar fringe position, the interferometer baseline vector is not stationary
over this time period, as its absolute length and orientation are
time--varying. This conflicts with the consistency condition necessary for
extracting the astrometric parameters which requires a stationary baseline
vector. This paper addresses how the time-varying baseline is ``regularized''
so that it may act as a single baseline vector for multiple stars, and thereby
establishing the fundamental operation of the instrument.Comment: 24 pages, 6 figure
Design-for-delay-testability techniques for high-speed digital circuits
The importance of delay faults is enhanced by the ever increasing clock rates and decreasing geometry sizes of nowadays' circuits. This thesis focuses on the development of Design-for-Delay-Testability (DfDT) techniques for high-speed circuits and embedded cores. The rising costs of IC testing and in particular the costs of Automatic Test Equipment are major concerns for the semiconductor industry. To reverse the trend of rising testing costs, DfDT is\ud
getting more and more important
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