29,383 research outputs found
Three-dimensional oblique water-entry problems at small\ud deadrise angles
This paper extends Wagner theory for the ideal, incompressible normal impact of rigid bodies that are nearly parallel to the surface of a liquid half-space. The impactors considered are three-dimensional and have an oblique impact velocity. A variational formulation is used to reveal the relationship between the oblique and corresponding normal impact solutions. In the case of axisymmetric impactors, several geometries are considered in which singularities develop in the boundary of the effective wetted region. We present the corresponding pressure profiles and models for the splash sheets
A note on oblique water entry
An apparently minor error in Howison, Ockendon & Oliver (J. Eng. Math. 48:321–337, 2004) obscured the fact that the points at which the free surface turns over in the solution of the Wagner model for the oblique impact of a two-dimensional body are directly related to the turnover points in the equivalent normal impact problem. This note corrects some results given in Howison, Ockendon & Oliver (2004) and discusses the implications for the applicability of the Wagner\ud
model
Droplet impact on a thin fluid layer
The initial stages of high-velocity droplet impact on a shallow water layer are described, with special emphasis given to the spray jet mechanics. Four stages of impact are delineated, with appropriate scalings, and the successively more important influence of the base is analysed. In particular, there is a finite time before which part of the water in the layer remains under the droplet and after which all of the layer is ejected in the splash jet
Nonuniqueness in a minimal model for cell motility
Two–phase flow models have been used previously to model cell motility, however these have rapidly become very complicated, including many physical processes, and are opaque. Here we demonstrate that even the simplest one–dimensional, two–phase, poroviscous, reactive flow model displays a number of behaviours relevant to cell crawling. We present stability analyses that show that an asymmetric perturbation is required to cause a spatially uniform, stationary strip of cytoplasm to move, which is relevant to cell polarization. Our numerical simulations identify qualitatively distinct families of travelling–wave solution that co–exist at certain parameter values. Within each family, the crawling speed of the strip has a bell–shaped dependence on the adhesion strength. The model captures the experimentally observed behaviour that cells crawl quickest at intermediate adhesion strengths, when the substrate is neither too sticky nor too slippy
Thinking territory historically.
BACKGROUND:
While the randomised controlled trial (RCT) is generally regarded as the design of
choice for assessing the effects of health care, within the social sciences there is
considerable debate about the relative suitability of RCTs and non-randomised
studies (NRSs) for evaluating public policy interventions.
// OBJECTIVES:
To determine whether RCTs lead to the same effect size and variance as NRSs of
similar policy interventions; and whether these findings can be explained by other
factors associated with the interventions or their evaluation.
// METHODS:
Analyses of methodological studies, empirical reviews, and individual health and
social services studies investigated the relationship between randomisation and
effect size of policy interventions by:
1) Comparing controlled trials that are identical in all respects other than the use of
randomisation by 'breaking' the randomisation in a trial to create non-randomised
trials (re-sampling studies).
2) Comparing randomised and non-randomised arms of controlled trials mounted
simultaneously in the field (replication studies).
3) Comparing similar controlled trials drawn from systematic reviews that include
both randomised and non-randomised studies (structured narrative reviews and
sensitivity analyses within meta-analyses).
4) Investigating associations between randomisation and effect size using a pool of
more diverse RCTs and NRSs within broadly similar areas (meta-epidemiology).
// RESULTS:
Prior methodological reviews and meta-analyses of existing reviews comparing
effects from RCTs and nRCTs suggested that effect sizes from RCTs and nRCTs
may indeed differ in some circumstances and that these differences may well be
associated with factors confounded with design.
Re-sampling studies offer no evidence that the absence of randomisation directly
influences the effect size of policy interventions in a systematic way. No consistent
explanations were found for randomisation being associated with changes in effect
sizes of policy interventions in field trials
Design considerations for a monolithic, GaAs, dual-mode, QPSK/QASK, high-throughput rate transceiver
A monolithic, GaAs, dual mode, quadrature amplitude shift keying and quadrature phase shift keying transceiver with one and two billion bits per second data rate is being considered to achieve a low power, small and ultra high speed communication system for satellite as well as terrestrial purposes. Recent GaAs integrated circuit achievements are surveyed and their constituent device types are evaluated. Design considerations, on an elemental level, of the entire modem are further included for monolithic realization with practical fabrication techniques. Numerous device types, with practical monolithic compatability, are used in the design of functional blocks with sufficient performances for realization of the transceiver
Three-Dimensional Propagation of Magnetohydrodynamic Waves in Solar Coronal Arcades
We numerically investigate the excitation and temporal evolution of
oscillations in a two-dimensional coronal arcade by including the
three-dimensional propagation of perturbations. The time evolution of
impulsively generated perturbations is studied by solving the linear, ideal
magnetohydrodynamic (MHD) equations in the zero-beta approximation. As we
neglect gas pressure the slow mode is absent and therefore only coupled MHD
fast and Alfven modes remain. Two types of numerical experiments are performed.
First, the resonant wave energy transfer between a fast normal mode of the
system and local Alfven waves is analyzed. It is seen how, because of resonant
coupling, the fast wave with global character transfers its energy to Alfvenic
oscillations localized around a particular magnetic surface within the arcade,
thus producing the damping of the initial fast MHD mode. Second, the time
evolution of a localized impulsive excitation, trying to mimic a nearby coronal
disturbance, is considered. In this case, the generated fast wavefront leaves
its energy on several magnetic surfaces within the arcade. The system is
therefore able to trap energy in the form of Alfvenic oscillations, even in the
absence of a density enhancement such as that of a coronal loop. These local
oscillations are subsequently phase-mixed to smaller spatial scales. The amount
of wave energy trapped by the system via wave energy conversion strongly
depends on the wavelength of perturbations in the perpendicular direction, but
is almost independent from the ratio of the magnetic to density scale heights.Comment: 27 pages, 11 figure
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