20,369 research outputs found
Laser velocimetry technique applied to the Langley 0.3 meter transonic cryogenic tunnel
A low power laser velocimeter operating in the forward scatter mode was used to measure free stream mean velocities in the Langley 0.3 Meter Transonic Cryogenic Tunnel. Velocity ranging from 51 to 235 m/s was measured. Measurements were obtained for a variety of nominal tunnel conditions: Mach numbers from 0.20 to 0.77, total temperatures from 100 to 250 K, and pressures from 101 to 152 kPa. Particles were not injected to augment the existing Mie scattering materials. Liquid nitrogen droplets were the existing liqht scattering material. Tunnel vibrations and thermal effects had no detrimental effects on the optical system
Curvature and Acoustic Instabilities in Rotating Fluid Disks
The stability of a rotating fluid disk to the formation of spiral arms is
studied in the tightwinding approximation in the linear regime. The dispersion
relation for spirals that was derived by Bertin et al. is shown to contain a
new, acoustic instability beyond the Lindblad resonances that depends only on
pressure and rotation. In this regime, pressure and gravity exchange roles as
drivers and inhibitors of spiral wave structures. Other instabilities that are
enhanced by pressure are also found in the general dispersion relation by
including higher order terms in the small parameter 1/kr for wavenumber k and
radius r. These instabilities are present even for large values of Toomre's
parameter Q. Unstable growth rates are determined in four cases: a
self-gravitating disk with a flat rotation curve, a self-gravitating disk with
solid body rotation, a non-self-gravitating disk with solid body rotation, and
a non-self-gravitating disk with Keplerian rotation. The most important
application appears to be as a source of spiral structure, possibly leading to
accretion in non-self-gravitating disks, such as some galactic nuclear disks,
disks around black holes, and proto-planetary disks. All of these examples have
short orbital times so the unstable growth time can be small.Comment: 30 pages, 5 figures, scheduled for ApJ 520, August 1, 199
A computer-assisted motivational social network intervention to reduce alcohol, drug and HIV risk behaviors among Housing First residents.
BackgroundIndividuals transitioning from homelessness to housing face challenges to reducing alcohol, drug and HIV risk behaviors. To aid in this transition, this study developed and will test a computer-assisted intervention that delivers personalized social network feedback by an intervention facilitator trained in motivational interviewing (MI). The intervention goal is to enhance motivation to reduce high risk alcohol and other drug (AOD) use and reduce HIV risk behaviors.Methods/designIn this Stage 1b pilot trial, 60 individuals that are transitioning from homelessness to housing will be randomly assigned to the intervention or control condition. The intervention condition consists of four biweekly social network sessions conducted using MI. AOD use and HIV risk behaviors will be monitored prior to and immediately following the intervention and compared to control participants' behaviors to explore whether the intervention was associated with any systematic changes in AOD use or HIV risk behaviors.DiscussionSocial network health interventions are an innovative approach for reducing future AOD use and HIV risk problems, but little is known about their feasibility, acceptability, and efficacy. The current study develops and pilot-tests a computer-assisted intervention that incorporates social network visualizations and MI techniques to reduce high risk AOD use and HIV behaviors among the formerly homeless. CLINICALTRIALS.Gov identifierNCT02140359
Monte Carlo cluster algorithm for fluid phase transitions in highly size-asymmetrical binary mixtures
Highly size-asymmetrical fluid mixtures arise in a variety of physical
contexts, notably in suspensions of colloidal particles to which much smaller
particles have been added in the form of polymers or nanoparticles.
Conventional schemes for simulating models of such systems are hamstrung by the
difficulty of relaxing the large species in the presence of the small one. Here
we describe how the rejection-free geometrical cluster algorithm (GCA) of Liu
and Luijten [Phys. Rev. Lett 92, 035504 (2004)] can be embedded within a
restricted Gibbs ensemble to facilitate efficient and accurate studies of fluid
phase behavior of highly size-asymmetrical mixtures. After providing a detailed
description of the algorithm, we summarize the bespoke analysis techniques of
Ashton et al. [J. Chem. Phys. 132, 074111 (2010)] that permit accurate
estimates of coexisting densities and critical-point parameters. We apply our
methods to study the liquid--vapor phase diagram of a particular mixture of
Lennard-Jones particles having a 10:1 size ratio. As the reservoir volume
fraction of small particles is increased in the range 0--5%, the critical
temperature decreases by approximately 50%, while the critical density drops by
some 30%. These trends imply that in our system, adding small particles
decreases the net attraction between large particles, a situation that
contrasts with hard-sphere mixtures where an attractive depletion force occurs.Comment: 11 pages, 10 figure
Critical Casimir interaction of ellipsoidal colloids with a planar wall
Based on renormalization group concepts and explicit mean field calculations
we study the universal contribution to the effective force and torque acting on
an ellipsoidal colloidal particle which is dissolved in a critical fluid and is
close to a homogeneous planar substrate. At the same closest distance between
the substrate and the surface of the particle, the ellipsoidal particle prefers
an orientation parallel to the substrate and the magnitude of the fluctuation
induced force is larger than if the orientation of the particle is
perpendicular to the substrate. The sign of the critical torque acting on the
ellipsoidal particle depends on the type of boundary conditions for the order
parameter at the particle and substrate surfaces, and on the pivot with respect
to which the particle rotates
Hierarchical Models for Independence Structures of Networks
We introduce a new family of network models, called hierarchical network
models, that allow us to represent in an explicit manner the stochastic
dependence among the dyads (random ties) of the network. In particular, each
member of this family can be associated with a graphical model defining
conditional independence clauses among the dyads of the network, called the
dependency graph. Every network model with dyadic independence assumption can
be generalized to construct members of this new family. Using this new
framework, we generalize the Erd\"os-R\'enyi and beta-models to create
hierarchical Erd\"os-R\'enyi and beta-models. We describe various methods for
parameter estimation as well as simulation studies for models with sparse
dependency graphs.Comment: 19 pages, 7 figure
Are there any good digraph width measures?
Several different measures for digraph width have appeared in the last few
years. However, none of them shares all the "nice" properties of treewidth:
First, being \emph{algorithmically useful} i.e. admitting polynomial-time
algorithms for all \MS1-definable problems on digraphs of bounded width. And,
second, having nice \emph{structural properties} i.e. being monotone under
taking subdigraphs and some form of arc contractions. As for the former,
(undirected) \MS1 seems to be the least common denominator of all reasonably
expressive logical languages on digraphs that can speak about the edge/arc
relation on the vertex set.The latter property is a necessary condition for a
width measure to be characterizable by some version of the cops-and-robber game
characterizing the ordinary treewidth. Our main result is that \emph{any
reasonable} algorithmically useful and structurally nice digraph measure cannot
be substantially different from the treewidth of the underlying undirected
graph. Moreover, we introduce \emph{directed topological minors} and argue that
they are the weakest useful notion of minors for digraphs
Development of sports turf systems suitable for Irish conditions.
End of Project ReportThe principal objective of the study was to establish scientific data in relation to the
nutritional requirements and best management practice for golf greens constructed to the
United States Golf Association (USGA) 1973 specification under Irish conditions.
The game of golf is one of the biggest sports industries in the world. Income from golf
tourism in Ireland has increased from £73 million in 1994 to £180 million in 1998. Good
quality turfgrass is required to underpin the promotion of golf tourism.
Traditionally, golf greens on Irish golf courses were constructed from local materials and
vary from green to green within a given golf course and also between different golf courses.
In recent years there is a perception that the quality of putting surfaces is superior on greens
constructed to the USGA specification. In addition, greens constructed to this specification
are similar one to the other and location to location. The principal features of the USGA
1973 specification could be summarised as follows: (1) A network of drainage pipes
installed in the underground soil covered with a carpet of peat gravel; (2) A blinding layer
of specifically graded sand placed on the peat gravel; (3) A root zone mixture of graded
sand (80%) and graded peat moss (20%) by volume. The particle size of the component
layers must comply to the exact specification in terms of size, diameter and shape. As sands
contain no nutrients, the management of greens constructed mainly of sand is more exacting
than the traditional soil constructed greens. The results from this project confirmed this
assumption.
Three major objectives were researched in this project: (a) the effect of micro nutrients,
when applied or omitted, on the quality and growth of grass on a green surface; (b) the
encroachment of Poa annua (annual meadow grass) onto the green; and (c) the comparison
of two nitrogen top dressing programmes on sand greens. The detailed results are given in
the text and in the conclusions of this report.European
Union Structural Funds (EAGGF
Electrophoretic Properties of Highly Charged Colloids: A Hybrid MD/LB Simulation Study
Using computer simulations, the electrophoretic motion of a positively
charged colloid (macroion) in an electrolyte solution is studied in the
framework of the primitive model. Hydrodynamic interactions are fully taken
into account by applying a hybrid simulation scheme, where the charged ions
(i.e. macroion and electrolyte), propagated via molecular dynamics (MD), are
coupled to a Lattice Boltzmann (LB) fluid. In a recent experiment it was shown
that, for multivalent salt ions, the mobility initially increases with
charge density , reaches a maximum and then decreases with further
increase of . The aim of the present work is to elucidate the behaviour
of at high values of . Even for the case of monovalent microions,
we find a decrease of with . A dynamic Stern layer is defined
that includes all the counterions that move with the macroion while subject to
an external electrical field. The number of counterions in the Stern layer,
, is a crucial parameter for the behavior of at high values of
. In this case, the mobility depends primarily on the ratio
(with the valency of the macroion). The previous contention that
the increase in the distortion of the electric double layer (EDL) with
increasing leads to the lowering of does not hold for high
. In fact, we show that the deformation of the EDL decreases with
increase of . The role of hydrodynamic interactions is inferred from
direct comparisons to Langevin simulations where the coupling to the LB fluid
is switched off. Moreover, systems with divalent counterions are considered. In
this case, at high values of the phenomenon of charge inversion is
found.Comment: accepted in J. Chem Phys., 10 pages, 9 figure
Large Attractive Depletion Interactions in Soft Repulsive-Sphere Binary Mixtures
We consider binary mixtures of soft repulsive spherical particles and
calculate the depletion interaction between two big spheres mediated by the
fluid of small spheres, using different theoretical and simulation methods. The
validity of the theoretical approach, a virial expansion in terms of the
density of the small spheres, is checked against simulation results. Attention
is given to the approach toward the hard-sphere limit, and to the effect of
density and temperature on the strength of the depletion potential. Our results
indicate, surprisingly, that even a modest degree of softness in the pair
potential governing the direct interactions between the particles may lead to a
significantly more attractive total effective potential for the big spheres
than in the hard-sphere case. This might lead to significant differences in
phase behavior, structure and dynamics of a binary mixture of soft repulsive
spheres. In particular, a perturbative scheme is applied to predict the phase
diagram of an effective system of big spheres interacting via depletion forces
for a size ratio of small and big spheres of 0.2; this diagram includes the
usual fluid-solid transition but, in the soft-sphere case, the metastable
fluid-fluid transition, which is probably absent in hard-sphere mixtures, is
close to being stable with respect to direct fluid-solid coexistence. From
these results the interesting possibility arises that, for sufficiently soft
repulsive particles, this phase transition could become stable. Possible
implications for the phase behavior of real colloidal dispersions are
discussed.Comment: 31 pages, 8 figures; version accepted for publication in the Journal
of Chemical Physic
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