1,988 research outputs found
Experimental verification of an Oseen flow slender body theory
Consider uniform flow past four slender bodies with elliptical cross-section of
constant ellipticity along the length of 0, 0.125, 0.25 and 0.375, respectively, for each
body. Here, ellipticity is defined as the ratio of the semiminor axis of the ellipse to
the semimajor axis. The bodies have a pointed nose which gradually increases in
cross-section with a radius of curvature 419mm to a mid-section which then remains
constant up to a blunt end section with semimajor axis diameter 160 mm, the total
length of all bodies being 800 mm. The bodies are side-mounted within a low-speed
wind tunnel with an operational wind speed of the order 30ms−1. The side force (or
lift) is measured within an angle of attack range of −3◦ to 3◦ such that the body is
rotated about the major axis of the ellipse cross-section. The lift slope is determined
for each body, and how it varies with ellipticity. It is found that this variance follows
a straight line which steadily increases with increasing ellipticity. It is shown that
this result is predicted by a recently developed Oseen flow slender body theory, and
cannot be predicted by either inviscid flow slender body theory or viscous crossflow
theories based upon the Allen and Perkins method
Collapse of an Instanton
We construct a two parameter family of collapsing solutions to the 4+1
Yang-Mills equations and derive the dynamical law of the collapse. Our
arguments indicate that this family of solutions is stable. The latter fact is
also supported by numerical simulations.Comment: 17 pages, 1 figur
Transit Use by Young People in Times of Financial Austerity: A Cause of Equity Concern?
This study investigates the effect of price and travel time fairness and spatial equity in transit provision on the perceived transit service quality, willingness to pay, and habitual frequency of use. Based on the theory of planned behavior, we developed a web-based questionnaire for revealed preferences data collection. The survey was administered among young people in Copenhagen and Lisbon to explore the transit perceptions and use under different economic and transit provision conditions. The survey yielded 499 questionnaires, analyzed by means of structural equation models. Results show that higher perceived fairness relates positively to higher perceived quality of transit service and higher perceived ease of paying for transit use. Higher perceived spatial equity in service provision is associated with higher perceived service quality. Higher perceived service quality relates to higher perceived ease of payment, which links to higher frequency of transit use
The relationship between young people's transit use and their perceptions of equity concepts in transit service provision
This study investigates the effect of price and travel mode fairness and spatial equity in transit provision on the perceived transit service quality, willingness to pay, and habitual frequency of use. Based on the theory of planned behavior, we developed a web-based questionnaire for revealed preferences data collection. The survey was administered among young people in Copenhagen and Lisbon to explore the transit perceptions and use under different economic and transit provision conditions. The survey yielded 499 questionnaires, analyzed by means of structural equation models. Results show that higher perceived fairness relates positively to higher perceived quality of transit service and higher perceived ease of paying for transit use. Higher perceived spatial equity in service provision is associated with higher perceived service quality. Higher perceived service quality relates to higher perceived ease of payment, which links to higher frequency of transit use
Towards exponentially-convergent simulations of extreme-mass-ratio inspirals: A time-domain solver for the scalar Teukolsky equation with singular source terms
Gravitational wave signals from extreme mass ratio inspirals are a key target
for space-based gravitational wave detectors. These systems are typically
modeled as a distributionally-forced Teukolsky equation, where the smaller
black hole is treated as a Dirac delta distribution. Time-domain solvers often
use regularization approaches that approximate the Dirac distribution that
often introduce small length scales and are a source of systematic error,
especially near the smaller black hole. We describe a multi-domain
discontinuous Galerkin method for solving the distributionally-forced Teukolsky
equation that describes scalar fields evolving on a Kerr spacetime. To handle
the Dirac delta, we expand the solution in spherical harmonics and recast the
sourced Teukolsky equation as a first-order, one-dimensional symmetric
hyperbolic system. This allows us to derive the method's numerical flux to
correctly account for the Dirac delta. As a result, our method achieves global
spectral accuracy even at the source's location. To connect the near field to
future null infinity, we use the hyperboloidal layer method, allowing us to
supply outer boundary conditions and providing direct access to the far-field
waveform. We document several numerical experiments where we test our method,
including convergence tests against exact solutions, energy luminosities for
circular orbits, the scheme's superconvergence properties at future null
infinity, and the late-time tail behavior of the scalar field. We also compare
two systems that arise from different choices of the first-order reduction
variables, finding that certain choices are numerically problematic in
practice. The methods developed here may be beneficial when computing
gravitational self-force effects, where the regularization procedure has been
developed for the spherical harmonic modes and high accuracy is needed at the
Dirac delta's location.Comment: 20 pages, 7 figures and 2 table
Interference with HIV infection of the first cell is essential for viral clearance at sub-optimal levels of drug inhibition
HIV infection can be cleared with antiretroviral drugs if they are administered before exposure, where exposure occurs at low viral doses which infect one or few cells. However, infection clearance does not happen once infection is established, and this may be because of the very early formation of a reservoir of latently infected cells. Here we investigated whether initial low dose infection could be cleared with sub-optimal drug inhibition which allows ongoing viral replication, and hence does not require latency for viral persistence. We derived a model for infection clearance with inputs being drug effects on ongoing viral replication and initial number of infected cells. We experimentally tested the model by inhibiting low dose infection with the drug tenofovir, which interferes with initial infection, and atazanavir, which reduces the cellular virion burst size and hence inhibits replication only after initial infection. Drugs were used at concentrations which allowed infection to expand. Under these conditions, tenofovir dramatically increased clearance while atazanavir did not. Addition of latency to the model resulted in a minor decrease in clearance probability if the drug inhibited initial infection. If not, latency strongly decreased clearance even at low latent cell frequencies. Therefore, the ability of drugs to clear initial but not established infection can be recapitulated without latency and depends only on the ability to target initial infection. The presence of latency can dramatically decrease infection clearance, but only if the drug is unable to interfere with infection of the first cells
Stochastic motion planning and applications to traffic
This paper presents a stochastic motion planning algorithm and its application to traffic navigation. The algorithm copes with the uncertainty of road traffic conditions by stochastic modeling of travel delay on road networks. The algorithm determines paths between two points that optimize a cost function of the delay probability distribution. It can be used to find paths that maximize the probability of reaching a destination within a particular travel deadline. For such problems, standard shortest-path algorithms don’t work because the optimal substructure property doesn’t hold. We evaluate our algorithm using both simulations and real-world drives, using delay data gathered from a set of taxis equipped with GPS sensors and a wireless network. Our algorithm can be integrated into on-board navigation systems as well as route-finding Web sites, providing drivers with good paths that meet their desired goals.National Science Foundation (U.S.) (grant EFRI-0710252)National Science Foundation (U.S.) (grant IIS-0426838
Resonances Width in Crossed Electric and Magnetic Fields
We study the spectral properties of a charged particle confined to a
two-dimensional plane and submitted to homogeneous magnetic and electric fields
and an impurity potential. We use the method of complex translations to prove
that the life-times of resonances induced by the presence of electric field are
at least Gaussian long as the electric field tends to zero.Comment: 3 figure
Expression of phosphorylcholine-specific B cells during murine development
The TEPC 15 (T15) clonotype, a putatively germline antibody specificity, does not appear in the neonatal B-cell repertoire until approximately 1 wk of age. This report extends this observation by the demonstration that (a) the T15 clonotype follows similar kinetics of appearance in germfree as well as conventionally-reared mice; (b) maternal influences and genetic background play a minor role in the development of the T15 clonotype since CBFI neonates raised by C57BL/6 or BALB/c mothers acquire the T15 clonotype at the same time in ontogeny as BALB/c neonates; (c) the lack of phosphorylcholine (PC)-specific B cells shortly after birth is reflected in a dearth of PC-binding cells in the neonate as well; and (d) no PC-specifc B cells are found in 19-day fetal liver or in bone marrow until 7 days of life, coincident with their appearance in the spleen. These findings, along with a previous report that PC-specific splenic B cells are tolerizable as late as day 10 after birth, confirm the invariant, late occurrence of the T15 clonotype and support a highly- ordered, rigorously predetermined mechanism for the acquisition of the B- cell repertoire. The results are discussed in light of other studies on the ontogeny of B-cell specificity, and in terms of the implications on the mechanism by which antibody diversity is generated
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