1,165 research outputs found
Capillary-gravity waves: The effect of viscosity on the wave resistance
The effect of viscosity on the wave resistance experienced by a 2d
perturbation moving at uniform velocity over the free surface of a fluid is
investigated. The analysis is based on Rayleigh's linearized theory of
capillary-gravity waves. It is shown in particular that the wave resistance
remains bounded as the velocity of the perturbation approches the minimun phase
speed, unlike what is predicted by the inviscid theory.Comment: Europhysics Letters, in pres
A note on leapfrogging vortex rings
In this paper we provide examples, by numerical simulation using the Navier-Stokes equations for axisymmetric laminar flow, of the 'leapfrogging' motion of two, initially identical, vortex rings which share a common axis of symmetry. We show that the number of clear passes that each ring makes through the other increases with Reynolds number, and that as long as the configuration remains stable the two rings ultimately merge to form a single vortex ring
Hydrodynamic and magnetohydrodynamic computations inside a rotating sphere
Numerical solutions of the incompressible magnetohydrodynamic (MHD) equations
are reported for the interior of a rotating, perfectly-conducting, rigid
spherical shell that is insulator-coated on the inside. A previously-reported
spectral method is used which relies on a Galerkin expansion in
Chandrasekhar-Kendall vector eigenfunctions of the curl. The new ingredient in
this set of computations is the rigid rotation of the sphere. After a few
purely hydrodynamic examples are sampled (spin down, Ekman pumping, inertial
waves), attention is focused on selective decay and the MHD dynamo problem. In
dynamo runs, prescribed mechanical forcing excites a persistent velocity field,
usually turbulent at modest Reynolds numbers, which in turn amplifies a small
seed magnetic field that is introduced. A wide variety of dynamo activity is
observed, all at unit magnetic Prandtl number. The code lacks the resolution to
probe high Reynolds numbers, but nevertheless interesting dynamo regimes turn
out to be plentiful in those parts of parameter space in which the code is
accurate. The key control parameters seem to be mechanical and magnetic
Reynolds numbers, the Rossby and Ekman numbers (which in our computations are
varied mostly by varying the rate of rotation of the sphere) and the amount of
mechanical helicity injected. Magnetic energy levels and magnetic dipole
behavior are exhibited which fluctuate strongly on a time scale of a few eddy
turnover times. These seem to stabilize as the rotation rate is increased until
the limit of the code resolution is reached.Comment: 26 pages, 17 figures, submitted to New Journal of Physic
Two-component {CH} system: Inverse Scattering, Peakons and Geometry
An inverse scattering transform method corresponding to a Riemann-Hilbert
problem is formulated for CH2, the two-component generalization of the
Camassa-Holm (CH) equation. As an illustration of the method, the multi -
soliton solutions corresponding to the reflectionless potentials are
constructed in terms of the scattering data for CH2.Comment: 22 pages, 3 figures, draft, please send comment
Sport, physical activity and the establishment of Health and Wellbeing Boards in Nottingham and Nottinghamshire
This paper will examine the emergence of Health and Wellbeing Boards in Nottinghamshire and the City of Nottingham and explore the implications for sport and physical activity. At the time of writing the transfer of responsibilities for Public Health and the establishment of Health and Wellbeing Boards in both the City of Nottingham and within Nottinghamshire County Council are considered to be relatively advanced by the Strategic Health Authorities, the respective local authorities and by the boards of the two Primary Care Trusts. "Shadow"ďż˝ Health and Well being Board have been established in both authorities and they have been meeting regularly for s everal months. Public health and commissioning staff have also been successfully relocated and new strategies and priorities are starting to emerge. Nottingham and Nottinghamshire have traditionally acknowledged the role of sport and physical activity to the wider determinants of public health and given a relatively high priority to the contribution that sport and physical activity can make to the ir preventative health and early intervention agendas. This paper will look at the transition to Health and Wellbeing boards to assess how the role of sport and physical activity may be changing and to identify opportunities for its contribution to policy and practise in the future. It will examine both the theory and practise behind the emerging governance arr angements, the strategic objectives and priorities, and the developing evidential base for future policy and delivery within the two areas
Laminin increases both levels and activity of tyrosine hydroxylase in calf adrenal chromaffin cells.
Impact of a Viscous Liquid Drop
We simulate the impact of a viscous liquid drop onto a smooth dry solid
surface. As in experiments, when ambient air effects are negligible, impact
flattens the falling drop without producing a splash. The no-slip boundary
condition at the wall produces a boundary layer inside the liquid. Later, the
flattening surface of the drop traces out the boundary layer. As a result, the
eventual shape of the drop is a "pancake" of uniform thickness except at the
rim, where surface tension effects are significant. The thickness of the
pancake is simply the height where the drop surface first collides with the
boundary layer.Comment: 5 pages, 3 figures, submitted to Physical Review Letter
Obesity: A Biobehavioral Point of View
Excerpt: If you ask an overweight person, “Why are you fat?’, you will, almost invariably, get the answer, “Because 1 eat too much.” You will get this answer in spite of the fact that of thirteen studies, six find no significant differences in the caloric intake of obese versus nonobese subjects, five report that the obese eat significantly less than the nonobese, and only two report that they eat significantly more
Regional-Specific Effects of Ovarian Hormone Loss on Synaptic Plasticity in Adult Human APOE Targeted Replacement Mice
The human apolipoprotein ε4 allele (APOE4) has been implicated as one of the strongest genetic risk factors associated with Alzheimer’s disease (AD) and in influencing normal cognitive functioning. Previous studies have demonstrated that mice expressing human apoE4 display deficits in behavioral and neurophysiological outcomes compared to those with apoE3. Ovarian hormones have also been shown to be important in modulating synaptic processes underlying cognitive function, yet little is known about how their effects are influenced by apoE. In the current study, female adult human APOE targeted replacement (TR) mice were utilized to examine the effects of human APOE genotype and long-term ovarian hormone loss on synaptic plasticity in limbic regions by measuring dendritic spine density and electrophysiological function. No significant genotype differences were observed on any outcomes within intact mice. However, there was a significant main effect of genotype on total spine density in apical dendrites in the hippocampus, with post-hoc t-tests revealing a significant reduction in spine density in apoE3 ovariectomized (OVX) mice compared to sham operated mice. There was also a significant main effect of OVX on the magnitude of LTP, with post-hoc t-tests revealing a decrease in apoE3 OVX mice relative to sham. In contrast, apoE4 OVX mice showed increased synaptic activity relative to sham. In the lateral amygdala, there was a significant increase in total spine density in apoE4 OVX mice relative to sham. This increase in spine density was consistent with a significant increase in spontaneous excitatory activity in apoE4 OVX mice. These findings suggest that ovarian hormones differentially modulate synaptic integrity in an apoE-dependent manner within brain regions that are susceptible to neurophysiological dysfunction associated with AD
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