522 research outputs found
Solar-like oscillations of semiregular variables
Oscillations of the Sun and solar-like stars are believed to be excited
stochastically by convection near the stellar surface. Theoretical modeling
predicts that the resulting amplitude increases rapidly with the luminosity of
the star. Thus one might expect oscillations of substantial amplitudes in red
giants with high luminosities and vigorous convection. Here we present evidence
that such oscillations may in fact have been detected in the so-called
semiregular variables, extensive observations of which have been made by
amateur astronomers in the American Association for Variable Star Observers
(AAVSO). This may offer a new opportunity for studying the physical processes
that give rise to the oscillations, possibly leading to further information
about the properties of convection in these stars.Comment: Astrophys. J. Lett., in the press. Processed with aastex and
emulateap
The underlying physical meaning of the relation
Asteroseismology of stars that exhibit solar-like oscillations are enjoying a
growing interest with the wealth of observational results obtained with the
CoRoT and Kepler missions. In this framework, scaling laws between
asteroseismic quantities and stellar parameters are becoming essential tools to
study a rich variety of stars. However, the physical underlying mechanisms of
those scaling laws are still poorly known. Our objective is to provide a
theoretical basis for the scaling between the frequency of the maximum in the
power spectrum () of solar-like oscillations and the cut-off
frequency (). Using the SoHO GOLF observations together with
theoretical considerations, we first confirm that the maximum of the height in
oscillation power spectrum is determined by the so-called \emph{plateau} of the
damping rates. The physical origin of the plateau can be traced to the
destabilizing effect of the Lagrangian perturbation of entropy in the
upper-most layers which becomes important when the modal period and the local
thermal relaxation time-scale are comparable. Based on this analysis, we then
find a linear relation between and , with a
coefficient that depends on the ratio of the Mach number of the exciting
turbulence to the third power to the mixing-length parameter.Comment: 8 pages, 11 figures. Accepted in A&
Viscoplastic boundary layers
In the limit of a large yield stress, or equivalently at the initiation of motion, viscoplastic flows can develop narrow boundary layers that provide either surfaces of failure between rigid plugs, the lubrication between a plugged flow and a wall or buffers for regions of predominantly plastic deformation. Oldroyd (Proc. Camb. Phil. Soc., vol. 43, 1947, pp. 383-395) presented the first theoretical discussion of these viscoplastic boundary layers, offering an asymptotic reduction of the governing equations and a discussion of some model flow problems. However, the complicated nonlinear form of Oldroyd's boundary-layer equations has evidently precluded further discussion of them. In the current paper, we revisit Oldroyd's viscoplastic boundary-layer analysis and his canonical examples of a jet-like intrusion and flow past a thin plate. We also consider flow down channels with either sudden expansions or wavy walls. In all these examples, we verify that viscoplastic boundary layers form as envisioned by Oldroyd. For each example, we extract the dependence of the boundary-layer thickness and flow profiles on the dimensionless yield-stress parameter (Bingham number). We find that, while Oldroyd's boundary-layer theory applies to free viscoplastic shear layers, it does not apply when the boundary layer is adjacent to a wall, as has been observed previously for two-dimensional flow around circular obstructions. Instead, the boundary-layer thickness scales in a different fashion with the Bingham number, as suggested by classical solutions for plane-parallel flows, lubrication theory and, for flow around a plate, by Piau (J. Non-Newtonian Fluid Mech., vol. 102, 2002, pp. 193-218); we rationalize this second scaling and provide an alternative boundary-layer theory
Viscoplastic boundary layers
In the limit of a large yield stress, or equivalently at the initiation of motion, viscoplastic
flows can develop narrow boundary layers that provide either surfaces of failure between
rigid plugs, the lubrication between plugged flow and a wall, or buffers for regions of
predominantly plastic deformation. (Oldroyd 1947, Proc. Camb. Phil. Soc. 43, 383 - 395)
presented the first theoretical discussion of these viscoplastic boundary layers, offering
an asymptotic reduction of the governing equations and a discussion of some model
flow problems. However, the complicated nonlinear form of Oldroydâs boundary-layer
equations has evidently precluded further discussion of them. In the current paper,
we revisit Oldroydâs viscoplastic boundary-layer analysis and his canonical examples of
a jet-like intrusion and flow past a thin plate. We also consider flow down channels
with either sudden expansions or wavy walls. In all these examples, we verify that
viscoplastic boundary layers form as envisioned by Oldroyd. For each example, we extract
the dependence of the boundary-layer thickness and flow profiles on the dimensionless
yield-stress parameter (Bingham number). We find that, while Oldroydâs boundary-layer
theory applies to free viscoplastic shear layers, it does not apply when the boundary
layer is adjacent to a wall, as has been observed previously for two-dimensional flow
around circular obstructions. Instead, the boundary-layer thickness scales in a different
fashion with the Bingham number, as suggested by classical solutions for plane-parallel
flows, lubrication theory and, for flow around a plate, by (Piau 2002, J. Non-Newtonian
Fluid Mech. 102, 193 - 218); we rationalize this second scaling and provide an alternative
boundary-layer theory
Cardiac amyloidosis in non-transplant cardiac surgery
Cardiac amyloidosis is a rare infiltrative cardiomyopathy that portends a poor prognosis. There is a growing recognition of co-existent aortic valve stenosis and transthyretin cardiac amyloidosis, with some studies suggesting that dual pathology may be associated increased risk of complication and mortality during surgical intervention. This review aims to evaluate the available literature on non-transplant cardiac surgical interventions in patients with cardiac amyloidosis, with particular focus on diagnosis, high surgical risk and areas of uncertainty that require further research
Non-Newtonian thin films with normal stresses: dynamics and spreading
The dynamics of thin films on a horizontal solid substrate is investigated in
the case of non-Newtonian fluids exhibiting normal stress differences, the
rheology of which is strongly non-linear. Two coupled equations of evolution
for the thickness of the film and the shear rate are proposed within the
lubrication approximation. This framework is applied to the motion of an
advancing contact line. The apparent dynamic contact angle is found to depend
logarithmically on a lengthscale determined solely by the rheological
properties of the fluid and the velocity of the contact line
Visco-plastic models of isothermal lava domes
Author Posting. © Cambridge University Press, 2000. This article is posted here by permission of Cambridge University Press for personal use, not for redistribution. The definitive version was published in Journal of Fluid Mechanics 403 (2000): 37-65, doi:10.1017/S0022112099006916.The dynamics of expanding domes of isothermal lava are studied by treating the lava as a viscoplastic material with the HerschelâBulkley constitutive law. Thin-layer theory is developed for radially symmetric extrusions onto horizontal plates. This provides an evolution equation for the thickness of the fluid that can be used to model expanding isothermal lava domes. Numerical and analytical solutions are derived that explore the effects of yield stress, shear thinning and basal sliding on the dome evolution. The results are briefly compared with an experimental study. It is found that it is difficult to unravel the combined effects of shear thinning and yield stress; this may prove important to studies that attempt to infer yield stress from morphology of flowing lava.The financial support
of an EPSRC Advanced Fellowship is gratefully acknowledged by R.V. C. N. J. B. was partially supported by the NSF
Grant OCE-9616017 and an EPSRC Visiting Fellowship Grant GR/M50409
Structure of the near-surface layers of the Sun: asphericity and time variation
We present results on the structure of the near-surface layers of the Sun
obtained by inverting frequencies of high-degree solar modes from "ring
diagrams". We have results for eight epochs between June 1996 and October 2003.
The frequencies for each epoch were obtained from ring diagrams constructed
from MDI Dopplergrams spanning complete Carrington rotations. We find that
there is a substantial latitudinal variation of both sound speed and the
adiabatic index Gamma_1 in the outer 2% of the Sun. We find that both the
sound-speed and Gamma_1 profiles change with changes in the level of solar
activity. In addition, we also study differences between the northern and
southern hemispheres of the Sun and find a small asymmetry that appears to
reflect the difference in magnetic activity between the two hemispheres.Comment: To appear in ApJ (January 2007
Isolation and characterization of glyoxylate dehydrogenase from the fungus Sclerotium rolfsii
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