2,214 research outputs found
Gravitational radiation from nonaxisymmetric spherical Couette flow in a neutron star
The gravitational wave signal generated by global, nonaxisymmetric shear
flows in a neutron star is calculated numerically by integrating the
incompressible Navier--Stokes equation in a spherical, differentially rotating
shell. At Reynolds numbers \Rey \gsim 3 \times 10^{3}, the laminar Stokes
flow is unstable and helical, oscillating Taylor--G\"ortler vortices develop.
The gravitational wave strain generated by the resulting kinetic-energy
fluctuations is computed in both and polarizations as a function
of time. It is found that the signal-to-noise ratio for a coherent,
-{\rm s} integration with LIGO II scales as for a star at 1 {\rm kpc} with angular velocity
. This should be regarded as a lower limit: it excludes pressure
fluctuations, herringbone flows, Stuart vortices, and fully developed
turbulence (for \Rey \gsim 10^{6}).Comment: (1) School of Physics, University of Melbourne, Parkville, VIC 3010,
Australia. (2) Departamento de Fisica, Escuela de Ciencias,Universidad de
Oriente, Cumana, Venezuela, (3) Department of Mechanical Engineering,
University of Melbourne, Parkville, VIC 3010, Australia. Accepted for
publication in The Astrophysical Journal Letter
Global three-dimensional flow of a neutron superfluid in a spherical shell in a neutron star
We integrate for the first time the hydrodynamic
Hall-Vinen-Bekarevich-Khalatnikov equations of motion of a -paired
neutron superfluid in a rotating spherical shell, using a pseudospectral
collocation algorithm coupled with a time-split fractional scheme. Numerical
instabilities are smoothed by spectral filtering. Three numerical experiments
are conducted, with the following results. (i) When the inner and outer spheres
are put into steady differential rotation, the viscous torque exerted on the
spheres oscillates quasiperiodically and persistently (after an initial
transient). The fractional oscillation amplitude () increases
with the angular shear and decreases with the gap width. (ii) When the outer
sphere is accelerated impulsively after an interval of steady differential
rotation, the torque increases suddenly, relaxes exponentially, then oscillates
persistently as in (i). The relaxation time-scale is determined principally by
the angular velocity jump, whereas the oscillation amplitude is determined
principally by the gap width. (iii) When the mutual friction force changes
suddenly from Hall-Vinen to Gorter-Mellink form, as happens when a rectilinear
array of quantized Feynman-Onsager vortices is destabilized by a counterflow to
form a reconnecting vortex tangle, the relaxation time-scale is reduced by a
factor of compared to (ii), and the system reaches a stationary state
where the torque oscillates with fractional amplitude about a
constant mean value. Preliminary scalings are computed for observable
quantities like angular velocity and acceleration as functions of Reynolds
number, angular shear, and gap width. The results are applied to the timing
irregularities (e.g., glitches and timing noise) observed in radio pulsars.Comment: 6 figures, 23 pages. Accepted for publication in Astrophysical
Journa
Magnetothermal instabilities in magnetized anisotropic plasmas
Using the transport equations for an ideal anisotropic collisionless plasma
derived from the Vlasov equation by the 16-moment method, we analyse the
influence of pressure anisotropy exhibited by collisionless magnetized plasmas
on the magnetothermal (MTI) and heat-flux-driven buoyancy (HBI) instabilities.
We calculate the dispersion relation and the growth rates for these
instabilities in the presence of a background heat flux and for configurations
with static pressure anisotropy, finding that when the frequency at which heat
conduction acts is much larger than any other frequency in the system (i.e.
weak magnetic field) the pressure anisotropy has no effect on the MTI/HBI,
provided the degree of anisotropy is small. In contrast, when this ordering of
timescales does not apply the instability criteria depend on pressure
anisotropy. Specifically, the growth time of the instabilities in the
anisotropic case can be almost one order of magnitude smaller than its
isotropic counterpart. We conclude that in plasmas where pressure anisotropy is
present the MTI/HBI are modified. However, in environments with low magnetic
fields and small anisotropy such as the ICM the results obtained from the
16-moment equations under the approximations considered are similar to those
obtained from ideal MHD.Comment: v3: 16 pages, 2 figures, fixed typos, added references and a final
note on related wor
Suppressed-scattering spectral windows for radiative cooling applications
The scattering of light by resonant nanoparticles is a key process for enhancing the solar reflectance in daylight radiative cooling. Here, we investigate the impact of material dispersion on the scattering performance of popular nanoparticles for radiative cooling applications. We show that, due to material dispersion, nanoparticles with a qualitatively similar response at visible frequencies exhibit fundamentally different scattering properties at infrared frequencies. It is found that dispersive nanoparticles exhibit suppressed-scattering windows, allowing for selective thermal emission within a highly reflective sample. The existence of suppressed-scattering windows solely depends on material dispersion, and they appear pinned to the same wavelength even in random composite materials and periodic metasurfaces. Finally, we investigate calcium-silicate-hydrate (CSH), the main phase of concrete, as an example of a dispersive host, illustrating that the co-design of nanoparticles and host allows for tuning of the suppressed-scattering windows. Our results indicate that controlled nanoporosities would enable concrete with daylight passive radiative cooling capabilities
Análisis numérico de la fisuración superficial de estructuras de hormigón armado por efecto de la corrosión
In this work the mechanical behavior of the covering is analyzed, using the finite element method, when the same one is put under a circumferential pressure in the zone of contact with the reinforcement. This pressure simulates the effect generated by the corrosion products and is valued from experimental studies of other authors.The crack process is associate with the evolution of the originated stresses. The zones that reach the permissible tension stresses, allow to locate the appearance of the cracks. In this work the covering/diameter (C/D) ratio and bar position influence is analyzed. The pressure necessary to generate cracks is obtained relating it to lost of reinforcement radius, obtained experimentally by other authors. This interrelation let to arrive to an expression that allow to consider this pressure as a function of the relation cover/reinforcement diameter and of the concrete quality.El proceso de corrosión de armaduras ubicadas dentro del hormigón da origen a la aparición de fisuras lo que reduce la capacidad portante y aumenta las deformaciones de los elementos afectados. Esto tiene directa relación con los estados lÃmites últimos y de servicio, afectando la durabilidad de las estructuras.Diversas variables influyen sobre dicho proceso corrosivo y sobre la generación y propagación de fisuras longitudinales en la superficie del elemento. Existen numerosos estudios experimentales y numéricos que abordan el tema, efectuando el seguimiento de dichas fisuras.El objetivo de este trabajo es estudiar la influencia de la relación adimensional recubrimiento de hormigón / diámetro de barra (C/D) en función de la posición de la barra, y estimar la presión necesaria para generar la fisura. Al efecto se emplea el Método de los Elementos Finitos utilizando un modelo elástico lineal, para simular el proceso de fisuración del recubrimiento en elementos de hormigón armado. La acción sobre el hormigón, provocada por los productos de corrosión, se materializa mediante una presión actuando radialmente sobre el hormigón adyacente a la barra. Se considera que las zonas en que la tensión alcanza la resistencia caracterÃstica de tracción del hormigón permiten establecer la ubicación de las fisuras y la presión que las origina. Esta última se ha podido relacionar con la pérdida de radio determinada empÃricamente por otros autores basados en resultados experimentales, permitiendo llegar a una expresión para estimar dicha presión en función de la relación C/D y de la calidad del hormigón
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