Página 18 dos resultados de 33122 itens digitais encontrados em 0.029 segundos

## Dynamical bar-mode instability in rotating and magnetized relativistic stars

Franci, Luca; De Pietri, Roberto; Dionysopoulou, Kyriaki; Rezzolla, Luciano
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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We present three-dimensional simulations of the dynamical bar-mode instability in magnetized and differentially rotating stars in full general relativity. Our focus is on the effects that magnetic fields have on the dynamics and the onset of the instability. In particular, we perform ideal-magnetohydrodynamics simulations of neutron stars that are known to be either stable or unstable against the purely hydrodynamical instability, but to which a poloidal magnetic field in the range of $10^{14}$--$10^{16}$ G is superimposed initially. As expected, the differential rotation is responsible for the shearing of the poloidal field and the consequent linear growth in time of the toroidal magnetic field. The latter rapidly exceeds in strength the original poloidal one, leading to a magnetic-field amplification in the the stars. Weak initial magnetic fields, i.e. $\lesssim 10^{15}$ G, have negligible effects on the development of the dynamical bar-mode instability, simply braking the stellar configuration via magnetic-field shearing, and over a timescale for which we derived a simple algebraic expression. On the other hand, strong magnetic fields, i.e. $\gtrsim 10^{16}$ G, can suppress the instability completely, with the precise threshold being dependent also on the amount of rotation. As a result...

## Potential vorticity dynamics in the framework of disk shallow-water theory: II. Mixed Barotropic-Baroclinic Instability

Umurhan, O. M.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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We extend exploration of potential vorticity instabilities in cold astrophysical disks whose mean states are baroclinic. In particular, we seek to demonstrate the potential existence of traditional baroclinic instabilities of meteorological studies in a simplified two-layer Philips Disk Model. Each disk layer is of constant but differing densities. The resulting mean azimuthal velocity profile shows a variation in the vertical direction implying that the system is baroclinic in the mean state. The stability of the system is treated in the context of disk shallow water theory wherein azimuthal disturbances are much longer than the corresponding radial or vertical scales. The normal-mode problem is solved numerically using two different methods. The results of a symmetric single layer barotropic model is considered and it is found that instability persists for models in which the potential vorticity profiles are not symmetric, consistent with previous results. The instaiblity is interpreted in terms of interacting Rossby waves. For a two layer system in which the flow is fundamentally baroclinic we report here that instability takes on the form of mixed barotropic-baroclinic type: instability occurs but it qualitatively follows the pattern of instability found in the barotropic models. Instability arises because of the phase locking and interaction of the Rossby waves between the two layers. The strength of the instability weakens as the density contrast between layers increases. (For full abstract see article.); Comment: Under consideration for publication in Astronomy and Astrophysics. Comments to the author welcome

## Spinning down newborn neutron stars: nonlinear development of the r-mode instability

Bondarescu, Ruxandra; Teukolsky, Saul A.; Wasserman, Ira
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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We model the nonlinear saturation of the r-mode instability via three-mode couplings and the effects of the instability on the spin evolution of young neutron stars. We include one mode triplet consisting of the r-mode and two near resonant inertial modes that couple to it. We find that the spectrum of evolutions is more diverse than previously thought. The evolution of the star is dynamic and initially dominated by fast neutrino cooling. Nonlinear effects become important when the r-mode amplitude grows above its first parametric instability threshold. The balance between neutrino cooling and viscous heating plays an important role in the evolution. Depending on the initial r-mode amplitude, and on the strength of the viscosity and of the cooling this balance can occur at different temperatures. If thermal equilibrium occurs on the r-mode stability curve, where gravitational driving equals viscous damping, the evolution may be adequately described by a one-mode model. Otherwise, nonlinear effects are important and lead to various more complicated scenarios. Once thermal balance occurs, the star spins-down oscillating between thermal equilibrium states until the instability is no longer active. For lower viscosity we observe runaway behavior in which the r-mode amplitude passes several parametric instability thresholds. In this case more modes need to be included to model the evolution accurately. In the most optimistic case...

## A search for non-pulsating, chemically normal stars in the $\delta$ Scuti instability strip using Kepler data

Murphy, Simon J.; Bedding, Timothy R.; Niemczura, Ewa; Kurtz, Donald W.; Smalley, Barry
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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We identify stars in the $\delta$ Sct instability strip that do not pulsate in p modes at the 50-$\mu$mag limit, using Kepler data. Spectral classification and abundance analyses from high-resolution spectroscopy allow us to identify chemically peculiar stars, in which the absence of pulsations is not surprising. The remaining stars are chemically normal, yet they are not $\delta$ Sct stars. Their lack of observed p modes cannot be explained through any known mechanism. However, they are mostly distributed around the edges of the $\delta$ Sct instability strip, which allows for the possibility that they actually lie outside the strip once the uncertainties are taken into account. We investigated the possibility that the non-pulsators inside the instability strip could be unresolved binary systems, having components that both lie outside the instability strip. If misinterpreted as single stars, we found that such binaries could generate temperature discrepancies of $\sim$300 K -- larger than the spectroscopic uncertainties, and fully consistent with the observations. After these considerations, there remains one chemically normal non-pulsator that lies in the middle of the instability strip. This star is a challenge to pulsation theory. However...

## Forming Planetesimals by Gravitational Instability: I. The Role of the Richardson Number in Triggering the Kelvin-Helmholtz Instability

Lee, Aaron T.; Chiang, Eugene; Asay-Davis, Xylar; Barranco, Joseph
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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Gravitational instability (GI) of a dust-rich layer at the midplane of a gaseous circumstellar disk is one proposed mechanism to form planetesimals, the building blocks of rocky planets and gas giant cores. Self-gravity competes against the Kelvin-Helmholtz instability (KHI): gradients in dust content drive a vertical shear which risks overturning the dusty subdisk and forestalling GI. To understand the conditions under which the disk can resist the KHI, we perform 3D simulations of stratified subdisks in the limit that dust particles are small and aerodynamically well coupled to gas. This limit screens out the streaming instability and isolates the KHI. Each subdisk is assumed to have a vertical density profile given by a spatially constant Richardson number Ri. We vary Ri and the midplane dust-to-gas ratio mu and find that the critical Richardson number dividing KH-unstable from KH-stable flows is not unique; rather Ri_crit grows nearly linearly with mu for mu=0.3-10. Only for disks of bulk solar metallicity is Ri_crit ~ 0.2, close to the classical value. Our results suggest that a dusty sublayer can gravitationally fragment and presumably spawn planetesimals if embedded within a solar metallicity gas disk ~4x more massive than the minimum-mass solar nebula; or a minimum-mass disk having ~3x solar metallicity; or some intermediate combination of these two possibilities. Gravitational instability seems possible without resorting to the streaming instability or to turbulent concentration of particles.; Comment: Accepted to ApJ

## Thermo-Resistive Instability of Hot Planetary Atmospheres

Menou, Kristen
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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The atmospheres of hot Jupiters and other strongly-forced exoplanets are susceptible to a thermal instability in the presence of ohmic dissipation, weak magnetic drag and strong winds. The instability occurs in radiatively-dominated atmospheric regions when the ohmic dissipation rate increases with temperature faster than the radiative (cooling) rate. The instability domain covers a specific range of atmospheric pressures and temperatures, typically P ~ 3-300 mbar and T ~ 1500-2500K for hot Jupiters, which makes it a candidate mechanism to explain the dayside thermal "inversions" inferred for a number of such exoplanets. The instability is suppressed by high levels of non-thermal photoionization, in possible agreement with a recently established observational trend. We highlight several shortcomings of the instability treatment presented here. Understanding the emergence and outcome of the instability, which should result in locally hotter atmospheres with stronger levels of drag, will require global non-linear atmospheric models with adequate MHD prescriptions.; Comment: 13 pages, 3 figures, accepted for publication in ApJL

## On the linear growth mechanism driving the stationary accretion shock instability

Guilet, Jerome; Foglizzo, Thierry
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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## The Role of the Radial Orbit Instability in Dark Matter Halo Formation and Structure

Bellovary, Jillian M.; Dalcanton, Julianne J.; Babul, Arif; Quinn, Thomas R.; Maas, Ryan W.; Austin, Crystal G.; Williams, Liliya L. R.; Barnes, Eric I.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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For a decade, N-body simulations have revealed a nearly universal dark matter density profile, which appears to be robust to changes in the overall density of the universe and the underlying power spectrum. Despite its universality, the physical origin of this profile has not yet been well understood. Semi--analytic models by Barnes et al. (2005) have suggested that the density structure of dark matter halos is determined by the onset of the radial orbit instability (ROI). We have tested this hypothesis using N-body simulations of collapsing dark matter halos with a variety of initial conditions. For dynamically cold initial conditions, the resulting halo structures are triaxial in shape, due to the mild aspect of the instability. We examine how variations in initial velocity dispersion affect the onset of the instability, and find that an isotropic velocity dispersion can suppress the ROI entirely, while a purely radial dispersion does not. The quantity sigma^2/vc^2 is a criterion for instability, where regions with sigma^2/vc^2 <~1 become triaxial due to the ROI or other perturbations. We also find that the radial orbit instability sets a scale length at which the velocity dispersion changes rapidly from isotropic to radially anisotropic. This scale length is proportional to the radius at which the density profile changes shape...

## Growth rate and the cutoff wavelength of the Darrieus-Landau instability in laser ablation

Modestov, Mikhail; Bychkov, Vitaly; Valiev, Damir; Marklund, Mattias
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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The main characteristics of the linear Darrieus-Landau instability in the laser ablation flow are investigated. The dispersion relation of the instability is found numerically as a solution to an eigenvalue stability problem, taking into account the continuous structure of the flow. The results are compared to the classical Darrieus-Landau instability of a usual slow flame. The difference between the two cases is due to the specific features of laser ablation: high plasma compression and strong temperature dependence of electron thermal conduction. It is demonstrated that the Darrieus-Landau instability in laser ablation is much stronger than in the classical case. In particular, the maximum growth rate in the case of laser ablation is about three times larger than that for slow flames. The characteristic length scale of the Darrieus-Landau instability in the ablation flow is comparable to the total distance from the ablation zone to the critical zone of laser light absorption. The possibility of experimental observations of the Darrieus-Landau instability in laser ablation is discussed.; Comment: 16 pages, 10 figures

## Equilibrium Electro-osmotic Instability

Rubinstein, Isaak; Zaltzman, Boris
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
26.47%
Since its prediction fifteen years ago, electro-osmotic instability has been attributed to non-equilibrium electro-osmosis related to the extended space charge which develops at the limiting current in the course of concentration polarization at a charge-selective interface. This attribution had a double basis. Firstly, it has been recognized that equilibrium electro-osmosis cannot yield instability for a perfectly charge-selective solid. Secondly, it has been shown that non-equilibrium electro-osmosis can. First theoretical studies in which electro-osmotic instability was predicted and analyzed employed the assumption of perfect charge-selectivity for the sake of simplicity and so did the subsequent numerical studies of various time-dependent and nonlinear features of electro-osmotic instability. In this letter, we show that relaxing the assumption of perfect charge-selectivity (tantamount to fixing the electrochemical potential in the solid) allows for equilibrium electro-osmotic instability. Moreover, we suggest a simple experimental test for determining the true, either equilibrium or non-equilibrium, origin of electro-osmotic instability.; Comment: 5 pages, 4 figures, submitted

## Dependence of the large-scale vortex instability on latitude, stratification and domain size

Mantere, Maarit J.; Käpylä, Petri J.; Hackman, Thomas
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
26.47%
In an earlier study, we reported on the excitation of large-scale vortices in Cartesian hydrodynamical convection models subject to rapid enough rotation. In that study, the conditions of the onset of the instability were investigated in terms of the Reynolds (Re) and Coriolis (Co) numbers in models located at the stellar North pole. In this study, we extend our investigation to varying domain sizes, increasing stratification and place the box at different latitudes. The effect of the increasing box size is to increase the sizes of the generated structures, so that the principal vortex always fills roughly half of the computational domain. The instability becomes stronger in the sense that the temperature anomaly and change in the radial velocity are observed to be enhanced. The model with the smallest box size is found to be stable against the instability, suggesting that a sufficient scale separation between the convective eddies and the scale of the domain is required for the instability to work. The instability can be seen upto the co-latitude of 30 degrees, above which value the flow becomes dominated by other types of mean flows. The instability can also be seen in a model with larger stratification. Unlike the weakly stratified cases...

## Rayleigh-Taylor Instability in a Relativistic Fireball on a Moving Computational Grid

Duffell, Paul C.; MacFadyen, Andrew I.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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We numerically calculate the growth and saturation of the Rayleigh-Taylor instability caused by the deceleration of relativistic outflows with Lorentz factor {\Gamma} = 10, 30, and 100. The instability generates turbulence whose scale exhibits strong dependence on Lorentz factor, as only modes with angular size smaller than 1/{\Gamma} can grow. We develop a simple diagnostic to measure the kinetic energy in turbulent fluctuations, and calculate a ratio of turbulent kinetic energy to thermal energy of .03 in the region affected by the instability. Although our numerical calculation does not include magnetic fields, we argue that small scale turbulent dynamo amplifies magnetic fields to nearly this same fraction, giving a ratio of magnetic to thermal energy of ~ .01, to within a factor of two. The instability completely disrupts the contact discontinuity between the ejecta and the swept up circumburst medium. The reverse shock is stable, but is impacted by the Rayleigh-Taylor instability, which strengthens the reverse shock and pushes it away from the forward shock. The forward shock front is unaffected by the instability, but Rayleigh-Taylor fingers can penetrate of order 10% of the way into the energetic region behind the shock during the two-shock phase of the explosion. We calculate afterglow emission from the explosion and find the reverse shock emission peaks at a later time due to its reduced Lorentz factor and modified density and pressure at the shock front. These calculations are performed using a novel numerical technique that includes a moving computational grid. The moving grid is essential as it maintains contact discontinuities to high precision and can easily evolve flows with extremely large Lorentz factors.; Comment: ApJ Accepted

## Stability of rotating magnetized jets in the solar atmosphere. I. Kelvin-Helmholtz instability

Zaqarashvili, T. V.; Zhelyazkov, I.; Ofman, L.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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Observations show various jets in the solar atmosphere with significant rotational motions, which may undergo instabilities leading to heat ambient plasma. We study the Kelvin-Helmholtz (KH) instability of twisted and rotating jets caused by the velocity jumps near the jet surface. We derive a dispersion equation with appropriate boundary condition for total pressure (including centrifugal force of tube rotation), which governs the dynamics of incompressible jets. Then, we obtain analytical instability criteria of Kelvin-Helmholtz instability in various cases, which were verified by numerical solutions to the dispersion equation. We find that twisted and rotating jets are unstable to KH instability when the kinetic energy of rotation is more than the magnetic energy of the twist. Our analysis shows that the azimuthal magnetic field of 1-5 G can stabilize observed rotations in spicule/macrospicules and X-ray/EUV jets. On the other hand, non-twisted jets are always unstable to KH instability. In this case, the instability growth time is several seconds for spicule/macrospicules and few minutes (or less) for EUV/X-ray jets. We also find that standing kink and torsional Alfven waves are always unstable near the antinodes due to the jump of azimuthal velocity at the surface...

## On Modeling the Kelvin--Helmholtz Instability in Solar Atmosphere

Zhelyazkov, I.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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In the present review article, we discuss the recent developments in studying the Kelvin--Helmholtz (KH) instability of magnetohydrodynamic (MHD) waves propagating in various solar magnetic structures. The main description is on the modeling of KH instability developing in the coronal mass ejections (CMEs), and contributes to the triggering of wave turbulence subsequently leading to the coronal heating. KH instability of MHD waves in coronal active regions recently observed and imaged in unprecedented detail in EUV high cadence, high-resolution observations by SDO/AIA, and spectroscopic observations by Hinode/EIS instrument, is posing now challenge for its realistic modeling. It is shown that considering the solar mass flows of CMEs as moving cylindrical twisted magnetic flux tubes, the observed instability can be explained in terms of unstable m = -3 MHD mode. We also describe the occurrence of the KH instability in solar jets. The obtained critical jet speeds for the instability onset as well as the linear wave growth rates are in good agreement with the observational data of solar jets.; Comment: 27 pages, 5 figures, 2 figures added, new references added, accepted. arXiv admin note: text overlap with arXiv:1310.8106, arXiv:1301.2736 by other authors

## Secular Gravitational Instability of a Dust Layer in Shear Turbulence

Michikoshi, Shugo; Kokubo, Eiichiro; Inutsuka, Shu-ichiro
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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We perform a linear stability analysis of a dust layer in a turbulent gas disk. Youdin (2011) investigated the secular gravitational instability of a dust layer using hydrodynamic equations with a turbulent diffusion term. We obtain essentially the same result independently of Youdin (2011). In the present analysis, we restrict the area of interest to small dust particles, while investigating the secular gravitational instability in a more rigorous manner. We discuss the time evolution of the dust surface density distribution using a stochastic model and derive the advection-diffusion equation. The validity of the analysis by Youdin (2011) is confirmed in the strong drag limit. We demonstrate quantitatively that the finite thickness of a dust layer weakens the secular gravitational instability and that the density-dependent diffusion coefficient changes the growth rate. We apply the obtained results to the turbulence driven by the shear instability and find that the secular gravitational instability is faster than the radial drift when the gas density is three times as large as that in the minimum-mass disk model. If the dust particles are larger than chondrules, the secular gravitational instability grows within the lifetime of a protoplanetary disk.; Comment: 32 pages...

## Instabilities and transport in magnetized plasmas

Rosin, Mark
Fonte: University of Cambridge; Department of Applied Mathematics and Theoretical Physics; Darwin College Publicador: University of Cambridge; Department of Applied Mathematics and Theoretical Physics; Darwin College
Tipo: Thesis; doctoral; PhD
EN
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In a magnetized plasma, naturally occurring pressure anisotropies facilitate in- stabilities that are expected to modify the transport properties of the system. In this thesis we examine two such instabilities and, where appropriate, their effects on transport. First we consider the collisional (fluid) magnetized magnetorotational instability (MRI) in the presence of the Braginskii viscosity. We conduct a global linear analysis of the instability in a galactic rotation profile for three magnetic field configurations: purely azimuthal, purely vertical and slightly pitched. Our analysis, numerical and asymptotic, shows that the first two represent singular configurations where the Braginskii viscosity?s primary role is dissipative and the maximum growth rate is proportional to the Reynolds number when this is small. For a weak pitched field, the Braginskii viscosity is destabilising and when its effects dominate over the Lorentz force, the growth rate of the MRI can be up to 2?2 times faster than the inviscid limit. If the field is strong, an over-stability develops and both the real and imaginary parts of the frequency increase with the coefficient of the viscosity. Second, in the context of the ICM of galaxy clusters, we consider the pressure-anisotropy-driven firehose instability. The linear instability is fast (? ion cyclotron period) and small-scale (ion Larmor radius ?i) and so fluid theory is inapplicable. We determine its nonlinear evolution in an ab initio kinetic calculation (for parallel gradients only). We use a particular physical asymptotic ordering to derive a closed nonlinear equation for the firehose turbulence...

## Magnetic effects on the low-T/|W| instability in differentially rotating neutron stars

Muhlberger, Curran D.; Nouri, Fatemeh Hossein; Duez, Matthew D.; Foucart, Francois; Kidder, Lawrence E.; Ott, Christian D.; Scheel, Mark A.; Szilágyi, Béla; Teukolsky, Saul A.
Fonte: American Physical Society Publicador: American Physical Society
Tipo: Article; PeerReviewed Formato: application/pdf; application/pdf
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Dynamical instabilities in protoneutron stars may produce gravitational waves whose observation could shed light on the physics of core-collapse supernovae. When born with sufficient differential rotation, these stars are susceptible to a shear instability (the “low-T/|W| instability”), but such rotation can also amplify magnetic fields to strengths where they have a considerable impact on the dynamics of the stellar matter. Using a new magnetohydrodynamics module for the Spectral Einstein Code, we have simulated a differentially-rotating neutron star in full 3D to study the effects of magnetic fields on this instability. Though strong toroidal fields were predicted to suppress the low-T/|W| instability, we find that they do so only in a small range of field strengths. Below 4×10^(13) G, poloidal seed fields do not wind up fast enough to have an effect before the instability saturates, while above 5×10^(14) G, magnetic instabilities can actually amplify a global quadrupole mode (this threshold may be even lower in reality, as small-scale magnetic instabilities remain difficult to resolve numerically). Thus, the prospects for observing gravitational waves from such systems are not in fact diminished over most of the magnetic parameter space. Additionally...

## Surge Instability on a Cavitating Propeller

Duttweiler, Mark E.; Brennen, Christopher E.
Fonte: Instituto de Tecnologia da Califórnia Publicador: Instituto de Tecnologia da Califórnia
Tipo: Conference or Workshop Item; PeerReviewed Formato: application/pdf
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This study details experiments investigating a previously unrecognized surge instability on a cavitating propeller in a water tunnel. The surge instability is furst explored through visual observation of the cavitation on the propeller blades and in the tip vortices. Similarities between the instability and previously documented cavitation phenomena are noted. Measurements of the radiated pressure are then obtained, and the acoustic signature of the instability is identified. The magnitudes of the fluctuating pressures are very large, presumably capable of producing sever hull vibration on a ship. The origins of the instability are explored through separate investigation of the cavitation dynamics and the response of the water tunnel to volumetric displacement in the working section. Experiments are conducted to quantify the dynamics of the propeller vacitation. Finally, a model is developed for the complete system, incorporating both the cavitation and facility dynamics. The model predicts active system dynamics (linked to the mass flow gain factor familiar in the context of pump dynamics) and therefore potentially unstable behavior for two distinct frequency ranges, one of which appears to be responsible for the instability.

## Surge Instability on a Cavitating Propeller

Duttweiler, M. E.; Brennen, C. E.
Fonte: Instituto de Tecnologia da Califórnia Publicador: Instituto de Tecnologia da Califórnia
Tipo: Article; PeerReviewed Formato: application/pdf