arXiv subjects

Xing Wei

Publications and source records attributed to Xing Wei.

At least 109 records · Page 6Linked to original sources

arXiv2019

Surface tide on a rapidly rotating body

By solving Laplace's tidal equations with friction terms we study the surface tide on a rapidly rotating body. When $ε=Ω^2 R/g$, the square of the ratio of dynamical timescale to rotational timescale, is very small for the Earth the asymptotic result was derived. When it is not so small, e.g. a rapidly rotating star or planet, we perform numerical calculations. It is found that when rotation is sufficiently fast ($ε$ reaches $0.1$) a great amount of tidal resonances appear. To generate the same level of tide, a faster rotation corresponds to a lower tidal frequency. Friction suppresses tidal resonances but cannot completely suppress them at fast rotation. The thickness of fluid layer can change tidal resonances but this change becomes weaker at faster rotation. This result is of help to understanding the tides in the atmosphere of a rapidly rotating star or planet or in the ocean of a neutron star.

astro-ph.SR↗

arXiv2018

Embedding Electronic Health Records for Clinical Information Retrieval

Neural network representation learning frameworks have recently shown to be highly effective at a wide range of tasks ranging from radiography interpretation via data-driven diagnostics to clinical decision support. This often superior performance comes at the price of dramatically increased training data requirements that cannot be satisfied in every given institution or scenario. As a means of countering such data sparsity effects, distant supervision alleviates the need for scarce in-domain data by relying on a related, resource-rich, task for training. This study presents an end-to-end neural clinical decision support system that recommends relevant literature for individual patients (few available resources) via distant supervision on the well-known MIMIC-III collection (abundant resource). Our experiments show significant improvements in retrieval effectiveness over traditional statistical as well as purely locally supervised retrieval models.

cs.IR↗

arXiv2018

Dynamo induced by time-periodic force

To understand the dynamo driven by time-dependent flow, e.g. turbulence, we investigate numerically the dynamo induced by time-periodic force in rotating magnetohydrodynamic flow and focus on the effect of force frequency on the dynamo action. It is found that the dynamo action depends on the force frequency. When the force frequency is near resonance the force can drive dynamo but when it is far away from resonance dynamo fails. In the frequency range near resonance to support dynamo, the force frequency at resonance induces a weak magnetic field and magnetic energy increases as the force frequency deviates from the resonant frequency. This is opposite to the intuition that a strong flow at resonance will induce a strong field. It is because magnetic field nonlinearly couples with fluid flow in the self-sustained dynamo and changes the resonance of driving force and inertial wave.

physics.flu-dyn↗

arXiv2018

Magnetic effect on dynamical tide in rapidly rotating astronomical objects

By numerically solving the equations of rotating magnetohydrodynamics (MHD), the magnetic effect on dynamical tide is studied. It is found that magnetic field has a significant impact not only on the flow structure, i.e. the internal shear layers in rotating flow can be destroyed in the presence of a moderate or stronger magnetic field (in the sense that the Alfvén velocity is at least of the order of 0.1 of the surface rotational velocity), but also on the dispersion relation of waves excited by tidal force such that the range of tidal resonance is broadened by magnetic field. A major result is that the total tidal dissipation scales as square of the field strength, which can be used to estimate the strength of internal magnetic field in the astronomical object of a binary system. Moreover, with a moderate or stronger field the ratio of the magnetic dissipation to the viscous dissipation is almost inversely proportional to magnetic Prandtl number (i.e. the ratio of viscosity to magnetic diffusivity), and therefore in the astrophysical situation at small magnetic Prandtl number magnetic dissipation dominates over viscous dissipation with a moderate or stronger field.

astro-ph.SR↗

arXiv2016

Numerical simulations of the Princeton magneto-rotational instability experiment with conducting axial boundaries

We investigate numerically the Princeton magneto-rotational instability (MRI) experiment and the effect of conducting axial boundaries or endcaps. MRI is identified and found to reach a much higher saturation than for insulating endcaps. This is probably due to stronger driving of the base flow by the magnetically rather than viscously coupled boundaries. Although the computations are necessarily limited to lower Reynolds numbers ($\Re$) than their experimental counterparts, it appears that the saturation level becomes independent of $\Re$ when $\Re$ is sufficiently large, whereas it has been found previously to decrease roughly as $\Re^{-1/4}$ with insulating endcaps. The much higher saturation levels will allow for the first positive detection of MRI beyond its theoretical and numerical predictions.

physics.flu-dyn↗

arXiv2016

The combined effect of precession and convection on the dynamo action

To understand the generation of the Earth's and planetary magnetic fields, we investigate numerically the combined effect of precession and convection on the dynamo action in a spherical shell. The convection alone, the precession alone and the combined effect of convection and precession are studied at the low Ekman number at which the precessing flow is already unstable. The key result is that although the precession or convection alone is not strong to support the dynamo action the combined effect of precession and convection can support the dynamo action because of the resonance of precessional and convective instabilities. This result may interpret why the geodynamo maintains for such a long history compared to the Martian dynamo.

astro-ph.EP↗

arXiv2016

Calculating rotating hydrodynamic and magneto-hydrodynamic waves to understand magnetic effects on dynamical tides

For understanding magnetic effects on dynamical tides, we study the rotating magneto-hydrodynamic (MHD) flow driven by harmonic forcing. The linear responses are analytically derived in a periodic box under the local WKB approximation. Both the kinetic and Ohmic dissipations at the resonant frequencies are calculated and the various parameters are investigated. Although magnetic pressure may be negligible compared to thermal pressure, magnetic field can be important for the first-order perturbation, e.g. dynamical tides. It is found that magnetic field splits the resonant frequency, namely the rotating hydrodynamic flow has only one resonant frequency but the rotating MHD flow has two, one positive and the other negative. In the weak field regime the dissipations are asymmetric around the two resonant frequencies and this asymmetry is more striking with a weaker magnetic field. It is also found that both the kinetic and Ohmic dissipations at the resonant frequencies are inversely proportional to the Ekman number and the square of wavenumber. The dissipation at the resonant frequency on small scales is almost equal to the dissipation at the non-resonant frequencies, namely the resonance takes its effect on the dissipation at intermediate length scales. Moreover, the waves with phase propagation perpendicular to magnetic field are much more damped. It is also interesting to find that the frequency-averaged dissipation is constant. This result suggests that in compact objects magnetic effects on tidal dissipation should be considered.

astro-ph.SR↗

arXiv2016

Superpixel Hierarchy

Superpixel segmentation is becoming ubiquitous in computer vision. In practice, an object can either be represented by a number of segments in finer levels of detail or included in a surrounding region at coarser levels of detail, and thus a superpixel segmentation hierarchy is useful for applications that require different levels of image segmentation detail depending on the particular image objects segmented. Unfortunately, there is no method that can generate all scales of superpixels accurately in real-time. As a result, a simple yet effective algorithm named Super Hierarchy (SH) is proposed in this paper. It is as accurate as the state-of-the-art but 1-2 orders of magnitude faster. The proposed method can be directly integrated with recent efficient edge detectors like the structured forest edges to significantly outperforms the state-of-the-art in terms of segmentation accuracy. Quantitative and qualitative evaluation on a number of computer vision applications was conducted, demonstrating that the proposed method is the top performer.

cs.CV↗

arXiv2016

Decay of isotropic flow and anisotropic flow with rotation or magnetic field or both in a weakly nonlinear regime

We investigate numerically the decay of isotropic, rotating, magnetohydrodynamic (MHD), and rotating MHD flows in a periodic box. The Reynolds number $Re$ defined with the box size and the initial velocity is $100$ at which the flows are in a weakly nonlinear regime, i.e. not laminar but far away from the fully turbulent state. The decay of isotropic flow has two stages, the first stage for the development of small scales and the second stage for the viscous dissipation. In the rapidly rotating flow, fast rotation induces the inertial wave and causes the large-scale structure to inhibit the development of the first stage and retard the flow decay. In the MHD flow, the imposed field also causes the large-scale structure but facilitates the flow decay in the first stage because of the energy conversion from flow to magnetic field. Magnetic Reynolds number $Rm$ is important for the dynamics of the MHD flow, namely a high $Rm$ induces the Alfvén wave but a low $Rm$ cannot. In the rotating MHD flow, slower rotation tends to convert more kinetic energy to magnetic energy. The orientation between the rotational and magnetic axes is important for the dynamics of the rotating MHD flow, namely the energy conversion is more efficient and the stronger wave is induced when the two axes are not parallel than when they are parallel.

physics.flu-dyn↗

arXiv2016

Linear and nonlinear responses to harmonic force in rotating flow

For understanding the dissipation in a rotating flow when resonance occurs, we study the rotating flow driven by the harmonic force in a periodic box. Both the linear and nonlinear regimes are studied. The various parameters such as the force amplitude $a$, the force frequency $ω$, the force wavenumber $k$, and the Ekman number $E$ are investigated. In the linear regime, the dissipation at the resonant frequency scales as $E^{-1}k^{-2}$, and it is much stronger than the dissipation at the non-resonant frequencies on the large scales and at the low Ekman numbers. In the nonlinear regime, at the resonant frequency the effective dissipation (dissipation normalised with the square of force amplitude) is lower than in the linear regime and it decreases with the increasing force amplitude. This nonlinear suppression effect is significant near the resonant frequency but negligible far away from the resonant frequency. Opposite to the linear regime, in the nonlinear regime at the resonant frequency the lower Ekman number leads to the lower dissipation because of the stronger nonlinear effect. This work implies that the previous linear calculations overestimated the tidal dissipation, which is important for understanding the tides in stars and giant planets.

astro-ph.SR↗

arXiv2015

On obliquely magnetized and differentially rotating stars

We investigate the interaction of differential rotation and a misaligned magnetic field. The incompressible magnetohydrodynamic equations are solved numerically for a free-decay problem. In the kinematic limit, differential rotation annihilates the non-axisymmetric field on a timescale proportional to the cube root of magnetic Reynolds number ($Rm$), as predicted by Rädler. Nonlinearly, the outcome depends upon the initial energy in the non-axisymmetric part of the field. Sufficiently weak fields approach axisymmetry as in the kinematic limit; some differential rotation survives across magnetic surfaces, at least on intermediate timescales. Stronger fields enforce uniform rotation and remain non-axisymmetric. The initial field strength that divides these two regimes does not follow the scaling $Rm^{-1/3}$ predicted by quasi-kinematic arguments, perhaps because our $Rm$ is never sufficiently large or because of reconnection. We discuss the possible relevance of these results to tidal synchronization and tidal heating of close binary stars, particularly double white dwarfs.

astro-ph.SR↗

arXiv2015

A simplified model of collision-driven dynamo action in small bodies

We investigate numerically the self-sustained dynamo action in a spinning sphere whose sense of rotation reverses periodically. This system serves as a simple model of a dynamo in small bodies powered by frequent collisions. It is found that dynamo action is possible in some intervals of collision rates. At high Ekman numbers the laminar spin-up flow is helical in the boundary layers and the Ekman circulation together with the azimuthal shear powers the dynamo action. At low Ekman number a non-axisymmetric instability helps the dynamo action. The intermittency of magnetic field occurs at low Ekman number.

physics.geo-ph↗

arXiv2014

Kinematic dynamo induced by helical waves

We investigate numerically the kinematic dynamo induced by the superposition of two helical waves in a periodic box as a simplified model to understand the dynamo action in astronomical bodies. The effects of magnetic Reynolds number, wavenumber and wave frequency on the dynamo action are studied. It is found that this helical-wave dynamo is a slow dynamo. There exists an optimal wavenumber for the dynamo growth rate. A lower wave frequency facilitates the dynamo action and the oscillations of magnetic energy emerge at some particular wave frequencies.

physics.flu-dyn↗

arXiv2014

DyCaPPON: Dynamic Circuit and Packet Passive Optical Network (Extended Version)

Dynamic circuits are well suited for applications that require predictable service with a constant bit rate for a prescribed period of time, such as cloud computing and e-science applications. Past research on upstream transmission in passive optical networks (PONs) has mainly considered packet-switched traffic and has focused on optimizing packet-level performance metrics, such as reducing mean delay. This study proposes and evaluates a dynamic circuit and packet PON (DyCaPPON) that provides dynamic circuits along with packet-switched service. DyCaPPON provides $(i)$ flexible packet-switched service through dynamic bandwidth allocation in periodic polling cycles, and $(ii)$ consistent circuit service by allocating each active circuit a fixed-duration upstream transmission window during each fixed-duration polling cycle. We analyze circuit-level performance metrics, including the blocking probability of dynamic circuit requests in DyCaPPON through a stochastic knapsack-based analysis. Through this analysis we also determine the bandwidth occupied by admitted circuits. The remaining bandwidth is available for packet traffic and we conduct an approximate analysis of the resulting mean delay of packet traffic. Through extensive numerical evaluations and verifying simulations we demonstrate the circuit blocking and packet delay trade-offs in DyCaPPON.

cs.NI↗

arXiv2013

Sign of helicity in the $α$-$Ω$ geodynamo

In this paper we discuss the sign of helicity in the $α$-$Ω$ dynamo and propose a wave-$Ω$ model in which the $α$ effect in the geodynamo is induced by helical wave but not helical flow as in the solar dynamo. We then tentatively interpret the mechanisms of the Earth's magnetic westward drift, tilt angle, and dipole reversals.

astro-ph.EP↗

arXiv2013

Local analysis of the magnetic instability in rotating magneto-hydrodynamics with the short-wavelength approximation

We investigate analytically the magnetic instability in a rotating and electrically conducting fluid induced by an imposed magnetic field with its associated electric current. The short-wavelength approximation is used in the linear stability analysis, i.e. the length scale of the imposed field is much larger than the wavelength of perturbations. The dispersion relationship is derived and then simplified to give the criteria for the onset of the magnetic instability in three cases of imposed field, namely the axial dependence, the radial dependence and the mixed case. The orientation of rotation, imposed field and imposed current is important for this instability.

astro-ph.EP↗

arXiv2013

Stratified precessional flow in spherical geometry

We investigate numerically in spherical geometry the interaction of stratification with precession. Both stable stratification and unstable stratification are studied. In the parameter regime we are concerned with, stable stratification suppresses the precessional instability, whereas unstable stratification and precession can either stablise or destablise each other at the different precession rates.

physics.flu-dyn↗

arXiv2012

Kinematic dynamo in spherical Couette flow

We investigate numerically kinematic dynamos driven by flow of electrically conducting fluid in the shell between two concentric differentially rotating spheres, a configuration normally referred to as spherical Couette flow. We compare between axisymmetric (2D) and fully three dimensional flows, between low and high global rotation rates, between prograde and retrograde differential rotations, between weak and strong nonlinear inertial forces, between insulating and conducting boundaries, and between two aspect ratios. The main results are as follows. Azimuthally drifting Rossby waves arising from the destabilisation of the Stewartson shear layer are crucial to dynamo action. Differential rotation and helical Rossby waves combine to contribute to the spherical Couette dynamo. At a slow global rotation rate, the direction of differential rotation plays an important role in the dynamo because of different patterns of Rossby waves in prograde and retrograde flows. At a rapid global rotation rate, stronger flow supercriticality (namely the difference between the differential rotation rate of the flow and its critical value for the onset of nonaxisymmetric instability) facilitates the onset of dynamo action. A conducting magnetic boundary condition and a larger aspect ratio both favour dynamo action.

astro-ph.EP↗