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Search Results: 1 - 10 of 78294 matches for " Xingming Chen "
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HOT ACCRETION DISKS WITH ADVECTION
Xingming Chen
Physics , 1995,
Abstract: The global structure of optically thin hot accretion disks with radial advection included has been investigated. We solve the full energy conservation equation explicitly and construct the radial structure of the disk. It is found that advection is a real cooling process and that there are two solutions co-exist for a given mass accretion rate less than a critical limit. One is fully advection cooling dominated and the other is dominated by local radiative cooling. The advection dominated accretion disks are hotter than the local cooling dominated disks; they are most probably in the two-temperature regime and effects such as electron-positron pair production and annihilation may need to be considered to study the microphysics of the hot plasma. However, the global disk structure will not be much affected by the local radiative process.
COLD ACCRETION DISKS WITH CORONAE AND ADVECTION
Xingming Chen
Physics , 1995, DOI: 10.1086/176008
Abstract: Cold optically thick accretion disks with hot coronae and radial advection have been investigated. Within the framework of $\alpha$-viscosity models, we assume that all the mass accretion and angular momentum transport take place in the cold disk, but that a fraction of the gravitational energy released is dissipated in the corona. Both the coronal energy dissipation and the advection heat transport have a stabilization effect on the thermal and viscous instabilities of the disk. If that more than $\sim 95$ percent of the total power is dissipated in the corona, then the locally unstable behavior of the disk is restricted to a relatively narrow spatial region and is found to lie in a small range of mass accretion rates. The global temporal variability of the disk can be very mild or may disappear, and which may be applicable to the low-frequency ($\sim 0.04$~Hz) quasi-periodic oscillations observed in black hole candidates Cyg X--1 and GRO J0422+32.
Instabilities of Advection-Dominated Accretion Flows
Xingming Chen
Physics , 1996,
Abstract: Accretion disk instabilities are briefly reviewed. Some details are given to the short-wavelength thermal instabilities and the convective instabilities. Time-dependent calculations of two-dimensional advection-dominated accretion flows are presented.
Safety Distance Determination for 500 kV AC Transmission Line’s Helicopter Inspection  [PDF]
Shuwei Wan, Xingming Bian, Lan Chen, Liming Wang, Zhicheng Guan
Energy and Power Engineering (EPE) , 2013, DOI: 10.4236/epe.2013.54B219
Abstract: As an efficient and advanced line inspection method, helicopter line patrol is gradually more and more used in transmission lines inspection, promoting the elaborate operation of transmission lines and reducing the management cost. However, as a 'floating-potential conductor' near to a high-voltage transmission line, the helicopter would be at a high electric field region; and bring security risk to equipment and operating personnel. In this paper, the electric field strength near the cabin at locations of different distance from transmission lines is investigated by calculation, and the field in the helicopter cabin is also calculated with finite element method (FEM). The result indicates that the potential difference becomes higher with the decrease of the distance between the helicopter and transmission line. Considering the discharge energy and the guarantee of the persons’ safety, the safety distance is determined as d≥15 m.
The Spectral States of Black Hole X-ray Binary Sources
Xingming Chen,Ronald E. Taam
Physics , 1996, DOI: 10.1086/177519
Abstract: A framework for the interpretation of the spectral states of black hole X-ray transients based on the diversity of accretion disk models is introduced. Depending on the mass accretion rate, it is proposed that the accretion disk is described by one or a combination of the following structures: optically thick disk, advection-dominated disk, corona-disk, and non-steady state disk. In particular, it is suggested that the very high, high, low, and off states are characterized by mass accretion rates of decreasing magnitude. The very high state corresponds to mass accretion rates near the Eddington limit in which an optically thin non steady inner region is surrounded by an optically thick structure. In the high state, the inner region is optically thin and advection-dominated or optically thick. The low hard state is interpreted in terms of a disk-corona system and the off state in terms of an optically thin disk dominated by advective energy transport into the black hole. The possible observational consequences of such a paradigm are discussed.
Variability of Accretion Disks Surrounding Black Holes ----- the Role of Inertial-Acoustic Mode Instabilities
Xingming Chen,Ronald E. Taam
Physics , 1994, DOI: 10.1086/175360
Abstract: The global nonlinear time-dependent evolution of the inertial-acoustic mode instability in accretion disks surrounding black holes has been investigated. The viscous stress is assumed to be proportional to the gas pressure only, i.e.\,, $\tau = - \alpha p_g$. It is found that an oscillatory instability exists in the inner regions of disks ($r < 10 r_g$, where $r_g$ is the Schwarzschild radius) for sufficiently large $\alpha$ ($\gapprox 0.2$), and for mass accretion rates less than about 0.3 times the Eddington value. The variations of the integrated bolometric luminosity from the disk, $\Delta L/L$, are less than 3\%. A power spectrum analysis of these variations reveals a power spectrum which can be fit to a power law function of the frequency $P \propto f^{-\gamma}$, with index $\gamma \sim 1.4-2.3$ and a low frequency feature at about 4 Hz in one case. In addition, a narrow peak centered at a frequency corresponding to the maximum epicyclic frequency of the disk at $\sim 100-130$ Hz and its first harmonic is also seen. The low frequency modulations are remarkably similar to those observed in black hole candidate systems. The possible existence of a scattering corona in the inner region of the disk and/or to other processes contributing to the power at high frequencies in the inner region of the accretion disk may make the detection of the high frequency component difficult.
Low Frequency Quasi-Periodic Oscillations in Low Mass X-Ray Binaries and Galactic Black Hole Candidates
Xingming Chen,Ronald E. Taam
Physics , 1994, DOI: 10.1086/174524
Abstract: We consider the inner regions of accretion disks surrounding black holes and neutron stars and investigate the nonlinear time dependent evolution of thermal-viscous instabilities. The viscous stress is assumed to be proportional to the gas pressure with the viscosity parameter formulated as $\alpha =\min[\alpha_0 (h/r)^n, \alpha_{\max}]$, where $h$ is the local scale height, $r$ is the distance from the central compact object, and $n$, $\alpha_0$ and $\alpha_{\max}$ are constants. It is found that the disk is unstable for $\alpha$ sufficiently sensitive to $h$ ($n \gapprox 1.2$). The instabilities are globally coherent in the entire unstable region of the disk and, depending on the viscosity parameters, the time variability of the mass accretion rates are manifested as periodic or quasi-periodic oscillations. We show that, the low frequency ($\sim 0.04$~Hz) quasi-periodic oscillations (QPOs) discovered recently in some of the black hole candidates (Cyg~X-1 and GRO~J0422+32) and a low mass X-ray binary (Rapid Burster MXB~1730--335) may be explicable by the thermal-viscous instabilities in accretion disks. The observations of QPOs place constraints on the viscosity parameters and suggest that $(n,\alpha_0)$ $\sim (1.6,30)$ for the Rapid Burster with a $1.4\,\msun$ neutron star. In the case of black hole candidates, the dependence of $\alpha$ on $h/r$ is less steep corresponding to $n \sim 1.2-1.3$ for black holes less than $10\,\msun$.
The Ultraviolet Detection of Corona Discharge in Power Transmission Lines  [PDF]
Lan Chen, Lin Lin, Mimi Tian, Xingming Bian, Liming Wang, Zhicheng Guan
Energy and Power Engineering (EPE) , 2013, DOI: 10.4236/epe.2013.54B246
Abstract:

Corona discharge is a common phenomenon in power transmission lines external insulation, and it may cause serious defect if without effective detection. The ultraviolet (UV) imagery technology has been widely used to detect the corona discharge in industry in recent years, but some influence factors’ functions are not definite. In this paper, the fracture aluminum strands which is common in power transmission lines were used as the electrode model while a SuperB ultraviolet imager were utilized to detect, the photon count rate was detected with different detect distance, electric field, aluminum strands length and UV gain were applied. Then the multivariate regression analysis (MRA) was taken to calculate the function between the photon count and the factors.

Thermal Instability of Advection-Dominated Disks against Local Perturbations
Shoji Kato,Marek A. Abramowicz,Xingming Chen
Physics , 1995, DOI: 10.1093/pasj/48.1.67
Abstract: Thermal instability is examined for advection-dominated one-temperature accretion disks. We consider axisymmetric perturbations with short wavelength in the radial direction. The viscosity is assumed to be sufficiently small for the vertical hydrostatic balance to hold in perturbed states. The type of viscosity is given either by the $\alpha$-viscosity or by a diffusion-type stress tensor. Optically thick disks are found to be in general more unstable than optically thin ones. When the thermal diffusion is present, the optically thin disks become stable, but the optically thick disks are still unstable. The instability of the advection-dominated disks is different from that of the geometrically thin disks without advection. In the case of no advection, the thermal mode behaves under no appreciable surface density change. In the case of advection-dominated disks, however, the thermal mode occurs with no appreciable pressure change (compared with the density change), when local perturbations are considered. The variations of angular momentum and of surface density associated with the perturbations lead to a thermal instability. The astrophysical implications of this instability are briefly discussed.
Accretion discs around black holes: two dimensional, advection cooled flows
Igor V. Igumenshchev,Xingming Chen,Marek A. Abramowicz
Physics , 1995, DOI: 10.1093/mnras/278.1.236
Abstract: Two-dimensional accretion flows near black holes have been investigated by time-dependent hydrodynamical calculations. We assume that the flow is axisymmetric and that radiative losses of internal energy are negligible, so that the disc is geometrically thick and hot. Accretion occurs due to the overflow of the effective potential barrier near the black hole, similar to the case of the Roche lobe overflowing star in a binary system. We make no pre-assumptions on the properties of the flow, instead our models evolve self-consistently from an initially non-accreting state. The viscosity is due to the the small-scale turbulence and it is described by the $\alpha$-viscosity prescription. We confirm earlier suggestions that viscous accretion flows are convectively unstable. We found that the instability produces transient eddies of various length-scales. The eddies contribute to the strength of the viscosity in the flow by redistributing the angular momentum. They also introduce low amplitude oscillatory variations which have a typical frequency about $100\; \msun/M$ Hz for a system of mass $M$. This may be relevant to the high frequency ($\sim 4-10$ Hz) quasi-periodic oscillations observed in the Galactic black hole candidate X-ray sources.
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