Absorption features in the quasar HS 1603 + 3820 II. Distance to the absorber obtained from photoionisation modelling     

《New Astronomy》2014

We present the photoionisation modelling of the intrinsic absorber in the bright quasar HS 1603 + 3820. We constructed the broad-band spectral energy distribution using the optical/UV/X-ray observations from different instruments as inputs for the photoionisation calculations. The spectra from the Keck telescope show extremely high Civ to Hi ratios, for the first absorber in system A, named A1. This value, together with high column density of Civ ion, place strong constraints on the photoionisation model. We used two photoionisation codes to derive the hydrogen number density at the cloud illuminated surface. By estimating bolometric luminosity of HS 1603 + 3820 using the typical formula for quasars, we calculated the distance to A1. We could find one photoionization solution, by assuming either a constant density cloud (which was modelled using cloudy), or a stratified cloud (which was modelled using titan), as well as the solar abundances. This model explained both the ionic column density of Civ and the high Civ to Hi ratio. The location of A1 is 0.1 pc, and it is situated even closer to the nucleus than the possible location of the Broad Line Region in this object. The upper limit of the distance is sensitive to the adopted covering factor and the carbon abundance. Photoionisation modelling always prefers dense clouds with the number density n₀ = 10¹⁰ − 10¹² cm⁻³, which explains intrinsic absorption in HS 1603 + 3820. This number density is of the same order as that in the disk atmosphere at the implied distance of A1. Therefore, our results show that the disk wind that escapes from the outermost accretion disk atmosphere can build up dense absorber in quasars.  相似文献   

A computational model for coupled multiphysics processes of CO2 sequestration in fractured porous media     

《Advances in water resources》2013

In this paper, a computational model for the simulation of coupled hydromechanical and electrokinetic flow in fractured porous media is introduced. Particular emphasis is placed on modeling CO₂ flow in a deformed, fractured geological formation and the associated electrokinetic flow. The governing field equations are derived based on the averaging theory and the double porosity model. They are solved numerically with a mixed discretization scheme, formulated on the basis of the standard Galerkin finite element method, the extended finite element method, the level-set method and the Petrov–Galerkin method. The standard Galerkin method is utilized to discretize the equilibrium and the diffusive dominant field equations, and the extended finite element method, together with the level-set method and the Petrov–Galerkin method, are utilized to discretize the advective dominant field equations. The level-set method is employed to trace the CO₂ plume front, and the extended finite element method is employed to model the high gradient in the saturation field front. The proposed mixed discretization scheme leads to a convergent system, giving a stable and effectively mesh-independent model. The accuracy and computational efficiency of the proposed model is evaluated by verification and numerical examples. Effects of the fracture spacing on the CO₂ flow and the streaming potential are discussed.  相似文献   

Investigation of nonlinear sloshing effects in seismically excited tanks     

《Soil Dynamics and Earthquake Engineering》2012

Nonlinear behavior of liquid sloshing inside a partially filled rectangular tank is investigated. The nonlinearity in the numerical modeling of the liquid sloshing originates from the nonlinear terms of the governing equations of the fluid flow and the liquid free surface motion as a not known boundary condition. The numerical simulations are performed for both linear and nonlinear conditions. The computed results using linear conditions are compared with readily available exact solution. In order to verify the results of the nonlinear numerical solution, a series of the shaking table tests on rectangular tank were conducted. Having verified linear and nonlinear numerical models, they are used for computation of near wall sloshing height at a series of real scale tanks (with various dimensions) under the both harmonic and earthquake base excitation. Finally, the nonlinear effects on liquid sloshing modeling are discussed and the practical limitations of the linear solution in evaluating the response of seismically excited liquids are also addressed.  相似文献   

A review of Space Climate and an introduction to the papers of the JASTP special issue on Space Climate     

Kalevi Mursula  Dan Marsh  Dibyendu Nandy  Ilya Usoskin 《Journal of Atmospheric and Solar》2011,73(2-3):179-181

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Calculating Method and Characteristics of the Distribution of Neutron Exposures in the Radiative S-process Nucleosynthesis Model for AGB Stars     

Zhang Feng-hua  Zhou Gui-de  Cui Wen-yuan  Zhang Bo 《Chinese Astronomy and Astrophysics》2013

A study on the distribution of neutron exposures in a low-mass asymptotic giant branch (AGB) star is presented, according to the s-process nucleosynthesis model with the ¹²C(α, n)¹⁶O reaction occurred under radiative conditions in the interpulse phases. The model parameters, such as the over- lap factor r of two successive convective thermal pulses, the mass ratio q of the ¹³C shell with respect to the He intershell, and the effective mass of ¹³C in the ¹³C shell, vary with the pulse number. Considering these factors, a calculating method for the distribution of neutron exposures in the He intershell has been presented. This method has the features of simplicity and universality. Using this method, the exposure distribution for the stellar model of a star with the mass of 3 M_? and the solar metallicity has been calculated. The results suggest that under the reasonable assumption that the number density of neutrons is uniform in the ¹³C shell, the ?nal exposure distribution approaches to an exponential distribution. For a stellar model with the de?nite initial mass and metallicity, there is a de?nite relation between the mean neutron exposure τ₀ and the neutron exposure Δτ of each pulse, namely τ₀ = 0.434λ(q1, q2, …, qm_max +1, …, r₁, r₂, …, rm_max +1)Δτ, where m_max is the total number of thermal pulses with the third dredge-up episode, and the proportional coeffcient λ(q₁, q₂, …, qm_max +1, …, r₁, r₂, …, rm_max +1) can be determined by an exponential curve ?tting to the ?nal exposure distribution. This new formula quantitatively uni?es the classical model with the s-process nu- cleosynthesis model by means of neutron exposure distribution, and makes the classical model continue to offer guidance and constraints to the s-process nu- merical calculations of stellar models.  相似文献   

AGB星辐射s-过程核合成模型中子辐照量分布的计算方法和特点     

张凤华  周贵德  崔文元  张波 《天文学报》2013,54(1):9-19

考虑到模型参量随脉冲数的变化,推导出13C辐射燃烧的低质量AGB星s-过程核合成模型中子辐照量分布的计算方法,该方法具有普适性和简洁性.利用该方法,计算了3 M☉、太阳金属丰度恒星模型的中子辐照量分布.结果表明,若合理假设13C薄层内中子数密度均匀分布,则辐射核合成模型最终的中子辐照量分布趋近于指数分布.对于初始质量一定、金属丰度一定的恒星模型,平均中子辐照量τ0与每个脉冲的中子辐照量△τ存在确定的关系:τ0=0.434λ(q1,q2…qmmax+1,r1,r2…rmmax+1)△τ,式中mmax是带挖掘的脉冲总数,比例系数λ(q1,q2…qmmax+1,r1,r2…rmmax+1)可通过对最终中子辐照量分布的指数拟合得到.该式从中子辐照量分布角度定量地将经典模型和辐射s-过程核合成模型统一起来,使经典模型能为恒星模型核合成数值计算提供指导和约束.  相似文献   

An all-pass filter for compensation of ionospheric dispersion effects on wideband GNSS signals     

Ningyan Guo  Yanhong Kou  Yan Zhao  Zhibin Yu  Yibo Chen 《GPS Solutions》2014,18(4):625-637

Compared with the traditional GPS L1 C/A BPSK-R(1) signal, wideband global navigation satellite system (GNSS) signals suffer more severe distortion due to ionospheric dispersion. Ionospheric dispersion inevitably introduces additional errors in pseudorange and carrier phase observations that cannot be readily eliminated by traditional methods. Researchers have reported power losses, waveform ripples, correlation peak asymmetries, and carrier phase shifts caused by ionospheric dispersion. We analyze the code tracking bias induced by ionospheric dispersion and propose an efficient all-pass filter to compensate the corresponding nonlinear group delay over the signal bandwidth. The filter is constructed in a cascaded biquad form based on the estimated total electron content (TEC). The effects of TEC accuracy, filter order, and fraction parameter on the filter fitting error are explored. Taking the AltBOC(15,10) signal as an example, we compare the time domain signal waveforms, correlation peaks, code tracking biases, and carrier phase biases with and without this all-pass filter and demonstrate that the proposed delay-equalization all-pass filter is a potential solution to ionospheric dispersion compensation and mitigation of observation biases for wideband GNSS signals.  相似文献   

Automated registration of dense terrestrial laser-scanning point clouds using curves     

《ISPRS Journal of Photogrammetry and Remote Sensing》2014

This paper proposes an automatic method for registering terrestrial laser scans in terms of robustness and accuracy. The proposed method uses spatial curves as matching primitives to overcome the limitations of registration methods based on points, lines, or patches as primitives. These methods often have difficulty finding correspondences between the scanned point clouds of freeform surfaces (e.g., statues, cultural heritage). The proposed method first clusters visually prominent points selected according to their associated geometric curvatures to extract crest lines which describe the shape characteristics of point clouds. Second, a deformation energy model is proposed to measure the shape similarity of these crest lines to select the correct matching-curve pairs. Based on these pairs, good initial orientation parameters can be obtained, resulting in fine registration. Experiments were undertaken to evaluate the robustness and accuracy of the proposed method, demonstrating a reliable and stable solution for accurately registering complex scenes without good initial alignment.  相似文献   

Column-generation kernel nonlocal joint collaborative representation for hyperspectral image classification     

《ISPRS Journal of Photogrammetry and Remote Sensing》2014

We propose a kernel nonlocal joint collaborative representation classification method based on column generation for hyperspectral imagery. The proposed approach first maps the original spectral space to a higher implicit kernel space by directly taking the similarity measures between spectral pixels as a feature, and then utilizes a nonlocal joint collaborative regression model for kernel signal reconstruction and the subsequent pixel classification. We also develop two kinds of specific radial basis function kernels for measuring the similarities. The experimental results indicate that the proposed algorithms obtain a competitive performance and outperform other state-of-the-art regression-based classifiers and the classical support vector machines classifier.  相似文献   

Towards 3D lidar point cloud registration improvement using optimal neighborhood knowledge     

《ISPRS Journal of Photogrammetry and Remote Sensing》2013

Automatic 3D point cloud registration is a main issue in computer vision and remote sensing. One of the most commonly adopted solution is the well-known Iterative Closest Point (ICP) algorithm. This standard approach performs a fine registration of two overlapping point clouds by iteratively estimating the transformation parameters, assuming good a priori alignment is provided. A large body of literature has proposed many variations in order to improve each step of the process (namely selecting, matching, rejecting, weighting and minimizing). The aim of this paper is to demonstrate how the knowledge of the shape that best fits the local geometry of each 3D point neighborhood can improve the speed and the accuracy of each of these steps. First we present the geometrical features that form the basis of this work. These low-level attributes indeed describe the neighborhood shape around each 3D point. They allow to retrieve the optimal size to analyze the neighborhoods at various scales as well as the privileged local dimension (linear, planar, or volumetric). Several variations of each step of the ICP process are then proposed and analyzed by introducing these features. Such variants are compared on real datasets with the original algorithm in order to retrieve the most efficient algorithm for the whole process. Therefore, the method is successfully applied to various 3D lidar point clouds from airborne, terrestrial, and mobile mapping systems. Improvement for two ICP steps has been noted, and we conclude that our features may not be relevant for very dissimilar object samplings.  相似文献