检索结果-南通市图书馆

A numerical study of self-gravitating protoplanetary disks

在线全文

学校读者我要写书评

暂无评论

Research in Astronomy and Astrophysics 2012年第3期12卷 331-344页

作者： kazem Faghei School of Physics Damghan University

The effect of self-gravity on protoplanetary disks is *** mechanisms of angular momentum transport and energy dissipation are assumed to be the viscosity due to turbulence in the accretion *** energy equation is considered in a situation where the released energy by viscosity dissipation is balanced with cooling *** viscosity is obtained by equality of dissipation and cooling functions,and is used to derive the angular momentum *** cooling rate of the flow is calculated by a prescription,du/dt = u/τ cool,where u and τ cool are the internal energy and cooling timescale,*** ratio of local cooling to dynamical timescales Ωτ cool is assumed to be a constant and also a function of the local *** solutions for protoplanetary disks show that in the case of Ωτ cool = constant,the disk does not exhibit any gravitational instability over small radii for a typical mass accretion rate,˙ M = 10 6 M yr 1,but when choosing Ωτ cool to be a function of temperature,gravitational instability can occur for this value of mass accretion rate or even less in small ***,by studying the viscosity parameter α,we find that the strength of turbulence in the inner part of self-gravitating protoplanetary disks is very *** results are qualitatively consistent with direct numerical simulations of protoplanetary ***,in the case of cooling with temperature dependence,the effect of physical parameters on the structure of the disk is *** solutions demonstrate that disk thickness and the Toomre parameter decrease by adding the ratio of disk mass to central object ***,the disk thickness and the Toomre parameter increase by adding mass accretion ***,for typical input parameters such as mass accretion rate 10 6 M yr 1,the ratio of the specific heat γ = 5/3 and the ratio of disk mass to central object mass q = 0.1,gravitational instability can occur over the whole radius of the disk excludin

关键词： accretion accretion disks—planetary systems:protoplanetary disks—planetary systems:formation

An SPH simulation for cooling and self-gravitating protoplanetary disks

在线全文

学校读者我要写书评

暂无评论

Research in Astronomy and Astrophysics 2013年第2期13卷 170-178页

作者： kazem Faghei School of Physics Damghan University

We investigate the effects of the cooling function in the formation of clumps of protoplanetary disks using two-dimensional smoothed particle hydrody- namic simulations. We use a simple prescription for the cooling rate of the flow, du/dt = -u/τcool, where u and %ool are the internal energy and cooling timeseale, respectively. We assume the ratio of local＇cooling to dynamical timescale, Ωτcool =β, to be a constant and also a function of the local temperature. We found that for the constantβ and γ = 5/3, fragmentation occurs only forβ ≤ 7. However, in the case ofβ having temperature dependence and γ = 5/3, fragmentation can also occur for larger values ofβ. By increasing the temperature dependence of the cooling timescale, the mass accretion rate decreases, the population of clumps/fragments increases, and the clumps/fragments can also form in the smaller radii. Moreover, we found that the clumps can form even in a low mass accretion rate, ≤10-7M⊙yr-1, in the case of temperature-dependentβ. However, clumps form with a larger mass accretion rate, 〉 10-7M⊙ yr-1, in the case of constantβ.

关键词： accretion, accretion disks -- planetary systems protoplanetary disks -- planetary systems formation

A dynamical model for radiatively inefficient accretion flows with convection

在线全文

学校读者我要写书评

暂无评论

Research in Astronomy and Astrophysics 2013年第9期13卷 1075-1086页

作者： kazem Faghei School of Physics Damghan University

We explore the time evolution of radiatively-inefficient accretion flows. Since these types of accretion flows are convectively unstable, we also study the ef- fects of convection in the present model. The effects of convection are applied to equations describing angular momentum and energy. In analogy to the traditional c^- prescription, we introduce the convection parameter c^e to study the influences of con- vection on physical quantities. The model is studied in two cases： the transport of angular momentum due to convection inward and outward. We found the physical variables are sensitive to the parameter C^c and are also dependent on the direction of angular momentum that is transported by convection. As for angular momentum transfer inward, the accretion flow can be convectively dominated and radial infall velocity becomes zero. Moreover, we found the radial dependence of the density and radial velocity takes an intermediate place between steady state radiatively-inefficient accretion flow and steady state advection-dominated accretion flow. This property is in accord with direct numerical simulation of radiatively-inefficient accretion flows.

关键词： accretion accretion disks -- convection -- hydrodynamics

The density and temperature dependence of the cooling timescale for fragmentation of self-gravitating disks

在线全文

学校读者我要写书评

暂无评论

Research in Astronomy and Astrophysics 2014年第6期14卷 648-666页

作者： kazem Faghei School of Physics Damghan University

The purpose of this paper is to explore the influences of cooling timescale on fragmentation of self-gravitating protoplanetary disks. We assume the cooling timescale, expressed in terms of the dynamical timescale Ω tcool, has a power-law dependence on temperature and density, Ω toool ∝∑-aT-b, where a and b are con- stants. We use this cooling timescale in a simple prescription for the cooling rate, du/dt = -u/tcoll, where u is the internal energy. We perform our simulations using the smoothed particle hydrodynamics method. The simulations demonstrate that the disk is very sensitive to the cooling timescale, which depends on density and tem- perature. Under such a cooling timescale, the disk becomes gravitationally unstable and clumps form in the disk. This property even occurs for cooling timescales which are much longer than the critical cooling timescale, Ω toool≥ 7. We show that by adding the dependence of a cooling timescale on temperature and density, the number of clumps increases and the clumps can also form at smaller radii. The simulations im- ply that the sensitivity of a cooling timescale to density is more than to temperature, because even for a small dependence of the cooling timescale on density, clumps can still form in the disk. However, when the cooling timescale has a large dependence on temperature, clumps form in the disk. We also consider the effects of artificial viscos- ity parameters on fragmentation conditions. This consideration is performed in two cases, where Ω tcool is a constant and Ω tcool is a function of density and temperature. The simulations consider both cases, and results show the artificial viscosity param- eters have rather similar effects. For example, using too small of values for linear and quadratic terms in artificial viscosity can suppress the gravitational instability and consequently the efficiency of the clump formation process decreases. This property is consistent with recent simulations of self-gravitating disks.

关键词： accretion accretion disks -- planetary systems: protoplanetary disks --planetary systems: formation

ASSESSMENT OF A CENTRAL DIFFERENCE FINITE VOLUME SCHEME FOR MODELING OF CAVITATING FLOWS USING PRECONDITIONED MULTIPHASE EULER EQUATIONS

在线全文

学校读者我要写书评

暂无评论

Journal of Hydrodynamics 2011年第3期23卷 302-313页

作者： HEJRANFAR kazem FATTAH-HESARY Kasra Aerospace Engineering Department Sharif University of Technology

A numerical treatment for the prediction of cavitating flows is presented and assessed. The algorithm uses the preconditioned multiphase Euler equations with appropriate mass transfer terms. A central difference finite volume scheme with suitable dissipation terms to account for density jumps across the cavity interface is shown to yield an effective method for solving the multiphase Euler equations. The Euler equations are utilized herein for the cavitation modeling, because some certain characteristics of cavitating flows can be obtained using the solution of this system of equations with relative low computational effort. In addition, the Euler equations are appropriate for the assessment of the numerical method used, because of the sensitivity of the solution to the numerical instabilities. For this reason, a sensitivity study is conducted to evaluate the effects of various parameters, such as numerical dissipation coefficients and grid size, on the accuracy and performance of the solution. The computations are performed for steady cavitating flows around the NACA 0012 and NACA 66 （MOD） hydrofoils and also an axisymmetric hemispherical fore-body under different conditions and the results are compared with the available numerical and experimental data. The solution procedure presented is shown to be accurate and efficient for predicting steady sheet- and super-cavitation for 2D/axisymmetric geometries.

关键词： cavitating flows preconditioned multiphase Euler equations interface capturing method central difference FVM

Spark Plasma Sintering of Ultrafine YSZ Reinforced Cu Matrix Functionally Graded Composite

在线全文

学校读者我要写书评

暂无评论

Acta Metallurgica Sinica(English Letters) 2016年第12期29卷 1169-1176页

作者： Jafar Mirazimi Parvin Abachi kazem Purazrang Department of Materials Science and Engineering Sharif University of Technology

Copper matrix composites have received more attentions as possible candidate for thermal and electrical conductive materials to be used in electrical contact applications. In this study, five-layered Cu/YSZ （yttria-stabilized zirconia） functionally graded material （FGM） and copper matrix composite specimens containing 3 and 5 vol% YSZ particles plus pure Cu specimen were synthesized using powder metallurgy （PM） route and spark plasma sintering （SPS） consolidation process. The microstructural and some physical, mechanical features of all specimens were characterized. Microscopic examinations showed that ultrafine YSZ particles were distributed in the copper matrix almost homoge- neously. An appropriate interface was observed at each layer of FGM. The density measurement indicated that the graded structure of the composite could be well densified after the SPS process. The microhardness values of various layers of Cu/ YSZ FGM specimen were gradually altered from 56.3 （pure copper side） to 75.2 HV （Cu-5 vol% YSZ side）. The increase of YSZ content resulted in a decrease in electrical conductivity. Additionally, thermal conductivity of Cu/YSZ FGM specimen [308.0 W/（m K）] was determined to be higher than that of the Cu-5 vol% YSZ composite specimen [260.7 W/ （m K）]. Accordingly, it can be concluded that the Cu/YSZ FGM can be a good candidate for the electrical applications, like sliding electrical contacts, where different material characteristics in the same component are required.

关键词： Functionally graded material (FGM) Spark plasma sintering (SPS) Composite Microstructure Interface

Pushover analysis of asymmetric-plan buildings based on distribution of the combined modal story shear and torsional moment

在线全文

学校读者我要写书评

暂无评论

Earthquake Engineering and Engineering Vibration 2014年第4期13卷 707-716页

作者： kazem Shakeri Karim Tarbali Mohtasham Mohebbi Faculty of Engineering University of Mohaghegh Ardabili International Institute of Earthquake Engineering and Seismology

A pushover procedure with a load pattern based on the height-wise distribution of the combined modal story shear and torsional moment is proposed to estimate the seismic response of 3D asymmetric-plan building frames. Contribution of the higher modes and torsional response of asymmetric-plan buildings are incorporated into the proposed load pattern. The proposed pushover method is a single-run procedure, which enables tracing the nonlinear response of the structure during the analysis and averts the elusiveness of conducting multiple pushover analyses. The proposed method has been used to estimate the response of two moment-resisting building frames with 9 and 20 stories. The obtained results indicate the appropriate accuracy and efficiency of the proposed procedure in estimating the trend of the drift profiles of the structures resulted from nonlinear time history analyses.

关键词： pushover analysis asymmetric-plan building torsional response story shear torsional moment

A soil quality index for evaluation of degradation under land use and soil erosion categories in a small mountainous catchment, Iran

在线全文

学校读者我要写书评

暂无评论

Journal of Mountain Science 2019年第11期16卷 2577-2590页

作者： kazem NOSRATI Adrian LCOLLINS Department of Physical Geography School of Earth SciencesShahid Beheshti University*** TehranIran Sustainable Agriculture Sciences Department Rothamsted ResearchNorth WykeOkehamptonEX202SBUK

Soil erosion and land use type have long been viewed as being particularly important drivers of soil *** objectives of this study,therefore,were to select a new soil quality index(SQI)which varies significantly with land use/soil erosion,and to evaluate the new SQI using expert *** total,18 soil physical,chemical,and biochemical properties(indicators)were measured on 56 soil samples collected from four land use/soil erosion categories(rangeland/surface erosion,rangeland/subsurface erosion,cultivated land/surface erosion and dry-farming land/surface erosion).Principal component and classification analysis(PCCA)identified five PCs that explained 77.7%of the variation in soil properties with the biochemical PC varying significantly with land use/soil *** discriminant analysis(GDA)selected urease and clay as the most sensitive properties distinguishing the land use/soil erosion *** GDA canonical scores for the new SQI were significantly correlated with expert opinion soil surface summed scores(for soil movement,surface litter,pedestalling,rills and flow pattern)derived using the *** of the Interior Bureau of Land Management(BLM)method.A forward stepwise general regression model revealed that the new SQI values were explained by soil movement,surface litter,and the summed values of the soil surface ***,this study confirmed that soil quality in the study area in Iran is controlled by land use and corresponding soil erosion.

关键词： Soil quality index Land use Erosion status Soil enzyme activities Multivariate statistical techniques

Qubit movement-assisted entanglement swapping

在线全文

学校读者我要写书评

暂无评论

Chinese Physics B 2020年第5期29卷 92-100页

作者： Sare Golkar Mohammad kazem Tavassoly Alireza Nourmandipour Atomic and Molecular Group Faculty of PhysicsYazd UniversityYazd 89195-741Iran Photonic Research Group Engineering Research CenterYazd UniversityYazd 89195-741Iran Department of Physics Faculty of ScienceSirjan University of TechnologySirjanIran

We propose a scheme to generate entanglement between two distant qubits(two-level atom)which are separately trapped in their own(in general)non-Markovian dissipative cavities by utilizing entangling swapping,considering the case in which the qubits can move along their cavity axes rather than a static state of *** first examine the role of movement of the qubit by studying the entropy evolution for each *** average entropy over the initial states of the qubit is *** by performing a Bell state measurement on the fields leaving the cavities,we swap the entanglement between qubit-field in each cavity into qubit-qubit and field-field *** entangling power is used to measure the average amount of swapped entanglement over all possible pure initial *** results are presented in two weak and strong coupling regimes,illustrating the positive role of movement of the qubits on the swapped *** is revealed that by considering certain conditions for the initial state of qubits,it is possible to achieve a maximally long-leaving stationary entanglement(Bell state)which is entirely independent of the environmental variables as well as the velocity of *** happens when the two qubits have the same velocities.

关键词： dissipative systems quantum entanglement entanglement swapping