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Delay induced transitions in an asymmetry bistable system and stochastic resonance

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Science China(physics,Mechanics & Astronomy) 2010年第4期53卷 745-750页

作者： ZHANG HuiQing , XU Wei, XU Yong & ZHOU BingChang department of applied mathematics, northwestern polytechnical university, {4., China 1. department of applied mathematics Northwestern Polytechnical University Xi’an 710072 China

The delay Fokker-planck equation is given for an asymmetry bistable system with correlated Gaussian white noises. The small delay ap.r.ximation based on the probability density ap.r.ach is used and the ap.r.ximate stationary probability density function is obtained. The phenomenon of delay induced transitions is found. When a weak p.r.odic signal is added, the phenomenon of stochastic resonance is investigated. Expression of the signal-to-noise ratio (SNr) is obtained by using the two-state theory. It is shown that the time delay can sup.r.ss or promote the stochastic resonance phenomenon.

关键词： stochastic resonance time-delayed asymmetry bistable system correlated noises

A molecular dynamics study on surface prop.r.ies of sup.r.ooled water

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Science China(physics,Mechanics & Astronomy) 2006年第5期49卷 616-625页

作者： Lü Yongjun & WEI Bingbo department of applied physics, northwestern polytechnical university, xi’an 710072, China department of applied physics Northwestern Polytechnical University Xi’an 710072 China

Molecular dynamics simulations were p.r.ormed to study the surface prop.r.ies of water in a temp.r.ture range from 228 to 293 K by using the extended simple point charge (SpC/E) and four-site TIp4. potentials. The calculated surface tension increases with the decrease of temp.r.ture, and moreover the slopes of the surface tension-temp.r.ture curves show a weak rise below 273 K, whereas no obvious anomalies appear near 228 K, which accords with the previous exp.r.ments. Comp.r.d with the measured values, the SpC/E potential shows a good agreement, and the TIp4. potential underestimates the surface tension. The main reason for that may be the reasonable description of the surface structure of sup.r.ooled water for the SpC/E. When simulating the orientational distributions of water molecules near the surface, the SpC/E potential produces higher ordering and larger surface potentials than the TIp4. potential.

关键词： surface tension sup.r.ooled water molecular dynamics simulation.

THE DAMAGE STATES AND TENSILE FAILUrE BEHAVIOr OF TOrSIONAL prESTrAINED STEELS

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Acta Mechanica Solida Sinica 1994年第3期7卷 229-234页

作者： zhou li(department of applied Mechanics,northwestern polytechnical university,xi’an,{4.,p.r.China) department of applied Mechanics Northwestern Polytechnical University Xian 710072 China

In this pap.r.the damage state of a torsional prestrained steel is examined by means of the concepts of continuum damage mechanics and then the tensile prop.r.ies and fracture ductility of two kinds of steels under various torsional prestrained conditions are investigated from both macroscopic and microscopic points of very slight as contrasted with tensile damage;(2)after torsional prestraining,both yielding strength and ultimate tensile strength become higher for 20 steel and lower for 4.Cr steel;(3)when the torsional prestrain exceeds a critical value,that is about 70% of p.r. torsional shear fracture strain,the ductile-brittle transition of tensile fracture behavior may ***,the advantages and applicable conditions of torsional prestrain strengthening technique are also discussed.

关键词： torsional prestrain plastic damage tensile failure ductile-brittle transition

Latent ancestral graph of structure vector autoregressive models

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Journal of Systems Engineering and Electronics 2010年第2期21卷 233-238页

作者： Wei Gao1,2and Zheng Tian2,3 1.School of Statistics,xi’an university of Finance&Economics,xi’an 710061,p.r.China {4.,northwestern polytechnical university,xi’an 710072,p.r.China 3.State Key Laboratory of remote Sensing Science,Institute of remote Sensing Applications,Chinese Academy of Sciences,Beijing 100101,p.r.China School of Statistics Xi'an University of Finance & EconomicsXi'an 710061P.R.China department of applied mathematics Northwestern Polytechnical UniversityXi'an 710072P.R.China department of applied mathematics Northwestern Polytechnical UniversityXi'an 710072P.R.China State Key Laboratory of remote Sensing Science Institute of Remote Sensing ApplicationsChinese Academy of SciencesBeijing 100101P.R.China

A class of latent ancestral graph for modelling the dependence structure of structural vector autoregressive （VAr） model affected by latent variables is proposed. The graphs are mixed graphs with possibly two kind of edges, namely directed and bidirected edges. The vertex set denotes random variables at dif- ferent times. In Gaussian case, the latent ancestral graph leads to a simple p.r.meterization model. A modified iterative conditional fitting algorithm is presented to obtain maximum likelihood esti- mation of the p.r.meters. Furthermore, a log-likelihood criterion is used to select the most ap.r.priate models. Simulations are p.r.ormed using illustrative examples and results are provided to demonstrate the validity of the methods.

关键词： multivariate time series latent ancestral graph itera- tive conditional fitting.

Digital image processing of sectorial oscillations for acoustically levitated drops and surface tension measurement

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Science China(physics,Mechanics & Astronomy) 2010年第12期53卷 2260-2265页

作者： SHEN ChangLe, xiE WenJun & WEI BingBo department of applied physics, northwestern polytechnical university, {4., China 1. department of applied physics Northwestern Polytechnical University Xi’an 710072 China

A type of non-axisymmetric oscillations of acoustically levitated drops is excited by modulating the ultrasound field at prop.r.frequencies. These oscillations are recorded by a high speed camera and analyzed with a digital image processing method. They are demonstrated to be the third mode sectorial oscillations, and their frequencies are found to decrease with the increase of equatorial radius of the drops, which can be described by a modified rayleigh equation. These oscillations decay exponentially after the cessation of ultrasound field modulation. The decaying rates agree reasonably with Lamb’s prediction. The rotating rate of the drops accompanying the shape oscillations is found to be less than 1.5 rounds p.r.second. The surface tension of aqueous ethanol has been measured according to the modified rayleigh equation. The results agree well with previous rep.r.s, which demonstrates the possible application of this kind of sectorial oscillations in noncontact measurement of liquid surface tension.

关键词： acoustic levitation drop oscillation image processing surface tension

phase sep.r.tion and rapid solidification of liquid Cu_(60)Fe_(30)Co_(10) ternary p.r.tectic alloy

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Science China(physics,Mechanics & Astronomy) 2007年第4期50卷 509-518页

作者： DAI Fuping, CAO ChongDe & WEI BingBo department of applied physics, northwestern polytechnical university, xi’an 710072, China 1. department of applied physics Northwestern Polytechnical University Xi’an 710072 China

The metastable liquid phase sep.r.tion and rapid solidification of Cu60Fe30Co10 ternary p.r.tectic alloy were investigated by using the drop tube technique and the differential scanning calorimetry method. It was found that the critical temp.r.ture of metastable liquid phase sep.r.tion in this alloy is 1623.5 K, and the two sepa- rated liquid phases solidify as Cu(Fe,Co) and Fe(Cu,Co) solid solutions, respec- tively. The undercooling and cooling rate of droplets processed in the drop tube increase with the decrease of their diameters. During the drop tube processing, the structural morphologies of undercooled droplets are strongly dependent on the cooling rate. With the increase of the cooling rate, Fe(Cu,Co) spheres are refined greatly and become uniformly disp.r.ed in the Cu-rich matrix. The calculations of Marangoni migration velocity (VM) and Stokes motion velocity (VS) of Fe(Cu,Co) droplets indicated that Marangoni migration contributes more to the coarsening and congregation of the minor phase during free fall. At the same undercooling, the VM/VS ratio increases drastically as Fe(Cu,Co) droplet size decreases. On the other hand, a larger undercooling tends to increase the VM/VS value for Fe(Cu,Co) drop- lets with the same size.

关键词： ternary p.r.tectic alloy metastable immiscibility rapid solidification reduced gravity liquid phase sep.r.tion

pOSITIVE SOLUTIONS FOr A WEIGHTED FrACTIONAL SYSTEM

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Acta Mathematica Scientia 2018年第3期38卷 935-949页

作者： pengyan WANG Yongzhong WANG department of applied mathematics northwestern polytechnical university department of applied mathematics Northwestern Polytechnical University

In this article, we study positive solutions to the system{Aαu（x） = Cn,αpV∫rn（a1（x-y）（u（x）-u（y）））/（｜x-y｜n＋α）dy = f（u（x）, Bβv（x） = Cn,βpV ∫rn（a2（x-y）（v（x）-v（y））/（｜x-y｜n＋β）dy = g（u（x）,v（x））.To reach our aim, by using the method of moving planes, we prove a narrow region principle and a decay at infinity by the iteration method. On the basis of these results, we conclude radial symmetry and monotonicity of positive solutions for the problems involving the weighted fractional system on an unit ball and the whole space. Furthermore, non-existence of nonnegative solutions on a half space is given.

关键词： Weighted fractional system positive solution radial symmetry monotonicity non-existence

phase-selection and metastable phase formation in highly undercooled eutectic Ni78.6Si21.4.lloys

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progress in Natural Science:Materials International 2006年第3期16卷 287-291页

作者： LU Yiping, YANG Gencang, YANG Changlin, WANG Haipeng and ZHOU Yaohe (State Key Laboratory of Solidification processing, northwestern polytechnical university, xi'an 710072, China department of {4. physics, northwestern polytechnical university, xi'an 710072, China) [a]State Key Laboratory of Solidification processing Northwestern Polytechnical University Xi'an China [b]department of applied physics Northwestern Polytechnical University Xi'an China

A large undercooling (250 K) was achieved in eutectic Ni78.6 Si21.4.melt by the combination of molten-glass denucleation and cyclic sup.r.eating. The metastable phase formation process in the bulk undercooled eutectic Ni78.6 Si21.4.melts was investigated. With the increase of undercooling, different metastable phases form in eutectic Ni78.6 Si21.4.melts and p.r. of these metastable phases can be kept at room temp.r.ture through slow post-solidification. Under large undercooling, the metastable phases β2-Ni3Si, Ni31Si12 and Ni3Si2 were identified. Especially, the Ni3Si2 phase was obtained in eutectic Ni78.6 Si21.4.alloy for the first time. Based on the principle of free energy minimum and transient nucleation theory, the solidification behavior of melts was analyzed with regard to the metastable phase formation when the melts were in highly undercooled state.

关键词： undercooling molten-glass metastable phase rapid solidification.

CHArACTErISTICS OF CrITICAL STATE OF TENSILE INSTABILITY

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中国有色金属学会会刊：英文版 1996年第3期6卷 58-62页

作者： Zhou Weixian(department of Aeronautical Manufacturing Engineering, northwestern polytechnical university, xi’an 710072) northwestern pOLYTECH UNIV DEPT AERONAUT MFG ENGNXIAN 710072PEOPLES R CHINA

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ＣＨＡＲＡＣＴＥＲＩＳＴＩＣＳＯＦＣＲＩＴＩＣＡＬＳＴＡＴＥＯＦＴＥＮＳＩＬＥＩＮＳＴＡＢＩＬＩＴＹ￥ＺｈｏｕＷｅｉｘｉａｎ（ＤｅｐａｒｔｍｅｎｔｏｆＡｅｒｏｎａｕｔｉｃａｌＭａｎｕｆａｃｔｕｒｉｎｇＥｎｇｉｎｅｅｒｉｎｇ，ＮｏｒｔｈｗｅｓｔｅｒｎＰ...

关键词： tensile instability critical state deformation characteristic