Showing papers by "Tongji University published in 1994"

Last glacial maximum in China: comparison between land and sea

Abstract: As a result of the sea-level drop at the last glacial maximum (about 100–120 m in the South China Sea, and 130–150 m in the East China Sea), the size and configurations of the China Seas changed greatly. The East China Sea with the Yellow Sea and Bohai Gulf was reduced into an elongated trough, while the South China Sea became a semi-enclosed gulf. Due to the southern shift of the polar front in the North Pacific and the reorganization of the surface current system, the winter temperature in the South China Sea was 6–10°C colder, and the seasonality was much stronger than it is now. The reduced sea area, and thus the increased distance to the coast, together with the declined sea surface temperature, led to intensified aridity on the land. The enhanced eolian processes related to the intensification of the winter monsoon caused an expansion of the loess deposition area reaching the southeastern part of China to the south of the Yangtze River and off the modern sea coast. Vegetation zones greatly shifted to the southeast. In north China, deserts and steppes were much more extensive than they are now and the modern areas of warm-temperate, broad-leaved forests were occupied by steppes or woodland. In south China, the deciduous, broadleaved forests were developed in areas now covered by evergreen forests, and the monsoon rain forest completely disappeared. The changes in the climate on land and the configurations of seas altered the sediment supply and sedimentation rate in the China Seas, giving rise to the Atlantic-type late Quaternary CaCO3 curve there.

Complex microstructural changes in as-cast eutectoid Zn-Al alloy

Abstract: Phase transformations and microstructural changes of an as-cast eutectoid Zn-Al alloy (ZnAl22Cu2) were investigated during isothermal holding. The typical dendritic structures consisted of α′s phase as a core with the edge of decomposed β′s phase and decomposed η′s in the interdendritic regions. A series of complex phase transformations was observed. Both decompositions of β′s and η′s were determined at an early stage of ageing and a four-phase transformation, αf+ɛ→T′+η, was observed at the boundaries of αf phase and the ɛ phase, instead of clearly observed at the boundaries of ɛ phase, in a solution-treated Zn-Al alloy during prolonged ageing.

Non-linear elastic theory of rectangular reticulated shallow shell structures

Abstract: Based on fundamental assumptions, an analysis of the constitutive relations between the internal forces and deformations of discrete rectangular reticulated structures is given. On the basis of this, an equivalent continuum model is adopted and the application of the principle of virtual work leads to non-linear governing equations and corresponding boundary conditions.

Extensions of simple modules for G2(p)

Abstract: Extensions of simple modules for the finite Chevalley group G2(p) with p >13 are completely determined in this note.

Non-linear vibration of rectangular reticulated shallow shell structures

Abstract: This paper deals with non-linear vibration of rectangular reticulated shallow shells by applying non-linear elastic theory of such structures established by the author. Using the assumed (generalized) Fourier series solutions for transverse deflection (lattice joint transverse displacement) and force function, weighted means of the trial functions lead to the relations among the coefficients related to the solutions and vibration equation which determines the unknown time function, and then the amplitude-frequency relations for free vibration and forced vibration due to harmonic force are derived with the aid of the regular perturbation emthod and Galerkin procedure, respectively. Numerical examples are given as well.

Image thresholding by histogram transformation

Abstract: A method for threshold selecting based on histogram transformation is proposed. New formulae for histogram transformation are introduced in the paper. After doing histogram transformation, the histogram became smooth. So, the optimal threshold is easy to detect. It is computationally fast and efficient. Experimental results for a set of images are presented.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Variational Gaussian wave functional and the (1+1)-dimensional sine-Gordon model.

Abstract: The (1+1)-dimensional sine-Gordon model is studied in the framework of the variational Gaussian wave functionals. The renormalized mass depends on two dimensionless parameters. In the phase diagram a first-order phase transition will replace the second-order phase transition for large bare mass. The spectrum of the two-particle states consists of a continuum of scattering states and the isolated values connected to the excitonlike bound states. The energy of the bound states and the phase shifts of the scattering states are explicitly calculated.

A Cylindrical Void Growth Model in Visco-Plastic Materials

Abstract: Based on the general macroscopic potential theory, a method to find macroscopic plastic potential and the corresponding constitutive relation is developed for visco-plastic materials containing cylindrical voids. The constitutive relation of the matrix has a generalized overstress form. A comparison of the exact macroscopic potential with its upper bound is made. The numerical results show little difference between the upper bound potential and the exact one. The macroscopic constitutive relation is investigated under triaxial loading conditions. When the strain rate sensitive parameter tends to zero, Gurson's model (1977) is retrieved. When the loading condition is axisymmetric, the pres ent result reverts to Licht and Suquet's result (1988). When the matrix becomes infinite, the result of Budiansky et al. (1982) is obtained. Finally, the macroscopic constitutive rela tion and the corresponding dynamic loading surface are investigated for the case of axi symmetric load. An approximate analytical expressi...

Phase diagram in a two-dimensional Coulomb-gas model.

Abstract: The phase diagram of a two-dimensional (2D) Coulomb-gas model is studied in the framework of quantum sine-Gordon theory in 2D Euclidean space The core structures of the Coulomb charge are considered by evaluating some distribution functions, and the phase diagrams related to the different distributions are compared

The Radiation Effect Mechanism of Some Rare Earth Doped BaF2 Crystal

Abstract: Barium fluoride (BaF{sub 2}) was discovered to be a fast inorganic scintillator in 1983. Considerable attention has been attracted in the fields of nuclear physics, nuclear medicine and high energy physics due to its high density, short radiation length, non-hygroscopic nature and high detection efficiency for {gamma}-rays. However, when BaF{sub 2} is exposed to a radiation environment, the fluorescence output will decrease, thus the study of the radiation damage mechanism of BaF{sub 2} is quite important. In this paper, the authors used the Discrete Variational X{sub {alpha}} (DV-X{sub {alpha}}) method to calculate the energy level of some rare earth doped BaF{sub 2} crystals and indicated that the Eu, Dy and Yb elements will change their valence from +3 to +2 under irradiation and thus change the optical properties of the crystals. The results show that this radiation effect model presented agree well with experiments.

ON q-COHERENT STATE OF q-DEFORMED OSCILLATOR

Abstract: In this letter, we discuss detailed properties of coherent state of deformed oscillator, and obtain some significant results.

Hough transform: it's application to the linearly moving point targets detection

Abstract: The Hough transform is applied to successfully detecting moving point targets in an image sequence. The behaviour and performance of the Hough transform are listed. Three realizable algorithms are provided. The complexity of the computations and noise performance are discussed. The computational simulation results are explained. >

An Investigation of Radiation Damage Induced by Hydroxyl and Oxygen Impurities In BaF2 Crystal

Abstract: Since the discovery of a fast emission component at 220 nm with a very short decay time of 0.6 ns, BaF{sub 2} has been widely used in nuclear physics and high energy physics due to its outstanding time resolution and high detection efficiency for {gamma}-ray. However, when BaF{sub 2} detector subjects to long term exposures of radiation, the scintillation light output will decrease. Therefore the radiation damage in BaF{sub 2} crystal becomes an extremely important problem for its applications. The radiation effect in hydrolyzed BaF{sub 2} was investigated through the changes in their optical absorption and EPR spectra before and after {gamma}-irradiation. The results demonstrated that hydroxyl and oxygen can be easily introduced into BaF{sub 2} by means of a hydrolysis, the most likely modes are OH{sup {minus}} ions substituting for fluorine and O{sup 2{minus}} ions substituting for fluorine associated with charge-compensating fluorine vacancies O{sub s}{sup 2{minus}} {minus} F{sup +}. Combining with the Hartree-Fock-Slater local-density discrete variational (HFS-X{sub {alpha}}-DV) cluster calculation on some possible defects related to hydrogen and oxygen impurities, the authors propose that the radiation damage observed in hydrolyzed BaF{sub 2} can be explained in terms of OH{sup {minus}} and O{sub s}{sup 2{minus}} {minus} F{sup +} dissociation throughmore » a radiolysis.« less

Charge-unbinding transition in the two-dimensional classical Coulomb-plasma model by use of a variational Gaussian wave functional.

Abstract: The charge-unbinding transitions in the two-dimensional classical Coulomb-plasma model are analyzed by use of the equivalence between the two-dimensional Coulomb plasma and (1+1)-dimensional quantum sine-Gordon models. The grand canonical potential and the screening length are obtained from the ground states of the (1+1)-dimensional quantum sine-Gordon model as derived by use of a variational Gaussian wave functional. We have shown that the charge-unbinding transition at small fugacity is a Kosterlitz-Thouless transition, while at large fugacity it is a first-order transition. The dependence of the potential on the mass is similar to that of the phenomenological Landau theory.

Ground-state wave function of antiferromagnetic s=1/2 heisenberg chains

Abstract: We have proposed an approach to solve the eigenvalue problem of the isotropic S=1/2 Heisenberg chain. Many interesting properties of the antiferromagnetic ground state have been reanalyzed by our technique. These results show our method is an appropriate technique.

Paleoceanography in China: Progress and Problems

Abstract: Paleoceanographic studies started in China early in the 1980’s with Quaternary carbonate cycles in the West Pacific (Wang and Zheng, 1981, 1982; Gao et al., 1982). Very soon the research interest moved to the China seas and the emphasis was laid on late Quaternary paleoceanography of the South China Sea (SCS) with a variety of topics including paleotemperature, isotopes, carbonate dissolution, coral reef history and others. There are ten volumes of scientific papers or monographs published or being published since 1987 on paleoceanography or related micro-paleontologic, sedimentologic topics of SCS (Second Institute of Oceanography, 1987, 1989; Feng et al., 1988; SOA, 1988; Multidisciplinary Expedition Team to the Nansha Islands, 1989, 1991; Hao et al. 1989; Zhang et al., 1989; Ye and Wang, 1992; Zheng and Chen, in press). Active research work has been carried out also in the Okinawa Trough.

Charge decay and transportation in teflon AF films

Abstract: Isothermal surface potential decay and thermally stimulated discharge (TSD) are performed with amorphous polymer teflon AF films, which is a new member of the teflon family with a lot of unique features The experimental results show that the charge retention in this material is quite good as long as the field across the films is not too high Heat treatment before and after charging changes the TSD peaks and consequently the charge stability for negative charging In the case of positive charging, a more significant effect occurs when the sample is charged at elevated temperatures