检索结果-南通市图书馆

boundary parametrization of planar self-affine tiles with collinear digit set

维普期刊数据库评论

在线全文

维普期刊数据库

学校读者我要写书评

暂无评论

Science China Mathematics 2010年第9期53卷 2173-2194页

作者： AKIYAMA Shigeki LORIDANT Benoit Department of Mathematics Faculty of ScienceNiigata UniversityIkarashi 28050 Niigata9502181Japan

We consider a class of planar self-affine tiles T = M-1 a∈D(T + a) generated by an expanding integral matrix M and a collinear digit set D as follows:M =(0-B 1-A),D = {(00),...,(|B|0-1)}.We give a parametrization S1 →T of the boundary of T with the following standard *** is H¨older continuous and associated with a sequence of simple closed polygonal approximations whose vertices lie on T and have algebraic *** derive a new proof that T is homeomorphic to a disk if and only if 2|A| |B + 2|.

关键词： self-similar tile boundary disk-likeness fractal parametrization

Evolution of boundary layer ozone in Shijiazhuang,a suburban site on the North China Plain

在线全文

学校读者我要写书评

暂无评论

Journal of Environmental Sciences 2019年第9期31卷 152-160页

作者： Wei Zhao Guiqian Tang Huan Yu Yang Yang Yinghong Wang Lili Wang Junlin An Wenkang Gao Bo Hu Mengtian Cheng Xingqin An Xin Li Yuesi Wang Nanjing University of Information Science and Technology Nanjing 210044 China State Key Laboratory of Atmospheric boundary Layer Physics and Atmospheric Chemistry fLAPC) Institute of Atmospheric Physics Chinese Academy of Sciences Beijing 100029 China Key Laboratory of Atmospheric Chemistry China Meteorological Administration Beijing 100081 China Department of Atmospheric Science School of Environmental Studies China University of Geosciences Wuhan 430074 China Weather Modification Office of Hebei Province Shijiazhuang 050021 China Key Laboratory of Meteorological Disaster Ministry ofEducation (KLME) Joint International Research Laboratory of Climate and Environment Change (ILCEC) Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters Key Laboratory for Aerosol-Cloud- Precipitation of China Meteorological Administration Nanjing University of Information Science and Technology Nanjing 210044 China Center for Excellence in Regional Atmospheric Environment Institute of Urban Environment Chinese Academy of Sciences Xiamen 361021 China University of Chinese Academy of Sciences Beijing 100049 China

The structure of the boundary layer affects the evolution of ozone(O3), and research into this structure will provide important insights for understanding photochemical *** this study, we conducted a one-month observation(from June 15 to July 14, 2016) of the boundary layer meteorological factors as well as O3 and its precursors in Luancheng County,Shijiazhuang(37°53′N, 114°38′E). Our research showed that photochemical pollution in Shijiazhuang is serious, and the mean hourly maximum and mean 8-hr maximum O3 concentrations are 97.9 ± 26.1 and 84.4 ± 22.4 ppbV, respectively. Meteorological factors play a significant role in the formation of O3. High temperatures and southeasterly winds lead to elevated O3 values, and at moderate relative humidity(40%–50%) and medium boundary layer heights(1200–1500 m), O3 production sensitivity occurred in the transitional region between volatile organic compounds(VOC) and nitrogen oxides(NOx) limitations,and the O3 concentration was the highest. The vertical profiles of O3 were also measured by a tethered balloon. The results showed that a large amount of O3 was stored in the residual layer, and the concentration was positively correlated with the O3 concentration measured the previous day. During the daytime of the following day, the contribution of O3 stored in the residual layer to the boundary layer reached 27%± 7% on average.

关键词： Ozone boundary layer Residual layer Vertical evolution Sensitivity

Improvement of microstructure and coercivity for Nd-Fe-B sintered magnets by boundary introducing low melting point alloys

在线全文

学校读者我要写书评

暂无评论

Journal of Rare Earths 2020年第4期38卷 395-401页

作者： Shuai Cao Xiaoqian Bao Jiheng Li Haijun Yu Kunyuan Zhu Xuexu Gao State Key Laboratory for Advanced Metals and Materials University of Science and Technology BeijingBeijing 100083China

Different from the grain boundary diffusion process(GBDP),which is suitable for modifying thin magnet,a green-pressing agents permeation process(GAPP)that uses low melting point alloys was applied to the Nd-Fe-B green compact with a thickness over 15 mm to reconstruct the boundary microstructure of a sintered Nd-Fe-B *** coercivity increases from 12.3 kOe for the sample free of Pr80Al20 to16.8 kOe for the sample with 2 wt%*** further increasing the Pr80Al20 content to 3 wt%,the coercivity increases slightly,but the remanence and Hk/Hcj deteriorate *** optimal comprehensive properties of Hcj=16.8 kOe,Br=13.4 kG and Hk/Hcj=0.975 are obtained at 2 wt%Pr80Al20,since matrix phase grains are separated by relatively continuous thin grain boundary layers,which weaken the magnetic coupling between adjacent *** coercivities of the samples from the GAPP that use2 wt%Pr80Al20,Pr70Al30 and Pr60Tb20Al20 alloys,respectively,can be enhanced to a large ***,the coercivity of the magnet reconstructed with Pr80Al20 is lower than that of the sample with Pr60Tb20Al20 but is higher than that of the sample reconstructed with Pr70Cu30 ***,the coercivity of the sample from the GAPP using 2 wt%Pr80Al20 is much higher than that of the sample from the GBDP,which is due to a nearly uniform boundary microstructure from the surface to the interior of the thick magnet from the GAPP,thus providing new insights into the fabrication of thick and bulky permanent magnets with high coercivity.

关键词： Nd-Fe-B sintered magnets Green-pressing agents permeation Grain boundary diffusion Pr-Al alloys Thick and bulky magnets Rare earths

An improved boundary element method for modelling a self-reacting point absorber wave energy converter

在线全文

学校读者我要写书评

暂无评论

Acta Mechanica Sinica 2018年第6期34卷 1015-1034页

作者： Qian-Long Xu Ye Li Zhi-Liang Lin State Key Laboratory of Ocean Engineering Collaborative Innovation Center for Advanced Ship and Deep-Sea ExplorationMuti-function Towing TankKey Laboratory of Hydrodynamics(Ministry of Education)School of Naval ArchitectureOcean and Civil EngineeringShanghai Jiao Tong UniversityShanghai 200240China

A numerical model based on a boundary element method (BEM) is developed to predict the performance of two-body selfreacting floating-point absorber (SRFPA) wave energy systems that operate predominantly in *** key numerical issues in applying the BEM are systematically *** particular,some improvements and simplifications in the numerical scheme are developed to evaluate the free surface Green's function,which is a main element of difficulty in the *** a locked SRFPA system,the present method is compared with the existing experiment and the Reynolds-averaged NavierStokes (RANS)-based method,where it is shown that the inviscid assumption leads to substantial over-prediction of the heave *** the unlocked SRFPA model we study in this paper,the additional viscous damping primarily induced by flow separation and vortex shedding,is modelled as a quadratic drag force,which is proportional to the square of body *** inclusion of viscous drag in present method significantly improves the prediction of the heave responses and the power absorption performance of the SRFPA system,obtaining results excellent agreement with experimental data and the RANS simulation results over a broad range of incident wave periods,except near resonance in larger wave height *** is found that the wave overtopping and the re-entering impact of out-of-water floating body are observed more frequently in larger waves,where these non-linear effects are the dominant damping sources and could significantly reduce the power output and the motion responses of the SRFPA system.

关键词： boundary element method Floating-point absorber Heave Green's function Viscous drag

Dynamics of an ultrasound contrast agent microbubble near spherical boundary in ultrasound field

维普期刊数据库评论

在线全文

维普期刊数据库

学校读者我要写书评

暂无评论

Chinese Physics B 2019年第11期28卷 204-209页

作者： Ji-Wen Hu Lian-Mei Wang Sheng-You Qian Wen-Yi Liu Ya-Tao Liu Wei-Rui Lei School of Mathematics and Physics University of South ChinaHengyang 421001China School of Physics and Electronics Hunan Normal UniversityChangsha 410081China

The goal of this article is to establish the conditions of excitation where one has to deal with ultrasound contrast agent(UCA) microbubbles pulsating near biological tissues with spherical boundary in ultrasound field for targeted drug delivery and cavitation-enhanced thrombolysis, etc., and contributes to understanding of mechanisms at play in such an interaction. A modified model is presented for describing microbubble dynamics near a spherical boundary(including convex boundary and concave boundary) with an arbitrary-sized aperture angle. The novelty of the model is such that an oscillating microbubble is influenced by an additional pressure produced by the sound reflection from the boundary wall. It is found that the amplitude of microbubble oscillation is positively correlated to the curve radius of the wall and negatively correlated to the aperture angle of the wall and the sound reflection coefficient. Moreover, the natural frequency of the microbubble oscillation for such a compliable wall increases with the wall compliance, but decreases with the reduction of the wall size, indicating distinct increase of the natural frequency compared to a common rigid wall. The proposed model may allow obtaining accurate information on the radiation force and signals that may be used to advantage in related as drug delivery and contrast agent imaging.

关键词： ultrasound contrast agent(UCA) microbubble spherical boundary ultrasound natural frequency

Leading-edge receptivity of boundary layer to three-dimensional free-stream turbulence

在线全文

学校读者我要写书评

暂无评论

Applied Mathematics and Mechanics(English Edition) 2019年第6期40卷 851-860页

作者： Luyu SHEN Changgen LU Xiaoqing ZHU School of Marine Sciences Nanjing University of Information Science & Technology

The laminar-turbulent transition has always been a hot topic of fluid mechanics. Receptivity is the initial stage and plays a crucial role in the entire transition process. The previous studies of receptivity focus on external disturbances such as sound waves and vortices in the free stream, whereas those on the leading-edge receptivity to the three-dimensional free-stream turbulence (FST), which is more general in the nature, are rarely reported. In consideration of this, this work is devoted to investigating the receptivity process of three-dimensional Tollmien-Schlichting (T-S) wave packets excited by the three-dimensional FST in a flat-plate boundary layer numerically. The relations between the leading-edge receptivity and the turbulence intensity are established, and the influence of the FST directions on the propagation directions and group velocities of the excited T-S wave packets is studied. Moreover, the leading-edge receptivity to the anisotropic FST is also studied. This parametric investigation can contribute to the prediction of laminar-turbulent transition.

关键词： leading-edge receptivity free-stream turbulence (FST) boundary layer three-dimensional Tollmien-Schlichting (T-S) wave

A study of boundary vorticity dynamics and identification of large-scale structures in flow field based on two-dimensional flow around a bluff body

在线全文

学校读者我要写书评

暂无评论

Journal of Hydrodynamics 2019年第2期31卷 231-248页

作者： Da-peng Zhang Yu-zhu Chen Xi-lin Xie Department of Aeronautics and Astronautics Fudan UniversityShanghai 200433China

A method based on decomposition of acceleration field and Lie derivative is introduced to identify shearing and rotational *** method is validated on two typical kinds of model *** dynamics of flow around bluff body is studied,illustrated by numerical examples of flow around an elliptic cylinder,a slanted elliptic cylinder and an elliptic cylinder with a pair of bumps on the front *** explain the generation of vortical structures and how they evolve into inner flow field,boundary vorticity dynamics analysis is *** vorticity flux as well as the enstrophy diffusion flux creates vorticity sources and vorticity sinks,which generate or consume boundary vorticity,then shearing layers are generated and interact with each other finally create *** results provide potential in accurate flow control by boundary deformation,and show that relevant theoretical conclusions can be effectively applied in revealing the flow mechanisms.

关键词： boundary vorticity dynamics large-scale structures Lie derivative

Numerical and experimental investigation into hypersonic boundary layer transition induced by roughness elements

维普期刊数据库评论

在线全文

维普期刊数据库

学校读者我要写书评

暂无评论

Chinese Journal of Aeronautics 2019年第3期32卷 559-567页

作者： Hao DONG Shicheng LIU Xi GENG Song LIU Liming YANG Keming CHENG Department of Aerodynamics Nanjing University of Aeronautics and Aeronautics Department of Mechanical Engineering National University of Singapore

In this work, the Direct Numerical Simulation(DNS) and Oil-Film Interferometry(OFI)technique are used to investigate the hypersonic boundary layer transition induced by single and double roughness elements at Mach number 5. For single roughness, the DNS results showed that both horseshoe vortices and hairpin vortices caused by shear layer instability can affect the boundary layer instability. The generation of the near-wall unstable structure is the key point of boundary layer transition behind the roughness element. At the downstream of the roughness element, the interaction between horseshoe vortices and hairpin vortices will spread in the spanwise *** double roughness elements, the effect of the spacing between roughness elements on the transition is studied. It is found that the case of higher spacing between roughness elements is more effective for inducing transition than the lower one. The interaction between two adjacent roughness elements can suppress the evolution of horseshoe vortices in the downstream and trigger the instability of shear layer. Thus, the transition will be suppressed accordingly.

关键词： boundary layer transition Hypersonic Direct numerical simulation (DNS) Oil-film interferometry Roughness elements

A new boundary element solution to evaluate the geometric effects of the canyon site on the displacement response spectrum

在线全文

学校读者我要写书评

暂无评论

Earthquake Engineering and Engineering Vibration 2019年第2期18卷 267-284页

作者： Reza Tarinejad Mohsen Isari Abbasali Taghavi Ghalesari Department of Civil Engineering University of Tabriz Department of Civil Engineering The University of Texas at El Paso 500 W University Ave.

This paper presents a step-by-step procedure using the three-dimensional boundary element approach to study the behavior of semi-circular canyons under seismic shear waves. The boundary element code TDASC allows utilization for various canyon geometries, evaluation of concurrent seismic waves and calculation of the ground motions on canyons due to an excitation at any arbitrary point of the incident field. Considering the widening ratio of the canyon(including prismatic, semi-prismatic and non-prismatic canyons), wave characteristics(wavelength, dimensionless period, direction) and maximum amplification pattern, the solution was applied to carry out a series of parametric studies. It was shown that canyon form can significantly affect the displacement amplification, especially at the points located on its edges. By increasing the wave dimensionless frequency(η > 1), the amplification pattern becomes more complex. On the basis of the results from a variety of considered cases, a new expression has been presented for the limiting wavelength beyond which the widening of the canyon will not have a major effect on the displacement amplification. To verify the reliability of the proposed approach, the obtained results, expressed in terms of displacement amplitude, were compared with those from the available published literature and a reasonably good agreement was observed.

关键词： canyon boundary element method widening ratio site effect wave scattering amplifi cation