大气非线性与湍流过程中动力学复杂性的研究与进展

通过实验数学方法、元胞自动机模型（并行算法与实际过程的模拟）、大型转盘模拟实验以及计算机数值模拟（用串行算法模拟时间演化的长期行为）来研究大气系统非线性过程与湍流过程中出现的混沌随时间演化的宏观特性、气溶胶粒子凝聚增长机制,湍流的拟序结构,系统状态在临界点的突变和多重平衡态以及台风的孤立子拓扑性质,促进学科新概念新方法的相互交叉与渗透,从分析、综合上升到整体研究（ＩｎｔｅｇｒａｔｉｖｅＳｔｕｄｙ）,在非线性动力学层次上认识大气系统复杂性的本质,这些都是大气科学领域中当前迅速发展的前沿课题,具有重要的科学意义和实际应用价值。     

湍流运动与非线性科学理论

本文较系统地介绍了近３０年来非线性科学理论的发展对湍流运动研究的贡献．它所涵盖的内容涉及到了当前非线性科学理论的大部分热点．从这里,可以隐约地看到一个正在形成中的湍流运动新理论的框架．     

第9届国际等离子体化学会议(ISPC-9)

概述了第9届国际等离子体化学会议的论文和活动情况,对这个领域的发展提出了一些看法。     

第9届国际等离子体化学会议(ISPC-9)

概述了第9届国际等离子体化学会议的论文和活动情况,对这个领域的发展提出了一些看法。     

IUTAM "第五届工程湍流模拟与测量"学术会议(ETMM-5)简介

位于地中海西部西班牙美丽的海岛马约卡的九月,风和日丽,景色宜人.蓝天和大海交相辉映,一幢幢白色的、浅黄色的或乳黄色的别墅掩映在郁郁葱葱的棕榈树与各类热带、亚热带植物之间.马约卡是欧洲大陆游客的旅游胜地,德国人、英国人尤其钟爱此地.     

第三届国际近海结构物性能讨论会(BOSS’82)

第三届国际近海结构物性能讨论会(Third International Conference on Behavior of Off—Shore—Structures,简称BOSS'82),将于1982年8月2—5日在美国麻省理工学院举行。BOSS讨论会由德耳夫特理工大学、麻省理工学院、挪威理工学院和伦敦大学联合主办,目的是讨论恶劣环境中复杂近海工程的关键性研究和设计问题。讨论会将讨论80年代建造新一代近海结构物     

第4届国际生物流变学会议(东京)及附属会议——国际心血管疾病血液流变学方法讨论会(大阪)

第4届国际生物流变学会议于1981年7月27日～8月1日在日本东京举行。25个国家240多名代表出席了会议。本届国际生物流变学协会主席深田荣一(E.Fukada,日本),副主席东健彦(T.AZuma,日本)、冯元桢(Y.C.B.Fung,美籍华裔学者)、H.H.Hartert(西德)、V.I.Vorob'ev(苏联)出席了会议。会议共组织6个大会邀请报告及30个分组会。由华中工学院、重庆大学、北京大学和复旦大学组成的我国代表团参加了会议并宣读了论文,受到了会议的热情接待和好评。     

第18届国际理论与应用力学大会(ICTAM 1992)

本届大会将于1992年8月22—28日在以色列海法市举行。会议范围涉及整个理论与应用力学领域。大会委员将邀请一定数量的报告。并接受约480篇论文以分组会报告或墙报的形式进行交流。以下三方面选题将作为本届大会中的小型会议(mini-symposia)的主题:①固体与结构力学中的不稳定性;②海洋表面力学及空气-海洋相互作用;③生物力学。     

第18届国际理论与应用力学大会(ICTAM-18)

<正> 由国际理论与应用力学联合会(IUTAM)召开的4年一次的第18届国际理论与应用力学大会(ICTAM-18),于1992年8月22—28日在以色列海法市的以色列工业学院举行。大会选定的3个主题是:固体与结构力学中的不稳定性;海面力学与海气相互作用;生物力学。设置主题分会的目的是对力学中的某些重要领域做综合介绍,以强调这领域的重要性,或对新领域起提倡作用。     

第十六届国际理论与应用力学大会(ICTAM)点滴

本刊已在1984年第6期中曾对我国力学工作者参加第16届ICTAM大会概况作了介绍。现再刊登一组文章,以飨读者。 ...     

Space temporal maps for vortical flow field construction

A class of models to generate a two-dimensional complex flow field, deriving from the coupled map lattice dynamics, is presented here. It automatically satisfies continuity equation for an incompressible fluid. The method is numerically implemented on a square lattice and some results relatively to a fully deterministic and a semi stochastic evolution are presented here. The qualitative similarity with two dimensional hydrodynamic turbulence is encouraging. In view of these first results, directions for advances are proposed.

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Sommario Si presenta una classe di modelli per generare un campo di moto complesso, derivante dai modelli dinamici a mappe accoppiate, in modo tale che l'equazione di continuita' per un fluido incomprimibile sia automaticamente soddisfatta. Il metodo e'implementato numericamente su una griglia quadrata. Si presentano alcuni risultati relativi ad un'evoluzione deterministica e semi stocastica. La somiglianza qualitativa con campi di moto idrodinamici turbolenti bidimensionali e'incoraggiante. Alla luce dei risultati ottenuti si propongono inoltre possibili direzioni per ulteriori sviluppi.

     

圆Couette流混沌现象的实验研究

本文用流动显示和激光散射功率谱观泌了圆Couette流中的混沌现象,发现有两条导致弱湍流激发的道路:一条是从准周期运动到混沌,另一条是阵发混沌,对于这两条道路在诸转变阈值上的巨大差别,作者用弱非线性作用局域有效的新观点给出物理解释。     

涡旋运动与混沌

本文综述了利用非线性动力学方法研究涡旋运动的最新进展。通过典型例子的分析,讨论了该领域的研究方法、主要困难和存在问题,并提出今后的可能研究方向。      

The effects of turbulence characteristics on sphere drag

Further results are reported in determining the drag coefficient of spheres in terms of the turbulence characteristics of the flow in which they are immersed. C_D contours are given on plots of turbulence scale versus intensity and show distinct regions where artificially low and high drag can be experienced. The results are shown to be in accordance with theory based on published results for flow conditions upstream and downstream of the spheres.

A Strouhal number (St) based on turbulence macroscale and the r.m.s. value of the fluctuating component of axial velocity is shown to be very appropriate in this application.     

Route to Chaos for Moderate Prandtl Number Convection in a Porous Layer Heated from Below

The route to chaos for moderate Prandtl number gravity driven convection in porous media is analysed by using Adomian's decomposition method which provides an accurate analytical solution in terms of infinite power series. The practical need to evaluate numerical values from the infinite power series, the consequent series truncation, and the practical procedure to accomplish this task, transform the otherwise analytical results into a computational solution achieved up to a desired but finite accuracy. The solution shows a transition to chaos via a period doubling sequence of bifurcations at a Rayleigh number value far beyond the critical value associated with the loss of stability of the convection steady solution. This result is extremely distinct from the sequence of events leading to chaos in low Prandtl number convection in porous media, where a sudden transition from steady convection to chaos associated with an homoclinic explosion occurs in the neighbourhood of the critical Rayleigh number (unless mentioned otherwise by 'the critical Rayleigh number' we mean the value associated with the loss of stability of the convection steady solution). In the present case of moderate Prandtl number convection the homoclinic explosion leads to a transition from steady convection to a period-2 periodic solution in the neighbourhood of the critical Rayleigh number. This occurs at a slightly sub-critical value of Rayleigh number via a transition associated with a period-1 limit cycle which seem to belong to the sub-critical Hopf bifurcation around the point where the convection steady solution looses its stability. The different regimes are analysed and periodic windows within the chaotic regime are identified. The significance of including a time derivative term in Darcy's equation when wave phenomena are being investigated becomes evident from the results.     

Local and Global Transitions to Chaos and Hysteresis in a Porous Layer Heated from Below

The routes to chaos in a fluid saturated porous layer heated from below are investigated by using the weak nonlinear theory as well as Adomian's decomposition method to solve a system of ordinary differential equations which result from a truncated Galerkin representation of the governing equations. This representation is equivalent to the familiar Lorenz equations with different coefficients which correspond to the porous media convection. While the weak nonlinear method of solution provides significant insight to the problem, to its solution and corresponding bifurcations and other transitions, it is limited because of its local domain of validity, which in the present case is in the neighbourhood of any one of the two steady state convective solutions. On the other hand, the Adomian's decomposition method provides an analytical solution to the problem in terms of infinite power series. The practical need to evaluate numerical values from the infinite power series, the consequent series truncation, and the practical procedure to accomplish this task transform the otherwise analytical results into a computational solution achieved up to a finite accuracy. The transition from the steady solution to chaos is analysed by using both methods and their results are compared, showing a very good agreement in the neighbourhood of the convective steady solutions. The analysis explains previously obtained computational results for low Prandtl number convection in porous media suggesting a transition from steady convection to chaos via a Hopf bifurcation, represented by a solitary limit cycle at a sub-critical value of Rayleigh number. A simple explanation of the well known experimental phenomenon of Hysteresis in the transition from steady convection to chaos and backwards from chaos to steady state is provided in terms of the present analysis results.     

Weak Turbulence and Chaos for Low Prandtl Number Gravity Driven Convection in Porous Media

Low Prandtl number convection in porous media is relevant to modern applications of transport phenomena in porous media such as the process of solidification of binary alloys. The transition from steady convection to chaos is analysed by using Adomian's decomposition method to obtain an analytical solution in terms of infinite power series. The practical need to evaluate the solution and obtain numerical values from the infinite power series, the consequent series truncation, and the practical procedure to accomplish this task, transform the analytical results into a computational solution evaluated up to a finite accuracy. The solution shows a transition from steady convection to chaos via a Hopf bifurcation producing a 'solitary limit cycle which may be associated with an homoclinic explosion. This occurs at a slightly subcritical value of Rayleigh number, the critical value being associated with the loss of linear stability of the steady convection solution. Periodic windows within the broad band of parameter regime where the chaotic solution persists are identified and analysed. It is evident that the further transition from chaos to a high Rayleigh number periodic convection occurs via a period halving sequence of bifurcations.     

Convective coupled map for simulating spatiotemporal chaos in flows

A coupled map lattices with convective nonlinearity or, for short, Convective Coupled Map (CCM) is proposed in this paper to simulate spatiotemporal chaos in fluid flows. It is found that the parameter region of spatiotemporal chaos can be determined by the maximal Liapunov exponent of its complexity time series. This simple model implies a similar physical mechanism for turbulence such that the route to spatiotemporal chaos in fluid flows can be envisaged. The study is supported by “Nonlinear Sciences Project” from the State Science and Technology Commission of China.