The role of impulse on the initiation of particle movement under turbulent flow conditions

Fundamental to our understanding of erosional and transport phenomena in earth-surface dynamics and engineering is knowledge of the conditions under which sediment motion will begin when subjected to turbulent flow. The onset criterion currently in use emphasizes the time-averaged boundary shear stress and therefore is incapable of accounting for the fluctuating forces encountered in turbulent flows. We have validated through laboratory experiments and analytical formulation of the problem a criterion based upon the impulse imparted to a sediment grain. We demonstrate that in addition to the magnitude of the instantaneous turbulent forces applied on a sediment grain, the duration of these turbulent forces is also important in determining the sediment grain's threshold of motion, and that their product, or impulse, is better suited for specifying such conditions.     

Experimental observation of nonlinear traveling waves in turbulent pipe flow

Transition to turbulence in pipe flow is one of the most fundamental and longest-standing problems in fluid dynamics. Stability theory suggests that the flow remains laminar for all flow rates, but in practice pipe flow becomes turbulent even at moderate speeds. This transition drastically affects the transport efficiency of mass, momentum, and heat. On the basis of the recent discovery of unstable traveling waves in computational studies of the Navier-Stokes equations and ideas from dynamical systems theory, a model for the transition process has been suggested. We report experimental observation of these traveling waves in pipe flow, confirming the proposed transition scenario and suggesting that the dynamics associated with these unstable states may indeed capture the nature of fluid turbulence.     

高分子聚合物的湍流减阻机理

对链状高分子聚合物进行球-棍模型简化,使得在描述其减阻机理时更加直观形象。利用流体流动控制方程和高分子本构方程进行直接数值模拟,根据模拟结果研究聚合物的主要减阻机理。在湍流条件下,聚合物反向扭矩从根本上抑制了湍流发卡型漩涡的迸发和再生,进而减少流体的湍流脉动及流动阻力损失。其中,聚合物的正向扭矩作用在发卡型漩涡的拱形头部和第2个发卡型漩涡的腿部,抑制湍流漩涡的迸发频率和强度,减少新的发卡型漩涡的生成;聚合物的反向扭矩作用在发卡型漩涡的卷曲方向,可以减小漩涡的弯曲率和倾斜角度。     

混流式水轮机导叶绕流的三维湍流数值模拟

在现有水轮机引水元件（包括导叶）内部流场数值计算方法和设计理论的基础上，基于Navier-Stokes方程和标准的κ-ε紊流模型，采用贴体坐标和交错网格系统，用SIMPLEC算法对一模型机的蜗壳、座环和导叶进行了联合计算。计算结果表明：混流式水轮机导叶叶间流道中的水流运动沿周向呈周期性变化．叶道中间水流速度较大，越靠近叶片速度越小；随着导叶开度的增大，水流切向速度减小，径向速度增大，导叶出口的水流角增大；导叶出口水流速度及环量分布不均匀，与工况变化有关，进行转轮及全流道三维湍流计算时应考虑这一因素；叶型表面水流速度和压力分布随导叶开度变化的规律比较复杂。     

CFD在水力机械湍流分析与性能预测中的应用

为了研究水力机械运行特性，将最新发展的计算流体动力学（CFD）理论与方法应用于水力机械湍流分析，给出了水力机械湍流流动控制方程，分析了求解湍流流动的Reynolds时均法和大涡模拟法的适用性，建立了通过湍流流场分布预估水力机械能量特性和空化特性的数学模型。通过基于k-ε双方程湍流模型的轴流泵全流道三维定常湍流计算，获得了与实际情况相符的流动细节。根据计算结果对轴流泵的能量、空化性能所做的预估．与试验结果基本一致。CFD理论和方法为探寻水力机械流场流动特性与结构动力特性间的复杂关系提供了一种新手段，为研究和开发高性能、大流量、高水头的水轮机和水泵提供理论和实际应用参考。     

长流胶分散凝固 集中加工试验

割胶制度改革后，以药代刀导致长流胶增多，过去长期形成的正常收送胶秩序补被打乱。为了确保橡胶的回收率，农场不惜动用大量人力、动力，将当天长流胶收集起来，及时送制胶厂按鲜胶乳的现行生产工艺及时加工，但数量太小，即要单独运送，而此时，中午的胶乳已凝固结束，也无法与正常鲜胶乳一道均匀混合后并流加酸凝固，使得收送胶及固环节的成本大大增加，当日产品的一致性都在当天下午高温环境中收集和运送，既增加了早保费用，也使工序质量和产品质量受到一定的影响，然而，若将长流胶在生产队制成凝块再送制胶厂加工，在云南垦区一些大型制胶厂，因受传统观念和做法的影响，如此加工出来的产品最多只能定为SCR10，严重损害了农场经济效益，为此东风农场在第三制胶厂，进行了长流胶分散凝固，集中加工这一制胶新途径的生产性试验，并以此验证胶园凝胶中胶块、胶团类胶料生产SCR5的适应性。     

模拟街道峡谷内的流场和污染物浓度的格子玻尔兹曼方法

构造了模拟街道内的流场及污染物扩散的格子玻尔兹曼模型,并且用这个模型来模拟了不同高宽比的街道峡谷流场和污染物扩散情况。模拟结果表明,地面附近的污染物浓度随着街道高宽比的增加而递增,这一结果和以往的研究结果有很好的一致性。     

近亲繁殖下一对等位基因群体的熵性质

研究了在近亲繁殖下一对等位基因群体的基因库信息熵和基因型信息熵 ,并讨论了在世代交替中基因型信息熵的性质、配子间的互信息性质及其与近交系数的关系 ,并定义了配子间的近交关联信息系数。结果表明 ,应用信息论模型研究群体遗传学与统计学方法具有一致性 ,而且还具有信息学含义。     

典范性状对的决策分析

【目的】对典范性状对进行决策分析,为多数量性状的选择提供一种简便的方法。【方法】运用通径分析的决策分析方法对典范组合性状对进行决策分析。【结果】建立了典范性状对的通径分析模型;分析得到典范性状对可以用以供选择的各性状xi(i=1,2,…,m)为因,典范目标性状Hig为果的中心线性回归模型表述;定义了各选择性状对典范目标性状的决策系数,各决策系数代表了各选择性状对典范目标性状的综合效用。【结论】通过各性状决策系数的确定,选择组合性状中的主载性状、限制性状和辅助性状,育种者就可根据个体的性状表现来选择,以达到对典范目标性状所期望的遗传进展。     

The context effect does not require a fourth base pair

The translational activity of a transfer RNA at a codon varies at different message sites, although the codon does not vary. The source of this effect, which may help to determine the level of gene expression, is generally agreed to be in nearby message sequences. By making every possible nucleotide combination between position 33 of the transfer RNA and the major context nucleotide of the message, it was shown that base-pairing between the two nucleotides is not the source of this context effect on translation in vivo.     

The role of inheritance in behavior

Inheritance plays a major role in behavior as shown by selection and strain studies for animal behavior and by twin and adoption studies for human behavior. Unlike simple Mendelian characteristics, genetic variance for behavioral dimensions and disorders rarely accounts for more than half of the phenotypic variance, and multiple genes with small effects appear to be involved rather than one or two major genes. Genetic research on behavior will be transformed by techniques of molecular biology that can be used to identify DNA sequences responsible for behavioral variation. However, the importance of nongenetic factors and the multigenetic control of behavior require new strategies to detect DNA markers that account for small amounts of behavioral variation.     

The onset of turbulence in pipe flow

Shear flows undergo a sudden transition from laminar to turbulent motion as the velocity increases, and the onset of turbulence radically changes transport efficiency and mixing properties. Even for the well-studied case of pipe flow, it has not been possible to determine at what Reynolds number the motion will be either persistently turbulent or ultimately laminar. We show that in pipes, turbulence that is transient at low Reynolds numbers becomes sustained at a distinct critical point. Through extensive experiments and computer simulations, we were able to identify and characterize the processes ultimately responsible for sustaining turbulence. In contrast to the classical Landau-Ruelle-Takens view that turbulence arises from an increase in the temporal complexity of fluid motion, here, spatial proliferation of chaotic domains is the decisive process and intrinsic to the nature of fluid turbulence.     

The role of intuition

"Intuition," as used by the modern mathematician, means an accumulation of attitudes (including beliefs and opinions) derived from experience, both individual and cultural. It is closely associated with mathematical knowledge, which forms the basis of intuition. This knowledge contributes to the growth of intuition and is in turn increased by new conceptual materials suggested by intuition. The major role of intuition is to provide a conceptual foundation that suggests the directions which new research should take. The opinion of the individual mathematician regarding existence of mathematical concepts (number, geometric notions, and the like) are provided by this intuition; these opinions are frequently so firmly held as to merit the appellation "Platonic." The role of intuition in research is to provide the "educated guess," which may prove to be true or false; but in either case, progress cannot be made without it and even a false guess may lead to progress. Thus intuition also plays a major role in the evolution of mathematical concepts. The advance of mathematical knowledge periodically reveals flaws in cultural intuition; these result in "crises," the solution of which result in a more mature intuition. The ultimate basis of modern mathematics is thus mathematical intuition. and it is in this sense that the Intuitionistic doctrine of Brouwer and his followers is correct. Modern instructional methods recognize this role of intuition by replacing the "do this, do that" mode of teaching by a "what should be done next?" attitude which appeals to the intuitive background already developed. It is in this way that understanding and appreciation of new mathematical knowledge may be properly instilled in the student.