Water Impingement on a Vertical Wall due to Discontinuity Decay Above a Drop

The paper reports experimental data on the height of water impingement on a vertical wall during wave reflection due to discontinuity decay above an even bottom and a bottom drop. It is shown that for a bore type wave with a roller in its head, propagating over finitedepth water, the impingement height is proportional to the initial difference in freesurface level. In the case of a dry bottom in the tail water, this is also true for other types of waves formed during discontinuity decay.     

Turbulent Channel Flow with an Artificial Two-Dimensional Wall Layer

Turbulent flow of an incompressible fluid in a plane channel with parallel walls is considered. The three-dimensional time-dependent Navier-Stokes equations are solved numerically using the spectral finite-difference method. An artificial force which completely suppresses lateral oscillations of the velocity is introduced in the near-wall zone (10 % of the channel half-width in the neighborhood of each wall). Thus, the three-dimensional flow zone, in which turbulent oscillations can develop, is separated from the wall by a fluid layer. It is found that the elimination of three-dimensionality in the neighborhood of the walls leads to a significant reduction in the drag. However, complete laminarization does not occur. The flow in the stream core remains turbulent and can be interpreted as a turbulent flow in a channel with walls located on the boundary of the two-dimensional layer and traveling at the local mean-flow velocity. The oscillations developing inside the two-dimensional layer, which have significant amplitude, distort the flow only in the adjacent zone. Beyond this zone the distributions of the mean characteristics and the structure of instantaneous fields completely correspond to ordinary turbulent flow in a channel with rigid walls. The results obtained confirm the hypothesis of the unimportance of the no-slip boundary conditions for the fluctuating velocity component in the mechanism of onset and self-maintenance of turbulence in wall flows.     

槽道湍流近壁区多尺度输运特性研究

利用槽道湍流直接数值模拟的数据库和离散正交子波,对近壁湍流的多尺度输运特性进 行了研究. 通过在流向和展向分别进行子波多尺度分解,得到了近壁区湍动能在流向和展向 多尺度传输的不同性质,发现流向传输以能量的反传为主,而在展向能量存在明显的正传, 并且当过滤尺度较大时以正传为主. 近壁湍流能量传输的各向异性为进一步构造各向异 性大涡模拟亚格子模式提供了必要的参考.     

基于壁面主动变形的湍流减阻控制研究

采用谱方法, 对反向控制下壁面主动变形的槽道湍流进行了直接数值模 拟研究. 结果表明, 在壁面最大变形量小于5倍黏性尺度条件下, 压差阻力可略, 摩擦阻力 降低7.6%. 施加控制后, 湍流强度和雷诺应力受到明显抑制, 平均速度剖面对数区上移. 受壁面法向运动的影响, 条带结构强度减弱、尺度变大; 流向涡外移且强度减弱, 其倾斜和 抬起的角度均有不同程度的减小. 壁面变形呈现流向拉长的凹槽结构, 其平均间距 为90倍黏性尺度.     

提高电化学法测试壁面剪应力精度的方法

应用电化学方法对流场壁面剪应力进行测试在生物力学中有着广泛的用途,静止极限电流在存在将影响壁面剪应力测试的精度,在测试时应根据具体的实验条件进行现场测试,实验表明在测试较低壁面剪应力时,静止极限电流的影响较大;在测试较高壁面剪应力时,则可忽略静止极限电流的影响,极化电压条件下测得极限电流是测试壁面剪应力的基础。为了测试的准确可靠,需对产生极化的外加电压进行实地测试,在测试时,对每个电极都需根据相应的测试条件进行系统标定。     

Genetic Algorithms for Optimizing the Remediation of Contaminated Aquifer

Produced water constitutes a large amount of waste fluids during the production operation of an oil field. Underground injection for disposing the wastewater from hydrocarbon production is an engineering problem due to the possibility of leakage of injected pollutant material from receiving medium to a drinking water source. This paper describes a method for optimization of polluted aquifer remediation design using one of the artificial intelligence optimization methods, namely Genetic Algorithms (GAs). As a case study, the contaminated area was created by using a groundwater transport simulator, which is based on Method of Characteristics (MOC). Then, the developed computer program was run to find the optimum solution for remediation, and the solution yielded from the program was verified by using a groundwater simulator. The plume was captured and the concentration level of chloride ion within the aquifer was diminished by using extraction wells. The analytical model approach provided different alternatives for appropriate isolation of plume. GAs were used as an optimization technique for making a decision among the alternatives, by considering operation time, number of wells, pumping rate and drawdown as decision variables and constraints.     

Spatial Direct Numerical Simulation of the Large Vortical Structures in Forced Plumes

Direct numerical simulation (DNS) of forced plumes arising frominput of both momentum and buoyancy into an ambient fluid is presented.The large vortical structures in the near field of thermal and reactiveplumes are investigated. Boundary conditions associated with the spatialDNS of open-boundary buoyant flows that are compatible with the modernnon-dissipative, high-order, finite-difference schemes have beendeveloped. The governing equations for flow and combustion at the plumecenterline are put into a special form to circumvent the singularity atthe axis associated with the cylindrical coordinates. Mixing is found tobe stronger in the planar thermal plume than in the axisymmetric case.An explanation is provided based on the vorticity budget. Axisymmetricreactive plumes with a one-step reaction governed by the Arrheniuskinetics have also been studied. The unsteady effects of chemical heatrelease and combustion-induced buoyancy on the flow structures areinvestigated. Budgets of the vorticity transport are examined to revealthe mechanisms leading to the formation and evolution of large vorticalstructures in forced plumes. It is found that volumetric expansion dueto chemical heat release tends to destroy vorticity, whilecombustion-induced buoyancy under the gravitational effect generatesvorticity. The gravitational term in the vorticity transport equation isfound to be the main mechanism for the buoyant flow instability and thedevelopment of counter-rotating vortices in reactive plumes.     

大气压直流激励空气等离子体羽发光的时空演化

在针-针电极结构的放电装置中以环境空气作为工作气体,大气压下产生了刷形等离子体羽。尽管使用的是直流电源,但放电发光呈现出脉冲性质,发光脉冲频率几乎不受气体流速的影响,但与电源输出功率成正相关关系。等离子体羽的长度与气体流速或者电源功率成正相关关系。通道出口附近,777.4 nm的氧原子谱线强度分布是非对称的,阴极附近处的谱线强度高于阳极附近处的谱线强度。远离通道出口位置,谱线强度逐渐趋于轴对称分布。电学特性和10 μs曝光高速影像结果表明,空气等离子体羽实际上是由拱形放电丝在远离通道出口的运动过程中叠加而成,同时放电从弧光放电丝向均匀辉光放电转化。     

沿面型介质阻挡放电喷枪的发光特性及气体温度研究

利用氩气作为工作气体,采用正弦电压驱动沿面型等离子体喷枪,在大气压空气环境中产生了均匀的等离子体羽。电学和光学测量结果表明,等离子体羽放电只存在于外加峰值电压的正半周期,并且正半周期的放电脉冲个数随气体流量的增加而增加。通过对正半周期不同位置的发光脉冲信号进行比较,发现等离子体羽均按子弹形式传播,其中每一个发光脉冲均对应一次等离子体子弹传播过程。通过对比放电电流和等离子体羽的发光信号,发现等离子体羽的发光脉冲滞后于放电电流脉冲,且该延迟时间基本服从正态分布。该延迟时间随着外加电压峰值及气体流量的增大而减小。利用光纤测温仪测量了等离子体羽的气体温度,发现气体温度随外加峰值电压的增大而升高,随工作气体流量的增大而降低。通过分析放电过程,对上述现象进行了定性解释。     

近壁面气泡的运动规律研究

基于势流假设,建立气泡与壁面耦合数值模型,运用边界积分法求解,并开发三维计算程序,计算值与实验值符合很好.从气泡与壁面相互作用的基本现象入手,基于开发的程序系统地研究了刚性壁面附近气泡的动力学特性,其中包括水平壁面及倾斜壁面,研究壁面的Bjerknes效应与各特征参数之间的关系,并将各种工况的计算结果与基于Kelvin-impulse理论的Blake准则进行对比分析讨论,得出偏射流方向及壁面压力与气泡的特征参数有密切的关系,同时给出了Blake准则的适用范围.旨在为相关的近壁面气泡动态特性研究提供参考. 关键词： 气泡 壁面 边界积分 Bjerknes效应