Classification of Precipitation Types Using Fall Velocity–Diameter Relationships from 2D-Video Distrometer Measurements

Fall velocity–diameter relationships for four different snowflake types（dendrite,plate,needle,and graupel） were investigated in northeastern South Korea,and a new algorithm for classifying hydrometeors is proposed for distrometric measurements based on the new relationships.Falling ice crystals（approximately 40 000 particles） were measured with a two-dimensional video disdrometer（2DVD） during a winter experiment from 15 January to 9 April 2010.The fall velocity–diameter relationships were derived for the four types of snowflakes based on manual classification by experts using snow photos and 2DVD measurements：the coefficients（exponents） for different snowflake types were 0.82（0.24） for dendrite,0.74（0.35） for plate,1.03（0.71） for needle,and 1.30（0.94） for graupel,respectively.These new relationships established in the present study（PS） were compared with those from two previous studies.Hydrometeor types were classified with the derived fall velocity–diameter relationships,and the classification algorithm was evaluated using 3 × 3 contingency tables for one rain–snow transition event and three snowfall events.The algorithm showed good performance for the transition event：the critical success indices（CSIs） were 0.89,0.61 and 0.71 for snow,wet-snow and rain,respectively.For snow events,the algorithm performance for dendrite and plate（CSIs = 1.0 and 1.0,respectively） was better than for needle and graupel（CSIs = 0.67 and 0.50,respectively）.

Model for seawater fouling and effects of temperature,flow velocity and surface free energy on seawater fouling

A kinetic model was proposed to predict the seawater fouling process in the seawater heat exchangers.The new model adopted an expression combining depositional and removal behaviors for seawater fouling based on the Kern–Seaton model.The present model parameters include the integrated kinetic rate of deposition(k d)and the integrated kinetic rate of removal(k r),which have clear physical signi ficance.A seawater-fouling monitoring device was established to validate the model.The experimental data were well fitted to the model,and the parameters were obtained in different conditions.SEM and EDX analyses were performed after the experiments,and the results show that the main components of seawater fouling are magnesium hydroxide and aluminum hydroxide.The effects of surface temperature,flow velocity and surface free energy were assessed by the model and the experimental data.The results indicate that the seawater fouling becomes aggravated as the surface temperature increased in a certain range,and the seawater fouling resistance reduced as the flow velocity of seawater increased.Furthermore,the effect of the surface free energy of metals was analyzed,showing that the lower surface free energy mitigates the seawater fouling accumulation.

Unsteady Liquid Film Flow with a Prescribed Free-Surface Velocity

A liquid film flow over a flat plate is investigated by prescribing the unsteady interface velocity. With this prescribed surface velocity, the governing Navier–Stokes(NS) equations are transformed into a similarity ordinary differential equation, which is solved numerically. The flow characteristics is controlled by an unsteadiness parameter S and the flow direction parameter Λ. The results show that solutions only exist for a certain range of the unsteadiness parameter, i.e., S≤1 for Λ =-1 and S≤-2.815877 for Λ = 1. In the solution domain,the dimensionless liquid film thickness β decreases with S for both the cases. The wall shear stress increases with the decrease of S for Λ =-1. However, for Λ =-1 the shear stress magnitude first decreases and then increases with the decrease of S. There are no zero crossing points for the velocity profiles for both the cases. The profiles of velocity stay either positive or negative all the time, except for the wall zero velocity. Consequently,the vertical velocity becomes a monotonic function. To maintain the prescribed velocity, mass transpiration is generally needed, but for the shrinking film case it is possible to have an impermeable wall. The results are also an exact solution to the full NS equations.

THE EFFECTS OF VELOCITY RATIO ON THE LARGE SCALE COHERENT STRUCTURES IN FREE SHEAR LAYERS

The velocity ratio of a free shear layer has an important influence on the spatial development of the large scale coherent structures in the layer. In this study, numerical simulations are performed to get an insight into this problem. The obtained numerical results agree quite well with those of a linear inviscid stability theory and the available experimental data.

High velocity flow simulation using lattice Boltzmann method with no-free-parameter dissipation scheme

A finite difference lattice Boltzmann method of second-order accuracy in time is developed based on non-oscillatory scheme with no-free-parameter dissipation （NND） difference scheme in this paper. The NND lattice Boltzmann method is used to simulate high-speed flows by constructing a new equilibrium distribution function of the lattice Boltzmann method. Compared with a variation of lattice Boltzmann method developed by Qu, et al., the present method can capture shock waves and handle oscillations of high velocity flows accurately in larger time steps and in shorter computing time. Numerical results indicate the correctness and capability of simulating shock wave interactions of the NND lattice Boltzmann method.

Numerical Simulation of the Water Entry of a Structure in Free Fall Motion

To solve the problems concerning water entry of a structure, the RANS equations and volume of fluid （VOF） method are used. Combining the user-defined function （UDF） procedure with dynamic grids, the water impact on a structure in free fall is simulated, and the velocity, displacement and the pressure distribution on the structure are investigated. The results of the numerical simulation were compared with the experimental data, and solidly consistent results have been achieved, which validates the numerical model. Therefore, this method can be used to study the water impact problems of a structure.

Effects of Degrees of Motion Freedom on Free-Fall of A Sphere in Fluid

Free-fall of a sphere in fluid is investigated at a Galileo number of 204 by direct numerical simulations(DNS). We mainly focus on the effects of different degrees-of-freedom(DOFs) of the sphere motion during free-fall. The characteristics of free-fall are compared with those of flow past a fixed sphere. Additional numerical tests are conducted with constraints placed on the translational or rotational DOFs of the sphere motion to analyze different DOFs of sphere motion. The transverse motion contributes significantly to the characteristics of free-fall; it results in the retardation of the vortex shedding, leading to the decrease of the Strouhal number. In addition, the transversal sphere motion exhibits the tendency to promote the sphere rotation. On the contrary, the effects of the sphere rotation and vertical oscillations during free-fall are negligible.

A 3D analytical model for vortex velocity field based on spiral streamline pattern

Vortices that develop over intakes are a hazardous hydraulic phenomenon.In this study, a 3D model was developed to study the flow field in air-core vortices.This model is based on the spiral pattern of streamlines and the analytical solution of the momentum and continuity equations for deriving the three components of velocity.The model provides equations for free surface profiles and 3D patterns of the streamlines.Moreover, a new relationship was suggested for calculating effective viscosity and its distribution across the vortex flow field.The performance of the proposed analytical model was compared with existing experimental data and the results of previous analytical models.The outcomes indicated that the proposed model could predict characteristics of the vortex flow with good accuracy.

An Analysis of Milton＇s Free Will Defense From the Two Falls in Paradise Lost

Paradise Lost is a great epic written by John Milton. Deeply influenced by Christian Humanism, Milton fully shows his free will defense in Paradise Lost. Based on the text of Paradise Lost, this paper mainly analyzes the causes of Milton＇s free will defense, his pursuit for free will shown in the two falls in the work： Satan＇s fall and Adam and Eve＇s fall. This paper also discusses the practical meaning of free will defense in Paradise Lost： it reveals his grief and anger of the failure of England revolution and shows the idea that people don＇t have the freedom to interfere or destroy other people＇s free will.

Optimal control of nonholonomic motion planning for a free-falling cat

The nonholonomic motion planning of a free-falling cat is investigated. Nonholonomicity arises in a free-falling cat subject to nonintegrable angle velocity constraints or nonintegrable conservation laws. When the total angular momentum is zero, the motion equation of a free-falling cat is established based on the model of two symmetric rigid bodies and conservation of angular momentum. The control of system can be converted to the problem of nonholonomic motion planning for a free-falling cat. Based on Ritz approximation theory, the Gauss-Newton method for motion planning by a falling cat is proposed. The effectiveness of the numerical algorithm is demonstrated through simulation on model of a free-falling cat.

Use of the Method of Guidance by a Required Velocity in Control of Spacecraft Attitude

We apply the method of guidance by a required velocity for solving the optimal control problem over spacecraft’s reorientation from known initial attitude into a required final attitude.We suppose that attitude control is carried out by impulse jet engines.For optimization of fuel consumption,the controlling moments are calculated and formed according to the method of free trajectories together with principle of iterative control using the quaternions for generating commands to actuators.Optimal solution corresponds to the principle“acceleration-free rotation-separate corrections-free rotation-braking”.Rotation along a hitting trajectory is supported by insignificant correction of the uncontrolled motion at discrete instants between segments of acceleration and braking.Various strategies of forming the correction impulses during stage of free motion are suggested.Improving accuracy of achievement of spacecraft's final position is reached by terminal control using information about current attitude and angular velocity measurements for determining an instant of beginning of braking(condition for start of braking based on actual motion parameters is formulated in analytical form).The described method is universal and invariant relative to moments of inertia.Developed laws of attitude control concern the algorithms with prognostic model,the synthesized control modes are invariant with respect to both external perturbations and parametric errors.Results of mathematical modeling are presented that demonstrate practical feasibility and high efficiency of designed algorithms.

仅有单个陀螺的Drag-free卫星初始速率阻尼模糊控制研究

研究了在仅有偏航姿态测量陀螺时,利用Drag-free卫星与其内部定向体间的相对状态观测值实现姿态确定的方法。针对Drag-free卫星的特点,建立了定向体的空间姿态观测模型和Drag-free卫星的姿态动力学方程,据此推导了用于姿态确定的Kalman滤波算法。为实现初始速率阻尼阶段Drag-free卫星的三轴姿态稳定,设计了模糊控制律。仿真结果验证了方法的有效性。

The anisotropy of free path in a vibro-fluidized granular gas

The free path of a vibro-fluidized two-dimensional（2D） inelastic granular gas confined in a rectangular box is investigated by 2D event-driven molecular simulation. By tracking particles in the simulation, we analyze the local free path.The probability distribution of the free path shows a high tail deviating from the exponential prediction. The anisotropy of the free path is found when we separate the free path to x and y components. The probability distribution of y component is exponential, while x component has a high tail. The probability distribution of angle between the relative velocity and the unit vector joined two particle centers deviates from the distribution of two random vectors, indicating the existence of the dynamic heterogeneities in our system. We explain these results by resorting to the kinetic theory with two-peak velocity distribution. The kinetic theory agrees well with the simulation result.

The Study and Investigation of Electron Trajectories in Free Electron Laser with Realizable Helical Wiggler and Ion Channel Guiding: A Computational Study

The following article has been retracted due to the investigation of complaints received against it. Mr. Mohammadali Ghorbani (corresponding author and also the last author) cheated the author’s name: Alireza Heidari. The scientific community takes a very strong view on this matter and we treat all unethical behavior such as plagiarism seriously. This paper published in Vol.3 No.3, 278-281, 2012, has been removed from this site.

孔洞增长层裂模型的改进及其在模拟不同加载波形层裂实验结果方面的应用

冲击波在靶板自由面反射导致靶板材料内部产生动态拉伸层裂损伤是材料的典型损伤破坏形式之一,材料的初始微结构、冲击加载的强度和应变率、温度等因素直接影响材料内部的损伤演化过程。靶板自由面速度曲线变化间接反映材料内部损伤的演化过程,在层裂损伤物理模型研究方面,目前采用适宜的层裂损伤模型较好地模拟不同冲击加载波形下靶板自由面速度曲线的相关文献很少,主要借助实验手段探讨加载波形与自由面速度曲线变化以及层裂损伤演化过程之间的关联。对于孔洞增长层裂损伤模型,通过解析加载应变率与层裂强度以及损伤模型初始损伤参数之间的相互关系,给出了模型初始损伤参数的计算方法,有效地将损伤模型初始损伤参数与加载应变率关联在一起。该计算方法不仅可以较好地模拟方波、三角波以及泰勒波冲击加载铝材料层裂实验的自由面速度曲线,同时,计算得到的层裂强度和层裂片厚度也与实验结果符合。此外,还进一步分析了靶板内部不同位置的初始损伤、层裂强度的分布与应变率之间的关联,以及其对自由面速度曲线的影响。

Smoothed Particle Hydrodynamic Modelling of Hydraulic Jumps: Bulk Parameters and Free Surface Fluctuations

A hydraulic jump is a rapid transition from supercritical flow to subcritical flow characterized by the development of large scale turbulence, surface waves, spray, energy dissipation and considerable air entrainment. Hydraulic jumps can be found in waterways such as spillways connected to hydropower plants and are an effective way to eliminate problems caused by high velocity flow, e.g. erosion. Due to the importance of the hydropower sector as a major contributor to the Swedish electricity production, the present study focuses on Smoothed Particle Hydrodynamic (SPH) modelling of 2D hydraulic jumps in horizontal open channels. Four cases with different spatial resolution of the SPH particles were investigated by comparing the conjugate depth in the subcritical section with theoretical results. These showed generally good agreement with theory. The coarsest case was run for a longer time and a quasi-stationary state was achieved, which facilitated an extended study of additional variables. The mean vertical velocity distribution in the horizontal direction compared favorably with experiments and the maximum velocity for the SPH-simulations indicated a too rapid decrease in the horizontal direction and poor agreement to experiments was obtained. Furthermore, the mean and the standard deviation of the free surface fluctuation showed generally good agreement with experimental results even though some discrepancies were found regarding the peak in the maximum standard deviation. The free surface fluctuation frequencies were over predicted and the model could not capture the decay of the fluctuations in the horizontal direction.

基于断面水边线定向的视频测流摄像机标定

针对现有基于直接线性变化法(DLT)的图像法测流技术依赖于地面控制点,存在效率低、风险高、宽断面天然河流操作难度大等问题,该文提出一种基于断面水边线定向的摄像机姿态角标定方法(CSWO)。该方法将标定过程分解为实验室内参标定和现场外参标定两步,其中后者又被划分为摄像机定位和定向两个环节。定向环节中首先在摄像机安装时将光轴与断面方向对齐,使方位角置零。然后利用无畸变图像中人工标注的平直断面水边线的斜率计算出横滚角。接下来通过计算水边线与图像纵轴交点作为其亚像素像方坐标。最后依据透视投影成像模型,联合水位与插值断面高程求得的物方坐标解算出俯仰角。该方法应用于时空图像测速法(STIV),实现了200 m宽河流的免像控表面流速测量。结果表明:起点距的最大绝对误差为0.59 m,最大相对误差为0.45%,表面流速的最大相对误差小于6.3%。

深海悬垂取水管振动特性研究

深海悬垂取水管的设计趋向于大直径、高内流流速,内流对取水管振动特性的影响不可忽略,分析内部流动引起的管道动态失稳行为以及评估失稳临界流速具有重要工程意义。基于小尺度模型试验,研究内流对深海悬垂取水管道振动特性的影响规律,结果表明:随着内流流速增加,取水管模型会发生动态失稳行为,表现为一种间歇性的近周期运动,该运动主要由管道一阶弯曲模态引起的不稳定性诱导。管道材料、顶端连接方式及底部配重块均影响其动态稳定性,顶端固定连接时质量比小的管道更易发生动态失稳;顶端变为铰接时,质量比大的管道失稳临界流速变小,质量比小的管道失稳临界流速变大。相比顶端边界条件,底部配重块对管道失稳临界流速影响不显著但可以减小其振动幅值。

明渠自由跌水水力特性及其在流量计量上的应用

【目的】明渠跌水作为衔接渠道上下游水流的有效设施,可应用于明渠流量计量,通过简单测量跌坎断面水深即可实现渠道流量的精确计量。为研究明渠自由跌水水力特性及其在流量计上的应用,【方法】采用分析对比法将国内外明渠自由跌水的研究成果进行整理,从自由跌水水力特性、跌水量水试验研究及理论研究3个方面进行了对比分析,【结果】发现明渠自由跌水水力特性的研究多集中在跌坎断面压力分布;明渠跌水量水试验研究集中在he/hc的影响因素(渠道底坡S、糙率n、渠道宽度、渠道断面形式、弗劳德数等),该值可基于能量方程、动量方程、N-S方程及其他数值模拟方法得出。【结论】结果表明:试验中he/hc值在矩形渠道中基本是一个常数,但其他断面形式的渠道中该结论未得到验证;该值与明渠底坡S和糙率n有关,底坡方面的研究主要集中在缓坡(S=0.002 6~0.097 3)渠道,陡坡研究较少而平坡研究更少,糙率n的研究范围在0.009 3~0.045 0,但实际工程中n值变化较大。因此有必要在其他形式渠道上对he/hc值做进一步验证,并在更大范围内研究he/hc值与底坡S和糙率n的关系,这样更有利于明渠跌水量水技术的推广和应用。

自落式贯入仪率相关系数标定方法

自落式贯入仪(free-falling penetrometer,FFP)作为一种新型地质勘察技术,可以高效快速地获取海底沉积物的力学性质参数。FFP贯入过程中锥尖阻力受贯入速率影响,数据处理时需借助速率因子将动贯入阻力转化为与静力触探试验(cone penetration test,CPT)等效的准静态贯入阻力,但目前速率因子中的率相关系数尚无明确测试方法。基于牛顿运动定律对率效应进行理论分析,并利用南海北部陆坡淤泥质沉积物开展室内贯入试验;并采用对数函数和幂次函数拟合锥尖阻力与贯入速率关系,提出了自落式贯入仪率相关系数的标定方法。研究结果表明,释放高度和探头质量都会影响最终贯入深度,但贯入过程中的锥尖阻力变化受探头质量影响较小,提出的率相关系数标定方法能够更准确获取FFP贯入的率相关系数。在实际应用中可采用室内试验标定的系数修正原位试验数据,为FFP实际应用提供技术支持。