Computation of natural frequencies and mode shapes of arch dams as an inverse problem

In this paper, a procedure is developed which can be used to identify the natural frequencies and natural modes of an Arch-Dam in a vacuum, from forced vibration testing data of partially filled reservoir, throughout removing the effect of hydrodynamic, by using an effective algorithm. The resonance frequency and the resonance mode of the dam–reservoir system have been computed from the in situ tested data. The solution strategy is verified by studying the dynamic response of simple structures, like beam with the fluid interaction effect taken into account, as the analytical results of its modal properties were available for comparison. The hydrodynamic pressure of the reservoir is calculated using the boundary element method. In the inverse problem procedure, the calculated resonance replaces the measured resonance, which has been obtained practically throughout the in situ testing. In this paper the strategies are extended to analyse the dynamic beam–fluid interaction problem with compressible water. The results derived by solving an inverse problem are compared with the exact analytical responses of the beam.     

Dynamic analysis of a water–soil–pore water coupling system

This paper explores the possible integration of the dynamic analyses of interaction of pore fluid flow and porous structure. The generalized fluid–structure interaction (FSI) algorithm in ADINA is applied for seismic analysis of a reservoir–earth dam–foundation system. In the given numerical example, water in the reservoir interacts with the earth dam on the upstream slope of the dam, and the foundation at the reservoir bottom. Pore water couples with soil particles throughout the earth dam and foundation. The reservoir water is modeled using both the Navier–Stokes equation based fluid element and the subsonic potential based fluid element. In the coupled analysis, hydrodynamic pressures and velocities are presented in the reservoir zone, while displacements, pore pressures and stresses are given for the earth dam and foundation domains at any earthquake time. The generalized FSI model is of great significance in performing soil liquefaction analysis if an appropriate soil plasticity model is employed in accounting for cyclic behavior of soils.     

Hydrodynamic loadings due to the motion of large offshore structures

A very effective non-reflecting boundary condition is proposed for the three-dimensional finite element analysis of hydrodynamic loads on offshore structures under motion due to earthquake, waves or machinery forces. The effect of surface waves is considered and the analysis is conducted in the frequency domain. The structure is assumed to be sufficiently large such that the non-linear effect of drag force is negligible. The unbounded extent of water surrounding the structure is assumed to be incompressible. The boundary condition, which is derived for the general analysis of structure-water interaction, is found to depend on the frequency of excitation, the location of the truncation boundary and the depth of water in the far field. Through some simplified numerical examples, it is shown that the proposed technique is very efficient for a wide range of the frequency of excitation. Incorporation of the boundary condition into a finite element program requires practically no extra effort.     

Driving mechanisms of CM-scaled PDMS boats of respective close and open reservoirs

A floating solid object may have a spontaneous motion on a water surface when a liquid, which has a surface tension lower than water, exits the solid object and has contact with water. This motion may be induced due to the difference between surface tensions of water and the liquid. In order to interpret this motion, it is important to know how the liquid exits the solid object. In this article, we reported in situ observation of exchange processes of water and isopropyl alcohol (IPA) inside both close and open reservoirs of cm-scaled polydimethylsiloxane (PDMS) boats. Based on this observation and force analysis, we interpreted driving mechanisms of these boats. We found that the exchange processes may be different in the PDMS boats of respective close and open reservoirs. The PDMS boat of a close reservoir might have a bubble trapped in its reservoir during the motion. This bubble slowed down the exiting process of the IPA from the reservoir, made the motion last longer, and enabled the boat to have a longer travel distance. Also, such a boat had gurgling-like motions (i.e., approximately periodic motions of deceleration and acceleration) after the majority of the IPA had exited the reservoir. The speeds of this boat had an order of 1?cm/s. On the other hand, neither bubbles nor gurgling-like phenomena were found in the motions of the PDMS boat of an open reservoir. The speeds of this boat were in the order of 1?cm/s as well. In addition, based on observed exiting processes of the IPA and experimentally determined speed-time relationships, we also set up a simple model to find force?Ctime relationships. Furthermore, we investigated the exiting process when the open reservoir of the cm-scaled PDMS boat was shallow. We found that, depending on the values of the height difference, h, between the bottom of the reservoir and the water surface outside the reservoir, three different phenomena might appear: (1) if h????1.33?mm, then the reservoir was empty at the end of the test and no water flowed into the reservoir during the test; (2) if 1.3?mm?<?h????2.3?mm, then the reservoir was empty at the end of the test while water flowed into the reservoir during the test; and (3) if 2.3?mm?<?h????3.0?mm, then the reservoir was filled by water at the end of the test. The difference in the filling results was induced by the interplay among Marangoni effect, free convection, and recovery of a hollow spot by surrounding water. The corresponding findings interpreted the exiting process of the IPA observed in the open reservoir of a mm-scaled SU-8 boat that we have previously developed, and also explained the filling result in the open reservoir of a PDMS boat.     

The response behaviour of a submerged cylindrical shell using the doubly asymptotic approximation method (DAA)

This paper presents experimentally measured and theoretically calculated responses of a flexible cylindrical shell subjected to an impulsive excitation in air and in water. The flat-ended, thin cylindrical shell of overall length 1284 mm, external radius 180 mm, thickness 3 mm is made of mild steel. The predicted responses are derived from a method using the doubly asymptotic approximation (DAA) models in order to uncouple the motion of the fluid from that of the structure. The DAAs enable the radiated pressure to be specified in terms of surface motion of the structure. The method, which employs the DAAs, is applied to determine the responses, i.e. acceleration, velocity and displacement, of the finite length cylindrical shell excited by an impulsive mechanical force. The calculated acceleration responses are compared with the experimental measurements.     

基于移动互联网的小型水库大坝安全巡查系统应用及研究

广东省小型水库数量众多、分散、管理难度大的特点给水利部门的管理工作带来诸多不便,而水库大坝现场巡查是大坝安全管理不可或缺、极为重要的环节。基于移动互联网和GIS技术,结合"一个登录门户,分级权限管理"的设计思想,以信息采集层、网络传输层、资源共享层、综合业务应用层的架构设计小型水库大坝安全巡查系统,分别介绍系统中的采用Java EE开发的水库大坝巡查管理平台,以及基于Android/iOS的移动巡查终端的业务流程、功能实现等内容。该系统能够明确水库大坝的巡查部位、频次和路线,规范巡查行为,进一步提升水库大坝安全管理标准化、专业化水平。     

Dynamic motion analysis of magnetic particles in microfluidic systems under an external gradient magnetic field

To gain an insightful understanding of motion behavior of paramagnetic particles suspended in a nonmagnetic fluid under a gradient magnetic field, a coupled fluid–structure model based on a direct numerical scheme is developed in this work. The governing equations of magnetic field, fluid flow field and particle motion are simultaneously solved using an Arbitrary Lagrangian–Eulerian method, taking into account magnetic and hydrodynamic interactions between particles in a fully coupled manner. The accuracy of the proposed method is validated using the magnetic particulate flows of two particles under a uniform magnetic field as the test problem and is then applied to investigate effects of magnetic and hydrodynamic interactions between particles on the particle motion behavior. Results show that neighboring magnetic particles are easy to form chain-like clusters along field direction due to magnetic interactions between particles and then move together toward the surface of magnetic source under the action of gradient magnetic force. More importantly, it has been found that both magnetic and hydrodynamic interactions between particles are conducive to the acceleration of particles and the chain formation of particles. The present method and results could help in understanding the basic mechanism underlying the low-gradient magnetophoretic separation process and designing magnetic aggregate-based microfluidic devices.     

Representation of radiation damping in a dam–reservoir interaction analysis based on a rational stiffness approximation

In this paper, the dam–reservoir interaction problem is solved directly in the time-domain by modelling a part of the reservoir as a semi-infinite fluid-channel. Radiation damping is taken into account rigorously using an analytical solution with respect to the direction of wave propagation. The key step of the method consists of approximating the resulting modal flux–pressure relationship by a rational function in iΩ, which can be replaced by a system of linear equations in the frequency-domain. Two algorithms for the calculation of the coefficients of the rational function are described and compared. Finally, a system of first-order differential equations in the time-domain is obtained which completely represents the semi-infinite fluid-channel and can be coupled to the equations of motion of a dam and a bounded reservoir. The proposed strategy is verified considering a rigid dam. Moreover, the results of an earthquake analysis of a coupled flexible dam–reservoir system are presented.     

Nonlinear secondary resonance of FG porous silicon nanobeams under periodic hard excitations based on surface elasticity theory

To impart desirable material properties, functionally graded (FG) porous silicon has been produced in which the porosity changes gradually across the material volume. The prime objective of this work is to predict the influence of the surface free energy on the nonlinear secondary resonance of FG porous silicon nanobeams under external hard excitations. On the basis of the closed-cell Gaussian-random field scheme, the mechanical properties of the FG porous material are achieved corresponding to the uniform and three different FG patterns of porosity dispersion. The Gurtin–Murdoch theory of elasticity is implemented into the classical beam theory to construct a surface elastic beam model. Thereafter, with the aid of the method of multiple time-scales together with the Galerkin technique, the size-dependent nonlinear differential equations of motion are solved corresponding to various immovable boundary conditions and porosity dispersion patterns. The frequency response and amplitude response associated with the both subharmonic and superharmonic hard excitations are obtained including multiple vibration modes and interactions between them. It is found that for the subharmonic excitation, the nanobeam is excited within a specific range of the excitation amplitude, and this range shifts to higher excitation amplitude by incorporating the surface free energy effects. For the superharmonic excitation, by taking surface stress effect into account, the excitation amplitude associated with the peak of the vibration amplitude enhances. Moreover, in the subharmonic case, it is demonstrated that by increasing the porosity coefficient, the value of the excitation frequency at the joint point of the two branches of the frequency-response curve reduces. In the superharmonic case, it is revealed that an increment in the value of porosity coefficient leads to decrease the peak of the oscillation amplitude and the associated excitation frequency.

     

鱼体尾频运动模型研究

鱼体运动特征能直观地反映水体环境质量状况,在生物水质监测中具有十分重要的意义.在分析鱼体运动模式的基础上,提出了基于鱼体骨架中心线的尾频活性模型,利用该模型获取鱼体尾频、速度、加速度等运动参数,表征鱼体游动在被监测水质中的运动活性程度.实验数据表明,模型较好地表征了鱼体当前的运动状态,为后期鱼体运动在生物水质监测中的应用提供了理论基础.     

Monte-Carlo simulation studies of the nonlinear imaging of a two dimensional surface wave field by a synthetic aperture radar

Abstract

The imaging of ocean surface waves by synthetic aperture radar (SAR) is investigated using two-dimensional Monte-Carlo simulations. The properties of the SAR imaging mechanism for windseas and swell in the Bragg scattering regime are discussed as a function of a few governing non-dimensional parameters formed from a combination of SAR and ocean wave parameters. The parameter ranges may be classified into three regimes corresponding to linear and weakly nonlinear, medium nonlinear and strongly nonlinear imaging. The nonlinearities are induced by motion effects (velocity bunching, velocity spread and acceleration smearing), while the real aperture radar (RAR) tilt and hydrodynamic modulation processes are regarded as linear. In the strongly nonlinear imaging regime, the velocity bunching mechanism causes a rotation of the spectral peak towards the range direction and a stretching of the peak wavelength. In addition, the azimuthal resolution is degraded through the Doppler spreading arising from the different facet velocities within a SAR resolution cell. The imaging properties in this regime are largely governed by two non-dimensional parameters, the velocity bunching and velocity smearing parameter. The nonlinear imaging distortions are strongest for broad spectra (windseas) and are significantly weaker for narrow-band swell. In the linear and weakly nonlinear imaging regime, the superposition of the hydrodynamic and tilt cross-section modulation and the velocity bunching transfer function normally produces a rotation of the spectral peak towards the azimuthal direction. The interference characteristics of these different modulation mechanisms depends on the wave propagation direction and can lead to a significant distortion of the image. This is often seen in large differences in the image modulation depths of waves propagating parallel and anti-parallel to the flight direction.     

Numerical simulation of morphological changes in rivers and reservoirs

For the computation of the morphological changes in river or reservoir beds, due to erosion or deposition, a hydrodynamic model is combined with the sediment continuity equation. The hydrodynamic model is based upon the equations of mass and momentum conservation. For the solution of the above system of three partial differential equations, the explicit numerical schemes of MacCormack and Lax-Wendroff are tested. The whole mathematical model is applied to two experimental cases and to a real case. The first experimental case concerns the aggradation in a laboratory channel due to sediment overloading, and the second one the sediment release from a reservoir after a dam break. The real case concerns the morphological changes in the torrent Mallero (Northern Italy) during a flood event.     

五坐标数控技术的联动原理

本文通过对五坐标数控技术的坐标运动规律进行分析研究,揭示了五坐标数控技术的联 动原理.即建立了从CAD中的理想轨迹的几何特性(包括位置、切矢、曲率)和五坐标联动的 运动动力学特性(包括位移、进给速度、各坐标理想速度和加速度)之间的通用数学模型.从 而可根据被加工零件表面的几何形状和机床的进给速度确定联动过程中各坐标的理想速度 和理想加速度.     

Investigation of density flow in dam reservoirs using a three-dimensional mathematical model including Coriolis effect

This paper aims to simulate and discuss the propagation of density current and divergence flow in a dam reservoir. The density plunging flow is modeled in three-dimensions through a dam reservoir with diverging and sloping bottom channels, and the plunging phenomenon has been reproduced in the present model. Nonlinear and unsteady continuity, momentum, energy and turbulence model equations are formulated in the Cartesian coordinates both in a sloping and in a diverging channel. For the turbulence viscosity, a k-ε turbulence model including buoyancy effects is used to reproduce the main flow characteristics. To investigate the Coriolis force effect on the density flow in a dam reservoir, Coriolis force parameter is also included in the governing equations. The equations of the model are solved based on the initial and boundary conditions of the dam reservoir flow for a range of bottom slopes and divergence angles. In this paper the main interest is the formation of separated flows, such as wall-jet and free-jet flows. The model successfully simulates the formation of attached flow, wall jets, and free jets in a negatively buoyant environment. The simulation results obtained from this study are compared with previous experimental work, and the mathematical model studies data on density current generated by the plunging of cold water in ambient warm water in a diverging channel, and is found to be of the same magnitude as the experimental measurements and followed the expected basic trend.     

Motion-induced interruptions and postural equilibrium in linear lateral accelerations

This study assesses lateral tipping motion-induced interruptions (MIIs) in a simulated motion environment. The objective is to revisit MII occurrence and sway motion relationship by focusing on the frequency and acceleration of the lateral motion stimulus. Results verify that MIIs increase with increasing peak sway acceleration, but the effect of sway frequency is not as clear as that of acceleration. Complex multidirectional motions create more tipping MIIs than unidirectional motion. Research should incorporate acceleration, frequency and motion complexity as factors influencing MII occurrence. To describe a temporary loss of balance without tipping, the term ‘probable’ MII is introduced. This term fills the gap between the theoretical definition and a human-centred perception of an MII where loss of balance is not a binary phenomenon. The ‘probable’ MIIs were 16–67% more common than the ‘definite’ MIIs. The developed mathematical model of MII occurrence versus sway acceleration (amplitude, frequency) approximated the observed MIIs with less than 9% difference.     

Numerical simulation of liquid sloshing phenomena in partially filled containers

In the simulation of the dynamic load excited by sloshing in a partially filled tank, appropriate boundary conditions need imposing to calculate the impact pressure. Traditionally, a thin artificial buffer zone is adopted near the tank ceiling and a linear combination of free surface dynamic and rigid wall boundary conditions are imposed inside the buffer zone. This investigation demonstrates that no special treatment is needed to describe the free surface, because a two-fluid approach based on a level set method is used to solve the Reynolds-averaged Navier-Stokes (RANS) equations in both water and air regions and the interface is treated as a variation of the fluid properties. All the boundary conditions adopted are those usually accepted in solutions of Navier-Stokes or Euler equations. Sloshing in a rectangular tank excited by a horizontal harmonic motion is assessed numerically at different filling levels and excitation frequencies. The dependency of numerical solution on grid resolution, time step size and the interface thickness are investigated. Further, numerical tests are conducted for a rectangular tank with both 45° and 60° chamfered ceiling corners subject to a harmonic rolling motion. The comparison of computed results with experimental data show the developed numerical method is capable of the simulation of dynamic pressure loads exerted on the tank walls and ceiling excited by fluid sloshing.     

Liquid-state motion sensing

This paper demonstrates a liquid droplet-based motion sensing system which has the advantages of simple fabrication, low power consumption and digital signal processing. The sensor consists of a dielectric substrate patterned with an array of microelectrodes, and a saline droplet as the proof mass. Once an external linear acceleration is applied, the inertial force moves the droplet on the micropatterned substrate. The acceleration is determined from the movement profile detected by the microelectrodes. In order to enhance the threshold and the sensitivity of motion sensing, two surface treatment approaches are utilized to create superhydrophobic surfaces. The result shows that the minimal sliding angle that can move a 20 μl droplet on the superhydrophobic surface is lower than 1°, corresponding to a threshold of lower than 0.017 g. A lumped-parameter model is developed to estimate the dynamic behavior of the proposed system. The result shows that the frequency response of the droplet-based sensor is more significant at low frequencies than at high frequencies, which is distinct from solid-state accelerometers. Measurement under a constant acceleration shows that the predicted value derived from the measurement has a good match with the actual applied acceleration, validating the proposed system as a viable alternative for motion sensing.     

基于库水位的坝体安全等级的可能性分析方法

针对库水位与尾矿坝安全等级的复杂对应关系,利用可能性理论将两者的模糊关系量化,并提出了一种基于库水位的尾矿坝安全等级的分析方法,通过构造库水位关于各安全等级的隶属函数、测量数据关于安全等级的可能性分布函数,计算测量数据关于各安全等级的信任匹配度,从而判断出尾矿坝当前的安全等级。通过实验仿真验证了该方法的有效性。     

超级单胎牵引车振动仿真分析和优化

针对某超级单胎牵引车的驾驶室振动问题,在MD Adams中建立该牵引车的刚柔耦合模型,采用平顺性随机试验测量的加速度值作为输入进行仿真,与试验结果对比验证该动力学模型的准确性.通过进行整车系统模态分析、车速灵敏度分析和频率响应分析,找出引起驾驶室振动加剧的主要原因,即在特定车速下路面激励频率与驾驶室固有频率接近,形成共振.对驾驶室悬架系统的减振器特性参数进行优化,降低驾驶室的振动加速度响应,实现该款牵引车平顺性的性能提升.     

GPU-acceleration for Moving Particle Semi-Implicit method

The MPS (Moving Particle Semi-implicit) method has been proven useful in computation free-surface hydrodynamic flows. Despite its applicability, one of its drawbacks in practical application is the high computational load. On the other hand, Graphics Processing Unit (GPU), which was originally developed for acceleration of computer graphics, now provides unprecedented capability for scientific computations.The main objective of this study is to develop a GPU-accelerated MPS code using CUDA (Compute Unified Device Architecture) language. Several techniques have been shown to optimize calculations in CUDA. In order to promote the acceleration by GPU, particular attentions are given to both the search of neighboring particles and the iterative solution of simultaneous linear equations in the Poisson Pressure Equation.In this paper, 2-dimensional calculations of elliptical drop evolution and dam break flow have been carried out by the GPU-accelerated MPS method, and the accuracy and performance of GPU-based code are investigated by comparing the results with those by CPU. It is shown that results of GPU-based calculations can be obtained much faster with the same reliability as the CPU-based ones.