利用简化PIV方法测量水底浮帘后方流场

粒子图像测速（PIV）是一种快速全流场测量方法。为克服PIV设备昂贵和操作复杂等缺点,文中介绍一种新的简化PIV方法。它是根据PIV测速原理,并利用现有的连续抓图软件,CAD和Origin等绘图软件对示踪粒子进行捕捉及图像处理,并将其应用于水底浮帘截留促淤装置的实验中,成功获得了浮帘后方回流区的瞬时流场分布图。     

内孤立波对小直径直立桩柱作用力的实验研究

内孤立波具有较大的振幅与较强的垂向剪切,能对海洋工程设施产生极大的破坏作用。本文设计实验研究了内孤立波与直立小直径桩柱的相互作用,采用粒子图像速度测量法(PIV)测量内孤立波的流速场,并采用自行设计的测力天平测量桩柱受力,测量分析了内孤立波对小直径直立桩柱产生作用力的实验值,与Morison公式计算的理论值比较。实验结果表明,第一模态内孤立波的流速方向以及作用力在桩柱的上下部分方向相反,产生很强的速度切变和扭力,对小直径直立桩柱造成破坏。通过与Morison公式计算的理论值比较,发现实验值与理论值的大小以及分布规律基本相同。     

Quantification of swash flows using video-based particle image velocimetry

Understanding of fluid flows and sediment transport in the foreshore has been severely hampered by the difficulty of obtaining swash flow velocity measurements in this dynamic and extremely shallow region. We present a digital imaging method, known as particle image velocimetry (PIV), to quantify the horizontal flow structure of swash. This technique exploits similar patterns of image intensity in multiple images sampled sequentially to identify spatial offsets corresponding with maximum correlations between image subregions. These offsets are used in conjunction with the sampling interval to derive velocity vectors describing the horizontal flow structure. Pre-processing methods to geo-rectify oblique imagery to a planar surface and post-processing methods of correcting spurious vectors are described. The PIV method overcomes many of the limitations of in situ sampling of swash flows and is shown consistent with results from a previously tested remote sensing technique for measuring swash edge velocities. In general, this technique provides a unique capability for spatially extensive and well-resolved quantification of swash flows.     

Laboratory investigation of damping of gravity-capillary waves on the surface of turbulized liquid

Investigation of damping of gravity-capillary waves (GCWs) in the presence of turbulence is a classical hydrodynamic problem which has important geophysical applications, one of which is related with the problem of forming a radar and optical image of a ship wake on wavy water surface. In this work a new method for the laboratory study of surface wave damping in turbulized liquid is described and the results are presented. The damping of standing GCWs by turbulence on the water surface in a tank mounted on a vibration table is studied. GCWs and turbulence are excited using a two-frequency mode of vibration table oscillations. A high-frequency small amplitude signal is used for parametric GCW excitation; a low-frequency large amplitude signal is used for generating turbulence due to water flowing through a fixed perforated grid submerged into the tank. The coefficient of GCW damping is determined by measured threshold of parametric excitation of the waves; turbulence characteristics are determined by the PIV and PTV techniques. Dependences of GCW damping coefficients on their frequency at different turbulence intensities are obtained, estimates for turbulent viscosity are presented, and a comparison with empirical models proposed earlier is performed.     

A moving PIV system for ship model test in a towing tank

A moving particle image velocimetry (PIV) system was successfully developed and used in a large towing tank for ship model tests to observe velocity fields near ship models. The experimental method involved adjustable optical devices for various test conditions and a special particle-seeding device. The streamwise and cross-streamwise flow fields of a yacht model and a tanker model were measured. Ship type, bottom shape, and towing speed were found to be the causes of problems affecting optical access and image quality. Possible solutions, deeper optical ducts, dark painting color, and pre-processed analysis method, were proposed and discussed.     

Noise Control Due to the Stator Wake Blade Interaction via Tail Articulation

The biologically inspired method of tail articulation is investigated as a means of reducing tonal noise due to wake deficit blade interaction in underwater vehicles. Experiments are carried out in a water tunnel under typical operating conditions for underwater vehicles. Tail articulation is implemented using a life scale stator model with a hinged flapping tail operating both in free-stream velocities corresponding to Reynolds number in the range 75000 < Re < 300000 and at frequencies up to 30 Hz to investigate the range of Strouhal number 0.0 < St < 0.35. Velocity measurements of the active stator wake are carried out by laser Doppler velocimetry (LDV) and particle image velocimetry (PIV) to investigate the effects of tail articulation on the stator wake. Time-averaged measurements of the stator wake by LDV show that of the tail articulation has a dominant effect on the time mean stator drag. Instantaneous phase-averaged measurements of the stator wake by PIV show a transition in the unsteady stator wake as is increased, from a deflected vortex sheet to a series of rolled up, discrete vortices. Measurements are made of the wake due to both sinusoidal and nonsinusoidal tail motion profiles, which show that significant wake alteration is achieved with tail articulation. A low-order model describing the creation and convection of vorticity by tail articulation is developed which describes wake phenomena observed in LDV and PIV measurements. Finally, a 3-D unsteady propeller simulation using both experimental wake velocity data by PIV and simulated wake velocity data generated with the reduced-order model are used to predict the effect of sinusoidal tail articulation on radiated noise. Results using simulated data indicate that a significant noise alteration is achieved in all cases, and noise reduction of 5-8 dB is achieved in some cases.     

近岸破碎波水体掺气及气泡输运过程研究

为准确探讨破碎波作用下气体如何卷入以及气泡的形成与输运特性, 文章结合粒子图像测速技术(particle image velocimetry, PIV)、高速相机和气泡测量系统, 以及基于Navier-Stokes方程的三维数值模型对气泡形成及其运动过程进行研究。研究结果表明: 文章建立的数值模型能合理地捕捉到破碎波作用下气体的卷入及其输运过程; 波浪的破碎会形成较大的气腔, 其破裂过程又将产生大量的气体微团; 气泡会增加水体的紊动, 造成水体与空气交界面附近形成大量的漩涡以及水体的飞溅; 气泡的破裂会消耗大量的水体能量, 同时发现较大的紊动动能与气泡的生成有关, 且气泡数随平均紊动动能的增加呈线性增长关系。     

Propagation of solitary waves over a submerged permeable breakwater

We study the interactions between a non-breaking solitary wave and a submerged permeable breakwater experimentally and numerically. The particle image velocimetry (PIV) technique is employed to measure instantaneous free surface displacements and velocity fields in the vicinity of a porous dike. The porous medium, consisting of uniform glass spheres, is mounted on the seafloor. Due to the limited size of each field of view (FOV) for high spatial resolution purposes, four FOVs are set in order to form a continuous flow field around the structure. Quantitative mean properties are obtained by ensemble averaging 30 repeated instantaneous measurements. The Reynolds decomposition method is then adopted to separate the velocity fluctuations for each trial to estimate the turbulent kinetic energy. In addition, a highly accurate two-dimensional model with the volume of fluid interface tracking technique is used to simulate an idealized volume-averaged porous medium. The model is based on the Volume-Averaged Reynolds Averaged Navier–Stokes equations coupled with the non-linear k–ε turbulence closure solver. Comparisons are performed between measurements and numerical results for the time histories of the free surface elevation recorded by wave gauges and the spatial distributions of free surface displacement with the corresponding velocity and turbulent kinetic energy around the permeable object imaged by the PIV system. Fairly good agreements are obtained. It is found that the measured and modeled turbulent intensities on the weather side are much larger than those on the lee side of the object, and that the magnitude of the turbulent intensity increases with increasing wave height of a solitary wave at a constant water depth. The verified numerical model is then used to estimate the energy reflection, transmission and dissipation using the energy integral method by varying the aspect ratio and the grain size of the permeable obstacle.     

内孤立波与平顶海山作用的能量耗散实验研究

内波破碎引起的能量耗散和混合是海洋内部的重要物理过程.通过在二维内波水槽进行实验室实验,分析内波与地形的作用,探究内孤立波与平顶海山地形作用时波要素、能量以及湍耗散率的时空变化.本实验利用重力塌陷法在两层流体中制造第一模态内孤立波,通过粒子图像测速技术(particle image velocimetry,PIV)获得...     

Study of nonlinear currents induced by coincident surface waves in liquid

This is an experimental study of the mixing induced by coincident surface waves in a liquid. The main mechanism leading to the emergence of mixing was shown to be the middle currents generated by coincident waves. The regime of these currents strongly depends on the amplitude of surface waves. For waves of near-critical amplitudes, an intense turbulization of middle currents is observed. Patterns of the velocity field were obtained using the Particle Imaging Velocimetry (PIV) technique for different amplitudes of surface waves. The results obtained can be used to estimate mixing in the near-surface oceanic layer.     

基于康达效应的海底集矿装置流场与集矿特性试验研究

海底集矿作为深海矿产资源开发的核心技术之一,直接影响到矿石采集效率和海底沉积物受扰动程度。基于康达效应的水力集矿装置因具有结构简单、可靠性高及对海床扰动小等优点,受到广泛关注。针对基于康达效应的水力集矿头模型进行3种类型的试验研究：1)利用粒子图像测速(PIV)技术测得集矿流场速度分布,试验发现流体质点在靠近凸曲面壁处流速和沿法线速度梯度较大,而远离凸曲面壁处则相反;2)通过高速摄像和图像处理获得球形矿粒运动轨迹和速度,试验发现了不同射流流速下5种典型矿粒运动形态;3)分析了射流流量对矿粒采集率的影响,试验得到了颗粒采集率关于射流流量的变化曲线,并根据变化特征划分为增长区、缓变区和跃升区。研究可为新式集矿装置的优化设计与性能分析提供参考。     

海量海洋遥感影像数据库建设研究

遥感影像数据库是用于存储、管理和检索遥感影像数据的数据库管理系统,是遥感数据应用系统中一个重要组成部分。以遥感影像数据库管理系统的建设为背景,针对实现遥感影像数据成果高效管理这一核心问题进行了深入研究,重点提出了解决海量影像数据的组织、存储、高效地获取和处理影像数据的方式和方法。     

The practical implementation of electronic stabilization for sectorscanning sonar displays

The natural motion of a ship at sea produces undesirable effects on sonar displays. To overcome this problem some form of stabilization is desirable. A practical sonar display system, which uses image processing techniques to stabilize the displayed sonar image against rotational motion is described. The stabilization method provides the capability of image enhancement as well as improved image interpretation due to an increased area of view and a geometrically correct conversion to polar display. Details of the three processing stages required in the system are provided. These processes are ship motion analysis, image integration, and polar display conversion. Stabilized results are given for simulated ship motion     

Surface manifestations of internal waves investigated by a subsurface buoyant Jet: 1. The mechanism of internal-wave generation

In a large test reservoir with artificial temperature stratification at the Institute of Applied Physics, Russian Academy of Sciences, we have performed a major laboratory simulation of the nonstationary dynamics of buoyant turbulent jets generated by wastewater flows from underwater collector diffusers. The interaction of buoyant jets with the pycnocline leads to an active generation of internal waves. An analysis of the dependence of wave amplitude on the control parameter proportional to the rate of liquid flow from the collector diffuser has indicated that this dependence is adequately described by a function that is characteristic for the presence in the Hopf bifurcation system, which occurs for a soft actuation mode of self-oscillations of the globally instable mode. To check the conditions for the actuation of the globally instable mode, we have performed an auxiliary experiment in a small reservoir with a salt stratification formulated similar to the experiment in the big reservoir. Using the particle image velocimetry (PIV) method, we have measured the velocity field in the buoyant jet and constructed the profiles of transverse velocity in several sections. When the jet approaches the pycnocline, a counterflow is generated at the edges. A stability analysis for the resulting profiles of flow velocities performed by the method of normal modes has revealed that, for the jet portions with counterflow, the condition of absolute instability by the Briggs criterion for axisymmetric jet oscillations is satisfied, which testifies to the fact that the globally instable mode is actuated. The estimates for oscillation frequencies of the globally instable mode are well consistent quantitatively with the measured spectrum of jet oscillations.     

波浪破碎下湍流混合的实验研究

波浪破碎过程产生的湍流动量和能量垂向输运对于加快海洋上混合层中垂向混合具有显著效果.采用二维实验室水槽中对波浪破碎过程进行模拟.对采集的波浪振幅时间序列采用希尔伯特变换定位破碎波位置,波浪的破碎率随有效波高的增加而增大,波浪谱分析得到的波浪基本周期与有效周期结果相似.实验中采用粒子图像测速技术(particle ima...     

下凹内孤立波致流场结构及其影响因素的实验研究

为进一步探究海洋内孤立波诱导流场对海洋工程结构物以及潜航器的影响,本文采用重力塌陷方法和粒子图像测速(Particle Image velocimetry,PIV)技术在大型分层流水槽中进行内孤立波造波以及内部流速场测量,定量分析了下凹型内孤立波诱导流场结构及其影响因素。研究表明:在密度分层流体中,PIV技术可实现对大幅面内孤立波诱导流场的精细测量以及波动结构特征的准确描述;水平流速在上下层方向相反且在跃层处最小,其剪切作用在波谷附近最强;垂向流动在波前和波后分别为上升和下沉流,两者流速值在距离波谷1/4~1/2波长位置达到最大;在相同内孤立波振幅条件下,上下层流体密度差越大、厚度比越小,则波致流场越强;随着振幅增大,流场结构与Kd V、e Kd V和MCC理论模型对应波幅适用范围的描述相吻合。     

光学在内波实验研究中的应用

内波的实验室模拟是目前物理海洋学和流体力学研究的热点之一。结合实验室内波的模拟实验,近代光学技术、数字图像获取和计算机数据处理技术及三者相结合的技术,介绍内波实验中流场图像化测量领域的研究进展。同时对传统的纹影仪（横式）、立式纹影仪、数字纹影仪和片光源染料粒子示踪技术进行综述。并比较了各自特点及其在内波研究中的优缺点。     

Lossless compression of hydroacoustic image data

Despite rapid progress in improving mass-storage density and digital communication system performance, compression of hydroacoustic image data is still significant in many engineering and research areas since it can overcome data storage and transmission bandwidth limitations. In this paper, we present a novel and effective approach for lossless compression of hydroacoustic image data which consists of two stages. The first stage reduces the information redundancy. We propose several new techniques to remove redundancy between data samples, data blocks, and data frames. The second stage uses a newly developed cascade coding scheme. This simple scheme can achieve an efficiency of 97%. A decomposition algorithm is presented for finding the optimal cascade coding parameters. The algorithm decomposes a multivariable optimization problem into a series of one-variable optimizations. Our two-stage algorithm offers a compression ratio of 2-3 and provides an exact recovery of the original data. Because of its simplicity, the algorithm can be incorporated into a variety of echo sounder systems. The compression algorithms can also be implemented using low-level assembly language to meet the requirements of real-time applications     

Laboratory observation of boundary layer flow under spilling breakers in surf zone using particle image velocimetry

A boundary layer flow under spilling breakers in a laboratory surf zone with a smooth bottom is investigated using a high resolution particle image velocimetry (PIV) technique. By cross-correlating the images, oscillatory velocity profiles within a viscous boundary layer of O(1) mm in thickness are resolved over ten points. Using PIV measurements taken for an earlier study and the present study, flow properties in the wave bottom boundary layer (WBBL) over the laboratory surf zone are obtained, including the mean velocities, turbulence intensity, Reynolds stresses, and intermittency of coherent events. The data are then used to estimate the boundary layer thickness, phase variation, and bottom shear stress. It is found that while the time averaged mass transport inside the WBBL is onshore in the outer surf zone, it changes to offshore in the inner surf zone. The zero Eulerian mass transport occurs at h/h_b ≈ 0.92 in the outer surf zone. The maximum overshoot of the streamwise velocity and boundary layer thickness are not constant across the surf zone. The bottom shear stress is mainly contributed by the viscous stress through mean velocity gradient while the Reynolds stress is small and negligible. The turbulence level is higher in the inner surf zone than that in the outer surf zone, although only a slight increase of turbulent intensity is observed inside the WBBL from the outer surf zone to the inner surf zone. The variation of phase inside and outside the WBBL was examined through the spatial velocity distribution. It is found the phase lead is not constant and its value is significantly smaller than previous thought. By analyzing instantaneous velocity and vorticity fields, a remarkable number of intermittent turbulent eddies are observed to penetrate into the WBBL in the inner surf zone. The size of the observed large eddies is about 0.11 to 0.16 times the local water depth. Its energy spectra follow the − 5/3 slope in the inertial subrange and decay exponentially in the dissipation subrange.     

Runup of solitary waves on a straight and a composite beach

Particle Image Velocimetry (PIV) and wave gauges have been used to investigate the runup of solitary waves at two different beaches. The first beach is straight with an inclination of 10°, whereas the second is a composite beach with a change in the 10° inclination to 4° at a vertex point above the equilibrium water level. Comparison with numerical simulations using a Navier–Stokes solver with zero viscosity has been performed for the composite beach. Four different amplitudes of incoming solitary waves are investigated.Measurements of the runup show that the composite beach gives a lower runup compared to the straight beach. Furthermore, the composite beach experiences a longer duration of the rundown compared to the straight beach. This is at least partially assumed to be a result of scaling effects, since the fluid above the vertex creates a relatively thinner runup tongue compared to the straight beach scenario.The appearance of a stagnation point at the beach boundary is clearly visible in both the PIV results and the numerical simulation. This stagnation point is originating at the lowermost part of the beach, and is moving upwards with time. It is found that the stagnation point moves faster upwards for the straight beach than for the composite beach. Further, the stagnation point is moving even faster in the numerical simulation, suggesting that the velocity with which the stagnation point moves is influenced by viscous scaling effects.Finally, the numerical simulation seems to capture the physics of the flow well, despite differences in the phase compared to the PIV results. This applies to both the flow field and the surface elevations.