Laboratory observations of the kinematics in the aerated region of breaking waves

An experimental technique has been developed to visualize the flow in the aerated upper part of breaking waves. This technique, in which ultraviolet light and fluorescent particles are used, reveals sequences of jet-splash motions in that upper part of breaking waves during the initial stages of breaking. These motions are found to be qualitatively similar for two different types of breakers. The jet motion consists of particles moving down along the forward face of the wave; subsequently a splash motion is formed by the impact of the jet and its penetration through the free surface. Both motions are found to be reproducible coherent structures.     

仿生机器鱼转向机动的水动力性能数值研究

将仿生机器鱼的C-型转向机动划分为弯曲阶段、保持阶段和伸展阶段,首先基于计算流体力学（CFD）方法建立了鱼体与流体相互作用的耦合求解方法,其次通过二次开发FLUENT实现机器鱼在C-型转向机动过程中鱼体的大变形运动,最后数值计算了机器鱼在C-型转向机动过程中的运动性能、水动力性能和流场涡结构。计算结果表明：仿生机器鱼在弯曲阶段和保持阶段进行快速的转向;在伸展阶段,机器鱼的艏向角速度和侧向速度快速的减小至零值,而前进速度则快速的增加,在伸展阶段结束时获得一个较大的前进速度;在滑行阶段,仿生机器鱼以获得的纵向速度向前滑行,并且纵向速度缓慢的减小。机器鱼的C-型转向机动能够实现小范围内的大角度转向;在弯曲阶段和伸展阶段鱼体的快速弯曲和伸展运动各产生一个涡环,每个涡环产生一个射流,射流产生作用于机器鱼上的水动力和力矩。     

Hydroelastic analysis of pontoon-type circular VLFS with an attached submerged plate

This paper is concerned with the hydroelastic analysis of a pontoon-type, circular, very large floating structure (VLFS) with a horizontal submerged annular plate attached around its perimeter. The coupled fluid–structure interaction problem may be solved by using the modal expansion method in the frequency domain. It involves, firstly, the decomposition of the deflection of a circular Mindlin plate with free edges into vibration modes that are obtained analytically. Then the hydrodynamic diffraction and radiation forces are evaluated by using the eigenfunction expansion matching method which can also be done in an exact manner. The hydroelastic equation of motion is solved by the Rayleigh–Ritz method for the modal amplitudes, and then the modal responses are summed up to obtain the total response. The effectiveness of the attached submerged annular plate in reducing the motion of VLFS has been confirmed by the analysis.     

仿生胸鳍的三维尾涡结构与参数影响分析

为研究仿胸鳍推进的机理和流体动力特性及缩小机器鱼与生物原型之间的性能差距,利用浸入边界法数值模拟了做耦合旋转运动胸鳍的非定常绕流问题。详细探讨胸鳍非定常运动的三维尾涡结构演化和推进机理,并开展胸鳍推进性能与尾涡结构的参数影响分析。结果表明:迎流面在背、腹侧边缘及鳍梢部显著涡旋结构的作用下所出现的低压力区,加之鳍表面和上游来流之间好的垂直度共同造成了在动力划水阶段的高推力;在恢复划水阶段的高升力与背侧边缘涡强度的持续增加,以及因鳍表面倾斜而引起的水动力被分解到竖直方向的比重提升有关;胸鳍尾流场被一个三维双环涡结构所支配;当前的模拟为仿胸鳍推进建立了一个最优的斯特劳哈尔数St范围(在0.55附近),在此之后平均推力仍随St的增大而增加,而推进效率则表现出一个缓慢降低的趋势;当前后拍动与纵倾运动之间的相位差为90度时,胸鳍同时取得最佳的推力和效率。     

挡潮闸矩形孔口式鱼道紊流结构及放鱼试验研究

对挡潮闸枢纽中矩形中孔、底孔鱼道中紊流结构进行了较为系统的试验研究,并做了放鱼试验。选择了一种鱼类偏爱流速所对应的流量作为典型流量,考虑了不同的孔口位置(中孔和底孔),用声学多普勒测速仪(ADV)量测了测点的三维瞬时流速及流向,分析了矩形孔口鱼道的三维时均流速分布特征、断面最大流速沿程变化规律、流速矢量场、紊动强度分布及雷诺应力分布。此外,还通过放鱼试验,利用在鱼体植入T形标签和高速摄影机观察了过鱼对象对中孔、底孔的反应情况,分析了过鱼对象与矩形孔口鱼道紊流结构的关系。试验结果表明:水流经中孔形成三维紊动自由射流,经底孔形成三维壁面射流,中孔纵向流速呈高斯分布,而底孔纵向流速则近似为高斯分布,流速由孔口向两侧逐渐减小;中孔和底孔横向流速在位于孔口范围内的纵剖面上沿程减小,孔口之外则变化较小;中孔和底孔垂向流速分布特征表现为在铅垂方向上均存在旋涡;在中孔水平面和纵剖面上,纵向最大流速均沿程衰减;中孔和底孔情形孔口处紊动强度和雷诺应力比非孔口处大得多,而非孔口处不同水深平面上紊动强度和雷诺应力变化趋于平缓;过鱼对象喜爱在紊动强度分布的峰值区和雷诺应力较大变幅区溯游。     

孤立波与升沉水平板相互作用数值模拟研究

合理的刚度和潜深设计可以使升沉水平板获得优异的消浪性能。基于考虑流体黏性的二维不可压缩Navier-Stokes方程,以高阶紧致插值CIP(constrained interpolation profile)方法求解方程对流项,采用VOF(volume of fluid)方法重构自由液面,构建二维数值波浪水槽。采用试验数据验证模型后,研究孤立波与升沉水平板相互作用,分析相对刚度K*、相对潜深d/h、相对波高H/h对于升沉板的消浪性能和运动响应的影响,揭示升沉板对孤立波的消浪机理。研究表明:在孤立波通过时,升沉板会经历一个先上升后下降的运动,随后非线性自由振动,板下方水体近似均匀流动,且水流的垂向流动与板的垂荡方向一致;升沉板主要通过不对称涡旋脱落、浅水变形、波浪反射与辐射波转化等方式消耗孤立波能量;一定条件下,采用最优相对刚度K*=4.0和最优相对潜深d/h=0.52可以取得良好的消浪效果,此时透射系数最小,同时升沉板的运动响应在合理的范围内。     

黑潮延伸体区域脱落涡旋的时空特征分析

本文利用1993-2015年AVISO卫星高度计融合数据,统计分析了从黑潮延伸体流轴脱落涡旋的空间分布特征、运动属性以及季节、年际和类年代际变化。研究结果表明,23年间共追踪到242个气旋涡,276个反气旋涡,脱落的涡旋主要分布在沙茨基海隆以西区域。从脱落涡旋的源地空间分布来看,气旋涡的形成区域有两个高值区,一个位于黑潮延伸体流轴稳定弯曲处,即144°~146°E之间的上游区域;另一个位于沙茨基海隆西侧156°E处。而反气旋涡的形成区域也有两个高值区,一个位于沙茨基海隆以西的下游区域,另一个位于148°E处。这些在上游和下游脱落的涡旋大多向西移动,其中有88%的涡旋再次被流轴吸收。脱落涡旋的数量显示出了明显的年际和类年代际变化。在流轴的上下游区域,类年代际和年际变化分别占主导地位。并且在上游区域,脱落涡旋的类年代际变化与黑潮延伸体的强度呈负相关。在季节变化上,夏季脱落形成的涡旋最多,冬季最少。     

Pulsatile vortex generators for low-speed maneuvering of small underwater vehicles

Compact zero-mass pulsatile jet actuators are proposed for low-speed maneuvering and station keeping of small underwater vehicles.¹ The flow field of such jets are initially dominated by vortex ring formation. Pinched-off vortices characterize the extremum impulse accumulated by the leading vortex ring in a vortex ring formation process. Relevant parameters in this process are identified in order to design simple and low cost zero-mass pulsatile jet actuators. Thrust optimization of synthetic jets for maximal thrust generation is achieved by enforcing the jet formation number to be around 4. Prototypes of such actuators are built and tested for underwater maneuvering and propulsion. The actuators could be used in two ways: (i) to improve the low-speed maneuvering and station keeping capabilities of traditional propeller driven underwater vehicles, and (ii) as a synthetic jet for flow control and drag reduction at higher cruising speeds. A model for calculating the rotation rate of the underwater vehicle is also proposed and verified.     

The asymptotic theory of the generation of long-wave structures in critical layers of weakly dissipative jet flows

The mechanisms of the large-scale vortex structures formation in zonal jet flows (atmospheric blockings, cyclonic, and anticyclonic vortices) is investigated. Nonlinear perturbations formed during the onset of barotropic instability of a long-wave mode in weakly-dissipative and weakly supercritical jet flows with a symmetric velocity profile are considered in the β-plane approximation. This analysis is performed within the framework of the asymptotic theory based on the concept of a nonlinear critical layer. The equations describing the interaction of a wave with vorticity perturbations in a critical layer are derived. The regimes of a quasi-stationary and nonstationary nonlinear critical layer are considered separately. Combined equations of evolution covering the principle regimes of instability development are proposed. The existence of autowave-type structures characterized by a balance between the energy receipt to the wave and its dissipation are obtained within the framework of a numerical simulation. The dependence of the parameters of generated autowave structures on the shape of the zonal jet profile and the flow supercriticality level is studied.     

赤道东太平洋弯曲急流区质点轨迹及混沌运动机制

利用运动学模型 ,研究了弯曲急流控制参数为定常和发生周期扰动时的流体质点运动轨迹以及混沌现象的产生机制。发现弯曲急流控制参数为定常时 ,流体质点的运动为规则的周期运动 ;而控制参数发生扰动时 ,可以发生流体质点在不同特征区的穿行 ,流体质点的运动具有混沌特性。弯曲急流的相速、振幅发生周期扰动均可使流体质点的运动出现混沌特性。作者从运动学上对弯曲急流区的漂流浮标混沌特性给出了解释。     

Steady streaming around a circular cylinder in an oscillatory flow

Steady streaming induced by an oscillatory flow around a circular cylinder is investigated using a numerical method. Two-dimensional Reynolds-averaged Navier-Stokes equations are solved using a finite element method with a k-ω turbulent model closure. The range of the Keulegan-Carpenter (KC) number investigated is between 2 and 40, which is substantially higher than those reported in literature related to steady streaming to date. A constant value of Stokes number (β) of 196 is chosen in this study. The steady streaming structures and velocity distribution are analysed in detail. It is found that the characteristics of steady streaming are strongly related to the vortex shedding flow regimes.     

Evaluating the effects of protection on fish predators and sea urchins in shallow artificial rocky habitats: a case study in the northern Adriatic Sea

Man-made defence structures (e.g., breakwaters, jetties) are becoming common features of marine coastal landscapes all around the world. The ecology of assemblages of species associated with such artificial structures is, however, poorly known. In this study, we evaluated the density and size of fish predators of echinoids (i.e., Diplodus sargus, Diplodus vulgaris, Sparus aurata), and the density of sea urchins (i.e., Paracentrotus lividus) at defence structures (i.e., breakwaters) inside and outside the marine protected area of Miramare (northern Adriatic Sea) in order to: (1) assess possible differences in fish predator density and size between protected and fished breakwaters; (2) assess whether fish predation may have the potential to affect sea urchin density in artificial rocky habitats. Surveys were carried out at four random times over a period of two years. Total density, and density of medium- and large-sized individuals of the three predatory fishes were generally greater at the protected than at the fished breakwaters, whereas no differences were detected in the density of small-sized individuals. Density of the sea urchin P. lividus did not show any difference between protected and fished breakwaters. The results of this study suggest that: (1) protection may significantly affect predatory fishes in artificial rocky habitats; (2) differences in predatory fish density, and size may be unrelated with the density of the sea urchin P. lividus; (3) protected artificial structures such as breakwaters, originally planned for other purposes, could represent a potential tool for fish population recovery and enhancement of local fisheries.     

CFD simulation of flow features and vorticity structures in tuna-like swimming

The theoretical research on the propulsive principle of aquatic animal becomes more important and attracted more researchers to make efforts on it.In the present study,a computational fluid dynamic (CFD) simulation of a three-dimensional traveling-wave undulations body of tuna has been developed to investigate the fluid flow features and vorticity structures around this body when moving in a straight line.The undulation only takes place in the posterior half of the fish,and the tuna-tail is considered as a lunate fin oscillating with the mode combined swaying with yawing.A Reynolds-averaged Navier-Stokes (RANS) equation is developed,employing a control-volume method and a k-omega SST turbulent model;meanwhile an unstructured tetrahedral grid,which is generated for the three-dimensional geometry,is used based on the deformation of the hind parts of the body and corresponding movement of the tail.We calculated the hydrodynamic performance of tuna-like body when a tuna swims in a uniform velocity,and compared the input power coefficient,output power coefficient and propulsive efficiency of the oscillating tuna-tail with or without body vortex shedding.Additionally,the load distribution on the body,flow features and vorticity structures around the body were demonstrated.The effect of interaction between the body-generated vortices and the tail-generated vorticity on the hydrodynamic performance can be obtained.     

Measurements of time-averaged intensity of water motion with plaster balls

Water motion is a very important environment surrounding marine organisms. It is, however, difficult to measure currents with a propeller type of current meters in seagrass or seaweed beds in shallow waters. Therefore, we elaborate the measuring method of time-averaged intensity of water motion with plaster balls invented by Muus (1968) to set them at several depths at short vertical intervals in shallow waters. Theoretical examination is also made on the relation between speed of steady flow and dissolution rate of plaster balls by experiments in a circuit tank. These experiments revealed that the temporal rate of decrease in the 1/2 power of wet weight of plaster balls excluding a rod supporting the plaster ball was proportional to the speed of steady flow in the tank. On the basis of this relation, we propose the method for estimating the time-averaged intensity of water motion by converting the loss of wet weight of plaster ball to the speed of steady flow bringing equivalent loss. Since this method is simple and convenient, it can be applied to obtaining environmental indices of water motion not only in seagrass or seaweed forests, but also in fishing grounds culturingporphyra, yellow tail (Seriola quinqueradiata) or oysters.     

Sediment transport by fishes in Harrington Sound,Bermuda

Harrington Sound, Bermuda, is a shallow subtropical lagoon with carbonate sediments. The most important fishes in this lagoon, in terms of biomass, are grunts (Haemulon aurolineatum, H. flavolineatum, H. sciurus) and a sea-bream (Diplodus bermudensis). These undertake diel feeding migrations from the shallow rocky zone towards the deeper sand and mud zones. When feeding on zoobenthos they cannot avoid swallowing carbonate sediment particles. These sediment particles pass through the alimentary canal of the fishes and are deposited again, after digestion of the food, as faeces in the shallow zones. Thus, the fishes transport the sediment in an unusual direction, from the deep to the shallow zones, in effect against the force of gravity. By recording the fish stock densities, digestion rates, and calcium carbonate content of fish stomach and guts, it was possible to estimate the amount of sediment transported by the fishes. In Harrington Sound, this amounts annually to 4530 kg calcium carbonate, 40% of which is deposited in the very shallow areas. The pH-values measured in fish stomachs seem to be acidic enough for the dissolution of carbonate sediment particles when transported by fishes.     

The effect of the horizontal structure for symmetric tropical cyclones on their movement

In this paper, using Holland's method, the effect of the horizontal structure of tropical cyclones on their motion is investigated. The "characteristic radius", r0 characterized as the horizontal structure of a tropical cyclone,in which m and p are the parameters of the vortex, has been found by the author. And then it has been shown that there is but one "characteristic radius" for each cyclone with horizontal structure. Two direct analytic solutions for the uniform and non-uniform basic flows in steady situations are presented with rc Results show that the change in the horizontal structure of the tropical cyclone itself will have obvious effect on the cyclone motion, on both its direction and speed. Therefore it must be considered in the research on the tropical cyclone motion.     

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.     

杭州湾海域春、秋季鱼类种类组成和数量分布

采用底层拖网法,用渔获率作为鱼类资源数量分布的指标,进行了杭州湾调查海域2012年5月(春季)、10月(秋季)2个季节的鱼类种类组成、区系特点、数量分布等研究。结果表明:(1)杭州湾海域春、秋两季共获得并鉴定出的鱼类有31种,隶属于9目15科,其中以石首鱼科和■虎鱼科的种类为最多,各有5种,分别占总种数的16.7%;鳀科鱼类次之,有4种,占总种数的13.3%,优势种有睛尾蝌蚪■虎鱼、龙头鱼、刀鲚、棘头梅童鱼;(2)根据鱼类适温性可将该海域的鱼类分为暖水性、暖温性和冷温性3种适温类型,以暖温性种类为主,暖水性种类次之,而冷温性种类极少,仅有1种;(3)根据其生态习性,可分为河口性、沿岸性和近海性3种生态类群;(4)渔获数量分布规律是春季在调查海域的内侧或者说中西部较高,而湾口较低,秋季则正好相反。     

分隔板对Spar平台涡激运动响应影响的分析研究

当流体流经单柱式结构物时,由于流体黏性和逆压梯度的存在,会产生非定常的升力、阻力和涡旋脱落,即涡激运动现象,对一座Truss Spar平台的硬舱部分附加长度为1.0D(D为硬舱直径)的刚性来流侧分隔板,基于开源平台OpenFOAM,采用基于剪切应力运输的延迟分离涡模拟(SST-DDES)方法,在不可压缩黏性流场中,对其进行三维数值模拟,以研究在不同流速下,来流侧分隔板对Spar平台涡激运动特性和运动响应的影响。结果表明：在较高流速下,横荡周期减小22.21%,横荡最大幅值减小81.3%。说明在较高流速下,附加分隔板有助于减少Spar平台的运动响应幅值。