A Comparative Study on Hydrodynamic Performance of Double Deflector Rectangular Cambered Otter Board

In the present work,the hydrodynamic performance of the double deflector rectangular cambered otter board was studied using wind tunnel experiment,flume tank experiment and numerical simulation.Results showed that the otter board had a good hydrodynamic performance with the maximum lift-to-drag ratio(K_(MAX) = 3.70).The flow separation occurred when the angle of attack(AOA) was at 45?,which revealed that the double deflector structure of the otter board can delay the flow separation.Numerical simulation results showed a good agreement with experiment ones,and could predict the critical AOA,which showed that it can be used to study the hydrodynamic performance of the otter board with the advantage of flow visualization.However,the drag coefficient in flume tank was much higher than that in wind tunnel,which resulted in a lower lift-to-drag ratio.These may be due to different fluid media between flume tank and wind tunnel,which result in the big difference of the vortexes around the otter board.Given the otter boards are operated in water,it was suggested to apply both flume tank experiment and numerical simulation to study the hydrodynamic performance of otter board.     

Studies on Factors Influencing Hydrodynamic Characteristics of Plates Used in Artificial Reefs

As a simplified model of artificial reefs, a series of plate models punched with square or circular openings are designed to investigate the effects of openings on the hydrodynamic characteristics of artificial reefs. The models are grouped by various opening numbers and opening-area ratios. They are physically tested in a water flume or used in the numerical simulation to obtain the drag force in the uniform flow with different speeds. The simulation results are found in good agreement with the experimental measurements. By the non-dimensional analysis, the drag coefficient specified to each model is achieved and the effects of openings are examined. It is found that the key factor affecting the drag coefficient is the open-area ratio. Generally, the drag coefficient is a linear function of the open area ratio with a minus slope. The empirical formulae for the square and circular openings respectively are deduced by means of the multiple regression analysis based on the measured and numerical data. They will be good references for the design of new artificial reefs. As a result of numerical simulation, the vorticity contours and pressure distribution are also presented in this work to better understand the hydrodynamic characteristics of different models.     

Combined action of uniform flow and oscillating flow around marine riser at low Keulegan-Carpenter number

With the increase of petroleum and gas production in deep ocean, marine risers of circular cylinder shape are widely used in the offshore oil and gas platform. In order to research the hydrodynamic performance of marine risers, the dynamic mesh technique and User-Defined Function （UDF） are used to simulate the circular cylinder motion. The motion of a transversely oscillat-ing circular cylinder in combination of uniform flow and oscillating flow is simulated. The uniform flow and oscillating flow both are in x direction. SIMPLE algorithm is used to solve the Navier-Stokes equations. The User-Defined Function is used to control the cylinder transverse vibration and the inlet flow. The lift and drag coefficient changing with time and the map of vorticity isolines at different phase angle are obtained. Force time histories are shown for uniform flow at Reynolds number （Re） of 200 and for the com-bination of uniform and oscillating flows. With the increase of amplitude of oscillating flow in combined flow, the change of lift am-plitude is not sensitive to the the change of cylinder oscillating frequency. Lift amplitude increases with the increase of oscillating flow amplitude in the combined flow, but there is no definite periodicity of the lift coefficient. The drag and inertia force coefficients change when the maximum velocity of the oscillating flow increases in the combined flow. The vortex shedding near the circular cylinder shows different characteristics.     

Wave uplift force on horizontal panels: a laboratory study

Accurate estimation of wave uplift force is essential to the designs of reliable coastal and marine structures. We presents a series of laboratory work here on the impact of regular waves on horizontal panels, from which an empirical formula to estimate accurately the wave uplift force on panels is established. The laboratory measurements show that the wave uplift force depends mainly on the incident wave height, the wave period, the wave length, the panel width, and the clearance between the subsurface of the panel and the still water level. Among these factors, the impact of the panel width on uplift forces is relatively complicated. Result shows that the relative panel width(i.e., the ratio of panel width to wave length) plays a more important role in estimating the wave uplift force. Based on our comprehensive laboratory measurements, we further developed an empirical formula to compute wave uplift force on horizontal panels through dimensionless analysis. Compared with other empirical formulas, this formula uses dimensionless variables of clear physical meanings, thus can describe the interaction between waves and the panels in a better way. In addition, the efficiency of the formula to estimate wave uplift force on horizontal panels is verified against existing works. Therefore, the findings in this study shall be useful for understanding the mechanism of wave uplift force on horizontal panels and numerical model validation.     

Application of Numerical Simulation for Analysis of Sinking Characteristics of Purse Seine

This study applies the mass-spring system to model the dynamic behavior of a submerged net panel similar to the shooting process in actual purse seine fishing operation. Modeling indicates that there is insufficient stretching with the net panel under the floatline in the prophase of the shooting process. Sinkers at different locations along the leadline descend successively after submergence, and the sinking speed decreases gradually with elapsed time until attainment of a stable state. Designs with different current speeds and sinker weights are executed to determine the dimensional shape and sinking characteristics of the net. The net rigged with greater sinker weight gains significantly greater sinking depth without water flow. Compared with the vertical spread of the net wall in static water, the middle part of the netting presents a larger displacement along the direction of current under flow condition. It follows that considerable deformation of the netting occurs with higher current speed as the sinkers affected by hydrodynamic force drift in the direction of current. The numerical model is verified by a comparison between simulated results and sea measurements. The calculated values generally coincide with the observed ones, with the former being slightly higher than the latter. This study provides an implicit algorithm which saves computational loads for enormous systems such as purse seines, and ensures the accuracy and stability of numerical solutions in a repetitious iteration process.     

Characteristics from a hydrodynamic model of a trapezoidal artificial reef

Flume experiments and numerical simulation were conducted to characterize the hydrodynamics of a trapezoid artificial reef. Measurements in particle image velocimetry were conducted to observe the formation of upwelling and vortices; and forces for the reef model were measured by load cell. The results of flume experiments agree well with the numerical data. In addition, the flow structure around a reef combining trapezoidal and cubic blocks was simulated numerically under two deployment schemes, showing a more complicated flow structure than that of a stand-alone reef. Relationship between drag coefficient and Reynolds number suggest that the degree of turbulence can be assessed from the value of drag coefficient downstream from the reef. The role of the reef in water flow is to reduce flow velocity and generate turbulence.     

Two-dimensional numerical simulation of flow around three-stranded rope

Three-stranded rope is widely used in fishing gear and mooring system. Results of numerical simulation are presented for flow around a three-stranded rope in uniform flow. The simulation was carried out to study the hydrodynamic characteristics of pressure and velocity fields of steady incompressible laminar and turbulent wakes behind a three-stranded rope. A three-cylinder configuration and single circular cylinder configuration are used to model the three-stranded rope in the two-dimensional simulation. The governing equations, Navier-Stokes equations, are solved by using two-dimensional finite volume method. The turbulence flow is simulated using Standard κ-ε model and Shear-Stress Transport κ-ω(SST) model. The drag of the three-cylinder model and single cylinder model is calculated for different Reynolds numbers by using control volume analysis method. The pressure coefficient is also calculated for the turbulent model and laminar model based on the control surface method. From the comparison of the drag coefficient and the pressure of the single cylinder and three-cylinder models, it is found that the drag coefficients of the three-cylinder model are generally 1.3–1.5 times those of the single circular cylinder for different Reynolds numbers. Comparing the numerical results with water tank test data, the results of the three-cylinder model are closer to the experiment results than the single cylinder model results.     

Numerical Simulation of Mechanical Characteristics of a Metal Net for Deep-Sea Aquaculture

The investigation on hydrodynamic characteristics of a cage is important for its application in the deep-sea aquaculture in our country. With finite element method, the beam element is used to simulate a three-dimensional metal chain net, and the connector element is introduced as the interaction between metal net lines. A mechanical model for the metal net is constructed to simulate the hydrodynamic characteristics of a metal net subjected to fluid current forces. The static simulation results show that the relative errors of the displacements are 2.13%, 4.19%, 6.64%, and 11.35% compared with static concentrated load tests under concentrated forces of 20, 40, 60, and 80 N, respectively. Both the transient hydrodynamic deformations and drag forces of the netting structures under different current velocities are obtained by solving the hydrodynamic equation of the netting structure. The average relative error of the current forces obtained by numerical simulations shows an 8.13% deviation from the drag tests of the metal nets in the tank under five current velocities. The effectiveness and precision of the simulation approach are verified by static and dynamic tests. The proposed simulation approach will provide a good foundation for the further investigation of the hydrodynamic characteristics of deep-sea aquaculture metal cages and the parameter design for the safety of such cage systems.     

The Influence of wave state and sea spray on drag coefficient from low to high wind speeds

Ocean waves alter the roughness of sea surface, and sea spray droplets redistribute the momentum flux at the air-sea interface. Hence, both wave state and sea spray influence sea surface drag coefficient. Based on the new sea spray generation function which depends on sea surface wave, a wave-dependent sea spray stress is obtained. According to the relationship between sea spray stress and the total wind stress on the sea surface, a new formula of drag coefficient at high wind speed is acquired. With the analysis of the new drag coefficient, it is shown that the drag coefficient reduces at high wind speed, indicating that the sea spray droplets can limit the increase of drag coefficient. However, the value of high wind speed corresponding to the initial reduced drag coefficient is not fixed, and it depends on the wave state, which means the influence of wave cannot be ignored. Comparisons between the theoretical and measured sea surface drag coefficients in field and laboratory show that under different wave ages, the theoretical result of drag coefficient could include the measured data, and it means that the new drag coefficient can be used properly from low to high wind speeds under any wave state condition.     

Dynamics of loose granular flow and its subsequent deposition in a narrow mountainous river

A large amount of loose debris materials were deposited on the slope of mountainous areas after the 2008 Ms 8.0 Wenchuan earthquake. During and after the earthquake, these loose debris deposits collapsed and slide into valleys or rivers, changing river sediment supply condition and channel morphology. To investigate the mechanisms of granular flow and deposition, the dynamics of slope failure and sediment transportation in typical mountainous rivers of different intersection angles were analyzed with a coupling model of Computational Fluid Dynamics and Discrete Element Method(CFD-DEM). The numerical results show that the change of intersection angle between the granular flow flume and the river channel can affect the deposit geometry and the fluid flow field significantly. As the intersection angle increases, the granular velocity perpendicular to the river channel increases, while the granular velocity parallel to the river channel decreases gradually. Compared to the test of dry granular flow, the CFD-DEM coupling tests show much higher granular velocity and larger volume of sediments entrained in the river. Due to the river flow, particles located at the edge of the deposition will move downstream gradually and the main section of sediments deposition moves from the center to the edge of the river channel. As a result, sediment supply in the downstream river will distribute unevenly. Under the erosion of fluid flow, the proportion of fine particles increases, while the proportion of coarse particles decreases gradually in the sediment deposition. The build-up of accumulated sediment mass will cause a significant increase in water level in the river channel, thus creating serious flooding hazard in mountainous rivers.     

无囊围网缩结系数研究

通过对缩结系数与网衣用量、下纲沉降速度的关系之分析，结合我国围网生产实际，提出了围网主网衣上、下边缘水平缩结系数的合理参考范围及其选取方法；通过对我国围网缩结系数统计资料的综合分析，提出了上缘网衣、下缘网衣、网头网衣或侧缘网衣的水平缩结系数等的选取参考范围。     

Field observations of debris-flow initiation processes on sediment deposits in a previous deep-seated landslide site

Although information regarding the initiation processes of debris flows is important for the development of mitigation measures,field data regarding these processes are scarce.We conducted field observations of debris-flow initiation processes in the upper Ichinosawa catchment of the Ohya landslide,central Japan.On 19 June 2012,our videocamera monitoring systems recorded the moment of debris-flow initiation on channel deposits(nine surges) and talus slopes(eight surges).The initiation mechanisms of these surges were classified into three types by analyzing the video images: erosion by the surface flow,movement of deposits as a mass,and upward development of the fluid area.The first type was associated with the progress of surface flow from the upper stream on unsaturated channel deposits.The second type was likely caused by an increase in the pore water pressure associated with the rising in the groundwater level in channel deposits;a continuous water supply from the upper stream by the surface flow might have induced this saturation.The third type was associated with changes in the downstream topography caused by erosion.The flow velocity of most surges was less than 3 m s~(-1) and they usually stopped within 100 m from the initiation point.Surges with abundant pore fluid had a higher flow velocity(about 3- 5 m s~(-1)) and could travel for alonger duration.Our observations indicate that the surface flow plays an important role in the initiation of debris flows on channel deposits and talus slopes.     

基于CUDA的地表水动态模拟并行方法

如何高效地动态监测、模拟和预测地表水过程是防灾减灾中亟待解决的问题,也是科学化的国土整治、区域规划、环境保护和水资源管理的基础。因此,本文利用统一计算设备架构(CUDA)对基于不规则三角网(TIN)的地表水动态模拟算法进行并行化改进,提出了一种基于CUDA的地表水动态模拟并行方法,旨在对任意时刻的地表水进行快速、高精度的动态模拟,从而满足实际的应用需求。该算法从高精度的数字高程模型(DEM)中提取地形特征点和流域线,生成受流域线约束的TIN。在此基础上,根据TIN表面的三角面坐标数据获取水流方向,再结合任意位置的降雨源点追踪得到流水线网络。基于曼宁公式,利用流水线流速计算核函数得到每条流水线上雨滴的流速,结合预设的时间,利用汇流量统计核函数得到该时刻的地表汇流量。具体的并行化过程包括数据的传输,CUDA中线程的划分和流水线流速计算核函数和汇流量统计核函数的实现。本文选取位于加拿大新布伦瑞克西北部的BBW(Black Brook Watershed)流域作为实验样区,将该算法与改进前的方法进行对比表明,该模型在保持精度的同时,大幅提高了模拟效率,加速比达到11.2;另外,通过与SWAT(Soil and Water Assessment Tool)模型进行对比,结果表明,该并行方法的Nash系数提高了88%,相关系数提高了5%。     

基于定向半变异函数的裂隙网络空间变异性分析

运用地质统计学中的定向半变异函数,研究了个旧高松矿田莲花山断裂和五指山背斜裂隙网络的空间变异性。首先基于ArcGIS将野外露头裂隙网络进行数字化处理并提取裂隙密度和强度,然后依托GSLIB对裂隙密度和强度图沿着0~175°方位角计算,创建了36个实验性半变异函数并进行克里金插值,最后创建归一化的半变异函数图,量化各个断裂强度和断裂密度的二维空间变异性。研究表明,越是靠近断层裂隙密度和强度的空间变化幅度越大,并且其空间变异性存在显著的各向异性。空间变异性最小的方向平行断层且靠近断层,最大方向垂直断层且靠近断层,在褶皱近端空间变异性最小的方向平行于褶皱轴面方向,最大的方向则垂直于褶皱轴面方向。裂隙网络空间变异性质反映了断层和褶皱对裂隙网络发育的不同影响,为构建矿田三维裂隙网络空间分布模型提供参考。      

Impacts of wave and current on drag coefficient and wind stress over the tropical and northern Pacific

By taking into consideration the effects of ocean surface wave-induced Stokes drift velocity U,w and current velocityU,c on the drag coefficient,the spatial distributions of drag coefficient and wind stress in 2004 are computed over the tropical andnorthern Pacific using an empirical drag coefficient parameterization formula based on wave steepness and wind speed.The globalocean current field is generated from the Hybrid Coordinate Ocean Model (HYCOM) and the wave data are generated from Wave-watch Ⅲ (WW3).The spatial variability of the drag coefficient and wind stress is analyzed.Preliminary results indicate that theocean surface Stokes drift velocity and current velocity exert an important influence on the wind stress.The results also show thatconsideration of the effects of the ocean surface Stokes drift velocity and current velocity on the wind stress can significantly im-prove the modeling of ocean circulation and air-sea interaction processes.     

Study on bottom stress under conditions of storm surge and tide

Sea bottom stress is conventionally assumed to be directly proportional to the square of the verticallyaveraged velocity,and the drag coefficient to be dependent on the speed and direction of the wind on the sea surface,the depth and dimension of the sea,the period of the tide and so on. In this paper a three-dimensional numerical model is used to discuss the relation the dragcoefficient and the above-mentioned factors.It can be shown from calculation that the relation, is valid,that the drag coefficient is a constant in a major part of a sea as thought conventionally,andthat there is a small area near the coast where the drag coefficient is far greater.We call it singular area. A number of conclusions on the relation between the drag coefficient and the speed and direction ofthe wind,the sea depth and so on,were obtained.     

Wind-sand movement characteristics and erosion mechanism of a solar photovoltaic array in the middle of the Hobq Desert,Northwestern China

The operation and power generation of utility-scale solar energy infrastructure in desert areas are affected by changes in surface erosion processes resulting from the construction of solar photovoltaic(PV) power stations. However, few studies have addressed the interactions between solar PV arrays and aeolian erosion processes. In this study, wind flow field characteristics and the vertical distribution of sediments were investigated in the near-surface transport layer at three different locations with respect to the solar PV arrays in a 200 WM-p PV power station in the central Hobq Desert, northwestern China. The results indicate that the sediment transport varied around the panels, with the greatest transport occurring between the panels, followed by behind and in front of the panels. The sediment fluxes of all of the observation sites obey an exponential function. The secondary flow field zones formed around the PV panels: the conflux accelerating zone between the panels, the resistance decelerating zone of the under panels, and the transition zone of the rapid velocity increase in front of and behind the panels. This resulted in a greater shear force in front of the panels under the downward flow diversion effect of PV panels, and the wind erosion depressions were finally formed here. The results of this study provide information for planning better technical schemes for wind-sand hazards at solar PV power stations, which would ensure operational stability and safety in desert areas.     

Evaluation of Wind Retrieval from Co-Polarization Gaofen-3 SAR Imagery Around China Seas

Gaofen-3(GF-3) is the first Chinese space-borne satellite to carry the C-band multi-polarization synthetic aperture radar(SAR). Marine applications, i.e., winds and waves retrieved from GF-3 SAR images, have been operational since January 2017. In this study, we have collected more than 1000 quad-polarization(vertical-vertical(VV); horizontal-horizontal(HH); vertical-horizontal(VH); horizontal-vertical(HV)) GF-3 SAR images, which were acquired around the China Seas from September 2016 to September 2017. Wind streaks were visible in these images in co-polarization(VV and HH) channel. Geophysical model functions(GMFs), including the CMOD5N together with polarization ratio(PR) model and C-SARMOD, were used to retrieve winds from the collected co-polarization GF-3 SAR images. Wind directions were directly obtained from GF-3 SAR images. Then, the SAR-derived wind speeds were compared with the measurements at a 0.25? grid from the Advanced Scatterometer on board the Metop-A/B and microwave radiometer WindSAT. Based on the analysis, empirical corrections are proposed to improve the performance of the two GMFs. Results of this study show that the standard deviation of wind speed is 1.63 m s^-1 with a 0.19 m s^-1 bias and 1.71 m s^-1 with a 0.26 m s^-1 bias for VV-and HH-polarization GF-3 SAR, respectively. Our work not only systematically evaluates wind retrieval by using the two advanced GMFs and PR models but also proposes empirical corrections to improve the accuracy of wind retrievals from GF-3 SAR images around the China Seas and thus enhance the accuracy of near real-time operational SAR-derived wind products.     

Effects of Roughness Elements Distribution on Overland Flow Resistance

Roughness elements are various in a mountain area; they include gravel and ground surface vegetation that often result in surface friction drag to resist overland flows. The variation and characteristics of flow resistance strongly impact the overland flow process and watershed floods. In view of the universal existence of natural vegetation, such as Chlorophytum malayense(CM) or Ophiopogon bodinieri(OB), and the sand-gravel bed of the river channel, it is important to understand the role of different types of roughness elements in flow resistance. This study was performed to investigate and compare through flume experiments the behaviors of overland flow resistance by the reaction of multi-scale configuration of different roughness elements. The result showed that the resistance coefficient gradually reduced versus the increase of flow rate in unit width and tended to be a constant when q = 3.0 l/s.m, Fr = 1.0, and Re = 4000 for slopes of 6 to 10 degrees. The gap of the vegetated rough bed and the gravel rough bed is limited to the same as the gap of the two types of vegetation, CM and OB. It was noted that the vegetation contributed to the increase in form resistance negatively and may lead to the mean resistance on decrease. To classify the flow pattern, the laminar flows were described by DarcyWeisbach's equation. In the study the f-Re equation of vegetated bed was developed with f ?5000 Re.The friction coefficient for laminar flows can be regarded as the critical value for identifying the transformation point of the flow pattern.     

Observation and numerical experiments for drag coefficient under typhoon wind forcing

This paper presents a study on drag coefficients under typhoon wind forcing based on observations and numerical experiments. The friction velocity and wind speed are measured at a marine observation platform in the South China Sea. Three typhoons: SOULIK(2013), TRAMI(2013) and FITOW(2013) are observed at a buoy station in the northeast sea area of Pingtan Island. A new parameterization is formulated for the wind drag coefficient as a function of wind speed. It is found that the drag coefficient(Cd) increases linearly with the slope of 0.083′10~(-3) for wind speed less than 24 m s~(-1). To investigate the drag coefficient under higher wind conditions, three numerical experiments are implemented for these three typhoons using SWAN wave model. The wind input data are objective reanalysis datasets, which are assimilated with many sources and provided every six hours with the resolution of 0.125?×0.125?. The numerical simulation results show a good agreement with wave observation data under typhoon wind forcing. The results indicate that the drag coefficient levels off with the linear slope of 0.012′10~(-3) for higher wind speeds(less than 34 m s~(-1)) and the new parameterization improvese the simulation accuracy compared with the Wu(1982) default used in SWAN.