Numerical Study on Aerodynamic Performance of Floating Dual-Rotor Wind Turbines in Heave and Surge Motions

Compared with the traditional wind turbine of a single rotor,dual-rotor wind turbines(DRWTs)have higher wind energy capture efficiency and a more complex structure.Therefore,the aerodynamic performance of the DRWT installed on the floating platform will be greatly affected by the motion caused by wind and wave loads.In this paper,5 MW and 750 kW single rotor wind turbines(SRWTs)are combined into a 5 MW-5 MW DRWT and a 5 MW-750 kW DRWT,and their power output and wake field characteristics in different motions are studied.The flow field is obtained by solving the Reynolds-averaged Navier–Stokes equation(RANS).The overset grid technique is employed to achieve the large-amplitude multiple-degree-of-freedom motion of the DRWT.The overall performance of the 5 MW single rotor wind turbine is determined by a numerical method.For the DRWTs,numerical results show that the surge motion and heave motion both have a negative effect on the power output of the DRWT.The surge motion is a critical factor that causes the power output of the DRWT to periodically change with motion.The average power output of the DRWT influenced by motion is lower than that of a DRWT with a fixed bottom.The surge motion significantly disturbs the wake of the DRWT due to the mutual interference between the upstream and downstream rotors.Under the influence of heave motion,low-velocity regions downstream of the blade tip are enhanced.This study indicates that attenuating the surge and heave motion of offshore DRWT is very significant for improving its efficiency and should be taken into consideration during the design procedure.

Numerical study of the splashing wave induced by a seaplane using mesh-based and particle-based methods

In recent years,forest fires and maritime accidents have occurred frequently,which have had a bad impact on human production and life.Thus,the development of seaplanes is an increasingly urgent demand.It is important to study the taxiing process of seaplanes for the development of seaplanes,which is a strong nonlinear fluid-structure interaction problem.In this paper,the smoothed particle hydrodynamics(SPH)method based on the Lagrangian framework is utilized to simulate the taxiing process of seaplanes,and the SPH results are compared with those of the finite volume method(FVM)based on the Eulerian method.The results show that the SPH method can not only give the same accuracy as the FVM but also have a strong ability to capture the splashing waves in the taxiing process,which is quite meaningful for the subsequent study of the effect of a splash on other parts of the seaplane.

Acoustic propagation uncertainty in internal wave environments using an ocean-acoustic joint model

An ocean-acoustic joint model is developed for research of acoustic propagation uncertainty in internal wave environments.The internal waves are numerically produced by tidal forcing over a continental slope using an ocean model.Three parameters(i.e.,internal wave,source depth,and water depth)contribute to the dynamic waveguide environments,and result in stochastic sound fields.The sensitivity of the transmission loss(TL)to environment parameters,statistical characteristics of the TL variation,and the associated physical mechanisms are investigated by the Sobol sensitivity analysis method,the Monte Carlo sampling,and the coupled normal mode theory,respectively.The results show that the TL is most sensitive to the source depth in the near field,resulted from the initial amplitudes of higher-order modes;while in middle and far fields,the internal waves are responsible for more than 80%of the total acoustic propagation contribution.In addition,the standard deviation of the TL in the near field and the shallow layer is smaller than those in the middle and far fields and the deep layer.

A preliminary study on the acoustic properties of seafloor sediment in the southern U-boundary of the South China Sea

The acoustic properties of seafloor sediment are essential parameters in the exploration of marine resources,ocean scientific research and ocean engineering.Seafloor sediment samples were collected at the southern U-boundary of the South China Sea(SCS),and the acoustic and physical properties were measured in the laboratory.The correlation between physical and sound speed ratio(SSR)was discussed,and SSR-physical property empirical regressions in the Sunda Shelf were established for the first time.Compared with the northern continental shelf of SCS,the Sunda Shelf are mainly silty and sand sediment,and the SSR ranges from 0.9949 to 1.0944,which has higher SSR than the northern continental shelf,implies that the Sunda Shelf is a high SSR area.Since the same kind of sediment has different physical properties,the single physical parameter of sediment cannot fully represent the acoustic properties of sediment,therefore,the multiple parameter prediction model should develop in the future to improve the prediction precision.

Effects of mesoscale eddies on the spatial coherence of a middle range sound field in deep water

Mesoscale eddies have a remarkable influence on the underwater sound field.Many previous studies have investigated the effects of eddies on transmission loss,the convergence zone,time delay,etc.However,the effects of eddies on spatial coherence are less well studied and remain unclear.In this paper,the effects of eddies on spatial coherence at the subsurface in deep water are investigated.The eddy environments are simulated with Gaussian eddy equations,the complex pressure field is obtained using a range-dependent parabolic equation model and the associated mechanism is analyzed based on ray theory and models.The results show that cold/warm mesoscale eddies affect spatial coherence in a high-intensity zone by changing the locations and width of the convergence zone.In the shadow zone,the horizontal correlation radius and the vertical correlation radius increase with range and decrease with depth,and they are increased by warm eddies and decreased by cold eddies,mainly caused by variation of the multipath structure.

强非线性规则波中内倾船的甲板上浪数值研究

Owing to the large amplitude and nonlinearity of extreme sea waves,sailing ships exhibit obvious large-amplitude motion and green water.For a tumblehome vessel,a low-tumblehome freeboard and wave-piercing bow make green water more likely.To study the green water of a wave-facing sailing tumblehome vessel in strong nonlinear regular waves,the computational fluid dynamics software STAR-CCM+was used.The Reynolds-averaged Navier–Stokes method was used for the numerical simulation,and the k-epsilon model was adopted to deal with viscous turbulence.The volume of the fluid method was used to capture the free surface,and overset grids were utilized to simulate the large-amplitude ship motion.This study delves into the influence of wave height on the ship motion response and a tumblehome vessel green water under a large wave steepness(0.033≤H/λ≤0.067)at Fr=0.22.In addition,the dynamic process of green water and the“wave run-up”phenomenon were evaluated.The results suggest that when the wavelength is equal to the ship length and the wave steepness increases to 0.056,the increase in the water height on the deck is obvious.However,the wave height had little effect on the green water duration.The wave steepness and“backwater”have a great impact on the value and number of the peak of the water height on the deck.When the wave steepness exceeded 0.056,the water climbed up,and the plunging-type water body was formed at the top of the wave baffle,resulting in a large water area on the deck.

Effect of sound speed profile on the structure of long-range acoustic pulse

The time arrival structure of acoustic pulse signals propagating in ocean waveguides is of great significance for underwater acoustic communication and navigation. Using the deep-sea sound propagation data from the experiments respectively conducted in the East Indian Ocean(EIO) and the South China Sea(SCS) with explosion sources near the sound channel axis(SCA), long-range transmission loss(TL) and time arrival structure of acoustic pulses for different sound speed profiles(SSPs) are compared. In the EIO environment, sound energy transmitting along the SCA is relatively large, and the corresponding signals arrive first, whereas signals propagating away from the SCA arrive late. In the full receiving depth, it shows a branch structure where the waveform near the SCA arrives earlier than other depths, which is totally different with the characteristics of the sound pulse in the SCS. Combined with the parametric mathematical model of deep-sea sound channel, the influence mechanism of SSP on the time arrival structure of long-range pulse propagation is theoretically analyzed, which well explains the phenomenon observed in the two experiments.

物体入水的轴对称光滑粒子流体动力学数值模拟研究

Water entry of marine structures has long been an important problem in ocean engineering.Among the different techniques to predict fluid-structure interactions during water entry,smoothed particle hydrodynamics(SPH)method gradually becomes a promising method that is able to solve the impact pressure and the splashing fluid jets simultaneously.However,for three-dimensional(3D)problems,SPH method is computationally expensive due to the huge number of particles that are needed to resolve the local impact pressure accurately.Therefore,in this work an axisymmetric SPH model is applied to solve different water entry problems with axisymmetric structures including spheres and cones with different deadrise angles.Importantly,the Volume Adaptive Scheme(VAS)is added to guarantee the homogeneousness of particle volumes during the simulation.The axisymmetric SPH model with VAS scheme will be introduced in detail and the numerical results will be sufficiently validated with experimental data to demonstrate the high robustness and accuracy of the SPH model for solving 3D axisymmetric water entry problems in an efficient way.

Shape optimization design of a heaving buoy of wave energy converter based on fully parametric modeling and CFD method

The hydrodynamic shape of the heaving buoy is an important factor of the motion response in waves and thus concerns the energy conversion efficiency for the point absorbers(PAs).The current experience-based designs are time consuming and not very efficient,hence,faster and smarter methods are desirable.An automated optimization method based on a fully parametric modeling method and computational fluid dynamics(CFD),is proposed in this paper.Using this method,a benchmark buoy is screen designed and then optimized by maximizing the heave motion response.The geometry is described parametrically and deformed by means of the free-form deformation(FFD)method.During the optimization process,the expansion factor of control points is the basis for the variations.A combination of the Sobol and the non-dominated sorting genetic algorithm II(NSGA-II)is used to search for the solutions.After several iterations,the heaving buoy shape with optimal heave motion response is obtained.The analyses show that the heave motion response has increased 55.3%after optimization.The developed methodology is valid and seems to be a promising way to design a novel buoy that can significantly improve the wave energy conversion efficiency of the PAs in future.

Collision Classification MAC Protocol for Underwater Acoustic Communication Networks Using Directional Antennas

Traditional underwater acoustic communication networks(UACNs)generally use omnidirectional transmission technology that causes a large number of data-packet collisions,thus resulting in low network throughput and high end-to-end delays.Compared with omnidirectional transmission technology,directional technology only sends and receives data packets in a specified direction.This can significantly reduce the probability of collisions and improve network performance.However,it also causes a deafness problem,which occurs when the sending node sends a data packet to the receiving node but the receiving node is unable to reply to the sender,because its antenna beam is closed.To resolve this issue,this study proposes a collision classification media access control(CC-MAC)protocol for UACNs.With this protocol,the underwater acoustic channel is divided into two subchannels,and the nodes transmit corresponding data types on them.The sending node can estimate the current status of the receiving node(i.e.,no collision,normal collision,deafness)according to the type of the data packet received and the sub-channel it arrived on,and it can choose correct options to improve network efficiency.Finally,we verify the performance of CC-MAC via simulations,showing that the protocol achieved higher network throughput and lower end-toend delays.

Numerical analysis of a ventilated supercavity under periodic motion of the cavitator

This paper presents a numerical simulation of the geometry and the pressure distribution of a ventilated supercavity at different cavitator amplitudes and periods of motion.The numerical method is validated by comparing with the results of a semi-empirical formula under specific conditions.It is shown that the simulation can capture the boundary fluctuations of the ventilated supercavity and its internal pressure variations in a cavitator motion cycle.The simulation results show that the supercavity boundary experiences wave-like deformations when the wavelength of the disturbance caused by the cavitator motion is comparable to the supercavity length.It is also shown that the supercavity closure changes in form between a re-entrant jet and a twin vortex owing to the variations of the pressure difference between the outside and the inside of the supercavity near the closure region.The maximum diameter of the ventilated supercavity exhibits periodic changes with a double peak in each cavitator motion cycle,caused by the corresponding changes of the difference between the internal and external pressures.With the increase of the amplitude of motion of the cavitator,the supercavity boundary has enhanced wave-like undulations,with an increased maximum diameter,and with fluctuations in the cavitation number.As the period of the cavitator motion increases,the wavelength of the disturbances caused by this motion becomes greater than the supercavity length,and so the wave-like undulations of the supercavity boundary and the maximum diameter of the supercavity gradually decrease,but the variations of the cavitation number increase.Moreover,with the increase of the periods,the delay effects on the variations of the characteristics of the supercavity geometry caused by cavitator motion gradually decrease,and they practically vanish for large periods.

光滑粒子流体动力学框架下的EoSB空化模型及空化流动数值模拟研究

过去十余年来,光滑粒子流体动力学(Smoothed Particle Hydrodynamics,SPH)研究领域一直在探索空化流动的数值模拟,但迄今为止依然没有精确且稳定的SPH空化模型.本文将阐述一种在SPH框架内实现空化流动模拟的数值方法.首先,为了捕捉空化初生及其发展,本文在SPH框架内提出一种基于状态方程的相变模型.特别地,本文还引入一种粒子体积自适应算法(Volume Adaptive Scheme,VAS),以保证空化区域流体剧烈膨胀时粒子体积分布的均一性.此外,本文还采用粒子位移修正技术(Particle Shifting Technique,PST)和张力不稳定性控制技术(Tensile Instability Control,TIC)以提高粒子分布质量和抑制数值空洞的产生.最后,本文还通过引入大涡模拟(Large Eddy Simulation,LES)模型以考虑湍流效应,提高本文提出的SPH空化模型对高雷诺数流动的适用性.研究结果表明,本文提出的基于SPH方法的新空化模型能实现空化区域和压力分布等问题的精确预报.

风暴在长江三角洲侵蚀和沉积物向南输运中的控制作用

Densely populated coasts are vulnerable to storm damage. Episodic storm-induced redistribution of coastal sediment is known to have major geological and ecological implications [1,2], but little is known about storm-driven delta erosion and longshore sediment transport. The Yangtze (Changjiang) Delta and Zhejiang–Fujian coasts (Fig. S1 online) are among the world’s largest coastal depositional systems, and play an important role in supporting China’s socioeconomic development [3,4].