Hydrodynamic Performance and Power Absorption of A Coaxial DoubleBuoy Wave Energy Converter

As an important wave energy converter(WEC),the double-buoy device has advantages of wider energy absorption band and deeper water adaptability,which attract an increasing number of attentions from researchers.This paper makes an in-depth study on double-buoy WEC,by means of the combination of model experiment and numerical simulation.The Response Amplitude Operator(RAO)and energy capture of the double-buoy under constant power take-off(PTO)damping are investigated in the model test,while the average power output and capture width ratio(CWR)are calculated by the numerical simulation to analyze the influence of the wave condition,PTO,and the geometry parameters of the device.The AQWA-Fortran united simulation sy stem,including the secondary developme nt of AQWA software coupled with the flowchart of the Fortran code,models a new dynamic system.Various viscous damping and hydraulic friction from WEC system are measured from the experimental results,and these values are added to the equation of motion.As a result,the energy loss is contained in the final numerical model the by united simulation system.Using the developed numerical model,the optimal period of energy capture is identified.The power capture reaches the maximum value under the outer buoy's natural period.The paper gives the peak value of the energy capture under the linear PTO damping force,and calculates the optimal mass ratio of the device.

Hydrodynamic Performance of A Porous-Type Land-Fixed Oscillating Water Column Wave Energy Converter

A hybrid, porous breakwater-Oscillating Water Column(OWC) Wave Energy Converter(WEC) system is put forward and its hydrodynamic performance is investigated using the fully nonlinear, open-source computational fluid dynamics(CFD) model, OpenFOAM. The permeable structure is positioned at the weather side of the OWC device and adjoined to its front wall. A numerical modelling approach is employed in which the interstices within the porous structure are explicitly defined. This permits the flow field development within the porous structure and at the OWC front wall to be observed. The WEC device is defined as a land-fixed, semi-submerged OWC chamber. A range of regular incident waves are generated at the inlet within the numerical tank. The OWC efficiency and the forces on the structure are examined. Results are compared for the simulation cases in which the porous component is present or absent in front of the OWC chamber. It is found that the incorporation of the porous component has minimal effect on the hydrodynamic efficiency of the OWC, reducing the efficiency by less than 5%. Nevertheless,the forces on the front wall of the OWC can be reduced by up to 20% at the higher wave steepness investigated,through inclusion of the porous structure at the OWC front wall. These findings have considerable implications for the design of hybrid OWC-breakwater systems, most importantly in terms of enhancing the durability and survivability of OWC WECs without significant loss of operational efficiency.

Numerical study on the aerodynamic and hydrodynamic performances of an ultra-high-speed AAMV

In order to investigate the resistance performance of an ultra-high-speed aerodynamically alleviated marine vehicle(AAMV),finite volume method(FVM)-based computational fluid dynamics(CFD)software STAR CCM+is used to simulate the forward motion of this vehicle.The calculated results are validated as they reach good agreement with experimental data.Comparing the motions of models with and without aero-wings,the hybrid aerodynamic and hydrodynamic mechanism of this novel hull is discussed.Study is subsequently performed that how step configuration,spray rail and deadrise angle act on hull behavior and resistance.The results show that models with double steps and spray rail possess better resistance characteristics at high speeds,and planing surface with variable deadrise angle could further improve the overall navigation performance.

Influence of bottom and sand curtain on hydrodynamic performance of otter boards

Numerical simulation is an important method for calculating the hydrodynamic performance of otter boards used in sea floor trawling.Although such simulations have been explored in prior studies,the effects of the proximity of the otter boards to the seafloor and the plume of upward-drawn sediment during bottom trawling have largely been ignored.In this study,we assessed these factors.The results show that within the angles of attack used during normal operations,the effect of the seafloor bottom boundary of the flow field on the hydrodynamic performance of an otter board is obvious.We found that when the ratio of the distance between the bottom of an otter board and the surface boundary of the flow field to the chord length of the board exceeds 0.4,the influence of the bottom boundary of the flow field on the hydrodynamic performance of the board is negligible.For values of less than 0.4,the seafloor bottom boundary has an increasingly obvious impact on the hydrodynamic performance as this ratio decreases.We also found that the turbid plume of ocean floor sediment raised during bottom trawling has an obvious effect on the lift and resistance coefficients of an otter board at high angles of attack.At low angles,this effect on the lift-to-drag ratio is reversed and less obvious.The simulation results show that the optimal lift-to-drag ratio decreases with an increase in the sediment concentration;however,beyond a certain threshold,an increasing concentration of sediments was not found to have an obvious impact on the lift-to-drag ratio.

Improving the hydrodynamic performance of the SUBOFF bare hull model: a CFD approach

The main aims of this study are to investigate the hydrodynamic performance of an autonomous underwater vehicle(AUV),calculate its hydrodynamic coefficients,and consider the flow characteristics of underwater bodies.In addition,three important parts of the SUBOFF bare hull,namely the main body,nose,and tail,are modified and redesigned to improve its hydrodynamic performance.A three-dimensional(3D)simulation is carried out using the computational fluid dynamics(CFD)method.To simulate turbulence,the k-ωshear stress transport(SST)model is employed,due to its good prediction capability at reasonable computational cost.Considering the effects of the length-to-diameter ratio(LTDR)and the nose and tail shapes on the hydrodynamic coefficients,it is concluded that a hull shape with bullet nose and sharp tail with LTDR equal to 7.14 performs better than the SUBOFF model.The final proposed model shows lower drag by about 14.9%at u=1.5 m·s^-1.Moreover,it produces 8 times more lift than the SUBOFF model at u=6.1 m·s^-1.These effects are due to the attachment of the fluid flow at the tail area of the hull,which weakens the wake region.

Influence of main-panel angle on the hydrodynamic performance of a single-slotted cambered otter-board

The effect of the main-panel angle of a single-slotted cambered otter-board was investigated using engineering models in a wind tunnel.Three different angles(0
,6
,and 12
)were evaluated at a wind speed of 28 m/s.Parameters measured included:drag coefficient Cx,lift coefficient Cy,pitch moment coefficient Cm,center of pressure coefficient Cp,and the liftedrag ratio Cy/Cx,over a range of angle of attack(0
e70
).These coefficients were used in analyzing the differences in the performance among the three otter-board models.Results showed that the maximum lift coefficient Cy of the otter-board model with a main-panel angle of 0
was highest(1.875 at a¼25
).However,when the angle of attack was smaller(0<a<22.5
),the lift coefficient of the otter-board increased as the angle of the main-panel increased.The maximum Cy/Cx of the otter-board with a main-panel angle of 12
was highest(7.417 at a¼2.5
),and the liftedrag ratio increased when the angle of the main-panel increased within the angle of attack at small angles(0<a<12.5
).A comparative analysis of Cm and Cp showed that the stability of the otter-board with a main-panel angle of 0
is better than those of the other models.Therefore,the comparative analysis of Cm and Cp,shows that a larger angle of the main-panel can reduce the stability of single-slotted otter-board.The findings of this study offer useful reference data for the structural optimization of otter-boards for trawling。

Hydrodynamic performance and structural response characteristics of the double-slotted vertical cambered V-Type otter board

Vertical cambered V-type otter boards are widely used in large and medium-sized trawlers for their good stability and adaptability to various water layers.However,limited numerical studies on the hydrodynamic performance and structural strength of this type of otter board have been published.In this study,we established the three-dimensional numerical model of the double-slotted vertical cambered V-type otter board according to its special structure and stress feature.We compare the hydrodynamic performance results of our model with those of previous experiments.Using this model,we analyzed the influence of parameters such as attack angle,aspect ratio,dihedral,and deflector angles on its hydrodynamic performance.Moreover,the structural response characteristics of the otter board under typical working conditions were studied.We believe our results will provide theoretical reference for the structural design and optimization of the vertical cambered V-type otter board.

Hydrodynamic Performance of Air-Filled Wave Attenuator for Wave Control:Experimental Study

Numerous types of floating breakwaters have been proposed,tested and commercialized in the past decades.The majority of these breakwaters are made of solid bodies;hence,they are relatively bulky and are not readily to be rapidly installed at the targeted sites when immediate wave protection of the coastal and offshore facilities is needed.Furthermore,the application of these hard floating structures at the recreational beaches is rather unlikely due to potential deadly marine traffic collision.To overcome these problems,a flexible air-filled wave attenuator(AFWA)has been developed in the present study.This floating breakwater is made of flexible waterproof membrane materials.The main body consists of a rectangular air-filled prism and is ballasted by sandbags located around the floating module.The objective of this study is to evaluate the wave transmission,wave reflection,energy dissipation,motion responses and mooring forces of the AFWA under the random wave actions using physical modelling.The test model located in a 20 m long wave flume was subjected to a range of wave heights and periods.The wave profiles in the vicinity of the test model were measured using wave probes for determination of wave transmission,reflection and energy loss coefficients.The motion responses in terms of heave,surge and pitch,and wave forces acting on the mooring lines were measured using a motion tracking system and load cells,respectively.The experimental results reveal that the AFWA is effective in attenuating up to 95%in the incoming wave height and has low-wave-reflection properties,which is commendable for floating breakwaters.

INVESTIGATION OF VORTEX SHEDDING INDUCED HYDRODYNAMIC VIBRATION IN VORTEX STREET FLOWMETER

Vortex street flowmeter has been used in steady flow measurement for about three decades The benefits of this type of flowmeter include high accuracy,good linearty,wide measuring range,and excellent reliability However,in unsteady flow measurement,the pressure disturbance as well as the noise from the system or surrounding can reduce the signal to noise ratio of the flowmeter seriously Aimed to use vortex street flowmeters in unsteady flow measurement,the characteristics of the vortex shedding induced hydrodynamic vibration around the prism bluff body in a vortex street flowmeter are investigated numerically and by expriments The results show that the hydrodynamic vibrations with 180° phase shift occur at the axisymmetric points of the channel around the bluff body The most intense vibration occurs at the points on the lateral faces close to the base of the prism The results provide therefore a useful reference for developing an anti interference vortex flowmeter using the different ial sensing technique.

半悬挂扭曲舵水动力及空化性能研究

In this study,we designed a new,semi-balanced,twisted rudder to reduce the surface cavitation problem of medium-high-speed surface warships.Based on the detached eddy simulation(DES)with the Spalart-Allmaras(SA)model(SA-DES)and the volume of fluid(VOF)method,the hydrodynamic and cavitation performances of an ordinary semi-balanced rudder and semi-balanced twisted rudder at different rudder angles were numerically calculated and compared using the commercial computational fluid dynamics(CFD)software STAR-CCM+with the whole-domain structured grid.The calculation results showed that,under the same working conditions,the maneuverability of the semi-balanced twisted rudder basically remained unchanged compared with that of the ordinary semi-balanced rudder.Furthermore,the surface cavitation range of the semi-balanced twisted rudder was much smaller,and the inception rudder angle of the rudder surface cavitation increased by at least 5°at the maximum speed.In conclusion,the semi-balanced twisted rudder effectively reduced the cavitation of the rudder surface without reducing the rudder effect and exhibited excellent anti-cavitation performance.

基于计算流体力学和人工神经网络对断阶式滑行艇的水动力性能预测

In the present paper,the hydrodynamic performance of stepped planing craft is investigated by computational fluid dynamics(CFD)analysis.For this purpose,the hydrodynamic resistances of without step,one-step,and two-step hulls of Cougar planing craft are evaluated under different distances of the second step and LCG from aft,weight loadings,and Froude numbers(Fr).Our CFD results are appropriately validated against our conducted experimental test in National Iranians Marine Laboratory(NIMALA),Tehran,Iran.Then,the hydrodynamic resistance of intended planing crafts under various geometrical and physical conditions is predicted using artificial neural networks(ANNs).CFD analysis shows two different trends in the growth rate of resistance to weight ratio.So that,using steps for planing craft increases the resistance to weight ratio at lower Fr and decreases it at higher Fr.Additionally,by the increase of the distance between two steps,the resistance to weight ratio is decreased and the porpoising phenomenon is delayed.Furthermore,we obtained the maximum mean square error of ANNs output in the prediction of resistance to weight ratio equal to 0.0027.Finally,the predictive equation is suggested for the resistance to weight ratio of stepped planing craft according to weights and bias of designed ANNs.

Numerical analysis of the performance of a three-bladed vertical-axis turbine with active pitch control using a coupled unsteady Reynolds-averaged Navier-Stokes and actuator line model

In this paper,we present a numerical model of a vertical-axis turbine(VAT)with active-pitch torque control.The model is based upon the Wind and Tidal Turbine Embedded Simulator(WATTES)and WATTES-V turbine realisations in conjunction with the actuator line method(ALM),and uses OpenFOAM to solve the unsteady Reynolds-averaged Navier-Stokes(URANS)equations with two-equation k-εturbulence closure.Our novel pitch-controlled system is based on an even pressure drop across the entire rotor to mitigate against dynamic stall at low tip speed ratio.The numerical model is validated against experimental measurements and alternative numerical predictions of the hydrodynamic performance of a 1:6 scale UNH-RM2 hydrokinetic turbine.Simulations deploying the variable pitch mechanism exhibit improved turbine performance compared to measured data and fixed zero-pitch model predictions.Near-wake characteristics are investigated by examining the vorticity distribution near the turbine.The pitch-controlled system is demonstrated to theoretically decrease turbulence generated by turbine rotations,mitigate the intensity of vortex shedding and size of detached vortices,and significantly enhance the performance of a vertical-axis hydrokinetic turbine for rated tip-speed ratios.

Study of Tunnel Thruster Performance and Flow by Quasi-Steady Reynolds-Averaged Navier-Stokes Simulation

A numerical approach based on the solution of the Reynolds-averaged Navier-Stokes(RANS) equations using the shear-stress transport(SST) turbulence model has been employed to investigate the hydrodynamic performance and flow of tunnel thrusters.The flow passages between adjacent blades are discretized with prismatic cells so that the boundary layer flow is resolved down to the viscous sub-layer.The hydrodynamic performances predicted by the quasi-steady approach agree well with the experimental data for three impellers covering a range of blade area and pitch.Through analysis of the flow field,the reason why the hub of impeller also contributes to thrust which can amount to 40%—60% of the impeller thrust,and the mechanism of the impeller inducing an axial force on the hull are elucidated.

The hydrodynamics of self-rolling locomotion driven by the flexible pectoral fins of 3-D bionic dolphin

The novel autonomous rolling performance is realized by the pair of pectoral fins of a three-dimensional(3-D)bionic dolphin in this paper numerically.3-D Navier-Stokes equations are employed to simulate the viscous fluid around the bionic dolphin.The effect of self-rolling manoeuvrability is ex-plored using the dynamic mesh technology and user-defined function(UDF).By varying the parameter ratios,the interaction of flexible pectoral fins is divided into two motion modes,amplitude differential and frequency differential mode.As the primary driving source,the differential motion of a pair of pec-toral fins can effectively provide the rolling torque,and the trajectory of the entire rolling process is approximately the clockwise spiral.The results demonstrate that the rolling angular velocity and driving torque in the steady state can be improved by increasing parameter ratios,and the rolling efficiency can reach the maximum under the optimal parameter ratio.Meanwhile,different parameter ratios do not af-fect the rolling radius of the self-rolling dolphin.The evolution process around the pair of pectoral fins is shown by the flow structures in self-rolling swimming,reasonably revealing that self-rolling locomotion is produced by the pressure and wake vortices surrounding the pair of pectoral fins,and the wake struc-tures depend primarily on the variation of parameter ratio.It properly turns out that the application of the pair of pectoral fins can realize the self-rolling performance through parameter differential modes.

Hydrodynamics study of dolphin’s self-yaw motion realized by spanwise flexibility of caudal fin

The hydrodynamic performance of the virtual underwater vehicle under self-yaw is investigated numerically in this paper,we aim to explore the fluid laws behind this plane motion achieved by the bionic flexibility,especially the spanwise flexibility of the caudal fin.The kinematics of the chordwise flexible body and the spanwise flexible caudal fin are explored through dynamic mesh technology and user-defined functions(UDF).The 3-D Navier-Stokes equations are applied to simulate the viscid fluid surrounding the bionic dolphin.The study focuses on quantitative problems about the fluid dynamics behind the specific motion law,including speed of movement,energy loss and working efficiency.The current results show that the self-yaw can be composed of two motions,autonomous propulsion and active steering.In addition,the degree of the flexible caudal fin can produce different yaw effects.The chordwise phase differenceФis dominant in the propulsion function,while the spanwise phase differenceδhas a more noticeable effect on the steering function.The pressure distribution on the surface of the dolphin and the wake vortex generated in the flow field reasonably reveal the evolution of self-yaw.It properly turns out that the dolphin can combine the spanwise flexible caudal fin and the chordwise flexible body to achieve self-yaw motion.