Effect of various winglets on the performance of marine propeller

The tip vortex cavitation (TVC) is an issue of increasing interest, because the TVC plays an important role in propeller radiated noise and cavitation erosion. The marine propeller with winglets, which is inspired by the winglets of airfoil, is numerically investigated in the present paper. The blade tip of newly designed propeller tilts toward the pressure side. The difference between six propellers is the change of the rake angle at r/R = 1.0. The pressure coefficient, TVC, axial velocity field and helicity are analyzed. The numerical results show that the winglets of newly designed propeller scarcely affect the efficiency of propeller. The thrust coefficient gradually decreases with the increase in rake angle. As for the suction side, the pressure coefficient (C_p) of winglets propellers is higher than the conventional propeller in general. In addition, the winglets are beneficial to generate less cavitation behavior when the rake angle is small. However, as the rake angle is further increased, the cavitation behavior of winglets propeller is also increased, even larger than the conventional propeller. Therefore, it can be deduced that the winglets can be used to effectively improve the TVC characteristics to some extent.     

Large-scale experimental investigation of the installation of suction caissons in silt sand

Offshore wind power is a rapidly growing area of electricity in China. In the present paper, interaction mechanisms between the caisson for wind turbines and saturated silt sand are investigated with laboratory tests based on two different installation methods, jacking installation and suction installation. For the jacking installation process, the results indicate that the soil pressures inner and outer the skirt of the caisson vary with a similar feature and the magnitudes of the two are nearly balanced. The tip resistance plays a key role in the total jacking installation resistance. This paper examines the predictive performance of q_c method and API approach for jacking installation resistance. It is demonstrated that the q_c method provides better predictions. The resistance coefficients are recommended. For the case of suction installation, however, the changes of soil pressures inner and outer the skirt are contrasting. Specifically, the inner pressure and tip resistance fall dramatically, but the outer pressure increases when suction is applied. Seepage effect is found to be an important mechanism for the installation of suction caisson. The reduction ratios of the inner friction and tip resistance follow a power-function with the normalized suction. Based on the test results, a prediction method for the required suction has been developed and evaluated.     

基于UDF的水平轴潮流能水轮机被动旋转水动力性能研究

针对水平轴潮流能水轮机被动旋转问题,基于Fluent 17.0,运用UDF(User Defined Function)控制滑移网格对网格进行动态调整,仿真研究水轮机在不同安放角下被动旋转的水动力特性。通过仿真分析,结果表明:潮流能水轮机随着叶片安放角度的增加,尖速比、输出功率、捕能系数都是先增大后减小,叶片安放角为6°时,叶轮前后速度差最大,对潮流能利用充分,且各项性能均达到最佳;通过分析叶片受力,叶尖叶素在安放角为2°时阻力最大,3°时升力最大,升阻比在6°时最大,此时叶尖叶素升阻比C_L/C_D=6.27、攻角α=3.06°。由仿真结果可知水平轴潮流能叶轮的自启动过程由5个阶段组成,即加速度增大的加速运动段—加速度减小的加速运动段—加速度反向增大的减速运动段—加速度反向减小的减速运动段—稳定运行段,这对潮流能水轮机的设计具有重要的指导意义。     

Numerical Study of A Generic Tidal Turbine Using BEM Optimization Methods

Three blade-geometry optimization models derived along with assumptions from the blade element momentum(BEM) approach are studied by using a steady BEM code to improve a small horizontal-axis rotor of three blades that has been previously used in experiments. The base rotor blade has linear-radially varying chord length and pitch angle, while the other three models noted as Burton, Implicit and Hansen due to their references and characteristics yield blades of non-linearly varying chord length and pitch angle. The aim is to compare these rapid models and study how assumptions embedded in them affect performance and induction factors. It is found that the model that has the least assumptions(Hansen) and which considers the blade-profile drag in its optimization procedure yields the highest power coefficient, C_P, at the optimal tip speed ratio(TSR), about 7% higher than the base one and also higher C_P at high TSR. It produces an axial induction factor distribution along the blade that is closest to the 1 D optimal value of 1/3. All optimized tangential induction-factor distributions along the blade closely vary as inverse to the square of the radial distance, while being mildly higher than the base distribution. It shows that sufficient swirl is necessary to increase power but at a level causing not too much energy loss in unnecessary swirl of the wake. At high TSR, all optimized rotors adversely produce higher thrust than the base one, but the one with most embedded assumptions(Burton) produces the highest thrust. Details of all three optimization models are given along with the distributions of the power, thrust, blade hydrodynamic efficiency and induction factors.     

Flow dynamics inside the rotor of a three straight bladed cross-flow turbine

In this work we study experimentally the flow dynamics inside the rotor of a three straight-bladed Cross-Flow Turbine (CFT). The CFT model used in the experiments is based on symmetric NACA-0015 profiles, with a chord to rotor diameter ratio of 0.16. The turbine model was designed in order to quantify the flow inside and around the rotor using planar Digital Particle Image Velocimetry (DPIV). Tests were made by forcing the rotation of the turbine with a DC motor, which provided precise control of the Tip Speed Ratio (TSR), while being towed in a still-water tank at a constant turbine diameter Reynolds number of 6.1 × 10⁴. The range of TSRs covered in the experiments went from 0.7 to 2.3.The focus is given to the analysis of the blade-wake interactions inside the rotor. The investigation has allowed us to relate the interactions with the performance differences in this type of turbines, as a function of the operational tip speed ratio.     

水平轴潮流能叶轮尖速比特性分析研究

针对设计尖速比对水平轴潮流能叶轮动力特性的影响问题开展研究工作。基于叶素-动量理论建立叶轮动力特性仿真模型,以叶片数量分别为2、3、4的叶轮为对象,考察设计尖速比对叶轮的功率系数的影响。研究结果表明:设计尖速比越大的叶轮,其最大效率也越高;叶片数量多的叶轮,其功率系数略优于叶片数量少的叶片。结合分析结果,给出了水平轴潮流能叶轮的设计尖速比的选择建议。     

水平轴海流能发电机叶片设计与性能分析

全球海洋蕴藏着丰富的海流能,合理利用海流能可以有效缓解能源危机。以额定工况下获能系数达到最大值为目标,利用叶素-动量理论设计了150 kW水平轴海流能发电机的叶片。使用叶素-动量理论结合普朗特修正和葛劳渥修正的方法,预测了海流机在不同尖速比以及不同桨距角下的水动力性能,分析了攻角和载荷沿着叶片径向的分布规律。使用CFD方法计算了海流机在不同尖速比下的水动力性能,并与理论方法的计算结果进行了比较。理论方法和CFD方法的结果均表明,所设计的海流机最大获能系数位于设计尖速比处,证明基于叶素-动量理论的水平轴海流机叶片设计方法是有效的。     

Stochastic modelling in wave power-equipment optimization: maximum energy production versus maximum profit

The paper presents an optimization study for the mechanical and electrical equipment of an oscillating-water-column (OWC) wave power plant of fixed shoreline or nearshore type, equipped with an air turbine. The plant’s structure geometry is assumed to be given and the corresponding hydrodynamic coefficients are known as functions of wave frequency. A stochastic model is adopted for the energy conversion process from wave to air turbine, it being assumed that the system is linear. The optimization concerns the turbine size, represented by its rotor diameter D. Two alternative criteria are used: (i) maximization of the produced electrical energy, (ii) maximization of the annual profit. An example calculation is presented, based on the hydrodynamic coefficients of the OWC on the island of Pico, Azores, and on the aerodynamic performance curves of its Wells turbine. The influence of the following parameters upon optimized turbine size and rated power output is analyzed: wave climate, capital costs of mechanical and electrical equipment, operation and maintenance costs, discount rate, equipment lifetime and price of electrical energy supplied to the grid.     

海底边界效应对海流发电水轮机水动力性能影响研究

对30 W海流能水平轴水轮机进行叶片设计,应用FLUENT软件对水轮机的水动力性能进行数值模拟,研究了边界效应对叶片表面压力、流场、湍流强度、获能和轴向力的影响。受海底边界效应影响,海流速度沿深度呈现梯度变化,底层流速较小,中上层流速较大。边界效应导致水轮机的水动力性能呈现周期性变化,降低了水轮机的获能和轴向力。机组布置时,宜选择水流稳定且流速较大的中上层区域。     

Modeling of twin-turbine systems with vertical axis tidal current turbine: Part II—torque fluctuation

We recently showed the advantage of using a numerical system to extract energy from tidal currents by developing a new twin-turbine model (Li and Calisal, 2010a). Encouraged by this result, we decided to use this model to study another important characteristic of the turbine system, torque fluctuation. This effort is summarized in this paper. The torque fluctuation is expected to reduce the fatigue life of tidal current turbines, though potentially it also may deteriorate the power quality of tidal current turbines. In this paper, after reviewing the twin-turbine model, we use it to predict the torque fluctuation of the system with the same configurations as we used to study the power output in Li and Calisal (2010a). Specifically, we investigate the torque fluctuation of twin-turbine systems with various turbine parameters (e.g., relative distance between two turbines and incoming flow angle) and operational condition (e.g., tip speed ratio). The results suggest that the torque of an optimally configured twin-turbine system fluctuates much less than that of the corresponding stand-alone turbine, under the same operating conditions. We then extensively compare the hydrodynamic interaction’s impact on the torque fluctuation and the power output of the system. We conclude that the hydrodynamic interactions pose more constructive impacts on the torque fluctuation than on the power output. The findings indicate that the optimally configured counter-rotating system should be a side-by-side system, and that the optimally configured co-rotating system should have the downstream turbine partially in the wake of the upstream turbine depending on the detailed configuration of the turbines. Furthermore, one must balance the optimal torque fluctuation against the optimal power output.     

砂土中吸力式导管架基础沉贯试验研究

吸力式导管架基础具备高承载力、高施工效率、高环境友好度、低造价等特点,使用其作为海上风电底部支撑结构有利于产业的规模化发展,实现上述愿景的前提在于使筒型基础顺利安装以满足设计要求。基于此,在不同初始施加泵压下,对吸力式导管架的沉贯效率特征值、基础内外围渗流变化、筒裙端部土压力特点进行探究。通过抱桩器使吸力式导管架在吸力安装过程中仅具备竖向自由度,从而进行纯沉贯试验。结果表明：初始泵压2 kPa时沉贯效率特征值最高;沉贯过程中基础外围渗流水压小于基础内围渗流水压;筒裙外侧所受土压要大于筒壁内侧所受土压力。     

基于LES的ROV导管推进器梢隙流动数值研究

导管推进器是一种普遍应用于无人遥控潜水器（ROV）等潜器中的特种推进器。在桨叶与导管之间的梢隙中存在非常复杂的流动，本研究基于大涡模拟（LES）对导管推进器的梢隙流动进行了数值模拟分析。通过对时间步长的收敛性研究，建立两套基于不同网格类型的计算模型。将计算结果与试验进行对比，比较两种不同类型网格模拟结果的差异发现，切割体网格能够更好地捕捉到泄涡的细节，并结合梢隙流场的原理分析泄涡发展的过程，梢隙涡的驱动力是吸力面与压力面之间的压差。此外，随着进速系数增大，梢隙周向的涡管轴向分布范围减小，主泄涡发生位置延后，泄出涡的长度和数量都有所减少。     

Numerical investigation on the performance of Wells turbine with non-uniform tip clearance for wave energy conversion

The performance of a Wells turbine with various non-uniform tip clearances was investigated using computational fluid dynamics (CFD). The investigation was performed on numerical models of a NACA0020 blade profile under steady flow conditions. The performance of turbines with uniform and non-uniform tip clearances was compared. The results were also compared with experimental results in literature. It was shown that the performance of turbine with non-uniform tip clearance is similar with that of turbine with uniform one in terms of torque coefficient, input power coefficient, and efficiency. However, the turbine with non-uniform tip clearance seems to have a preferable overall performance. An investigation on the flow-field around the turbine blade was performed in order to explain the phenomena.     

Wave climate scatter performance of a cycloidal wave energy converter

A lift based cycloidal wave energy converter (WEC) was investigated using potential flow numerical simulations in combination with viscous loss estimates based on published hydrofoil data. This type of wave energy converter consists of a shaft with one or more hydrofoils attached eccentrically at a radius. The main shaft is aligned parallel to the wave crests and submerged at a fixed depth. The operation of the WEC as a wave-to-shaft energy converter interacting with straight crested waves was estimated for an actual ocean wave climate. The climate chosen was the climate recorded by a buoy off the north-east shore of Oahu/Hawaii, which was a typical moderate wave climate featuring an average annual wave power P_W = 17 kWh/m of wave crest. The impact of the design variables radius, chord, span and maximum generator power on the average annual shaft energy yield, capacity factor and power production time fraction were explored. In the selected wave climate, a radius R = 5 m, chord C = 5 m and span of S = 60 m along with a maximum generator power of P_G = 1.25 MW were found to be optimal in terms of annual shaft energy yield. At the design point, the CycWEC achieved a wave-to-shaft power efficiency of 70%. In the annual average, 40% of the incoming wave energy was converted to shaft energy, and a capacity factor of 42% was achieved. These numbers exceeded the typical performance of competing renewables like wind power, and demonstrated that the WEC was able to convert wave energy to shaft energy efficiently for a range of wave periods and wave heights as encountered in a typical wave climate.     

The effects of yawing motion with different frequencies on the hydrodynamic performance of floating vertical-axis tidal current turbines

Under real sea conditions, the hydrodynamic performance of floating vertical-axis tidal current turbines is affected by waves and currents. The wave circular frequency is a significant factor in determining the frequencies of the wave-induced motion responses of turbines. In this study, the ANSYS-CFX software (manufacturer: ANSYS Inc., Pittsburgh, Pennsylvania, United States) is used to analyse the hydrodynamic performance of a vertical-axis turbine for different yawing frequencies and to study how the yawing frequencies affect the main hydrodynamic coefficients of the turbine, including the power coefficient, thrust coefficient, lateral force coefficient, and yawing moment coefficient. The time-varying curves obtained from the CFX software are fitted using the least-squares method; the damping and added mass coefficients are then calculated to analyse the influence of different yawing frequencies. The simulation results demonstrate that when analysing non-yawing turbines rotating under constant inflow, the main hydrodynamic coefficient time-varying curves of yawing turbines exhibit an additional fluctuation. Furthermore, the amplitude is positively correlated with the yawing frequency, and the oscillation amplitudes also increase with increasing yawing frequency; however, the average values of the hydrodynamic coefficients (except the power coefficient) are only weakly influenced by yawing motion. The power coefficient under yawing motion is lower than that under non-yawing motion, which means that yawing motion will cause the annual energy production of a turbine to decrease. The fitting results show that the damping term and the added mass term exert effects of the same level on the loads and moments of vertical-axis turbines under yawing motion. The results of this study can facilitate the study of the motion response of floating vertical-axis tidal current turbine systems in waves.     

Effect of caudal on hydrodynamic performance of flapping foil in fish-like swimming

In the present study the effect of caudal length on hydrodynamic performance of flapping foil is investigated. According to reality of swimming of fishes, the kinematics of their oscillation tail is involved with two rotational motions where one of them causes the tail to move in circular direction and the other leads the tail to pitch around its pitch axis. With this concept, a generalized kinematic model is considered. According to simulation of the motion trajectory of flapping foil, it is shown that the length of caudal may affect the hydrodynamic performance. It is shown that at lower and higher Strouhal numbers (St < 0.2 and St > 0.6) the hydrodynamic performance of flapping foil is optimum when the length of caudal is infinitive. It should be noted that at higher caudal length the variation of propulsive efficiency and produced thrust are stopped and these hydrodynamic parameters are kept at constant values. Additionally, it is demonstrated that there is the possibility of improving propulsive efficiency at moderate Strouhal numbers (0.2 < St < 0.6) by manipulation of caudal length. Furthermore, it is shown that in some cases the manipulation of caudal length may increase thrust coefficient as the propulsive efficiency is also increased.     

水平轴潮流能发电装置结构对其水动力性能的影响

为研究水平轴潮流能发电装置结构对其水动力性能的影响,运用格子玻尔兹曼(LBM)方法,建立水平轴潮流能发电装置的数值模拟分析模型,对水轮机在不同尖速比工况下的水动力性能进行模拟。将模拟结果与同工况水池拖曳实验得到的数据相对比,二者捕获能系数误差在2%左右,验证了LBM方法的可行性和准确性。在此基础上利用LBM方法研究机舱和立柱结构对水轮机特性的影响,得到其对水轮机捕获效率的影响规律。     

潮流能水轮机仿真及尾流场分析

为了研究潮流能水轮机尾流场的变化规律,本文在水平轴水轮机现有理论基础上,采用计算流体力学技术建立了数值模型,计算了水轮机的功率特性,进行了网格独立性验证,比较了模型中旋转区域不同对结果的影响。仿真结果与试验结果进行了对比分析,验证了数值模型的有效性。研究了全工况下尾流场速度衰减随尖速比和距离变化的规律,结果显示在小尖速比下尾流场速度恢复情况相对大尖速比下较好;靠近水轮机轮毂中心线位置处,衰减相对较大。研究建议水轮机在同等功率表现下尽量选择在小尖速比下工作,水轮机组在实际排列时避免采用成列布置,水轮机设计阶段应综合考虑功率特性和尾流场衰减。本文研究内容可以为潮流能水轮机的研究以及阵列式应用提供参考。     

Numerical and Experimental Analysis of A Vertical-Axis Eccentric-disc Variable-Pitch Turbine(VEVT)

A combined experimental and numerical investigation is carried out to study the performance of a vertical-axis eccentric-disc variable-pitch turbine(VEVT). A scheme of eccentric disc pitch control mechanism based on doubleblock mechanism is proposed. The eccentric control mechanism and the deflection angle control mechanism in the pitch control structure are designed and optimized according to the functional requirements of the turbine, and the three-dimensional model of the turbine is established. Kinematics analysis of the eccentric disc pitch control mechanism is carried out. Kinematics parameters and kinematics equations which can characterize its motion characteristics are derived. Kinematics analysis and simulation are carried out, and the motion law of the corresponding mechanical system is obtained. By analyzing the force and motion of blade of VEVT, the expressions of the important parameters such as deflection angle, attack angle and energy utilization coefficient are obtained. The lateral induced velocity coefficient is acquired by momentum theorem, the hydrodynamic parameters such as energy utilization coefficient are derived, and the hydrodynamic characteristics of VEVT are also obtained. The experimental results show that the turbine has good energy capture capability at different inflow velocities of different sizes and directions, which verifies that VEVT has good self-startup performance and high energy capture efficiency.     

Model testing and performance comparison of plastic and metal tidal turbine rotors

Experimental model tests were conducted to predict the performance of two sets of metal and plastic bi-directional tidal turbine rotors. This test programme aims to provide reliable and accurate measurement data as references for developers, designers and researchers on both model and full scale. The data set presented in this paper makes available the detailed geometry and motion parameters that are valuable for numerical tools validation. A rotor testing apparatus that was built using an off-the-shelf K&R propeller dynamometer, its configuration, testing set-up, calibration of the apparatus and data acquisition are described. Comparison analysis between the metal and plastic rotors hydrodynamic performance in terms of torque, drag and derived power and drag coefficients are also presented. The results show a substantial decrease in maximum power performance for the plastic rotors – about 40% decrease at a tip speed ratio of around 3.0, compared with rigid metal rotors. The plastic rotors have also a much higher cut-in speed. It showed that materials for rotor models with low rigidity such as polyamide (nylon) produced by selective laser sintering (SLS) systems may substantially under-predict power generation capacity. As a result, they are considered unsuitable for rotor model performance evaluation.