Self-Starting Analysis of an OWC Axial Impulse Turbine in Constant Flows: Experimental and Numerical Studies

In recent times, self-rectifying axial-flow air turbines are being widely employed in oscillating water column (OWC) wave energy converters (WEC). The steady performance of air turbines has been systematically investigated in previous studies. However, there still exists a lack of information on their unsteady performance, such as in the self-starting characteristics and subsequent running behavior. In this study, the unsteady behavior of impulse turbine under various constant-flow conditions is investigated. Experimental studies were conducted to investigate the effects of constant-load on the variations in the rotation speed, the pressure drop and the torque output of the turbine starting from rest. A fully passive flow-driving numerical model is employed for further detailed analysis of the flow and pressure fields. Followed by a well-agreed validation using the corresponding experimental data, the three dimensional (3D) transient model is used to study the effects of the air-flow velocity magnitude and the rotors’ moment of inertia on the self-starting performance of the turbine. Except for the variations in the rotation speed, the pressure drop and the pneumatic torque, the distributions of the flow-field and the pressure over the blades at specific time-points are analyzed.     

考虑水-桩-土相互作用的单桩式海上风机结构系统自振频率解析解

海上风机结构系统频率是海上风机结构和基础设计考虑的关键因素之一,桩-土相互作用对海上风机结构系统频率影响显著。基于欧拉-伯努利梁理论和传递矩阵方法,考虑水-桩-土相互作用及塔筒变截面特性,建立单桩式海上风机结构系统横向振动自振频率特征方程;将桩-水相互作用等效为附加质量、桩-土相互作用等效为线性弹簧,变截面塔筒等效为多段均匀梁,利用MATLAB中fsolve函数求解固有频率。通过与有限元分析结果进行对比,验证本文方法精度与有效性,并将本文方法应用于实际工程中,研究桩基础埋深、上部质量、转动惯量和桩-水相互作用对单桩式海上风机结构系统自振频率的影响。     

高桩-混凝土承台式海上风电塔强振分析

对某个风电场的23座高桩-混凝土承台式海上风电塔进行了1年的强振监测,统计了其在台风、偏航冲击等作用力下的强振特性。监测数据分析发现:机舱对风向偏航时会引起塔筒强烈振动,且此类型强振现象持续时间达几十秒,有时振动加速度可超过10 m/s^2,每个月多达几百次;在某次台风作用下,塔筒的振动加速度接近10 m/s^2;施工船靠船时的碰撞引起塔筒的强振幅值接近15 m/s^2;通过分析风电塔1年运营期间的塔筒固有频率值,发现前3阶固有频率值、阻尼比未发现变化。监测结果表明:高桩-混凝土承台式风电塔在台风、机舱偏航,施工船碰撞时都会产生强烈振动。因此,机舱偏航和施工船碰撞引起风电塔的强振现象过于频繁发生,是风电塔疲劳损伤的重要因素。本文的研究成果可为此类型风电塔设计、运营安全监测及损伤诊断提供参考。     

Design of a prototype ocean current turbine—Part I: mathematical modeling and dynamics simulation

The “C-Plane” is a submerged variable depth ocean current turbine that is tethered to the sea floor and uses sustained ocean currents to produce electricity. As part of the development of a $\frac{1}{30}$th scale physical model of the C-Plane, a mathematical model and dynamics simulation of the prototype was developed and is presented in this paper. This three-dimensional mathematical model represents the C-Plane as a rigid body with moveable control surfaces that is moored with three linear elastic cable elements. Gravitational, buoyancy, hydrodynamic, cable, gyroscopic, and inertial forces are included and a PC-based dynamics simulation is created. The simulation demonstrates that the C-Plane is stable and capable of changing depth in all expected operating conditions. The C-Plane prototype can fly level from a height of 3 to 6 m using the configuration suggested in this paper. The maximum ascent rates of the C-Plane with a water speed of 0.3 m/s are 0.015 m/s when the pitch is fixed at 0° and 0.030 m/s when the pitch is fixed at 4°. The maximum descent rates of the C-Plane are 0.018 m/s when the pitch is held at 0° and 0.031 m/s if the pitch is held at −4°.     

振荡水柱波能装置冲击式空气透平优化数值模拟研究

冲击式空气透平是振荡水柱式波能发电装置的二级能量转换装置,具有自启动性能好、在大流量系数区保持较高效率等优势,近年来应用越来越广泛.有学者提出在冲击式透平动叶片尖端安装环结构的设计,可以改善动叶片叶尖间隙处的气流流动形态,提高透平的工作性能.依托于此观点,构建了安装有环结构的冲击式透平的三维定常数值模型,并通过网格数量...     

Wake studies of a 1/30th scale horizontal axis marine current turbine

A 0.4 m diameter (1:30th scale) horizontal axis marine current turbine (MCT) was tested in a circulating water channel. The turbine performance and wake characteristics were determined over a range of flow speeds and rotor thrust coefficients. Measurements of the water surface elevation profiles indicated increasing variation and surface turbulence with increasing flow speeds. Blockage-type effects (where the measured point velocity was greater than the inflow velocity) occurred around the sides of the rotor for all flow speeds. Although the effects were exaggerated at model scale, it is expected that reasonable variations in water level and flow velocity could also occur over a full scale MCT array.     

用于Wells透平性能分析的稳态实验台研制

文中利用二维模型分析了威尔斯透平（Wells)的气动力性能，并以此为基础，设计并研制出多用途吸风式稳态透平试验台，该试验台一方面可用来研究稳态流动透平的总体气动性能以及稠密度、翼型等参数对透平性能的影响，另一方面又还可以测量透平前后的速度场、压力场，为分析透平气动损失、优化透平设计、提高波浪能转换效率提供手段。     

Coupled hydrodynamic and geotechnical analysis of jacket offshore wind turbine

This paper compares the response of a jacket-supported offshore wind turbine (OWT) under wave loading, when (a) soil–structure interaction (SSI) is ignored and (b) SSI is considered. The jacket is placed in a water depth of 70 m and soil conditions off the west coast of India are used in the study. The rotor of the OWT is considered to be parked, under a survival average wind speed of 50 m/s, significant waver height Hs=16 m and peak spectral period Tp=18 s. The significance of includng SSI in OWT studies is investigated by means of pushover analyses and irregular-wave dynamic analyses. Modal studies are performed to determine the variation in the free-vibration response of the two cases. It is observed that ignoring SSI tends to over-estimate the ultimate strength characteristics of the OWT by 3–60% in various modes or increase the tower top displacement above serviceable limit. For dynamics analysis, the wave elevation is generated using wave superposition method. The JONSWAP wave spectrum is discretized using constant area method which introduces additional uncertainty. The analysis shows that approximately 200 frequencies are necessary using constant area method to capture the tail region appropriately. Also the statistical uncertainty in the generation of wave elevation for dynamic analyses is quantified by means of sample convergence studies. The results show that approximately 20–40 samples are required in order to obtain reasonable statistics.     

海上风机分段斜壁桶形基础的水平承载力计算方法

在部分浅海由于大型施工船舶无法进入,海上风电工程中传统的重力式基础或桩基础不再适用,为此提出了一种新型基础--分段斜壁桶形基础。基于极限平衡法对分段斜壁桶形基础的水平承载力进行了系统性研究,提出了水平承载力计算方法,并进一步考察了斜壁倾角、桶高、桶径、土性参数等对水平承载力的影响。计算表明,水平承载力随着上部桶壁倾角、下部桶壁倾角、地基反力比例系数、桶基顶部直径、桶基高度的增大而增大,随着海床深度的增加而减小。基于体积压缩率的概念,论证并推荐优先使用上部桶壁倾斜、下部桶壁直立的桶形基础构型。建议发展针对海床地基反力比例系数的准确确定方法。研究结果有助于对桶形基础进行优化设计。     

Natural vibration frequency and damping of slender structures founded on monopiles

Offshore wind turbine (OWT) is a typical example of a slender engineering structure founded on large diameter rigid piles (monopiles). The natural vibration characteristics of these structures are of primary interest since the dominant loading conditions are dynamic. A rigorous analytical solution of the modified SSI eigenfrequency and damping is presented, which accounts for the cross coupling stiffness and damping terms of the soil–pile system and is applicable but not restrictive to OWTs. A parametric study was performed to illustrate the sensitivity of the eigenfrequency and damping on the foundation properties, the latter being expressed using the notion of dimensionless parameters (slenderness ratio and flexibility factor). The application of the approximate solution that disregards the off diagonal terms of the dynamic impedance matrix was found to overestimate the eigenfrequency and underestimate the damping. The modified SSI eigenfrequency and damping was mostly affected by the soil–pile properties, when the structural eigenfrequency was set between the first and second eigenfrequency of the soil layer. Caution is suggested when selecting one of the popular design approaches for OWTs, since the dynamic SSI effects may drive even a conservative design to restrictive frequency ranges, nonetheless along with advantageous – from a designers perspective – increased damping.