Un- and reloading stiffness of monopile foundations in sand

The eigenfrequency of offshore wind turbine structures is a crucial design parameter, since it determines the dynamic behavior of the structure and with that the fatigue loads for the structural design. For offshore wind turbines founded on monopiles, the rotational stiffness of the monopile-soil system for un- and reloading states strongly affects the eigenfrequency. A numerical model for the calculation of the monopile’s behavior under un- and reloading is established and validated by back-calculation of model and field tests. With this model, a parametric study is conducted in which pile geometry, soil parameters and load conditions are varied. It is shown that of course the rotational stiffness varies with mean load and magnitude of the considered un- and reloading span, but that for most relevant load situations the initial rotational stiffness of the monopile system, i.e. the initial slope of the moment-rotation curve for monotonic loading, gives a good estimate of the actual stiffness. Comparisons of different p–y approaches show that the ordinary API approach considerably underestimates the initial stiffness, whereas the recently developed ‘Thieken’ approach and also the ‘Kallehave’ approach give a much better prediction and thus might be used in the design of monopiles in sand.     

山东半岛沿岸海域悬浮体时空分布及形成机制分析

依据2015年GOCI(geostationary ocean color imager)卫星影像反演的悬浮体浓度数据,分析了山东半岛沿岸海域表层悬浮体质量浓度和锋面月变化特征,揭示该海域悬浮体的分布特征和扩散格局,并结合风速、波高以及海表温度数据,对其控制因素进行初步探讨。结果显示:研究区内悬浮体质量浓度整体表现为冬季最高,春秋次之,夏季最低的分布特征;悬浮体扩散过程可以划分为4个阶段,冬季稳定外输,春季向岸退缩,夏季近岸贮存,秋季向外扩散。此外,山东半岛近岸存在一条悬浮体质量浓度高于10 mg/L的浑浊带,该浑浊带同样表现出季节变化,它在秋季开始形成,其悬浮体含量、幅宽及延伸范围在冬季达到最大,春季减弱,夏季消失。研究认为山东半岛沿岸海域的表层悬浮体来源主要是海底沉积物的再悬浮。风场、海浪以及沿岸流的强弱变化对悬浮体分布和输运的季节变化有重要的控制作用:风场和海浪影响海水混合搅拌强度,改变海底沉积物再悬浮作用的临界深度,进而影响表层海水悬浮体浓度,致使悬浮体浓度与风浪的月际变化趋势基本一致;沿岸流携带高浓度悬浮体沿山东半岛输运形成沿岸浑浊带,沿岸流的强度变化直接控制浑浊带的季节变化。     

Improving extreme wind speed prediction for North Sea offshore oil and gas fields

The paper adresses accuracy improving of statistical prediction, extracted from a shorter stochastic process, using the information provided by another synchronous highly correlated stochastic process that has been measured for a longer time. As an example, the specific issue of improving extreme wind speed prediction has been addressed. For this purpose, an efficient transfer of information is necessary between two synchronous, highly correlated stochastic processes. To illustrate the efficiency of the proposed technique, two time series of measured wind speed data from North sea oil and gas fields were used.     

Wave run-up on slender circular cylindrical foundations for offshore wind turbines in nonlinear random waves

A practical method for estimating the wave run-up height on a slender circular cylindrical foundation for wind turbines in nonlinear random waves is provided. The approach is based on the velocity stagnation head theory and Stokes second order wave theory by assuming the basic harmonic wave motion to be a stationary Gaussian narrow-band random process. Comparisons are made with measurements by De Vos et al. (2007), and some of the highest wave run-up events that were predicted agree with those measured.     

基于VAP方法的多普勒雷达风场反演研究

采用VAP(Velocity Azimuth Processing)方法反演多普勒雷达风矢量场,在利用各个距离圈径向速度随方位角分布的廓线推算风向和风速的设计中,要求基数据速度资料中没有明显的脉动.介绍了在多普勒天气雷达风场反演设计中,直接将方格法的速度资料处理过程融入到方格法风场反演的过程中,利用距离方格内保持中尺度特性的特点,不改变原始资料的同时保证风场反演的正确性.     

风浪对海-气界面动量通量估计的影响

利用实验室和有代表性的外海观测数据综合分析表明,海面粗糙度对波龄的依赖性与是否将实验室和外海数据一起考虑有关,而风应力拖曳系数与此无关,且随波龄增大而减小.利用Toba-3/2指数律和风浪成长关系的分析表明,风应力拖曳系数为常数或随波龄的增大而增大,与上述结果定性上相矛盾,说明风浪对风应力拖曳系数影响问题需要进一步研究.     

Non-Stationary Rayleigh-Taylor Instabilities in Pulsar Wind Interaction with a Supernova Shell

The Rayleigh-Taylor instability (RTI) plays an important role in the dynamics of several astronomical objects, in particular, in the supernovae (SN) evolution. In the present paper we examine the dynamics of a shell (representing a type II SN remnant) blown-up by a wind emitted by a central pulsar. The shell is accelerated by the pulsar wind and its inner surface experiences the RTI. We develop an analytical approach by using a specific transformation into the coordinate frame co-moving with the SN ejecta. We first derive a non-stationary spherically symmetric solution describing an expansion of a gas shell under the pressure of a central source (pulsar). Then, we analyze its 3D stability with respect to a small perturbation on the inner shell surface. The dispersion relation is derived in the co-moving reference frame. The growth rate of the perturbation is found and its temporal evolution is discussed. We compare our result with the previous published studies and apply it to the Crab nebula evolution.     

基于四种再分析资料的1988-2015年中国近海月均风速特征评估

This study investigates the long-term changes of monthly sea surface wind speeds over the China seas from 1988 to 2015. The 10-meter wind speeds products from four major global reanalysis datasets with high resolution are used: Cross-Calibrated Multi-Platform data set(CCMP), NCEP climate forecast system reanalysis data set(CFSR),ERA-interim reanalysis data set(ERA-int) and Japanese 55-year reanalysis data set(JRA55). The monthly sea surface wind speeds of four major reanalysis data sets have been investigated through comparisons with the longterm and homogeneous observation wind speeds data recorded at ten stations. The results reveal that(1) the wind speeds bias of CCMP, CFSR, ERA-int and JRA55 are 0.91 m/s, 1.22 m/s, 0.62 m/s and 0.22 m/s, respectively.The wind speeds RMSE of CCMP, CFSR, ERA-int and JRA55 are 1.38 m/s, 1.59 m/s, 1.01 m/s and 0.96 m/s,respectively;(2) JRA55 and ERA-int provides a realistic representation of monthly wind speeds, while CCMP and CFSR tend to overestimate observed wind speeds. And all the four data sets tend to underestimate observed wind speeds in Bohai Sea and Yellow Sea;(3) Comparing the annual wind speeds trends between observation and the four data sets at ten stations for 1988-1997, 1988–2007 and 1988–2015, the result show that ERA-int is superior to represent homogeneity monthly wind speeds over the China seaes.     

小麦玉米田耕作模式的防风蚀效果

通过风洞实验，在5个风速下对6种耕作方式下农田土壤风蚀速率、0~20 cm风沙流结构进行了模拟研究。结果表明：保护性耕作土壤风蚀速率较传统耕作平均降低20%~40%；保护性耕作和传统耕作条件下土壤风蚀速率均随风速的增大呈幂函数递增，但在传统耕作条件下递增较快；风速14 m·s^-1是荒漠绿洲农田土壤风蚀加剧转折点，当风速>14 m·s^-1时保护性耕作下风蚀速率较传统耕作明显降低；0~20 cm内，传统耕作和保护性耕作下输沙率与高度分别呈线性和指数关系，保护性耕作下0~4 cm输沙量和输沙量百分比（Q_0~4/Q_0~20）均低于传统耕作。     

荒漠草原区风力发电场建设前后的植被变化

调查了甘肃景泰荒漠草原区一处风电场基础设施建设和运行对植被的影响。结果表明：荒漠草原区百兆瓦级风力发电场建设造成直接植被生产力损失量为6.79 g·m^-2·a^-1，间接植被生产力损失量为91.43 g·m^-2·a^-1。随着扰动程度增加，物种丰富度降低，均匀度减小。在工程运行后，经历近10 a的自然恢复，核心扰动区虽然物种单一，但植被生产力是次级扰动区的两倍以上，为间接扰动区和未扰动区的50%和33%左右；而间接扰动区植被生产力仅为未扰动区的50%。总体上，风电工程扰动对区域植被的影响，不仅在于建设期对局地群落植被生产力的完全丧失，更重要的是在运行期间对周边大面积间接影响区群落植被生产力的大幅度降低，而且这种影响是长期和持续性的。