遥感与地球系统科学

地球作为一个高度复杂的非线性系统,各圈层(大气、海洋、陆地、生物、冰雪圈、固体地球)尤其是人类活动等任何组成成份的变化,都会引起地球系统的变化。人类可持续发展面临的巨大科学挑战之一是认识人类赖以生存的、复杂变化的地球系统,认识地球系统如何变化及主要驱动因素,认识地球系统未来变化趋势及如何提高对全球变化的适应能力。卫星独特的全球覆盖和日尺度的观测改变了地球科学的研究方法,它强调所能探测到的多时空尺度上的物理动力过程,在全球范围应对气候变化、能源和环境挑战具有重要作用,揭开了地球系统多学科交叉的新纪元。以地球系统的视野,抓住驱动地球系统的关键循环过程(如能量、水、生物化学循环),是当前地球系统科学的发展趋势。地球系统科学(全球变化)研究需要长期稳定、准确性较高的卫星观测数据,以水循环为例,卫星遥感具备获取全球范围水循环关键参数能力,但是系统性综合观测能力不足,整体精确性受到综合化的可靠空间数据集的限制。目前中国正在积极研制发展新型水循环卫星WCOM(Water Cycle Observation Misssion),并寄希望以此为核心传感器发起全球分布式水循环观测星座系统,进一步提高中国在国际水循环观测与地球系统科学研究方面的话语权与领先能力。     

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.     

Analysis of wave groups by wave envelope-phase and the Hilbert Huang transform methods

The wave groups are studied by both conventional wave analysis methods and by the non-stationary Hilbert Huang Transform (HHT) method. Full-scale wave records containing abnormal waves are used. Instantaneous quantities, such as envelope, phase and frequency, are adopted to study the wave grouping. A refined definition of wave group is proposed considering that the wave process is simultaneously amplitude and frequency modulated. The validation of the proposed definition is conducted by analysis of numerical simulation data. Group parameters are proposed based on the time-frequency distribution of energy. An attempt is made to find the relationship between the characteristics of abnormal waves and the group characteristics.     

Experimental and theoretical study of a cylindrical oscillating water column device with a quadratic power take-off model

The wave power extraction by a cylindrical oscillating water column (OWC) device with a quadratic power take-off (PTO) model was studied experimentally and theoretically. In the experiment, a scaled model OWC was tested in a wave flume, with an orifice being used to simulate a quadratic PTO mechanism. In the theoretical analysis, the quadratic PTO model was linearized based on Lorenz's principle of equivalent work, which allows us to perform a frequency domain analysis using an eigen-function matching method. The effects of higher harmonic components and the spatial non-uniformity of the surface velocity inside the chamber were discussed. A semi-analytical model was proposed to understand the viscous loss affecting the measured capture length. Our treatment of the quadratic PTO model was validated by comparing quasi-linear theoretical capture length and the laboratory measurement. Our results also showed that the effects of spatial non-uniformity and viscous loss could be noticeable for shorter waves.     

Compression limit state of HVAC submarine cables

An industry accepted standard does not currently exist for determination of compression limits in a subsea cable. This has resulted in most manufacturers specifying that subsea cables are not permitted to be axially loaded in compression.Additionally industry guidance does not exist regarding the consequences of inducing compression forces within subsea cables and the resulting effect on cable integrity. Industry recommended practice and guidance also does not have any information regarding experimental test arrangements to determine allowable compression levels within a subsea cable. This lack of modelling/testing guidance along with manufacturer recommendations of zero compressive loads within subsea cables results in overly conservative and restrictive design parameters for subsea cable installation and use.Due to the complex interaction within a subsea cable structure, such as contact interaction and friction between cable strands, theoretical modelling has been unable to provide reliable stress predictions and therefore an experimental testing regime is required if compression limits within the cable are to be appropriately determined. This paper describes combined axial and bending test arrangements that can be used as a guideline for determination of allowable compression limits for subsea cables.     

Differentiation in reproductive traits of geminate mangrove crabs of the genus Aratus (Decapoda: Brachyura) across the Central American Isthmus

The mangrove crab Aratus pisonii was considered to have an amphi‐American distribution; however, a recent study revealed that the Eastern Tropical Pacific populations were genetically distinct, thus representing a new species: Aratus pacificus. These species separated by the Isthmus of Panama have diverged under different environmental conditions that may have influenced their reproductive biology. As the available information about this genus concerns almost exclusively the Caribbean species, the aim of the present study was to analyse and compare reproductive aspects of A. pacificus and A. pisonii obtained from both the Pacific and Caribbean coasts of Costa Rica. Females were collected from April 2011 to April 2012, and reproductive features such as breeding season, size distribution of ovigerous females, fecundity, reproductive output, embryo volume and embryo water content were assessed. Both species produced embryos during the entire sampled period. Most females of A. pacificus carrying embryos close to hatching were found during the rainy season. Ovigerous females of A. pisonii were substantially larger and reached sexual maturity at a larger size than females of A. pacificus. Embryo production started in A. pacificus at a smaller female size than in A. pisonii. As fecundity increased with female size, the average fecundity was lower in A. pacificus. Females of A. pisonii produced larger embryos, which might be related to lower food availability: higher energy content in the embryo enhances the chances of larval survival. These data regarding reproductive features of the Pacific and Caribbean species support the conclusion to separate A. pacificus from A. pisonii.     

CFD investigations of OXYFLUX device,an innovative wave pump technology for artificial downwelling of surface water

No other environmental variable of such ecological importance to estuarine and coastal marine ecosystems around the world has changed so drastically, in such a short period of time, as dissolved oxygen in coastal waters. The prevalent methods for counteracting anoxic sea events are indirect measures which aim to cut-down anthropic loads introduced in river and marine environments. To date, no direct approaches, like artificial devices have been investigated except the WEBAP and OXYFLUX devices. The present paper adopts a numerical approach to the analysis of the pumped surface water as well as the analysis of the dynamic response of the OXYFLUX device. By means of a CFD-RANS code and through the application of overset grid method, the 1/16 OXYFLUX model’s dynamic response and pumping performance are evaluated. The appropriate grid is selected after an extensive sensitivity analysis carried out on 9 different grids. The CFD model is validated by comparing numerical and physical results of heave decay test, heave response, and surface water discharge under the action of regular waves. The extensive comparison with experimental results shows consistently accurate predictions. The main findings of the study show that nonlinear effects remarkable reduce the dynamic behaviour of the OXYFLUX and generate an unexpected second harmonic for pitch response intensifying the overtopping discharge also for small waves caused by the summer's low intensity winds.     

琼州海峡潮流能资源的数值模拟评估

近年来,我国能源消耗量不断的增长使我们更加重视可再生能源的开发利用,而我国近海拥有复杂的海岸线和广阔的大陆架,其中许多海域蕴藏着丰富的潮流能资源。潮流能资源评估则是其电站站址选择、发电量预测等工程设计的首要工作。结合两个站位的潮流实测数据,本文利用FVCOM海洋环流数值模式较好的模拟了琼州海峡潮波传播状况,分析了该海域潮流能资源水平分布规律和时间变化特征,初步估算了该水道的潮流能的理论蕴藏量,并采用FLUX方法对该水道的技术可开发量进行了评估。结果表明,琼州海峡中心海域功率密度高,两岸资源低;可能最大流速、大潮年平均最大功率密度、小潮年平均功率密度和年平均功率密度等特征值分布基本相似;其丰富区域出现在海峡东口南部海域以及海峡中部海域,其中东口南部海域可能最大流速可达4.6 m/s,表层流大潮年平均最大功率密度为5996 W/m²,小潮平均最大功率密度仅为467 W/m²,年平均功率密度为819 W/m²,代表点超过0.7 m/s的潮流流速年统计时间约为4717 h;海峡潮流能资源理论蕴藏量为189.55MW,利用FLUX、FARM、GC方法得到该水道的潮流能可开发量分别为249GW/yr、20.2GW/yr和263GW/yr。     

黑潮延伸体上游中尺度涡场的年代际振荡及其相关机制

黑潮延伸体上游区域的中尺度涡场的涡动能和涡特征尺度存在显著地年代际振荡,和黑潮延伸体路径的年代际变化有很好的相关性。当黑潮延伸体路径比较稳定时,其上游区域涡动能比较高,涡特征尺度比较大,反之相反。通过对黑潮延伸体上游区域的中尺度涡场进行集合分析发现:当黑潮延伸体处于稳定状态时,上游涡场几乎是各向均匀地,有轻微的径向伸长;而当黑潮延伸体处于不稳定状态时,上游的中尺度涡场有显著地纬向伸长。对与中尺度涡场的产生相关的线性斜压不稳定和正压不稳定进行了计算分析,结果显示,线性斜压不稳定不是控制中尺度涡场年代际变化的机制,而正压不稳定对中尺度涡场的年代际变化有积极的贡献。不稳定产生的中尺度涡之间存在非线性涡-涡相互作用。     

南大洋太平洋扇区中尺度涡旋的统计特性及其变化

中尺度涡旋在南大洋海洋动力学中具有重要地位,其对气候变化的响应表现也引起了海洋学家与气候学家的广泛关注。本文利用涡动动能与涡旋自动探测技术两种方法对南大洋太平洋扇区的涡旋特性及其变化进行了分析。与前人结果相一致的是,高值的涡动动能主要集中在南极极锋海区,并且自西向东逐渐减弱。在过去的20年里,涡动动能在太平洋扇区的显著增强也集中在中西部海域,这里也是南极绕极流斜压性较强的海域。涡旋统计特性揭示了涡动动能的空间分布及其年际变化主要归因于涡旋振幅与旋转速度,而并非涡旋个数或者涡旋半径。这些结果进一步确认了对应于南半球环状模正位相的绕极西风异常改变了南大洋的涡旋特性,从而表现出涡旋活跃性增强。