On the influence of changes in the drag relation on surface wind speeds and storm surge forecasts

In this study it is investigated how uncertainties in the magnitude of the drag coefficient translate into uncertainties in storm surge forecasts in the case of severe weather. A storm surge model is used with wind stress data from a numerical weather prediction (NWP) model, to simulate several recent storms over the North Sea. For a fixed wind speed, the wind stress is linear in the drag coefficient. However, in the NWP model the wind speed is not fixed and increasing the drag in the NWP model results into reduced wind speeds. The results from simulations show that for given increase in the drag coefficient, the weakening of the 10-m wind field reduces the increase in the stress considerably. When the Charnock parameter is increased in the NWP model, the resulting relative changes in the wind stress are almost independent of the wind speed. This is related to the fact that the depth of the surface boundary layer depends on the wind speed. The ratio between relative changes in the wind stress and relative changes in the drag coefficient depends on the wind speed. For 10-m wind speeds larger than 20?m?s^?1 the ratio is 0.52; for lower wind speed criteria the ratio is somewhat larger (??0.60). Approximately 36% of the relative change in the drag coefficient translates into a relative change in the surge in stations at the Dutch coast. The relative increase in the storm surge is approximately 68% of the relative increase in the stress.     

寒区灌丛与积雪关系研究进展

在气候变化的背景下, 寒区灌丛与积雪的相互关系成为寒区水文循环研究的重要环节. 综述近几十年来寒区灌丛-积雪相互关系的国内外研究现状, 并对未来研究提出了展望. 寒区灌丛过去几十年来覆盖面积和生物量等呈现增加趋势, 灌丛的增加可截留积雪, 改变积雪重分布, 影响积雪消融过程; 积雪可增加灌丛区地温, 制约灌丛区融雪时空变化过程, 影响寒区灌丛的生理生态过程. 灌丛与积雪同为寒区自然生态系统和环境的重要组成部分, 二者相互作用使地面太阳辐射和地表水分分配过程复杂化, 从而间接地影响寒区冻土环境变化. 最后, 指出了未来研究需要重点关注的几个问题: 寒区灌丛区积雪分布的精确估计; 灌丛-积雪-冻土连续体的研究; 耦合灌丛-积雪作用的寒区水文模型的构建.     

北方浅水湖泊冬季结冰对风生流的影响

为了研究北方浅水湖泊冬季结冰对风生流的影响,采用MIKE21构建山东省聊城市东昌湖水动力学模型,分析真实风场作用下6种风应力拖曳系数对应的模拟流速与实测数据的差异,进一步讨论了冰盖面积对模拟结果产生的影响。结果表明,风应力拖曳系数为随风速连续变化比设置为常数模拟精度提高20%左右,其中采用微风条件下的风应力拖曳系数表达式模拟效果最好。此外,冬季水体结冰对风生流数值模拟影响较大,尤其是在被冰盖所覆盖的水域,风应力对水体流动的作用减弱甚至被抵消。与传统的风生流模拟相比,在北方浅水湖泊冬季风生流模拟中,引入冰场或是对风应力拖曳系数进行相应调整是有必要的。     

大型浅水湖泊水动力模型不确定性和敏感性分析

选取国内外常用的水动力学模型(EFDC)和典型的浅水湖泊(太湖),采用拉丁超立方取样(LHS),研究湖泊水动力模块中常用的5个重要参数(风拖曳系数、床面粗糙高度、涡粘性系数、紊流扩散系数以及风遮挡系数)对湖体水位和流速的影响。结果表明:针对大型浅水湖泊,湖泊岸线形状和湖底地形、湖泊周围地形、湖泊水面风场对模拟结果产生决定性影响。尤其是在湖湾区和周边地形比较复杂的地区,风场参数对水动力模拟结果不确定性的贡献率最大。在垂向上,表层流速受到参数不确定性的影响最大,底层次之,中层最小。床面粗糙高度对水动力模拟结果不确定性贡献率较风场参数要小,水体涡粘滞系数和扩散系数影响则更小。故在选择大型浅水湖泊水动力模型参数时,要充分考虑湖泊岸线和周围地形,着重率定风场参数以及床面粗糙高度。     

水流拖曳力对斜坡浅层土稳定性的影响分析

为研究强降雨条件下地表径流和裂隙水流的拖曳力作用对斜坡稳定性的影响,建立斜坡地表径流和地下水渗流耦合分析模型。采用Navier-Stokes方程描述地表径流和裂隙水流,Brinkman-extended Darcy方程描述土体和岩石中的渗流,并根据不同介质交界面处流速相等及剪应力连续的边界条件,推求各介质中的流速分布。应用Newton内摩擦定律求得水流对斜坡产生的拖曳力,进而将拖曳力嵌入刚体极限平衡理论对斜坡稳定性进行分析。典型斜坡实例计算结果表明,当未考虑拖曳力效应时,斜坡的安全系数为1.164,考虑水流对斜坡土体产生拖曳力效应时,斜坡的安全系数为1.089,斜坡安全系数降低了6.44%。这表明拖曳力作用对斜坡的稳定性存在不利影响,斜坡土体在近乎临界稳定状态下拖曳力将对斜坡失稳起到决定性作用。最后分别讨论了考虑拖曳力和不考虑拖曳力情况下斜坡的安全系数与径流水深、斜坡倾角、土层厚度之间的关系。分析表明,在其他条件相似的条件下,斜坡越陡,斜坡土体的安全系数下降越明显;斜坡安全系数随着斜坡倾角增加而不断降低,随着土层厚度的增加而不断降低。     

植被作用下土壤抗剪强度和径流侵蚀力的耦合效应

利用野外径流小区动态监测和人工模拟降雨试验,阐明了草被和灌木的减流减沙效应,从力学层面揭示了坡面侵蚀产沙的过程机理。结果表明,与裸地相比,野外坡面草地和灌木地径流量分别减少28.1%~56.5%和85.7%~100%、产沙量分别减少84.9%~90.7%和98.5%~100%;在人工模拟降雨强度下,草地和灌木地径流量分别减少51.9%~90.9%和61.7%~80.6%、产沙量分别减少93.6%~99.2%和95.5%~99.2%;植被具有明显的增强土壤抗剪强度的作用,不同植被坡面抗剪强度与剪切面上的法向压力成正比,且符合库仑定律;不同下垫面条件下土壤黏聚力与坡面径流量和侵蚀产沙量呈显著的负相关关系,随着黏聚力的增大,径流量和侵蚀产沙量呈下降趋势;草地和灌木地坡面侵蚀临界径流切应力分别为裸地的2.64~3.16倍和2.44~3.18倍,建立了不同被覆坡面临界径流切应力与土壤抗剪强度和黏聚力的关系。研究结果对定量评价植被减蚀作用和深化土壤侵蚀力学过程有一定的参考意义。     

A simple model for the prediction of the deflation threshold shear velocity of dry loose particles

A very important parameter in aeolian equations is the deflation threshold shear velocity, which quantifies the instant of particle motion. In this paper, a simple model is presented for the prediction of the threshold shear velocity of dry loose particles. It has the same functional form as the widely used models of Bagnold (1941) and Greeley & Iversen (1985), but differs in its treatment of the so‐called threshold parameter. As the new expression was based on the moment balance equation used by Greeley & Iversen, it includes a function for the aerodynamic forces, including the drag force, the lift force and the aerodynamic moment force, and a function for the interparticle forces. The effect of gravitation is incorporated in both functions. However, rather than using an implicit function for the effect of the aerodynamic forces as in the Greeley & Iversen model, a constant aerodynamic coefficient was introduced. From consideration of the van der Waals' force between two particles, it was also shown that the function for the interparticle cohesion force is inversely proportional to the particle diameter squared. The model was calibrated on data reported by Iversen & White (1982). The new expression compared, at least for terrestrial conditions, very well with the Greeley & Iversen model, although it is much simpler. It was finally validated with data from wind‐tunnel experiments on different fractions of dune sand and sandy loam soil aggregates. The soil aggregates were treated as individual particles with a density equal to their bulk density. The good agreement between observations and predictions means that, when predicting mass transport of particles above a given soil, minimally dispersed particle‐size distributions should be considered rather than the granulometric composition of the soil.     

考虑钙质砂细观颗粒形状影响的液体拖曳力系数试验

钙质砂作为南海岛礁填筑常用的岩土材料,其渗透性很大程度上决定着填筑后土体的固结和沉降。拖曳力系数是表达流体对土体颗粒表面力的参数,也是表征颗粒状土体渗透能力的一个重要参数,目前国内外对钙质砂拖曳力系数的研究十分有限。首先引入一个修正的三维参数 对钙质砂这种天然非规则颗粒材料的形状进行定量描述,然后开展一系列单个钙质砂颗粒在液体中沉降试验,利用高速相机记录颗粒沉降过程,结合图像处理技术获得颗粒沉降平衡速度Ut,进而计算出拖曳力系数CD和雷诺数Re,最后拟合出包含CD、Re及 三个参数的钙质砂拖曳力系数半经验模型。结果发现,在相同雷诺数条件下钙质砂的形状系数 越大,拖曳力系数越小。通过与其他研究结果对比发现,其表面微孔隙越发育,拖曳力系数越小的规律。该模型能够考虑不规则颗粒形状对拖曳力系数的影响,从而提高对土体渗透性预测的精度,对南海岛礁填筑工程中钙质砂固结和沉降的计算也具有重要意义。     

塔克拉玛干沙漠南缘柽柳冠下枯枝落叶沉积特征

柽柳是分布广泛的耐盐耐旱植物,温带干旱区重要物种。观测证明柽柳冠下有机凋落物的沉积作用是显著的,具有有机碳沉积埋藏的重要意义。有机质凋落沉积量与柽柳当年生长量关系不密切,但受当地水分条件、气温变化和风沙作用影响显著。在策勒绿洲前缘,柽柳冠下有机质凋落物年沉积量达到400g/m2。当多枝柽柳冠丛受到损伤或生长受到胁迫时,沉积明显增加,年生长量与凋落量之比(回归量)﹥40%; 在时间序列上,有机凋落物与无机碎屑的主要沉积过程是明显分开的,有机凋落物沉积主要发生在冬季,占年沉积量的75%以上,而同期无机碎屑沉积量不足5%。沉积交替过程对有机凋落物形成了良好的埋藏保存作用; 在空间范围上,有机凋落物并不是在冠下均匀或对称分布的。在风的影响下,冠丛下风侧有机凋落物被显著集中。冠丛丘的顶部只是无机碎屑的沉积中心区而并非有机凋落物。     

拖曳锚与安装缆绳相互作用力计算方法研究

深水系泊系统及其系泊基础的研发已成为国际海洋工程领域的前沿和关键课题,拖曳锚因其在承载力和深水安装中的诸多优势而具有良好的发展前景。在拖曳锚安装过程中,锚板与安装缆绳在系缆点处发生复杂的相互作用,分析二者之间的相互作用可等价为分析系缆点拖曳力的大小,研究拖曳锚与安装缆绳相互作用力的计算方法对认识锚板在海床土中的受力以及确定拖曳锚安装船的吨位具有重要指导意义。分别基于嵌入缆绳和拖曳锚在海床土中的力学模型,首次推导出适用于黏性土和无黏性土的系缆点拖曳力表达式,并开展模型试验验证,对比两类拖曳力计算方法的精度和稳定性,结果表明,两类计算方法均能合理预测系缆点拖曳力,但基于拖曳锚受力模型获得的表达式更为精确和稳定;通过参数考察,探究系缆点拖曳力预测模型中各参数的影响效应;经过与试验对比,给出关键参数的建议取值,该取值对研究结构与土体的相互作用问题具有重要参考价值。     

The influence of wind speed on surface layer stability and turbulent fluxes over southern Indian peninsula station

Surface to atmosphere exchange has received much attention in numerical weather prediction models. This exchange is defined by turbulent parameters such as frictional velocity, drag coefficient and heat fluxes, which have to be derived experimentally from high-frequency observations. High-frequency measurements of wind speed, air temperature and water vapour mixing ratio (eddy covariance measurements), were made during the Integrated Ground Observation Campaign (IGOC) of Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) at Mahabubnagar, India (16^°44^′N, 77^°59^′E) in the south-west monsoon season. Using these observations, an attempt was made to investigate the behaviour of the turbulent parameters, mentioned above, with respect to wind speed. We found that the surface layer stability derived from the Monin–Obukhov length scale, is well depicted by the magnitude of wind speed, i.e., the atmospheric boundary layer was under unstable regime for wind speeds >4 m s^?1; under stable regime for wind speeds <2 m s^?1 and under neutral regime for wind speeds in the range of 2–3 m s^?1. All the three stability regimes were mixed for wind speeds 3–4 m s^?1. The drag coefficient shows scatter variation with wind speed in stable as well as unstable conditions.     

Research Advance of Shrub Dendrochronology in the Cold and Arid Regions of China

Along with the booming of dendrochronology in China, the woody species for the tree-ring study have expanded gradually from tree species to the shrub and dwarf shrub species in the last decades. The zonal woody species in the vast alpine mountains, arid desert areas and arid regions in China are mostly shrubs and semi-shrubs, which is very important to understand the process of regional evolution, environmental protection and ecological recovery. In this paper, the shrub species which have been studied on tree rings in cold and arid areas of China were collected and sorted, and the fundamental research advances were presented, which include the shrub tree ring identification, radial growth characteristics, and chronology construction by different parameters. The applications of shrub dendrochronology to the subjects in eco-response, paleoclimate reconstruction, hydrological process, ecological study of artificial forest and ecological restoration were also presented. The prospect of shrub dendrochronology in the future was also discussed.     

Parameterization of sea surface drag under varying sea state and its dependence on wave age

Momentum and energy exchange at air–sea interface through wind stress is very important for air–sea interaction studies, ocean modeling, and climate studies. The accurate representation of wind stress, in terms of drag coefficient, is a key factor in estimating the momentum transfer at the interface. The drag coefficient, in general, estimated using bulk formulae does not take into account the influence of wave age. This study examines the dependence of wave age on computed surface drag coefficient obtained by combining the Toba 3/2-power law with Froude number scaling, resulting in a new drag formulation (hereafter referred as RP formulation). We demonstrate that our proposed formulation is in good conjunction with established theories for both young and mature waves. Our investigation shows the theoretical formulation advocated earlier by Guan and Xie (hereafter referred as GX) overestimated the surface drag for mature waves as wind speed tends to increase. In addition, the formulation by GX was not verified by observational data. In the present work, for validation purpose, we use time series measurement of meteorological and oceanographic data from a deep water location in the Indian Ocean which was tested with both RP and GX formulations. We find that the proposed RP formulation, which embeds the 3/2 power of wave-age, shows a better match for both young and mature waves with the results of Janssen compared to the hypothesis of conventional wave age used by GX.     

On the effect of moisture bonding forces in air-dry soils on threshold friction velocity of wind erosion

Wind erosion is a dominant geomorphological process in arid and semi-arid regions with major impacts on regional climate and desertification. The erosion process occurs when the wind speed exceeds a certain threshold value, which depends on a number of factors including surface soil moisture. The understanding and modelling of aeolian erosion requires a better understanding of the soil erodibility associated with different moisture conditions. In arid regions during the dry season, the atmospheric humidity plays an important role in determining the surface moisture content and the threshold shear velocity. By a series of wind tunnel tests and theoretical analyses, this dependence of threshold velocity on near surface air humidity is shown for three soils of different textures: sand, sandy loam, and clay loam. The results show that the threshold shear velocity decreases with increasing values of relative humidity for values of relative humidity between about 40% and 65%, while above and below this range the threshold shear velocity increases with air humidity. A theoretical framework is developed to explain these dependencies assuming an equilibrium between the surface soil moisture and the humidity of the overlying atmosphere. The conditions under which soil-atmosphere equilibrium occurs were tested experimentally in the laboratory for different soils in order to determine the effect of grain surface area and texture on the time required to reach equilibrium starting from different initial conditions.     

The initiation of particle movement by wind

When air blows across the surface of dry, loose sand, a critical shear velocity (fluid threshold, ut), must be achieved to initiate motion. However, since most natural sediments consist of a range of grain sizes, fluid threshold for any sediment cannot be defined by a finite value but should be viewed as a threshold range which is a function of the size, shape, sorting and packing of the surface sediment. In order to investigate the initiation of particle movement by wind a series of wind-tunnel tests was carried out on a range of pre-screened fluvial sands and commercially available glass beads with differing mean sizes and sorting characteristics. A sensitive laser-monitoring system was used in conjunction with a high speed counter to detect initial grain motion and to count individual grain movements. Test results indicate that when velocity is slowly increased over the sediment surface the smaller or more exposed grains are first entrained by the fluid drag and lift forces either in surface creep (rolling) or in saltation (bouncing or hopping downwind). As velocity continues to rise, larger or less exposed grains may also be moved by fluid drag. On striking the surface saltating grains impart momentum to stationary grains. This impact may result in the rebound of the original grain as well as the ejection of one or more stationary grains into the air stream at shear velocities lower than that required to entrain a stationary particle by direct fluid pressure. As a result, there is a cascade effect with a few grains of varying size initially moving over a range of shear velocities (the fluid threshold range) and setting in motion a rapidly increasing number of grains. Results of the tests showed that the progression from fluid to dynamic threshold, based on grain movement, can be characterized by a power function, the coefficients of which are directly related to the mean size and sorting characteristics of the sediment.     

Simulations of Dynamical Gas–Dust Circumstellar Disks: Going Beyond the Epstein Regime

Abstract—In circumstellar disks, the size of dust particles varies from submicron to several centimeters, while planetesimals have sizes of hundreds of kilometers. Therefore, various regimes for the aerodynamic drag between solid bodies and gas can be realized in these disks, depending on the grain sizes and velocities: Epstein, Stokes, and Newton, as well as transitional regimes between them. This means that simulations of the dynamics of gas–dust disks require the use of a drag coefficient that is applicable for a wide range for sizes and velocities for the bodies. Furthermore, the need to compute the dynamics of bodies of different sizes in the same way imposes high demands on the numerical method used to find the solution. For example, in the Epstein and Stokes regimes, the force of friction depends linearly on the relative velocity between the gas and bodies, while this dependence is non-linear in the transitional and Newton regimes. On the other hand, for small bodies moving in the Epstein regime, the time required to establish the constant relative velocity between the gas and bodies can be much less than the dynamical time scale for the problem—the time for the rotation of the disk about the central body. In addition, the dust may be concentrated in individual regions of the disk, making it necessary to take into account the transfer of momentum between the dust and gas. It is shown that, for a system of equations for gas and monodisperse dust, a semi-implicit first-order approximation scheme in time in which the interphase interaction is calculated implicitly, while other forces, such as the pressure gradient and gravity are calculated explicitly, is suitable for stiff problems with intense interphase interactions and for computations of the drag in non-linear regimes. The piecewise drag coefficient widely used in astrophysical simulations has a discontinuity at some values of the Mach and Knudsen numbers that are realized in a circumstellar disk. A continuous drag coefficient is presented, which corresponds to experimental dependences obtained for various drag regimes.     

草原农垦区土地沙质荒漠化过程的生物学机制

以沙质荒漠化正处在发展过程的河北坝上地区和沙质荒漠化严重的科尔沁沙地流沙治理区为例,结合国内外已有的研究成果,探讨草原沙质荒漠化的主要生物学机制。指出在草原农垦区开垦和过度放牧引起的土壤细颗粒物质、养分损失,促进了灌木侵入。灌木侵入又加剧了土壤养分、水分空间格局的改变,在灌丛周围形成一个包括微生物在内的土壤肥力富集区,即灌丛"土壤肥岛",而在丛间发生土壤风蚀和水蚀,导致斑块状流沙甚至大面积流沙的出现。诱发荒漠化的不合理耕作、灌溉、盐渍化、过度放牧等过程同样影响菌根菌的密度和接种成功率,从而降低草地生产力。在草原地区的流沙上建立各种固沙植被不仅使流沙地土壤养分、水分异质性加强,而且使沙地景观异质性增加。还提出用土壤颗粒分形维数来表征土地沙质荒漠化的程度,以空间上不同样点土壤属性变异系数（方差与平均值之比）作为沙质荒漠化土壤属性异质性的测度。     

Breaking pattern and critical breaking condition of Japanese pine trees on coastal sand dunes in huge tsunami caused by Great East Japan Earthquake

Coastal vegetation is widely recognized to reduce tsunami damage to people and buildings, and it has been studied recently because it requires relatively little capital investment compared with artificial measures, provides human-friendly beach fronts, and enhances inter-relationships with other ecological systems. However, the tsunami caused by the Great East Japan Earthquake at 14:46 JST on March 11, 2011, with a magnitude of 9.0 and epicenter 129?km east of Sendai, broke most of the sea wall (tsunami gates, large embankments) and caused catastrophic damage to coastal forests in the Tohoku and Kanto districts of Japan. A field survey was conducted to elucidate the critical breaking condition of Japanese coastal pine trees. Tree-trunk breakage was observed when the sea embankment was washed out or when there was no sea embankment and the tree was under strong inertia force or impact force by debris. Even though the trunk bending and breaking phenomena are different, statistical analysis showed that the critical diameters for trunk bending and trunk breaking were not very different. The overturning phenomenon is a little more complex than trunk breaking because the resistive force is a function of the substrate and root anchorage. An equation to determine the critical diameters for trunk bending, trunk breaking, and overturning was derived as a function of tsunami water depth, soil-root strength, and the hydrodynamic parameter (H _D ) formulated by Froude number, drag coefficient, and the ratio of impact force to drag force considering the physical mechanisms to resist the tsunami. Trunk bending and breaking were closely related to tsunami water depth and the hydrodynamic parameter (H _D ), but tree overturning was found to be more site specific, and the root-soil strength greatly affected the critical value. The proposed critical diameter equation and its coefficient are useful for the design of an inland forest of pine trees that can trap large trees, cars, debris, etc., to its breaking limit. The trapping function should be utilized more in the future designs of inland forests, if possible, on embankments.     

Unsteady flow over offshore wind turbine airfoils and aerodynamic loads with computational fluid dynamic simulations

The first notable megawatt class wind turbine, which was the pioneer of improvement in the blade performance in large wind turbines, appeared in Vermont. Nowadays, modern wind turbines are using blades with multi-airfoils at different sections. In this study, in order to indicate the best airfoil profile for the optimum performance in different sections of a blade, five popular airfoils, including S8xx, FFA and AH series, were studied. On the large-scale profile, shear stress transport K–ω model was applied for the simulation of horizontal axis wind turbines for different wind speeds. The aerodynamic simulation was accomplished using computational fluid dynamic method, which in turn is based on the finite volume method, and semi-implicit method for pressure-linked equations algorithm is used for pressure–velocity coupling. The governing equations applied in this simulation are the unsteady Reynolds-averaged Navier–Stokes equations. The aerodynamic coefficients of lift and drag were calculated at different angle of attacks and different wind speeds. The results were validated by EPPLER code, XFOIL and experimental data of the US National Renewable Energy Laboratory. The results showed that S818 profile is the best profile in terms of gaining the highest lift coefficient with the lowest angle of attack at the root of the blades. The findings also indicated that the selected model can predict the exact geometry with a high precision.     

喀斯特地区不同坡位条件下优势灌木根际与非根际土壤养分与pH的分布特征

对广西喀斯特地区2种优势灌丛植物子凌蒲桃与火棘在不同坡位条件下根际与非根际土壤养分与pH值的分布特征进行研究,揭示植物种类及其立地条件对植物根际养分与pH值分布特征的影响。结果表明,2种植物根际养分存在一定程度的富集现象,其中,火棘根际有机碳与速效磷含量富集明显,子凌蒲桃根际有机碳与全氮含量富集明显;根际与非根际土壤养分含量在不同坡位下存在显著性差异（p<0.05）;根际土壤有机碳含量与全氮含量呈显著正相关（p<0.05）,与速效磷含量呈显著负相关（p<0.05）;非根际土壤有机碳含量与速效磷、全氮含量呈显著正相关（p<0.05）。双因素方差分析显示,与植物种类相比,坡位对根际土壤养分含量影响更大。以上的研究表明,喀斯特灌丛优势植物有利于提高该地区土壤养分利用,但应该考虑植物立地条件对根际养分利用的影响。