Dependence of sea surface drag coefficient on wind-wave parameters

The relationships between sea surface roughness z 0 and wind-wave parameters are analyzed,and spurious self-correlations are found in all of the parameterization schemes.Sea surface drag coefficient C D is fitted by four wind-wave parameters that are wave age,wave steepness,windsea Reynolds number R B and R H ,and the analyzed data are divided into laboratory,field and combined data sets respectively.Comparison and analysis of dependence of C D on wind-wave parameters show that R B can fit the C D most appropriately.Wave age and wave steepness are not suitable to fit C D with a narrow range data set.When the value of wave age has a board range,R H is not suitable to fit C D either.Three relationships between C D and R B are integrated into the bulk algorithm COARE to calculate the observational friction velocity,and the results show that the relationship between C D and R B which is fitted with field data set can describe the momentum transfer in the open ocean,under low-moderate wind speed condition,most appropriately.

Impacts of Wave and Current on Drag Coefficient and Wind Stress over the Tropical and Northern Pacific

By taking into consideration the effects of ocean surface wave-induced Stokes drift velocity Uw and current velocity Uc on the drag coefficient,the spatial distributions of drag coefficient and wind stress in 2004 are computed over the tropical and northern Pacific using an empirical drag coefficient parameterization formula based on wave steepness and wind speed. The global ocean current field is generated from the Hybrid Coordinate Ocean Model (HYCOM) and the wave data are generated from Wave-watch Ⅲ (WW3). The spatial variability of the drag coefficient and wind stress is analyzed. Preliminary results indicate that the ocean surface Stokes drift velocity and current velocity exert an important influence on the wind stress. The results also show that consideration of the effects of the ocean surface Stokes drift velocity and current velocity on the wind stress can significantly im-prove the modeling of ocean circulation and air-sea interaction processes.

Observation and Numerical Experiments for Drag Coefficient Under Typhoon Wind Forcing

This paper presents a study on drag coefficients under typhoon wind forcing based on observations and numerical experiments. The friction velocity and wind speed are measured at a marine observation platform in the South China Sea. Three typhoons: SOULIK(2013), TRAMI(2013) and FITOW(2013) are observed at a buoy station in the northeast sea area of Pingtan Island. A new parameterization is formulated for the wind drag coefficient as a function of wind speed. It is found that the drag coefficient(Cd) increases linearly with the slope of 0.083′10^(-3) for wind speed less than 24 m s^(-1). To investigate the drag coefficient under higher wind conditions, three numerical experiments are implemented for these three typhoons using SWAN wave model. The wind input data are objective reanalysis datasets, which are assimilated with many sources and provided every six hours with the resolution of 0.125?×0.125?. The numerical simulation results show a good agreement with wave observation data under typhoon wind forcing. The results indicate that the drag coefficient levels off with the linear slope of 0.012′10^(-3) for higher wind speeds(less than 34 m s^(-1)) and the new parameterization improvese the simulation accuracy compared with the Wu(1982) default used in SWAN.

Wind tunnel experiment of drag of isolated tree models in surface boundary layer

For very sparse tree land individual tree was the basic element of interaction between atmosphere and the surface. Drag of isolated tree was preliminary aerodynamic index for analyzing the atmospheric boundary layer of this kind of surface. A simple pendulum method was designed and carried out in wind tunnel to measure drag of isolated tree models according to balance law of moment of force. The method was easy to conduct and with small error The results showed that the drag and drag coefficient of isolated tree increased with decreasing of its permeability or porosity. Relationship between drag coefficient and permeability of isolated tree empirically was expressed by quadric curve.

Experiment about Drag Reduction of Bionic Non-smooth Surface in Low Speed Wind Tunnel

The body surface of some organisms has non-smooth structure, which is related to drag reduction in moving fluid. To imitate these structures, models with a non-smooth surface were made. In order to find a relationship between drag reduction and the non-smooth surface, an orthogonal design test was employed in a low speed wind tunnel. Six factors likely to influence drag reduction were considered, and each factor tested at three levels. The six factors were the configuration, diameter/bottom width, height/depth, distribution, the arrangement of the rough structures on the experimental model and the wind speed. It was shown that the non-smooth surface causes drag reduction and the distribution of non-smooth structures on the model, and wind speed, are the predominant factors affecting drag reduction. Using analysis of variance, the optimal combination and levels were obtained, which were a wind speed of 44 m/s, distribution of the non-smooth structure on the tail of the experimental model, the configuration of riblets, diameter/bottom width of i mm, height/depth of 0.5 mm, arranged in a rhombic formation. At the optimal combination mentioned above, the 99% confidence interval for drag reduction was 11.13% to 22.30%.

Analytical investigation on 3D non-Boussinesq mountain wave drag for wind profiles with vertical variations

A new analytical model was developed to predict the gravity wave drag （GWD） induced by an isolated 3-dimensional mountain, over which a stratified, nonrotating non-Boussinesq sheared flow is impinged. The model is confined to small amplitude motion and assumes the ambient velocity varying slowly with height. The modified Taylor-Goldstein equation with variable coefficients is solved with a Wentzel-Kramers-Brillouin （WKB） approximation, formally valid at high Richardson numbers. With this WKB solution, generic formulae of second order accuracy, for the GWD and surface pressure perturbation （both for hydrostatic and non-hydrostatic flow） are presented, enabling a rigorous treatment on the effects by vertical variations in wind profiles. In an ideal test to the circular bell-shaped mountain, itwas found that when the wind is linearly sheared, that the GWD decreases as the Richardson number decreases. However, the GWD for a forward sheared wind （wind increases with height） decreases always faster than that for the backward sheared wind （wind deceases with height）. This difference is evident whenever the model is hydrostatic or not.

某SUV气动减阻优化及其流场机制

为了提高整车燃油经济性,本文以某款SUV车型为研究对象,将仿真与试验相结合改善汽车行驶过程中的气动阻力系数。首先通过风洞试验确定对整车气动阻力有重要影响的区域或部件,其次对气动阻力系数贡献值较大的部件或区域进行减阻优化。结果表明,前轮阻风板、尾灯和尾翼对整车气动阻力系数贡献值较大。对前轮阻风板的改型,有效降低正压区面积以及减弱车轮干扰阻力;对尾灯和尾翼的优化设计,改善了尾部负压区,缩短了分离流在后窗上部的再附着的距离。基于本征正交分解方法进行局部流场信息的提取和分析可知,1阶与2阶模态主要构成了该SUV尾流场的关键流态。经试验与仿真验证,相比于初始方案,气动优化组合设计减阻率可达7.5%。本文为新一代SUV改型与升级换代提供了理论基础与技术支持。

基于节段模型测力方法的塔式起重机风荷载特性研究

塔式起重机被广泛应用于铁道工程和土木工程施工过程,属于高耸结构体系,风荷载是塔式起重机结构设计的控制性荷载。为完善《塔式起重机设计规范》(GB/T 13752—2017)和《建筑结构荷载规范》(GB 50009—2012)中的塔式起重机斜风向设计风荷载计算方法,解决风向、吊臂、平衡臂位置等因素对塔式起重机遮挡和干扰引起的设计风荷载计算问题。选取平头塔式起重机为研究对象,对塔身、吊臂、平衡臂进行多工况的节段模型测力风洞试验,研究湍流强度、节段干扰和遮挡等因素对塔式起重机节段风荷载特性的影响规律,研究不同风向下节段阻力系数分布规律,对比标准化阻力系数与当前主要规范的异同,进而探讨节段斜风向标准化阻力系数计算方法。研究结果表明湍流强度对节段阻力系数影响较小,可忽略,而平衡臂和吊臂相邻节段的干扰会减小测试节段的阻力系数;在斜风向下,由于吊臂和平衡臂之间存在相邻节段干扰,国内外规范方法的计算值均会大于干扰工况的节段风洞试验值;当前主要国家规范规定的垂直风向塔身、吊臂、平衡臂阻力系数与风洞试验测得的阻力系数值较为接近,基于风洞试验结果给出典型风向的塔式起重机各节段的风荷载系数取值可用于结构设计;基于风洞试验测试结果推导的塔式起重机斜风向风荷载计算方法具有较好的拟合效果。研究成果可推荐相关工程应用,可为相关规范的修改或补充提供参考。

螺旋扭曲升阻复合型垂直轴风力机气动性能分析

提出了一种新型升阻复合型垂直轴风力机。阻力型风轮是由双层典型的萨沃纽斯型风轮组成,而升力型风轮是由螺旋扭曲一定角度的达里厄NACA0018翼型风轮组成。阻力型风轮在内,升力型风轮在外,二者用传动轴和离合器相连接。在低风速时由阻力型风轮带动升力型风轮旋转,高风速时二者各自旋转,互不影响。首先,基于计算流体力学(Computational fluid dynamics,CFD)软件对其启动及气动性能进行了仿真,分析了风轮表面的压力和风速分布。然后,分别对复合型风力机和单达里厄型风力机的启动风速和输出功率进行了测量。结果表明:复合型风力机的启动风速要小于单达里厄型的启动风速,而输出功率的差别并不大。最后,探讨了单达里厄风力机不同风速下叶尖速比与转矩系数和风能利用率的关系,确定了在叶尖速比为1.9时可将阻力型风轮和升力型风轮分离,在风速为8 m/s且叶尖速比为3时风力机的风能利用率最高,为40.6%。

边界层和压力滞后对翼型动态失速性能的影响

为优化动态失速模型经验常数,提升动态失速发生时翼型气动性能预测精度,该文基于B-L动态失速模型,结合内蒙古工业大学风能太阳能利用技术教育部重点实验室风洞实验数据,探究压力滞后及边界层滞后时间常数对翼型动态失速性能的影响。主要结论如下:压力滞后与边界层滞后时间常数对动态升力系数的影响较大且与平均攻角有关。当平均攻角相对较小且气流处于附着流动与分离流动之间时,适当减小时间常数可使动态失速模型计算结果更接近实验值;当平均攻角相对较大,气流处于分离流动与完全分离流动时,可适当增大时间常数值。压力滞后与边界层滞后时间常数对动态阻力系数的影响不显著。动态升力系数仅在攻角逐渐减小的完全分离流动过程中,随着边界层滞后时间常数的增大而减小。

仿生鲨鱼皮复合微纳减风阻结构的仿真与制备

仿生学与减阻技术的结合,为减阻开辟了重要的研究方向,在航空航天领域有着潜在的发展与应用前景。为提高降低风阻效果,本文对复合微纳减风阻结构进行了研究,基于仿生学原理,采用CFD仿真及激光微纳制造技术,建立了减阻结构组合模型,并在飞行器的大气传感器半球头体模型表面制造仿生鲨鱼皮复合微纳结构,即在仿生鲨鱼皮鳞片结构的基础上,通过激光干涉扫描二级微沟槽,以进一步提升减阻效果。采用CFD仿真与风洞实验相结合的方式,对减阻机理进行理论分析,完成了复合结构的微纳制造,减阻率最高可达10.3%。

迎风角对跳台滑雪运动员飞行气动性能影响的研究

以探究迎风角对跳台滑雪运动员与滑雪板组成的多体系统的飞行气动性能的影响为目的,采用CFD数值模拟方法,在风速为29 m/s情况下,研究了多体系统迎风角为15°、20°、25°、30°、35°、40°六种情况下的气动性能变化情况。并在保证跳台滑雪运动员姿态以及飞行速度不变的情况下,以多体系统的升阻比作为衡量标准,对迎风角对多体系统气动性能的影响情况进行了评估。研究得出:多体系统升阻比随迎风角的增大呈递减趋势,且不同迎风角之间多体系统升阻比差异较大。在多体系统迎风角取为15°时,整个多体系统的升阻比取得最大值,有利于运动员取得良好的气动性能。此外,迎风角对跳台滑雪运动员气动性能影响的研究可为运动员比赛技巧相关的训练提供一定的指导作用。

单叶风阀调节性能试验研究

文章主要研究了不同规格的单叶风阀在不同开度或风量下的调节性能。本文在保持风量不变的情况下,通过改变风阀的开度,测试了风管内部压力、风速、阻力等参数,以及在开度不变的情况下改变风量,测试了不同开度下风管内部压力、风速、阻力等参数。找到了不同工况下,风管内部气流从距离阀门6倍管径处开始稳定,以及风阀阻力、风速随风阀开度变化的规律基本呈“指数型”,风阀阻力随风量变化的规律基本呈“线性”,风阀调节对管路影响的不敏感区处于0°~45°,敏感区域处于45°~80°。以上结论在200×200 mm与300×300 mm规格的方形风管与阀门中均成立。

砂土中筒型基础临界吸力计算方法研究

开展砂土中筒型基础吸力沉贯的系列模型试验,测试筒型基础下沉过程中吸力的发挥过程,并通过反分析得出筒内外土体渗透系数比的变化规律。结果表明:当筒内土体出现管涌现象时,筒内负压会瞬间降低;随着沉贯深度的增加,临界吸力值也逐渐增大;此外,筒内外土体的渗透系数比呈指数函数型增长;假设在沉贯过程中土体的渗透系数不变,则临界吸力的预测结果均小于实测值。根据试验结果改进砂土中筒型基础沉贯过程临界吸力的计算公式,并进行工程案例验证。

某SUV底盘空气动力性能优化研究

汽车底盘空气动力性能优化对降低整车空气阻力和升力有重要影响,针对底盘区域进行空气动力性能优化设计是提升整车动力经济性的重要途径。针对某款运动型多用途汽车(Sport Utility Vehicle,SUV)开展整车外流场计算流体动力学(Computational Fluid Dynamics,CFD)仿真分析,对比了Realizable k-ε和SSTk-ω两种湍流模型仿真精度,选用Realizable k-ε湍流模型进行底盘空气动力学方案设计。结合该SUV底盘特征及仿真流场分析结果,在尽可能降低整车成本的前提下,设计前保险杠下部阻风板、前轮挡板斜梯、副车架后部导流板、底盘中部护板以及尾部消音器造型优化共5种空气动力学方案,并进行了实车风洞试验验证。结果表明,5种方案对整车空气阻力性能提升均有不同程度的贡献,其中尾部消音器造型优化减阻效果明显,实车空气阻力系数降低2.99%;综合采用5种底盘空气动力学方案后空气阻力系数共降低5.16%,升力系数降低21.00%,有效实现节能降阻,并有助于提高整车行驶稳定性。

bZ3纯电动轿车的空气动力性能开发

作为比亚迪丰田电动车科技有限公司第1款纯电动轿车,bZ3的能耗要求极高,也给空气动力学性能开发带来了很大的挑战。为实现这一目标,通过采用计算流体力学(Computational Fluid Dynamics,CFD)仿真和风洞试验相结合的方法,对车身造型、车底、前舱进气管理、车轮、密封等部位进行了持续的优化验证。最终bZ3在风洞中进行实车试验验收,空气阻力系数达到0.218,在同级别车型中处于领先水平。通过两种不同仿真方法对比研究发现格子玻尔兹曼(Lattice Boltzmann Method,LBM)方法整体精度较高,但对于底部气流的模拟精度还有待提升。

某无人机多点式起落架风洞试验研究

降低起落架阻力是提高飞机气动性能的关键技术之一。以某型无人机多点式起落架为研究对象,基于风洞试验技术对比分析前/后起落架的零升阻力系数,并提出改善起落架阻力特性的方法。试验结果表明,前起落架构型零升阻力系数最大,相比于干净构型,零升阻力系数增加81%,最大升阻比降低31%。取消前/后机轮作为对照组,获得机轮的零升阻力系数,研究发现前机轮零升阻力系数为0.014,后机轮零升阻力系数为0.001,后续可以给前起落架安装整流罩以降低阻力。

电力机车走行风相变散热器风洞试验研究

文章基于电力机车走行风相变散热器的结构模型,建立了其等效热阻网络,分析其传热特性,推导了风速影响下散热量的修正系数关系式;通过风洞试验,研究了散热器在不同风速、不同水流量下散热量和热阻的变化特性,以及散热器在不同风速下的热启动性能及阻力特性。结果表明:散热器散热量随风速及水流量的增大而不断增大,而散热器热阻则刚好相反;散热器静态启动时间和散热启动时间均小于4 min;单个散热器产生阻力仅为机车牵引力的0.17‰,对整车阻力影响很小。

尺度适应重力波拖曳方案在高分辨率数值预报中的应用研究

随着数值模式分辨率越来越高,在复杂地形区域仅考虑高层重力波拖曳作用和地形阻塞作用具有局限性,WRFv4.3模式中新型重力波拖曳方案在原有大尺度重力波拖曳和地形阻塞作用基础上新加入了边界层小尺度重力波拖曳作用,且对湍流地形拖曳作用增加了描述,从而可以达到尺度适应。为了验证新的重力波拖曳方案的效果,本文基于华北区域3 km对流可分辨模式开展了2021年冬季重力波拖曳参数化方案应用试验,对比分析了三组试验方案:不开启重力波拖曳、原有的仅考虑大尺度重力波拖曳和阻塞作用的方案以及新型的尺度适应方案的模拟结果。试验结果表明:原有重力波拖曳方案仅考虑重力波拖曳和大尺度地形阻塞,在3 km模式中对复杂地形附近整层风场影响较大,但是正效果有限;新型尺度适应重力波拖曳方案在3 km分辨率模式中大尺度重力波拖曳和阻塞拖曳作用为0,仅由边界层内小尺度重力波拖曳和湍流地形拖曳起作用,能够达到尺度适应效果;由于高层重力波拖曳为0,开启新型重力波拖曳会降低低空尤其是边界层内风场预报正偏差,对高空风场无影响;对模拟区域的统计检验结果表明,新型重力波拖曳方案可以有效减少地形复杂区域近地面风场预报的误差,但对温度等其它要素的改善有限。

Effects of gravel mulch on aeolian transport:a field wind tunnel simulation

The shape,size and coverage of gravels have significant impacts on aeolian sand transport.This study provided an understanding of aeolian transport over the gravel mulching surfaces at different wind velocities by means of a mobile wind tunnel simulation.The tested gravel coverage increased from 5% to 80%,with a progressive increment of 5%.The gravels used in the experiments have three sizes in diameter.Wind velocities were measured using 10 sand-proof pitot-static probes,and mean velocity fields were obtained and discussed.The results showed that mean velocity fields obtained over different gravel mulches were similar.The analysis of wind speed patterns revealed an inherent link between gravel mulches and mean airflow characteristics on the gravel surfaces.The optimal gravel coverage is considered to be the critical level above or below which aeolian transport characteristics differ strongly.According to the present study,the optimal gravel coverage was found to be around 30% or 40%.Threshold velocity linearly increased with gravel coverage.Sand transport rate first increased with height above the wind tunnel floor（Hf）,reaching a peak at some midpoint,and then decreased.