Requirements for modeling of a snowdrift

In an experiment of a model snowdrift formed behind each of two model snow fences, which are the same in shape but different in size, the independence of the wind speed ratio between them from their length ratio was observed. This phenomenon can be explained not by the Froude number, but by the effect of wakes behind an obstruction and movement characteristics of snow particles. The wind speed ratio and the duration of a storm in which the model snowdrifts of glass beads and activated clay particles become similar to each other, were examined. It follows from the result that the friction speed ratio is equal to the ratio of the threshold friction speed of both model snow particles, and the ratio of storm duration can be calculated by comparing the volume of both model snowdrifts. This result suggests strongly that the same relationships hold between the model and the prototype snowdrift.     

Snow deflector built at the edge of a road cut

This paper describes experiments with a solid snow fence for snowdrift prevention on a road cut.Solid fence has seldom been used for prevention of snowdrifts because the fence soon becomes saturated and ineffective. However, a modeling experiment with a solid fence built at the shoulder edge of a road cut indicated the effect of the fence on blowing snow particles upward. Therefore, it was easily predicted that the effect could prevent the snowdrift formation on a full-scale road cut. The result of the field experiment using the same type of snow fence showed the same effect as shown in the modeling experiment.     

风致积雪漂移堆积效应的研究进展

屋盖表面的雪颗粒在风力作用下发生复杂的漂移堆积运动,对建筑结构、交通、环境等方面造成很大的影响。雪粒在风力作用的运动形式包含蠕移、跃移、悬移运动以及雪粒与地表的碰撞运动。从实地观测、风洞试验和数值方法等方面详细评述了风致积雪的漂移堆积效应研究进展:通过实地观测细致分析了临界摩阻速度,并建立了计算雪流量的理论模型;风雪两相运动的风洞试验涉及相似参数多,该文根据前人经验总结了可以放松的相似参数,并指出了试验中的技术难点;数值研究方面一般基于Euler-Euler方法进行模拟,越来越多的因素在数值模拟中得到考虑。通过上述三方面的总结分析:指出了目前风雪研究中的不足之处,给出了今后研究的建议。     

Wet snow accretion on overhead lines with French report of experience

This article deals with the accretion mechanisms of wet snow observed both on a stranded conductor sample in wind tunnel conditions and on real power lines in natural weather conditions. It summarizes the theory and practical observations. Report of experience from France are given in order to demonstrate the efficiency of this passive preventive method based on the increase of torsional stiffness of conductors and to introduce wet snow risk map and countermeasures in France.     

不同护栏透风率下桥面雪飘移的数值模拟

采用计算流体动力学(Computational Fluid Dynamics, CFD)方法，对典型桥梁断面上的雪飘移进行了数值模拟，得到桥面上风致积雪的重分布。为验证该文数值模拟方法的正确性，以平屋面风吹雪为案例，将该文数值模拟方法得到的结果与风洞试验结果进行了对比。在桥面雪飘移的数值模拟过程中，考虑了桥梁护栏的影响，对比分析了不同护栏透风率下桥面风致积雪重分布形式。研究发现：当护栏透风率大于50%时，桥面上不会出现显著的积雪沉积；当护栏透风率小于50%时，桥面护栏附近出现了较显著的积雪沉积，且在迎风端护栏的背风侧沉积最大。为减小桥面风致积雪堆积对交通的不利影响，建议在桥梁设计时采用高透风率的护栏。     

拉格朗日方法在风雪运动模拟中的应用

采用拉格朗日方法,模拟空旷场地中雪颗粒从开始运动至整个流域达到平衡状态的过程。分析了雪颗粒的运动特性,结合文献中的击溅函数,追踪每个跃移雪颗粒的运动状态,同时考虑了跃移雪颗粒对风场的修正作用。考察了在风雪流发展过程中跃移雪颗粒的数目、质量传输率和摩擦速度的变化规律,以及流域达到平衡状态后跃移雪颗粒质量通量沿高度的分布。同时将数值模拟结果与文献中的实测结果进行了详细的对比,两者吻合 较好。     

拱形屋盖结构不平衡雪荷载的模拟研究

该文利用基于有限面元(Finite Area Element,FAE)方法对拱形屋面的不均匀积雪荷载进行了研究。首先设计制作了缩尺比为1∶100、矢跨比为0.125的拱形屋面风洞试验模型,通过风洞试验测得屋面平均风速大小,结合CFD数值模拟确定平均风速方向,再基于FAE方法利用风速-雪通量经验公式计算得到屋面的积雪分布系数;并分析了不同风速和不同风向角对屋面积雪分布系数及不平衡雪荷载的影响。其次,对基于FAE方法计算得到的积雪分布系数、基于两相流的CFD数值模拟结果以及我国现行《建筑结构荷载规范》GB 50009-2012的结果进行了比较。最后,总结了屋面雪荷载分布规律,给出了针对这类大跨结构雪荷载设计的一些建议。     

A numerical study of mechanisms of accretion for rotating snow sleeves evolving on conductor rods

It is the purpose of this paper to construct a mathematical/computational model for the accretion of snow on rotating snow sleeves attached to conductor rods, and to consider some of the limitations in the use of the assumption of perfect cylindrical-sleeve growth, as currently employed to provide estimates of snow-loading intensity. For purposes of illustration, three rotating snow accretion regimes are considered, in which the direction of the airflow is always taken as perpendicular to the axis of the conductor rod. Firstly, a horizontal conductor rod is free to rotate and is initially bare. Rotation of the system (conductor rod and accreted snow) is initiated by snow torque, due to eccentric snow loading on the windward side of the conductor, and is driven by aerodynamic torque generated by the windflow past the evolving, rotating, asymmetrical sleeve growth. Secondly, a vertical conductor rod is free to rotate and the bare rod is given an initial small angular velocity; the rotation of the accreting system is then driven by aerodynamic torque alone. Thirdly, a horizontal rod is fixed and is initially covered by an annulus of snow, supported by a thin layer of (unfrozen) water existing between the conductor and the snow sleeve. In this case, the rotation of the snow sleeve is due to the combined snow and aerodynamic torques and is now retarded by the viscous torque due to the rotating thin film of water. Asymptotic solutions obtained for small accretion time supplement numerical results derived from the computational models. Numerical results on these rotating non-periodic snow accretion regimes are presented as a function of the wind speed, snow precipitation rate and diameter of the conductor rod.     

积雪漂移对空间结构动力稳定性的影响

以一单层柱面网壳为例,以Budiansky-Roth准则为动力失稳判别准则,研究积雪漂移对风雪耦合作用下空间结构动力稳定性的影响。由风洞试验获得结构表面的风荷载时程,通过数值方法模拟风力下积雪在结构表面的漂移。结果表明,风力下网壳顶部的积雪发生侵蚀而迎风与背风面的积雪发生沉积,沉积区和侵蚀区之间形成与风向角一致的分界线。所有风向角下积雪漂移均使单层柱面网壳的动力稳定性降低。风吹雪时间越长,结构的动力稳定性越差,但影响程度随时间逐渐降低。随着初始积雪厚度的增加,结构的动力稳定性降低。     

防雪栅与路基间距对路基积雪分布影响规律的数值模拟研究

防雪栅是目前道路风吹雪灾害地区主要的防护设施,该研究结合数值模拟和现场实测方法,研究防雪栅与路基的布置间距对路基周围积雪分布的影响规律,进而分析积雪堆积机理。结果表明:数值模拟结果与现场实测结果有很好的对应关系,验证了结果的正确性;随着路堤与防雪栅间距的增大,路堤两个坡脚处的风速减小区范围均呈先增大后减小的趋势。路堤表面各部分的剪切速度均呈先增大后减小又增大的趋势,透风率为60%的防雪栅与路堤的最不利组合间距为16.67倍的防雪栅高度;随着路堑与防雪栅间距的增大,路堑内部积雪不易消除,因此防雪栅对路堑的挡雪效果不佳;运用剪切速度能够清晰地判断路基表面的积雪堆积与侵蚀,建议在风吹雪灾害频发地区修建铁路前可先采用该方法对工程表面的积雪堆积与侵蚀进行预判断,可合理确定工程中防雪栅布置的位置。     

雪荷载规范比较与风致雪漂移风洞试验方法研究

风致雪飘移在复杂屋面上产生的不均匀雪荷载是多雪地区结构设计中需要重点考虑的问题之一。该研究对比了我国和美国、加拿大、欧洲四本关于雪荷载的规范,比较了雪荷载的计算表达式、屋面形式等内容,通过风洞试验,采用食用盐、硅砂、明矾、干松木屑等材料,对雪飘移问题进行了试验研究。结果发现：四本规范中都是将基本雪压与各种系数的乘积作为雪荷载;相对其他三本规范,我国规范对影响雪荷载的各种环境参数考虑相对较少;明矾因为颗粒间作用力较大,在风洞试验中不易产生连续的漂移运动;干松木屑因为密度太小,模拟效果也不佳;大密度的食用盐颗粒和硅砂的模拟效果相对较好;为了在实际工程中应用风洞试验的方法,需要进行多参数的现场观测、收集完整的气象资料、研究试验的相似关系和试验参数。     

Caractérisation de la neige produite en soufflerie climatiqueCharacterization of snow produced in a climatic wind tunnel

Using the experimental devices of the Jules Verne climatic wind tunnel, it is possible to make snow that has a specified liquid water content. Measurements of the density of the snow mantle and of the shape and size of the snow grains show that the snow produced is equivalent to -day-old natural snow or snow drifted by wind. This paper describes a theoretical model of the snow-making process. The model describes the injection of a high pressure water/air mixture with “snow guns” into low-temperature ambient air. This model uses the Goertler theory for the air jet, the classic ballistic laws for the water droplets and the heat and mass transfer laws in forced convection and evaporation for the freezing mechanisms of the water droplets. This model provides good agreement between the flying time and the freezing time of the water droplets within certain limitations. The variation of the liquid water content as a function of the distance from the snow gun is also well described.     

Failure analysis on collapsed towers of overhead electrical lines in the region Münsterland (Germany) 2005

End of November 2005 strong south-west wind and heavy snowfall were predominant in the region Münsterland, north-western part of Germany. This led to accretion of a considerable quantity of wet snow to overhead electrical lines in form of snow rolls on the conductors. Eighty-two transmission towers failed catastrophically, most of them by buckling, however some by brittle fracture. As a consequence nearly 250,000 people have been cut off from electrical power supply for several days with major media attention.This paper describes the forensic analysis in order to investigate the failure cause. Therefore extensive materials investigations, mechanical testing of original components and specimens thereof, estimations for the real wind and snow loads and their combinations, structural analyses as well as detailed evaluations on the basis of previous investigations, literature and regulations were conducted. It was revealed that some of the examined components were manufactured from Thomas steel which was partially in embrittled condition. The investigated towers fulfilled the design codes valid at the time of erection. However the present line loads of the wet snow rolls on the conductors exceeded by far the ones given in the design codes valid at that time.The load case leading to failure was reconstructed by the derived positions of loads mainly caused by unequal and asymmetric distribution of snow rolls on left and right electrical system. The loads and corresponding stresses acting on the structure before failure were estimated. By comparison with the fracture forces from mechanical testing of original members of the collapsed tower the component that primarily failed was localised. The primary fracture occurred on a diagonal member under tension made of Thomas steel which was weakened by embrittlement. The failure cause was a combination of heavy weather conditions (storm, approx. 0 °C and wet snowfall leading to heavy snow rolls on conductors), asymmetric loading conditions and the usage of Thomas steel which was partially embrittled. Finally, recommendations for avoiding future failures are given.     

风雪耦合作用下单层柱面网壳的动力稳定

强风带动积雪漂移可能使结构处于稳定最不利状态,而目前在这方面的研究很少。该文研究一单层柱面网壳在风雪耦合作用下的动力稳定性。首先阐述风雪耦合作用下动力稳定性的研究方法,然后介绍风雪耦合作用的数值模拟,并通过风洞试验获得网壳表面的非定常气动力,最后分析了风雪耦合作用下单层柱面网壳的动力稳定。研究表明,斜风作用和不均匀分布...     

Modelling the effects of a snowdrift-preventing forest and a snow fence by means of activated clay particles

This paper presents experimental results obtained in a wind tunnel concerning the drift geometry of snow formed around models of a snowdrift-preventing forest and a snow fence, using activated clay particles as a substitute for natural snow. They gave a well defined simulation of the drift geometry of snow, whereas other materials such as MgO₂, Al₂O₃, sawdust and mica flakes were unsuccessful in providing good results.Moiré photography was used for measuring the two dimensional distribution of depth of clayparticle deposits formed around structures without causing any disturbances to the topography of the snow surface.     

A wind tunnel study of the aerodynamic roughness associated with drifting snow

Characteristics of wind profiles observed with drifting snow in a cold wind tunnel are similar to those observed in the field. The aerodynamic roughness increases with wind velocity during drifting.Equations are developed to describe the relationship among various characteristics of the trajectories of saltating snow particles, as measured from photographs and cinefilm. The momentum flux by saltating particles is estimated from measurements of mass flux, and it is shown that the apparent change in surface roughness is caused by the particles in the lowest layers of saltation. The observed effect of surface hardness on the roughness change can be explained by the proposed equation for momentum flux.     

风雪绕流数值模拟的积雪预测模型研究

为研究钝体风雪绕流效应下的地表积雪现象,根据壁面剪切机制及空间混合理论共同确立了积雪预测模型,采用欧拉框架单流体方法对1 m高度立方体模型周边的风雪两相流动情况及空间雪相浓度进行了求解。通过对比不同积雪模型下沉积预测指标在绕流区域的分布情况,及以此得到的积雪预测形态与实测结果的差异发现：该文采用的积雪模型不完全依赖于当地摩阻风速与临界起动风速的相对关系,一定程度上避免了模型参数的取值局限,对沉积现象的反映更为直观;由于综合了两套理论并引入沉积量估计的双重动态指标,该文对于雪面侵蚀及沉积区域的界定划分较为清晰,对积雪形态的预测包括雪面极值位置的捕捉、坡面起伏规律的再现,都较已有数值计算结果更为合理。模型方法可用于其他外形地物绕流下的地表积雪现象模拟。     

Simulating wet snow loads on power line cables by a simple model

A wet snow event in the northwest part of Germany caused extensive damage to electric power lines in November 2005. Maximum wet snow loads of approximately 50 Nm⁻¹ were estimated by evaluating the snow deposit diameter from photographs and the snow density from meteorological data. A simple wet snow accretion model was applied to reconstruct the snow accretion process. The maximum wet snow loads estimated by the model are in good agreement with the observations when the model input parameter, visibility, was adjusted by a constant factor for night-time observations. The improved wet snow accretion model was also applied to historical weather data for the area demonstrating the power and weaknesses of the extreme value analysis of wet snow events.     

Estimation of combined ice and wind load on overhead transmission lines

In recent years, the design of high-voltage transmission lines has been increasingly optimized. In areas where ice accretion on conductors is possible, the load resulting from added weight or increased aerodynamic forces becomes an important design parameter. At present, calculations for combined ice-wind loads do not take the effect of accretion shape into account. In this study, ice accretions are formed on a conductor in a wind tunnel for three types of ice: soft rime, hard rime and glaze. Aerodynamic vertical and horizontal forces are then measured for different wind velocities. It is shown that when the wind remains in the direction of ice build-up, the aerodynamic force due to the asymmetric shape is responsible for a significant increase in the total force and that the present combined ice-wind load calculations can dangerously underestimate the risk of an overload.     

基于PIV技术的平屋盖表面分离泡流动结构研究

建筑屋面的摩擦速度是决定其风致迁移雪荷载的重要因素。该文采用CFD (computational fluid dynamics)方法模拟了平屋面的摩擦速度,并结合其分布特征分析了屋面积雪传输率、侵蚀通量和暴露系数随跨度的变化趋势。为验证CFD方法的湍流模型对平屋面摩擦速度模拟的影响,将数值模拟结果与Irwin探头风洞试验进行了对比,结果表明Realizable k-ε湍流模型的模拟结果与风洞试验最为接近。根据4种不同跨度屋面的CFD模拟结果发现,当屋面跨度与高度之比为4时,屋面平均摩擦速度最小;并且随着跨度的增大,屋面摩擦速度最小值所在的位置和积雪沉积的区域均逐渐向迎风侧屋檐靠近。同时发现,屋面积雪的平均侵蚀通量随着跨度的增大而减小,屋面积雪传输率和雪荷载暴露系数则随着跨度的增大而增大。