Effects of sand-fixing and windbreak forests on wind flow:a synthesis of results from field experiments and numerical simulations

Sand-fixing and windbreak forests are widely used to protect or/and improve the ecological environments in arid and semi-arid regions. A full understanding of wind flow characteristics is essential to arranging the patterns of these protective forests for enhancing the effectiveness. In this study, the wind velocity over the underlying surface with sand-fixing forests and windbreak forests at the heights of 1–49 m was monitored from two 50-m high observation towers in an oasis of Minqin, Gansu Province of China. The wind velocities were simulated at different locations over these protective forests between those two towers by a two-dimensional Computational Fluid Dynamics（CFD） model. The results showed that at the heights of 1–49 m, the wind velocity profiles followed a classical logarithm law at the edge of the oasis and a multilayer structure inside the oasis. With increasing number of sand-fixing forest and windbreak forest arrays, the wind velocity at the heights of 1–49 m generally decreased along the downstream direction of the prevailing wind. Specifically, below the height of windbreak forests, the wind velocity decelerates as the airflow approaches to the windbreak forests and then accelerates as the airflow passes over the windbreak forests. In contrast, above the height of windbreak forests, the wind velocity accelerates as the airflow approaches to the windbreak forests and then generally decelerates as the airflow passes over the windbreak forests. Both the array number and array spacing of sand-fixing and windbreak forests could influence the wind velocity. The wind protection effects of sand-fixing forests were closely related to the array spacing of windbreak forests and increased with the addition of sand-fixing forests when the array of the forests was adequately spaced. However, if the array spacing of windbreak forests was smaller than seven times of the heights of windbreak forests, the effects were reduced or completely masked by the effects of windbreak forests. The results could offer theoretical guidelines on how to systematically arrange the patterns of sand-fixing and windbreak forests for preventing wind erosion in the most convenient and the cheapest ways.

Effects of Desertification Control Project Based on Windbreak System in Daxing County, Beijing

Windbreak system consisting of the forms of net, belt, sheet and point, has improved local eco environment greatly and promoted the level of social economy and local people's living. The result of environment impact assessment shows that the quality of eco environment has been improved obviously and the impact process is mainly positive after building of windbreak system in Daxing County of Beijing, a dry sub humid region. Effects of windbreak system and their environmental impacts are analyzed scientifically and reasonably in this paper. The study will enrich the theory of environment impact assessment and extend the scope of research, and will provide experience for the construction of windbreak system in the region.

Evaluation on Regional Ecological Benefits of Windbreak Systems

This paper studied how dynamically to evaluate regional ecological benefits of windbreak systems in a county scale. Data of air temperature. humidity. wind speed, crop yield and forest cover percentage were investigated and collected in some of counties of Heilongjiang Province, where windbreak systems were established in the last thirty years. Through analysis, an evaluation index system estimating regional ecological benetits of windbreak systems was set up, which consists of both a vegetation index system and a climate index systeln. The former includes a forest cover percentage index and a crop yield index; the later does three variable indexes of wind speed, air temperature and humidity. An integrated regional ecological benefit index.was developed through combinations of some coined evaluation indexes from the evaluation system.And to take Zhaozhou County and Anda County as a case study, the regional ecological benetits since establishment of windbreak systems of the two counties were evaluated dynamically using the integrated index.

Plant water use strategies in the Shapotou artificial sand-fixed vegetation of the southeastern margin of the Tengger Desert, northwestern China

Stable oxygen and hydrogen isotopic compositions(δ^(18)O and δD) of plant xylem water and its potential water sources can provide new information for studying water sources, competitive interactions and water use patterns of plants. The contributions of different water sources to three plants, Hedysarum scoparium(HS), Caragana Korshinskii(CK) and Artemisia ordosica(AO), were investigated in the artificial sand-fixed vegetation of Shapotou, the southeastern margin of the Tengger Desert of northwestern China, based on meteorological data and δ^(18)O and δD values of precipitation, groundwater, soil water and xylem water of HS, CK and AO. Our results indicated that soil water infiltration through precipitation was the main water source of the artificial sand-fixed vegetation. Obvious differences in soil water content and in δ^(18)O of soil water and xylem water were found among different seasons. No relationship was found between the δ^(18)O in plant xylem water and in soil water in January. The same water use patterns were found in CK, HS and AO in May, suggesting they have the same water sources. The different water sources of CK, HS and AO in August indicate that water competition occurred. In addition, the main water sources of CK, HS and AO in August mainly come from shallow soil water, while they use relatively deep soil water in May. This phenomenon is related to the differences of soil water content throughout soil profile, precipitation, transpiration and water competition under different growth periods. The water use patterns of CK, HS and AO respond to soil water content throughout the soil profile and their competition balance for water uptake during different growth season. The results indicate that these sandfixed plants have developed into a relatively stable stage and they are able to regulate their water use behavior as a response to the environmental conditions, which reinforces the effectiveness of plantation of native shrubs without irrigation in degraded areas.

Wind-proof and sand-fixing effects of Artemisia ordosica with different coverages in the Mu Us Sandy Land,northern China

Wind erosion is a key global environmental problem and has many adverse effects.The Mu Us Sandy Land of northern China is characterized by an arid climate,where vegetation patches and bare sand patches are usually distributed mosaically,and aeolian activities occur frequently.Vegetation plays a significant role in controlling wind erosion.Artemisia ordosica is the most dominant native plant species in the Mu Us Sandy Land.It is urgent to study the wind-proof and sand-fixing effects of Artemisia ordosica in the Mu Us Sandy Land.This study analyzed the wind-proof and sand-fixing effects of Artemisia ordosica based on the field data of wind regimes,aeolian sediment transport,and surface change of Artemisia ordosica plots with four coverages(denoted as site A,site B,site C,and site D)in the Mu Us Sandy Land during the period from 1 June 2018 to 29 June 2019.The coverages of Artemisia ordosica at site A,site B,site C,and site D were 2%,16%,29%,and 69%,respectively.The annual average wind speeds at 2.0 m height above the ground for site A,site B,site C,and site D were 3.47,2.77,2.21,and 1.97 m/s,respectively.The annual drift potentials were 193.80,69.72,15.05,and 6.73 VU at site A,site B,site C,and site D,respectively.The total horizontal aeolian sediment fluxes during the period from 2-3 June 2018 to 6 June 2019 at site A,site B,site C,and site D were 4633.61,896.80,10.54,and 6.14 kg/m,respectively.Site A had the largest surface changes,and the surface changes at site B were significantly weaker than those at site A,whereas the surface changes at site C and site D were minimal.The results indicated that Artemisia ordosica significantly reduced the wind speed,drift potential,aeolian sediment transport,and surface changes.The higher the coverage of Artemisia ordosica is,the more obvious the effects of wind-proof and sand-fixing.Wind erosion would be effectively controlled in the Mu Us Sandy Land if the coverage of Artemisia ordosica is greater than 29%.These results provide a scientific basis for evaluating the ecosystem service function of Artemisia ordosica and the vegetation protection and construction projects in the Mu Us Sandy Land.

Dynamic behaviors and mitigation measures of a train passing through windbreak transitions from ground to cutting

In this paper,the effects of a right-angle windbreak transition(RWT)from the flat ground to cutting on train aerodynamic and dynamic responses were investigated,then a mitigation measure,an oblique structure transition(OST)was proposed to reduce the impact of RWT on the train aerodynamic and dynamic performance.The results showed that in the RWT region,the airflow was divided into two parts.One part of the airflow induced a strong backflow in the flat ground position,and the other part of the airflow induced a strong backflow in the cutting position.Therefore,there were two lateral impacts on the train.For the head car with the OST,the drop ratios of the peak-to-peak values compared with RWT were 47%,40%,and 52%for the side force coefficient C_(Fy),lift force coefficient C_(Fz) and overturning moment coefficient C_(Mx),respectively.For the peak-to-peak value of the dynamic parameters,the drop ratios of OST compared with RWT were all larger than 50%.The maximum dynamic overturning coefficients for RWT and OST were 0.75 and 0.3,respectively.

Numerical Analysis of Different Nose Shapes on the Train Aerodynamic Performance at a Windbreak Transition under Crosswinds

Based on the Unsteady Reynolds-Averaged Navier-Stokes (URANS) method, this paper studied the effect of the nose shape on the aerodynamic performance when the high-speed train subjected to a windbreak transition under crosswinds. The windbreak transition generated by the irregular terrain from the flat ground to the cutting. The results showed that with the height of the front window increased from Z ? 2 to Z + 2 (the dimensionless height), the side force coefficient Cy and rolling moment co-efficient Cmx increased by 26% and 27% for the head car, respectively. The flow structures around the lower front window were smoother than that around the higher front window. The flow structures in the higher front window resulted in more considerable positive pressure on the windward side (WWS) and top of the nose region.

Preparation and Water Retention Properties of Clay-based Sand-fixing and Grass-planting Materials

Targeting the problem of available water conservation in sand fixation, the sand-fixing and grass-planting materials were prepared with clay modified by emulsifying vegetable waxes and octylphenol polyoxyethylene ether （OP4）. The water retention property was studied in simulating desertification environmental climate and the materials were characterized by means of UV-Vis, SEM, FTIR, XRD and TGA measurements. The experimental result showed that the materials had excellent water retention properties, due to that vegetable waxes adhered evenly to clay particle surfaces, made the clay pores changing from hydrophilic to hydrophobic and so inhibited the water evaporation. Grass-planting experiment showed that, with reasonable mass ratio of clay, vegetable waxes and surfactant, the materials not only inhibited water evaporation but also maintained sound air permeability so shat the germination rate and survival rate of grass were significantly improved.

Sand-fixing characteristics of Carex brunnescens and its application with straw checkerboard technique in restoration of degraded alpine meadows

Carex brunnescens（Pers.）, a typical clonal species, is considered to be the only herb found to date that can develop on sand dunes in Maqu alpine region of northwestern China. However, the characteristics that C. brunnescens resists to harsh alpine environment have not been documented. In this study, we conducted a field investigation to determine the morphological, reproductive, and sand-fixing characteristics of C. brunnescens. Concomitantly, we transplanted the belowground rhizomes of C. brunnescens to sand dunes and compared the abilities to restore degraded alpine meadows among sand dunes that had no further treatment（SD＋N）, sand dunes that had straw checkerboard technique but no transplanted rhizomes of C. brunnescens（SD＋SCT）, and sand dunes that had both SCT and transplanted rhizomes of C. brunnescens（SD＋SCT＋P）. We found that belowground vertical rhizomes and horizontal rhizomes（including branching rhizomes and main rhizomes） of C. brunnescens were highly developed and that population reproduction was dominated by horizontal rhizomes. C. brunnescens exhibited a significant sand-fixation effect under following conditions： population density was 145–156 ramets/m^2, vegetation cover was 31.2%–39.3%, total length of belowground rhizomes was 11,223 cm/m^2, total length of belowground first-order roots was 9161–10,524 cm/m^2, fresh weight of aboveground part was 198.5–212.6 g/m^2, and fresh weight of belowground part was 578.8–612.4 g/m^2. It should be particularly noted that SD＋SCT＋P treatment（sand dunes that had both straw checkerboard technique and transplanted rhizomes of C. brunnescens） was the best and SD＋N（sand dunes that had no further treatment） was the worst in terms of following biotic indicators： total number of reproductive ramets, total number of belowground rhizomes, and fresh weight of aboveground and belowground parts of C. brunnescens, contents of soil organic carbon, available nitrogen, microbial biomass carbon, and microbial biomass nitrogen. It implies that applying SCT in sand dunes and transplanting belowground rhizomes to sand dunes with SCT could improve both soil fertility and growth of C. brunnescens. These results suggest that the SCT-promoted high reproductive abilities of belowground rhizomes of C. brunnescens can successfully facilitate the establishment of ramets and can thus be an effective strategy to restore degraded vegetation in Maqu alpine region of northwestern China.

A PRELIMINARY SPECTRUM ANALYSIS ON TURBULENCE IN LEESIDE OF WINDBREAKS

The spectrum characteristics of turbulence such as period of fluctuation, frequency and amplitude of oscillation were discussed by using the method of spectral analysis of turbulence in this paper. The results show that the spectrum density in the range of low frequency is sharply variance with the distance from the windbreaks, but it follows the power index law of -5 / 3 and satisfies the theory of local isotropic turbulence in the range of high frequency. It also shows by connecting with Eulerium autocorrelation analysis that there are small scale,high frequency and short period of turbulence near the lecsidc of windbreaks,but often large scale, low frequency and long period of turbulence far from the windbreaks.

WINDBREAKS FOR SAND DUNE FIXATION IN JILIN PROVINCE

There are 0.78 million hm2 of sand dune areas in Jilin Province. The effects of two types of windbreaks. shelterbelts and small areas ofartdicial forests, on sand dune fixation were discussed. The temperature, humidity and reduced situation of wind speed in the two types of windbreaks wer systematically studied.

THE DISTRIBUTION CHARACTERISTICS OF TURBULENCE ON LEESIDE OF WINDBREAKS

The distributive characteristics of turbulence, turbulent velocity, turbulent intensity and Reynolds stress, in horizontal and vertical direction on leeward side of windbreak were discussed in this paper. The results show that all the three kinds of parameters of turbulence have the similar distributive patterns in lee, their peaks were around 10H for horizontal and Z/ H = 1 for vertical. The windbreak is important barrier to affect the structure and distribution of wind speed and turbulence. Porosities of windbreaks could alter the patterns of turbulence, therefore, affect the abilities and functions of windbreaks.

ECOLOGICAL LANDSCAPE EFFECTS OF WINDBREAK SYSTEMS IN DESERT ZONE OF WESTERN CHINA

This paper, by applying the theories of landscape ecology, illustrates the role and mechanism of windbreak system in the establishment and maintenance of oasis ecosystem on the basis of systematic analysis of characteristics and ecological crises of China's desert, and especially of desert zone in western China. Furthermore, direct economic benefits are summarized.

Design of Tall Cable-Supported Windbreak Panels

The design and construction of tall and slender steel structures is always challenging. This paper discusses several design aspects (structural information, analysis methods, applied loads, cost optimisation) and a case study regarding the design and construction of 10-m-tall windbreak panels for a Greek electricity producer. The purpose of the panels is to reduce wind turbulence and improve the performance of the electricity producer’s air-cooled condenser. In this case, the main wind load acts in the longitudinal direction, with friction inducing only a small amount of wind load in the transverse direction. The steel columns are constructed from 10-m-tall hot-rolled IPE 270 (S235) cross-sections, and are supported by cables in the longitudinal direction and bracing systems in the transverse direction. Concrete anchorages and concrete footings are used for the cables and steel columns, respectively. System optimisation is investigated in terms of the steel weight, cable length, and overall cost, and practical issues are explained regarding technical decisions. Furthermore, the construction details, construction methods, and cost estimation are discussed.

Effects of Different Wind Speeds behind Windbreak Wall of Fattening Cattle Farm on Farm Environment and Performance of Beef Cattle in Zhangye District

[Objective]The paper was to study the wind-reducing effect of windbreak wall at different sheltering distances and the effects of wind speed on cattle performance.[Method]The 40 cattle with insignificant difference in initial weight were randomly distributed in four selected stalls,and were divided into two groups according to the distance between stalls and windbreak wall to measure their daily gain.According to the actual conditions and weather conditions of fattening cattle farm,the wind speed,temperature and other environmental indexes of measuring point and heat production of beef cattle were carried out field measurement and analysis.[Result]The windbreak wall in fattening cattle farm reduced the wind speed by 40%-70%within a horizontal distance of 10 times of the wall height at a height of 1.2 m.The best sheltered area behind windbreak wall was 2-4 times of the height of windbreak wall.Within the distance of 6 times of the wall height,the wind speed reduction effect was obvious and the maximum reduction rate reached 70%.With an external WCI of-15.61 and average beef cattle weight of 480 kg,the WCI of fattening cattle farm equipped with windbreak wall reached-9.00,and the heat loss reduced by each cow was 3.31 MJ/h.Meantime,due to the difference in distance from windbreak wall,the daily gain difference of beef cattle was 0.15 kg/head day,but the difference was not significant(P>0.05).

基于多孔介质的高速铁路透风式挡风墙关键参数研究

合理的匹配参数是风区铁路沿线防风设施研究的关键问题。针对风区铁路沿线部分区段混凝土实心挡风墙出现的环境风越过挡风墙顶导致严重的弓网离线、无法正常升弓,危及铁路行车安全问题,提出改“堵”为“疏”,通过一定的透风率来调控挡风墙后的线路风场,从而为缓解上述现象提供一种新思路,其关键问题在于透风式挡风墙关键参数与防风效果的合理匹配。为此,基于多孔介质的自身特性,提出一种基于多孔介质的挡风墙风场新研究方法,将挡风墙透风率与防风效果的合理匹配问题转化为多孔介质的阻力系数与列车气动载荷的关系研究。首先,通过比较数值模拟结果与兰新高铁挡风墙风场现场试验结果,修正并验证数值模拟方法的准确性。然后,通过数值模拟研究进一步探究多孔介质阻力参数与挡风墙透风率之间的关系,获得风区铁路沿线透风式挡风墙关键参数最优区间。研究结果表明:可以采用多孔介质代替挡风墙几何进行挡风墙风场研究,能够显著提升研究效率;多孔介质的黏性阻力系数在挡风墙风场研究中可以忽略,多孔介质的惯性阻力系数与挡风墙的透风率呈幂函数关系;风区铁路沿线透风式挡风墙高度为3.5 m,透风率在24.4%~28.7%区间时,对一线、二线列车的防风效果最好。

Analysis of the Application of Windbreak and Sand Fixation Technology in Desert Roads

In order to solve the problems in the construction of desert roads and improve the quality of desert roads in China,this paper discusses the current windbreak and sand fixation technologies in desert areas in China,and puts forward relevant suggestions and corresponding solutions for each problem.This paper is written with hopes of contributing to the development of windbreak and sand fixation technologies in China as well as the development of the economy,society and environment in China's deserts.

大风区高速铁路路基双侧挡风墙周围沙粒沉积数值模拟研究

兰新高速铁路横穿多个大风区,对其安全运营造成严重影响。为降低强风对线路的危害,在大风区修筑大量防风构筑物,其中以路基挡风墙最为常见,挡风墙降低了风害的影响,但是带来严重的线路积沙问题。通过数值模拟研究不同风速下路基双侧挡风墙周围的流场特征,利用风产生的壁面剪切应力与沙粒剪切应力的比值确定地表沙粒沉积与侵蚀类型。研究发现,气流在两道挡风墙之间分别形成一个顺时针涡流和一个逆时针涡流,在第二道挡风墙背风侧形成一个顺时针大涡,随着来流风速增大,漩涡尺寸和涡流强度均在增大;由于涡流的存在,部分沙粒被卷走,路堤背风坡附近以及路基面上的沙粒沉积显著减少,当来流风速为24、30 m/s时,南北侧线路道床面均出现沙粒反向侵蚀;单双侧挡风墙的积沙规律对比结果表明,设置双侧挡风墙时,线路积沙显著减少。研究揭示了大风区铁路双侧挡风墙周围路基面上的沙粒沉积规律,对未来风区铁路的防护建设具有指导意义。

槽式太阳能阵列挡风增速效果数值模拟

采用开源计算流体力学软件OpenFOAM中的Simple求解器对流场进行数值模拟,研究5.00 m/s风速下槽式太阳能阵列挡风墙挡风效果。对由槽式太阳能阵列组成的6个不同高度的挡风墙对整体流场速度的影响以及不同高度挡风墙背风侧30,60,90 m处的速度进行了分析。研究发现,当气流通过挡风墙时,低于墙高的位置速度大幅衰减,而高于墙高的位置开始加速,存在明显的分区作用。当气流流过挡风墙后,会在其后形成明显的速度衰减区,其面积和延伸与挡风墙的高度正相关。在挡风墙背风侧较远处,高于墙高的气流开始减速,低于墙高的气流开始加速,并随着距离的增加风速逐渐趋于平稳。

木麻黄基干林带对台风“杜苏芮”的防风效应

为了解海防林对台风的防护效应,以惠安赤湖国有防护林场监测站收集的“杜苏芮”台风登陆前后的气象数据为材料,分析了台风登陆前后近地层的风速、风向、湍流强度、防风效能和气流加速率等的特征规律。结果表明:(1)台风登陆前后的平均风速和风向时程变化特征满足了台风眼区经过的条件,平均风速曲线呈“M”形双峰分布,通过采集到的台风眼区、眼壁和外围强风的完整风况数据,完整展示了台风经过林带前后风速、风向的演变过程。(2)湍流强度的时程整体变化趋势随着台风靠近和离去呈现增大-减小-剧增-回落的过程,尤其台风后风圈影响的区域,湍流强度剧烈增大,强度可增大2倍以上。(3)向岸风在垂直方向上,弱风或强风的防风效能随高度增加而降低,高度相同,林带后弱风的防风效能小于强风;水平方向上,林内及林带后1H和5H处防风效能最好,在高度3 m平均防风效能为49.57%~93.93%,在高度9 m平均防风效能为44.04%~90.10%。离岸风在垂直方向上,随着气流靠近林带,弱风或强风的气流加速率随高度增加而增大,林带后1H高度3 m气流加速率平均为0.2193,高度9 m气流加速率平均为0.3242,林内气流加速率达到最低值;而风越过林带后,弱风风速恢复到88%左右,强风的气流则有个加速的过程。