首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
More than 4000 hourly wind profiles measured on three topographically different foredunes are analysed and discussed. Wind flow over the foredunes is studied by means of the relative wind speed: the ratio between wind speed at a certain location and the reference wind speed at the same height. Relative wind speeds appear to be independent of general wind speed but dependent on wind direction. For perpendicular onshore winds the flow over the foredune is accelerated due to topographic changes and decelerated due to changes in surface roughness. Accelerations dominate over decelerations on the seaward slope. The pattern of acceleration and deceleration in relation to wind direction is more or less comparable for different foredunes, but the magnitudes differ. An increase in foredune height from 6 to 10m leads to an increase in speed-up near the top of the seaward slope from 110 to 150 per cent during onshore wind, but further increase of foredune height from 10 to 23m appears to have little effect, due to increased roughness and deflection of flow. Topography also influences the direction of the flow. Between beach and top, the flow deflects in the direction of the normal during onshore winds. During offshore winds the flow is deflected to the parallel. Near the dunefoot, deflection is always in the direction of the parallel, and increases with steeper topography. The maximum deflection near the dunefoot was 90°, over a 23 m high dune, observed during offshore winds. Patterns of erosion and sedimentation resulting from winds from different directions can be explained by the observed accelerations and decelerations. Owing to speed-up on the seaward front of the foredune, sand transport capacity of the wind increases, which results in erosion if vegetation is absent. During strong onshore wind, sand is lifted near the dunefoot and moves over the foredune in suspension. During weaker winds, vertical wind velocities do not exceed fall velocities of the sand grains, and most of the sand is deposited near the dunefoot.  相似文献   

2.
This paper presents results from a study designed to explore the effects of beach surface moisture and fetch effects on the threshold of movement, intensity of sand transport by wind and mass flux. The experiment was carried out over a period of five weeks at Greenwich Dunes, Prince Edward Island, Canada in May and June 2002. Moisture content was measured with a Delta‐T moisture probe over a 50 m by 25 m grid established on the beach. Measurements of wind speed and direction were made with arrays of cup anemometers and a two‐dimensional sonic anemometer. Transport intensity was measured at a height of 2–4 cm above the bed using omnidirectional saltation probes which count the impact of saltating grains on a piezoelectric crystal. Anemometers and saltation probes were sampled at 1 Hz. Sand transport was measured with vertical integrating sand traps over periods of 10–20 minutes. Results show that where there is a considerable supply of dry sand the saltation system responds very rapidly (1–2 s) to fluctuations in wind speed, i.e. to wind gusts. Where sand supply from the surface is limited by moisture, mean transport rates are much lower and this reflects in both a reduction in the instantaneous transport rate and in a transport system that becomes increasingly intermittent. Threshold wind speed is significantly correlated with an increase in surface moisture content near the upwind end of the beach fetch, but the relationship is not significant at the downwind end where sediment transport is initiated primarily by saltation impact from upwind. Mass flux increases with increasing fetch length and the relationship is described best by a power function. Further work is necessary to develop a theoretical function to predict the increase in transport with fetch distance as well as the critical fetch distance. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

3.
Wind flow and sand transport intensity were measured on the seaward slope of a vegetated foredune during a 16 h storm using an array of sonic anemometers and Wenglor laser particle counters. The foredune had a compound seaward slope with a wave‐cut scarp about 0.5 m high separating the upper vegetated portion from the lower dune ramp, which was bare of vegetation. Wind direction veered from obliquely offshore at the start of the event to obliquely onshore during the storm peak and finally to directly onshore during the final 2 h as wind speed dropped to below threshold. Sand transport was initially inhibited by a brief period of rain at the start of the event but as the surface dried and wind speed increased sand transport was initiated over the entire seaward slope. Transport intensity was quite variable both temporally and spatially on the upper slope as a result of fluctuating wind speed and direction, but overall magnitudes were similar over the whole length. Ten‐minute average transport intensity correlates strongly with mean wind speed measured at the dune crest, and there is also strong correlation between instantaneous wind speed and transport intensity measured at the same locations when the data are smoothed with a 10 s running mean. Transport on the beach for onshore winds is decoupled from that on the seaward slope above the small scarp when the wind angle is highly oblique, but for wind angles <45° from shore perpendicular some sand is transported onto the lower slope. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

4.
沙尘暴电效应的实验观测研究   总被引：7，自引：0，他引：7       下载免费PDF全文
利用国内大型风沙物理风洞实验模拟沙尘暴电现象,研究风沙起电机理,结果表 明,不同风速下不同沙粒会产生不同极性的电场强度和电位效应,风沙电随风速增大而增强 ,且随沙粒度增大而减小. 在沙漠区的16m,8m,4m和1m高度上观测到27次不同沙尘暴天气 过程的电场和风速随时间变化. 结果表明, 在晴天4个高度上的电场均为小正电场值,电场 随高度降低而减小,最大电场强度在5kV/m以下,日风速变化对各层电场起伏没有较大影响 . 有沙尘天气,各高度上的电场强度随风速变化而变化. 16m高度上电场均为负值,平均值 为 -20kV/m;中层8m 电场一般为较高正电场值,达到10～40kV/m,与16m高度上电场呈反相 关;下层1m 电场值变化一般很小,在1kV/m以下. 在强沙尘暴天气4个高度上的电场均为负 电值,电场值随高度降低而减小,16m高度上最大平均电场强度达到-200kV/m以上,瞬时值 超过 -2500kV/m,与晴天电场矢量相反.  相似文献   

5.
A stochastic model for entrainment of sand grains by wind is presented through analysis of the forces exerted on a single spherical grain, coupled with fluctuations of wind velocity and the change in grain position on the surface. The structure of the stochastic model is consistent with experimental data in the literature. The probability of initial motion increases first, and then decreases, with grain size. It reaches a maximum at diameters of about 0·9 mm. Some sand grains are still in motion at less than the conventional threshold velocity, even at very low velocities. The probability of sand grain movement reaches unity at twice the conventional threshold velocity. Considerable discrepancies amongst conventional threshold formulae may result from the different probabilities of initial movement implied in these formulae. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

6.
Conditional sampling is used herein to examine the effect of fetch, stability, and surface roughness changes on wind speeds in the coastal zone. Using data from an offshore wind farm it is shown that at a distance of 1.2–1.7 km from the coast, up to a height of 20 m above the surface, differences in wind speed distributions from onshore and offshore masts are statistically significant for flow moving offshore under all stability conditions. In contrast, differences between the distribution of wind speeds at 38 and 48 m at masts located at the coast and in the coastal zone are not significant for flow moving offshore, indicating that flow at these heights is not fully adjusted to the change in surface roughness (land to sea). These findings are in accordance with calculations of the internal boundary layer (IBL) height which indicate that the IBL would frequently be below the two upper measurement heights at 1.2–1.6 km from the coast. The analyses presented here indicate that the wind speed distribution at a potential offshore wind farm site is not solely dependent on fetch (distance from the coast) but also depends on the stability climate.  相似文献   

7.
下击暴流是雷暴天气产生的近地面短时强风,具有和大气边界层近地风显著不同的风场特征,其风荷载数值模拟方法也和常规风荷载模拟方法不同。在将下击暴流风速表达为时变的平均风速和调制的非平稳脉动风速之和的前提下,本文采用基于改进的OBV模型（Oseguera and Bowles／Vicroy（OBV））的方法来模拟平均风速,其平均风速直接由改进的OBV模型表达,而不同于已往研究中将平均风速表达为竖向风剖面与时间函数的乘积,并采用结合快速傅里叶变化的谐波叠加法来模拟调制的非平稳脉动风速。改进的OBV模型考虑了下击暴流的风暴中心移动和强度演化的影响,更准确地描述了下击暴流的实际物理过程。通过实例仿真,说明结果准确合理。  相似文献   

8.
This paper presents results from one of the few scientific studies to examine the physical characteristics of aeolian sediment transport in an alpine area, where topographically reinforced foehn winds initiate dust storm events. The major objective of this study is to improve knowledge of aeolian processes in mid-latitude alpine regions experiencing extreme wind speeds. Of particular interest is the role of surface characteristics in contributing to the unusually deep saltation layer which is seen to form over fluvio-glacial deposits in the Southern Alps of New Zealand. Sediment was collected at several heights (0ċ5, 1, 2 and 4 m) and locations over a large alpine braided river delta, and standard laboratory techniques used to examine grain size characteristics. An image processing technique was also used to evaluate grain roundness. Grains filtered from the airstream at 0ċ5 m and 1 m above such surfaces were found to display a mean grain size of approximately 300 to 435 μm, resembling grain size characteristics of saltation clouds previously observed in high latitude, cold climate locations, in contrast to desert and prairie environments. Samples collected at 2 and 4 m above the surface were found to consist of 60 to 65 per cent sand-sized material, with some grains exceeding 1–1ċ5 mm in diameter. Grain shape analysis conducted on silt- and clay-sized grains filtered from the airstream above mixed sand and gravel surfaces showed such grains to display an increase in grain roundness with height. This characteristic is thought to reflect the airstream's shape-sorting ability and has important implications with respect to the often observed increase in grain roundness in aeolian deposits with increasing distance from source areas. Namely, if more rounded grains are preferentially carried higher into the airstream and therefore into regions of higher wind speed, they should theoretically be transported further from the entrainment zone before being deposited. The high wind speeds observed, often exceeding 30 m s−1, are seen to transport significantly larger sediment than reported in the literature for desert and prairie environments. In addition, the mixture of grain sizes, and especially the pebble- and cobble-sized clasts that dominate the fluvio-glacial deposits associated with the braided rivers in this mountain region, also appear to increase significantly the trajectory height of saltating sand grains. As a result of these two factors, the depth of the saltation cloud often exceeds 1 m. Observations made in this study therefore highlight the need for field and laboratory aeolian process studies to be extended to examine grain transport over coarse-grained beds during much higher wind velocities than typically reported in the literature. Such studies would provide a valuable insight into aeolian processes in high latitude/altitude environments, such as loess genesis. © 1997 by John Wiley & Sons, Ltd.  相似文献   

9.
Particle–turbulence interaction has been a research focus in the field of pneumatic transport, especially in aeolian environments. However, knowledge regarding the effect of saltating particles on the turbulence characteristics is very limited. In this article, a process of sand-laden flow from forming sand streamers to stability is investigated via a coupled mathematical model of wind-blown sand that includes the spatiotemporal development. The variations in the turbulence characteristics, such as the mean velocity and turbulence intensity in clean air or sand-laden flow field, are analyzed. The results show that the splash process of sand grains near the wall decrease the wind speed in the saltation layer and destroy the low-speed streaks. Moreover, the profiles of streamwise turbulence intensity exhibit a transition from ‘decreasing’ to ‘increasing’ and approximately intersect at an ‘intensity focus’, which is presented for the first time. Furthermore, it was found that saltating particles could enhance the Reynolds stress. Meanwhile, it was also noticed that the shear stress at the wall surface is greater than the impact threshold and that there is a tendency towards the impact threshold. Therefore, saltation makes the particle Reynolds number of sand-laden flow higher than that under non-saltation conditions, thus changing the particles’ effect on the turbulence intensity. Gravity-dominated saltation is probably the most essential difference between wind-blown sand and other traditional two-phase flows. © 2020 John Wiley & Sons, Ltd.  相似文献   

10.
Conventional aeolian sand transport models relate mass transport rate to wind speed or shear velocity, usually expressed and empirically tested on a 1-s time scale. Projections of total sand delivery over long time scales based on these models are highly sensitive to any small bias arising from statistical fitting on empirical data. We analysed time series of wind speed and sand transport rate collected at 14 independent measurement stations on a beach during a prior field experiment. The results show that relating total sand drift to cumulative above-threshold wind run yields models which are more statistically robust when fitted on empirical data, generating smaller prediction errors when projected to longer time scales. Testing of different power exponents indicates that a linear relationship between sand drift and above-threshold wind run yields the best results. These findings inspire a speculative novel phenomenological model relating the mass flow of air in the boundary layer to the mass transport of sand over the surface. © 2020 John Wiley & Sons, Ltd.  相似文献   

11.
Airflow patterns through a saucer blowout are examined from wind speed and direction measurements made during a chinook wind event. The blowout long‐axis is oriented east–west with a broad depositional apron on the east side. Wind directions during the event rotated from south‐westerly to westerly, permitting an assessment of oblique and axis‐parallel flows. Results show that airflow passing over the windward rim of the saucer blowout expands and decelerates, leading to flow separation and a small re‐circulation zone on sheltered lee slopes. Near the deflation basin, airflow re‐attaches to the blowout surface and accelerates up to a small opening in the east rim, where it can be up to 50% faster than on the windward edge. Beyond the downwind rim the airflow expands and decelerates and sand is deposited onto a broad apron. Similar to coastal trough blowouts, the degree of airflow steering and acceleration along the deflation basin is determined by the angle of incidence between the approach wind and the long‐axis of the blowout. As the angle of incidence increases wind speed accelerates at 0·3 m above the surface of the deflation basin and the degree of airflow steering increases. Overall, a two‐fold process is identified, where south‐westerly flows have greater potential for eroding the deflation basin, while westerly flows have greater potential for evacuating sand from within the blowout. Visual observations indicate that sand eroded from the deflation basin during south‐westerly flows is re‐distributed to adjacent zones of low wind speed until axis‐parallel winds evacuate the sand through the opening in the east rim. Morphometric changes since 1994 indicate that the blowout morphology has remained relatively constant, suggesting a persistent interplay between oblique and axis‐parallel wind erosion events. Collectively, these findings indicate that the angle of approach winds is an important control on saucer blowout morphodynamics. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

12.
不同生态系统CO2通量和浓度特征分析研究   总被引：3，自引：0，他引：3       下载免费PDF全文
本文利用1993～1994年日本国家农业环境研究所与中国科学院沙漠研究所合作在内蒙古奈曼地区实测的7种不同生态系统（沙丘、轻度放牧草原、中度放牧草原、重度放牧草原、无放牧草原、玉米田和大豆田）的净辐射、土壤热通量、两个高度的CO2浓度、温度、湿度和风速等资料，采用空气动力学方法，计算了CO2通量及其与环境和人为干扰因子的关系，并分析了不同下垫面的光合作用特征. 结果表明：各种下垫面CO2通量的共同特点是：在白天，CO2通量和梯度的输送方向是从大气向植被，在中午（11～13时）输送达到负的最大值； 在夜间，CO2通量和梯度输送方向与白天相反，是从植被向大气，在早晨（3～5时）达到正的最大值. 植被覆盖率及生物量不同的下垫面光合作用强度有明显差异，天气状况对光合作用也有一定影响.  相似文献   

13.
Evidence from a field study on wind flow and sediment transport across a beach–dune system under onshore and offshore conditions (including oblique approach angles) indicates that sediment transport response on the back‐beach and stoss slope of the foredune can be exceedingly complex. The upper‐air flow – measured by a sonic anemometer at the top of a 3·5 m tower located on the dune crest – is similar to regional wind records obtained from a nearby meteorological station, but quite different from the near‐surface flow field measured locally across the beach–dune profile by sonic anemometers positioned 20 cm above the sand surface. Flow–form interaction at macro and micro scales leads to strong modulation of the near‐surface wind vectors, including wind speed reductions (due to surface roughness drag and adverse pressure effects induced by the dune) and wind speed increases (due to flow compression toward the top of the dune) as well as pronounced topographic steering during oblique wind approach angles. A conceptual model is proposed, building on the ideas of Sweet and Kocurek (Sedimentology 37 : 1023–1038, 1990), Walker and Nickling (Earth Surface Processes and Landforms 28 : 111–1124, 2002), and Lynch et al. (Earth Surface Processes and Landforms 33 : 991–1005, 2008, Geomorphology 105 : 139–146, 2010), which shows how near‐surface wind vectors are altered for four regional wind conditions: (a) onshore, detached; (b) onshore‐oblique, attached and deflected; (c) offshore, detached; and (d) offshore‐oblique, attached and deflected. High‐frequency measurements of sediment transport intensity during these different events demonstrate that predictions of sediment flux using standard equations driven by regional wind statistics would by unreliable and misleading. It is recommended that field studies routinely implement experimental designs that treat the near‐surface wind field as comprising true vector quantities (with speed and direction) in order that a more robust linkage between the regional (upper air) wind field and the sediment transport response across the beach–dune profile be established. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

14.
This paper discusses a two-dimensional second-order closure model simulating air flow and turbulence across transverse dunes. Input parameters are upwind wind speed, topography of the dune ridge and surface roughness distribution over the ridge. The most important output is the distribution of the friction velocity over the surface. This model is dynamically linked to a model that calculates sand transport rates and the resulting changes in elevation. The sand transport model is discussed in a separate paper. The simulated wind speeds resemble patterns observed during field experiments. Despite the increased wind speed over the crest, the friction velocity at the crest of a bare dune is reduced compared to the upstream value, because of the effect of stream line curvature on turbulence. These curvature effects explain why desert dunes can grow in height. In order to obtain realistic predictions of friction velocity it was essential to include equations for the turbulent variables in the model. In these equations streamline curvature is an important parameter. The main flaw of the model is that it cannot deal with flow separation and the resulting recirculation vortex. As a result, the increase of the wind speed and friction velocity after a steep dune or a slipface will be too close to the dune foot. In the sand transport model this was overcome by defining a separation zone. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

15.
Blowouts are depressions that occur on coastal dunes, deserts and grasslands. The absence of vegetation in blowouts permits high speed winds to entrain and remove sediment. Whereas much research has examined patterns of wind flow and sediment transport on the stoss slopes and lee of sand dunes, no study has yet investigated the connections between secondary air‐flow structures and sediment transport in a blowout where zones of streamline compression, expansion and steering are less clearly delineated. In this study we investigated the variability of sediment flux and its relation to near‐surface wind speed and turbulence within a trough blowout during wind flow that was oblique to the axis of the blowout. Wind flow was measured using six, three‐dimensional (3D) ultrasonic anemometers while sediment flux by eight sand traps, all operating at 25 Hz. Results demonstrated that sediment flux rates were highly variable throughout the blowout deflation basin, even over short distances (< 0.5 m). Where flow was steadiest, flux was greatest. Consequently the highest rates of sediment transport were recorded on the erosional wall crest where flow was compressed and accelerated. The strength of correlation between sediment flux and wind parameter improved with an increase in averaging interval, from 10 seconds to 1 minute. At an interval of 10 seconds, however, wind speed correlated best with flux at seven of eight traps, whereas at an interval of one minute Turbulent Kinetic Energy (TKE) provided the best correlation with flux at six of the eight traps. Correlation between sediment flux and wind parameters was best in the centre of the blowout and poorest on the erosional wall crest. The evidence from this paper suggests, for the first time, that TKE may be a better predictor of sediment transport at minute scale averaging intervals, particularly over landforms where wind flow is highly turbulent. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

16.
Sand columns, sand cones, sand mushrooms and other striking sand forms are frequently observed in the Dutch and German beach and dune sands. This paper aims to clarify the mechanism of sand column formation. Recently it has become evident that homogeneous beach and dune sands often become irregularly wetted by infiltrating rainwater. In otherwise dry sandy soils, wet preferential flow paths (‘fingers’) may develop. At two test sites the volumetric soil moisture content varied between 0·2 and 12·0 per cent. The wet fingers represent the premature state of sand columns. When the dry sand in between these fingers is blown away by the wind, the more resistant wet sand of the fingers will remain in its place and appear as sand columns at the surface. As a result of wind and erosive sand drifts, striking sand forms may be formed.  相似文献   

17.
The sound-producing mechanism of booming sand has long been a pending problem in the blown sand physics. Based on the earlier researches, the authors collected some silent sand samples from Teng- ger Desert, Australian Desert, Kuwait Desert, beaches of Hainan Island and Japanese coast as well as the soundless booming sand samples from the Mingsha Mountain in Dunhuang to make washing ex- periments. In the meantime the chemical corrosion experiment of glass micro-spheres, surface coating experiment and SEM examination were also conducted. The experimental results show that the sound production of booming sand seems to have nothing to do with the presence of SiO2 gel on the surface of sand grains and unrelated to the surface chemical composition of sand grains but is related to the resonance cavities formed by porous (pit-like) physical structure resulting from a number of factors such as wind erosion, water erosion, chemical corrosion and SiO2 gel deposition, etc. Its resonance mechanism is similar to that of Hemholz resonance cavity. Under the action of external forces, nu- merous spherical and sand grains with smooth surface and porous surface are set in motion and rub with each other to produce extremely weak vibration sound and then become audible sound by human ears through the magnification of surface cavity resonance. However the booming sands may lose their resonance mechanism and become silent sand due to the damping action caused by the invasion of finer particles such as dust and clay into surface holes of sand grains. Therefore, clearing away fine pollutants on the quartz grain surface is an effective way to make silent sand emit audible sound.  相似文献   

18.
This paper deals with the effect of rainfall on the process of wind erosion of beach sands and presents results from both field and wind tunnel experiments. Although sediment transport by splash is of secondary importance on coastal dunes, splash–saltation processes can move sediments in conditions where no motion is predicted by aeolian processes. The effect of raindrop impact on the movement of soil particles by wind was measured on a sand beach plain using an acoustic sediment sampler. In general, an increase of particle movement by wind at the sensor heights was observed during rainfall. Rainfall also affected the wind erosion process during and after rain by changing the cohesive conditions of the surface. The influence of the surface moisture content on the initiation of wind erosion and on the vertical distribution of transported sand particles was studied in a wind tunnel. Moisture significantly increased threshold wind velocities for the initiation of sediment transport and modified vertical sediment profiles.  相似文献   

19.
Summary The main characteristics of surface winds are tabulated for 34 Antarctic stations. Using these data, supplemented by traverse records, the average wind flow is interpolated for each region and presented as a map showing the pattern of surface wind flow for the whole continent. Attention is focused on the flow in relation to surface contours. Statistics are presented for surface slope, wind speed, temperature, seasonal variations of speed and temperature, diurnal variation (including power spectra) of the wind speed and times of maximum and minimum speed at coastal and inland stations, wind frequency versus direction, the occurrence of calms, the deviation of the plateau wind from the downslope direction, the wind direction near the front of ice shelves, the proportion of cloud cover, and wind chill factors. In all cases data are grouped according to the environs of the stations in an attempt to isolate systematic differences depending on location: coastal stations near the foot of the ice slope and fully exposed to katabatic flow, coastal stations on offshore islands, coastal stations on peninsulas, coastal stations on extensive rock areas, ice shelf stations and inland stations.  相似文献   

20.
Accurate knowledge of the surface roughness and the resultant wind speed are important for many applications, such as climatic models, wind power meteorology, agriculture and erosion hazards, especially on sand dunes in arid and semi‐arid environments, where vegetation cover is scarce. In this study we aimed at quantifying the effects of vegetation cover and topography on surface roughness over a stabilizing dune field on the southern coast of Israel. Forty‐six wind measurements were made at various distances from the coastline, ranging from 10 to 2800 m, and z0 values were calculated from the wind measurements based on the ratio between the wind gust and the average wind speed. We estimated vegetation cover using the soil adjusted vegetation index (SAVI) from Landsat satellite images for the upwind sector at various lengths, ranging from 15 to 400 m, and based on digital elevation models and differential GPS field measurements we calculated the topographic variable of the relative heights of the stations. z0 values were positively correlated with the winter SAVI values (r = 0·87 at an upwind length of 200 m) and negatively correlated with the relative height (r = ?0·68 at an upwind length of 200–400 m for the inland dune stations). Using these variables we were able to create a map of estimated z0 values having an accuracy of over 64%. Such maps provide a better understanding of the spatial variability in both wind speed and sand movement over coastal dune areas. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号