Effects of rock fragment size and cover on overland flow hydraulics,local turbulence and sediment yield on an erodible soil surface   总被引：1，自引：0，他引：1  

Kristin Bunte  Jean Poesen 《地球表面变化过程与地形》1994,19(2):115-135

The interactions between overland flow hydraulics and sediment yield were studied in flume experiments on erodible soil surfaces covered by rock fragments. The high erodibility of a non-cohesive fine sediment (D₅₀ + 0·09mm) permitted the effects of local turbulence and scour on sediment yield to be examined. Overland flow hydraulics and sediment yield were compared for experiments with pebble (D₅₀ + 1·5cm) and cobble (D₅₀ + 8·6cm) rock fragment covers. Cover percentages range from 0 to 99 per cent. Rock fragment size strongly affects the relations between flow hydraulics and rock fragment cover. For pebbles spatially-averaged hydraulic parameters (flow velocity, flow depth, effective flow width, unit discharge, total shear stress, Darcy-Weisbach friction factor, percentage grain friction and grain shear stress) vary most rapidly within cover percentages at low covers (power functions). In contrast, for cobbles these parameters vary most rapidly within cover percentages at high covers (exponential functions). As the type of the function that describes the relation between flow hydraulics and cover percentage can be deduced from the ratio of rock fragment height to flow depth, the continuity equation can be employed to determine the actual coefficients of the functions, provided the regression of one hydraulic parameter (e.g. flow velocity) with cover percentage is known and a good estimate exists for two values of another hydraulic variable for a low and a high cover percentage. The variation of sediment yield with cover percentage is also strongly dependent on rock fragment size, but neither the convex-upward relation for pebbles, nor the positive relation for cobbles can be solely attributed to the spatially averaged hydraulics of sheet-flow. Rock fragments induce local turbulence that leads to scour hole development on the stoss side of the rock fragments while deposition commonly occurs in the wake. This local scour and deposition substantially affects sediment yield. However, scour dimensions cannot be predicted by spatially averaged flow hydraulics. An adjustment of existing scour formulas that predict scour around bridge piers is suggested. Sediment yield from non-cohesive soils might then be estimated by a combination of sediment transport and scour formulas.  相似文献   

Spatial Variability of Both Turbulent Fluxes and Temperature Profiles in an Urban Roughness Layer     

M. Kanda  R. Moriwaki  F. Kasamatsu 《Boundary-Layer Meteorology》2006,121(2):339-350

The spatial variability of both turbulent flow statistics in the roughness sublayer (RSL) and temperature profiles within and above the canopy layer (CL) were investigated experimentally in a densely built-up residential area in Tokyo, Japan. Using five towers with measuring devices, each tower isolated from the others by at least 200 m, we collected high-frequency measurements of velocity and temperature at a height z=1.8 z _H, where z _H, the mean building height in the area, is 7.3 m. Also, temperature profiles were measured from z=0.4 to 1.8 z _H. The ‘areal mean’ geometric parameters that were obtained for the areas within 200 m of each tower were fairly homogeneous among the tower sites. The main results are as follows: (1) The spatial variability of all RSL turbulent statistics, except the sensible heat flux, was comparable to that reported in a pine forest. Also, the variability decreased with increasing friction velocity. (2) The spatial variability of the RSL sensible heat flux was larger than that reported in a pine forest. Also, the variability depended on the time of the day and became larger in the morning. The difference among the sites was well related to the areal fraction of vegetation. (3) The spatial variability of the CL temperature profile depended on the time of the day and became larger in the morning. Nevertheless, the spatial standard deviation of CL temperature was always below 0.7 K. (4) It is suggested that the “warming-up” process in the morning when heat storage is dominant increases the spatial variation of RSL sensible heat flux and CL temperature according to the local properties around each tower and the variation decreases once there is further convective mixing in the midday  相似文献   

Dispersion dependence on retardation in a real fracture geometry using lattice-gas cellular automaton     

V. Pot  A. Genty 《Advances in water resources》2007

Fractures have been recently identified in potential host rock for high level nuclear waste disposal, like indurated argilite formations. These fractures appear as potential rapid pathways for radionuclides transport and hydrodynamic properties of the transport inside these systems must thus be characterized. Miscible non-sorbing and sorbing tracers displacements were performed on a 2-D model derived from a real fracture geometry observed in the Tournemire argilite formation with a lattice-gas cellular automaton (LGA). LGA was shown to easily handle the complex geometry of such a fracture. The numerical breakthrough curves obtained were inverted with the 1-D CDE and MIM transport models. Two main conclusions were drawn: (i) at the length scale of the study, the non-sorbing tracer transport in our fracture geometry was more accurately interpreted in terms of the MIM model rather than in terms of the classical CDE model; (ii) in order to correctly model the sorbing tracers migration, the hydrodynamic dispersion coefficient value was found to increase with the increase of the retardation factor. A semi-empirical relation based on the Taylor–Aris theory was then used to describe this dependency.  相似文献   

Velocity and concentration profiles of particle movement in sheet flows     

Yu-Hai Wang  Guo-Hua Yu 《Advances in water resources》2007

Sheet flows occur widely in natural free-surface flows including rivers in flood, tidal estuaries and coastal waters in storm conditions when bed shear stress becomes sufficiently high. Particle volumetric concentration in sheet flows normally follows a linear distribution with the Rouse [Rouse H. Modern conceptions of the mechanics of fluid turbulence. Trans ASCE, 1937;102:463–543] distribution applicable in the dilute water column above the sheet-flow layer. However, a well-verified formula for determining particle velocity distribution in sheet flows is still lacking. This paper presents formulas to describe the particle velocity profile in steady or oscillatory sheet flows. They compare well with measured data. In particular, the novel formula for determining the particle velocity at the top of bedload–sediment-dominated sublayer in sheet flows is also well verified with measured data.  相似文献   

THREE DIMENSIONAL NUMERICAL MODELLING OF FLOW AND SEDIMENT TRANSPORT IN RIVERS     

M. A. MEKONNEN B. DARGAHI 《国际泥沙研究》2007,22(3):188-198

The 3D numerical model, ECOMSED (open source code), was used to simulate flow and sediment transport in rivers. The model has a long history of successful applications to oceanic, coastal and estuarine waters. Improvements in the advection scheme, treatment of river roughness parameterization and shear stress partitioning were necessary to reproduce realistic and comparable results in a river application. To account for the dynamics of the mobile bed boundary, a model for the bed load transport was included in the code. The model reproduced observed secondary currents, bed shear stress distribution and erosion-deposition patterns on a curved channel. The model also successfully predicted the general flow patterns and sediment transport characteristics of a 1-km long reach of the River Klar?lven, located in the north of the county of V?rmland, Sweden.  相似文献   

Wind profiles,momentum fluxes and roughness lengths at Cabauw revisited   总被引：1，自引：1，他引：1  

J. W. Verkaik  A. A. M. Holtslag 《Boundary-Layer Meteorology》2007,122(3):701-719

We describe the results of an experiment focusing on wind speed and momentum fluxes in the atmospheric boundary layer up to 200 m. The measurements were conducted in 1996 at the Cabauw site in the Netherlands. Momentum fluxes are measured using the K-Gill Propeller Vane. Estimates of the roughness length are derived using various techniques from the wind speed and flux measurements, and the observed differences are explained by considering the source area of the meteorological parameters. A clear rough-to-smooth transition is found in the wind speed profiles at Cabauw. The internal boundary layer reaches the lowest k-vane (20 m) only in the south-west direction where the obstacle-free fetch is about 2 km. The internal boundary layer is also reflected in the roughness lengths derived from the wind speed profiles. The lower part of the profile (< 40 m) is not in equilibrium and no reliable roughness analysis can be given. The upper part of the profile can be linked to a large-scale roughness length. Roughness lengths derived from the horizontal wind speed variance and gustiness have large footprints and therefore represent a large-scale average roughness. The drag coefficient is more locally determined but still represents a large-scale roughness length when it is measured above the local internal boundary layer. The roughness length at inhomogeneous sites can therefore be determined best from drag coefficient measurements just above the local internal boundary layers directly, or indirectly from horizontal wind speed variance or gustiness. In addition, the momentum and heat fluxes along the tower are analysed and these show significant variation with height related to stability and possibly surface heterogeneity. It appears that the dimensionless wind speed gradients scale well with local fluxes for the variety of conditions considered, including the unstable cases.  相似文献   

盘锦芦苇湿地空气动力学参数动态特征及其影响因素分析   总被引：1，自引：0，他引：1  

何奇瑾  周广胜  周莉  王云龙 《气象与环境学报》2007,23(4):7-12

应用盘锦芦苇生态系统观测场的涡度相关通量观测系统和小气候梯度系统的观测资料,估算了芦苇下垫面的空气动力学参数,并分析了其影响因素。结果表明:芦苇湿地零平面位移d和粗糙度Z0的季节变化总体呈现先增加后减少的单峰曲线变化规律。Z0和d均在9月达到最大,分别为0.24 m和1.85 m;12月达到最小,分别为0.03 m和0.02 m。芦苇湿地的Z0和d的季节变化主要受叶面积指数(LAI)、株高(h)及风速的影响。d随h的增加而单调增加,Z0随着h的变化遵循二次曲线,d/h、Z0/h与LAI之间的复相关系数分别为0.99和0.78。研究亦表明,大气稳定度和风向也对d和Z0有一定的影响。  相似文献   

Velocity and Surface Shear Stress Distributions Behind a Rough-to-Smooth Surface Transition: A Simple New Model   总被引：1，自引：1，他引：0  

Leonardo P. Chamorro  Fernando Porté-Agel 《Boundary-Layer Meteorology》2009,130(1):29-41

A simple new model is proposed to predict the distribution of wind velocity and surface shear stress downwind of a rough-to-smooth surface transition. The wind velocity is estimated as a weighted average between two limiting logarithmic profiles: the first log law, which is recovered above the internal boundary-layer height, corresponds to the upwind velocity profile; the second log law is adjusted to the downwind aerodynamic roughness and local surface shear stress, and it is recovered near the surface, in the equilibrium sublayer. The proposed non-linear form of the weighting factor is equal to ln(z/z ₀₁)/ln(δ _i /z ₀₁), where z, δ _i and z ₀₁ are the elevation of the prediction location, the internal boundary-layer height at that downwind distance, and the upwind surface roughness, respectively. Unlike other simple analytical models, the new model does not rely on the assumption of a constant or linear distribution for the turbulent shear stress within the internal boundary layer. The performance of the new model is tested with wind-tunnel measurements and also with the field data of Bradley. Compared with other existing analytical models, the proposed model shows improved predictions of both surface shear stress and velocity distributions at different positions downwind of the transition.  相似文献   

Boundary-Layer Stress Instabilities in Neutral,Rotating Turbulent Flows     

Enrico Ferrero  Luca Mortarini  Massimigliano Manfrin  Arnaldo Longhetto  Rita Genovese  Renato Forza 《Boundary-Layer Meteorology》2009,130(3):347-363

Boundary-layer instabilities are studied by analyzing the results of laboratory simulations of wall turbulence in a shear-driven rotating flow. The experiments were carried out in the Turin University Laboratory rotating water tank, where a circular flow was generated by either increasing (spin-up) or decreasing (spin-down) the rotation speed of the platform. The flow was measured using a Particle Image Velocimetry technique and the developed turbulence analyzed. Two cases were accounted for, in the former the measurements were performed over a smooth surface (bottom of the tank), while in the latter a rough-to-smooth transition was considered. The turbulent boundary layer developed inside the tank is analyzed by means of vertical profiles of mean and turbulent quantities and on the basis of drag coefficients. Then turbulent structures developed in the different cases are shown and discussed in terms of the vorticity fields. Finally, an analysis based on the concept of swirling strength was carried out to select among the vortex extremes those associated with a coherent structure.  相似文献   

高大建筑物影响城市粗糙副层流场特征的数值研究   总被引：1，自引：1，他引：0  

刘罡  孙鉴泞  王霄雪 《气象科学》2012,32(4):385-394

用数值模拟方法研究了高大建筑物对城市粗糙副层气流场特征的影响。数值模式采用基于雷诺平均纳维—斯托克斯方程组的应用计算流体力学FLUENT软件,次网格湍流参数化选用k-ε闭合方案。建筑物用立方体表示,并规则排列于模拟区域内。通过改变高大建筑物的数量与位置,对建筑物阵列内及其上空的气流特征进行了多个算例的数值模拟。依据模拟结果计算获得建筑物区域的面积平均风速廓线,结果表明各算例的粗糙副层风速廓线各不相同。运用动力学方法由风速廓线计算出各算例的零平面位移高度和粗糙度,并与几种计算零平面位移高度和粗糙度的形态学方法进行了比较检验。结果表明两种形态学方法(Ba、Ma)计算所得的零平面位移高度与动力学方法计算结果很接近,但对于粗糙度而言,几种形态学方法的计算结果都明显偏高。  相似文献