The orientation of helical flow in curved channels

The planform patterns of meandering submarine channels and subaerial fluvial bends show many similarities that have given rise to strong analogies concerning the fluid dynamics of these channel types. Existing models of helical motion in open‐channel bends depict flow that is characterized by surface flow towards the outer bank, and basal flow towards the inner bank. This paper investigates and compares, through an analytical model and physical experiment, flows within fluvial meanders, and submarine channel bends that contain density‐driven gravity currents. The results indicate that the sense of helical motion can be reversed in submarine bends that contain density currents when compared with fluvial bends, and that the orientation of the helical flow is dependent on the vertical distribution of downstream velocity. Specifically, the sense of helical motion is reversed in bends when the maximum downstream velocity is near the bed, resulting in near‐bed flow towards the outer bank. These findings suggest that the dynamics of sediment transport and deposition in curved channels with such velocity profiles will be fundamentally different to those currently assumed from sinuous open‐channels.     

基于X形真实裂隙通道的煤层瓦斯渗流模拟

使用常规X射线或CT扫描进行图像处理,研究裂隙结构时,主裂隙不易提取,其特征不明显,不能直接反映主裂隙构造与流体速度关系。运用高清相机拍摄井下煤壁X形真实裂隙进行数字化处理,并运用AutoCAD软件提取裂隙特征,将图片矢量化导入Comsol Mutiphysics仿真模拟软件进行计算,模拟得到瓦斯分布压力场和渗流速度场云图。结果表明:含X形裂隙煤样中,瓦斯自入口开始,渗流压力从左至右递减,裂隙通道内瓦斯压力均匀分布,是一段压力缓冲区;非裂隙区渗流速度场分布不均匀,X形裂隙支流处较汇流处瓦斯渗流更为活跃,瓦斯的流向和裂隙走向的夹角对渗流速度有明显的影响,瓦斯自下边界流入时的最大渗流速度是自左边界流入的29.5倍;裂隙通道内的渗流速度与裂隙的尺度成单调递减对数函数关系,裂缝尺度越大,达西渗流速度越低,当裂隙尺度为0.68~1.23 mm时对渗流速度影响最明显。研究成果可直观地了解煤裂隙内瓦斯渗流特征。      

Large eddy simulation of hydrocyclone—prediction of air-core diameter and shape

Hydrocyclones are widely used in the mining and chemical industries. An attempt has been made in this study, to develop a CFD (computational fluid dynamics) model, which is capable of predicting the flow patterns inside the hydrocyclone, including accurate prediction of flow split as well as the size of the air-core. The flow velocities and air-core diameters are predicted by DRSM (differential Reynolds stress model) and LES (large eddy simulations) models were compared to experimental results. The predicted water splits and air-core diameter with LES and RSM turbulence models along with VOF (volume of fluid) model for the air phase, through the outlets for various inlet pressures were also analyzed. The LES turbulence model led to an improved turbulence field prediction and thereby to more accurate prediction of pressure and velocity fields. This improvement was distinctive for the axial profile of pressure, indicating that air-core development is principally a transport effect rather than a pressure effect.     

Flow and sediment dynamics in a low sinuosity, braided river: Calamus River, Nebraska Sandhills

The interaction between channel geometry, flow, sediment transport and deposition associated with a midstream island was studied in a braided to meandering reach of the Calamus River, Nebraska Sandhills. Hydraulic and sediment transport measurements were made over a large discharge range using equipment operated from catwalk bridges. The relatively low sinuosity channel on the right-hand side of the island carries over 70% of the water discharge at high flow stages and 50–60% at low flow stages. As a result, mean velocity, depth, bed shear stress and sediment transport rate tend to be greater here than in the more strongly curved left-hand channel. The loci of maximum flow velocity, depth and bed shear stress are near the centre of the channel upstream of the island, but then split and move towards the outer banks of both channels downstream. Variations in these loci depend on the flow stage. Topographically induced across-stream flows are generally stronger than the weak, curvature-induced secondary circulations. Water surface topography is controlled mainly by centrifugal accelerations and local changes in downstream flow velocity. The averaged water surface slope of the study reach varies very little with discharge, having values between 0·00075 and 0·00090. As bed shear stress generally varies in a similar way to mean velocity, friction coefficients vary little, normally being in the range 0·07–0·13. These values are similar to those in straight channels with sandy dune-covered beds. Bedload is moved mainly as dunes at all flow stages. Grain size is mainly medium sand with coarse sand moved in thalwegs adjacent to the cut banks, and with fine sand at the downstream end of the island. These patterns of flow velocity, depth, water surface topography, bed shear stress, bedload transport rate and mean grain size can be accurately predicted using theoretical models of flow, bed topography and sediment transport rate in single river bends, applied separately to the left and right channels. During high flow stages deposition occurs persistently near the downstream end of the island, and cut banks are eroded. Otherwise, erosion and deposition occurs only locally within the channel as discharge varies. Abandonment and filling of a strongly curved channel segment may occur by migration of an upstream bar into the channel entrance at a high flow stage.     

Coupled DEM-CFD investigation on the formation of landslide dams in narrow rivers

Large-scale landslide dams can induce significant hazards to human lives by blocking the river flows and causing inundation upstream. They may trigger severe outburst flooding that may devastate downstream areas once failed. Thus, the advancement in understanding the formation of landslide dams is highly necessary. This paper presents 3D numerical investigations of the formation of landslide dams in open fluid channels via the discrete element method (DEM) coupled with computational fluid dynamics (CFD). By employing this model, the influence of flow velocity on granular depositional morphology has been clarified. As the grains settle downwards in the fluid channel, positive excess water pressures are generated at the bottom region, reducing the total forces acting on the granular mass. In the meantime, the particle sedimentations into the fluid channel with high impacting velocities can generate fluid streams to flow backwards and forwards. The coupled hydraulic effects of excess water pressure and fluid flow would entrain the solid grains to move long distances along the channel. For simulations using different flow velocities, the larger the flow velocity is, the further distance the grains can be transported to. In this process, the solid grains move as a series of surges, with decreasing deposit lengths for the successive surges. The granular flux into the fluid channel has very little influence on the depositional pattern of particles, while it affects the particle–fluid interactions significantly. The results obtained from the DEM-CFD coupled simulations can reasonably explain the mechanisms of granular transportation and deposition in the formation of landslide dams in narrow rivers.     

Downstream grain‐size changes associated with a transition from single channel to anabranching

Downstream variation in grain size associated with changes in river pattern is a topic that interests multiple disciplines. How grain size varies between adjacent reaches with strongly contrasting river pattern is an outstanding question. This study presents a combined field and numerical modelling investigation of a river with a downstream planform change from single channel to anabranching, where the planform is controlled by a change in underlying lithology. This approach enabled exploration of the controls on sedimentology in a river for which there is very limited opportunity to collect flow and sediment transport data. This study shows that the surficial grain size decreases as a result of the downstream change in planform. This is because of a decrease in flow velocity and shear stress associated with a decrease in channel depth related to the planform change. Channel geometries in both the field and modelling data fit into distinct groups based on channel depth, the deepest being the single channel reach and the shallowest being the anabranching. This downstream reduction in channel dimension (depth) is caused because the total discharge is split from one channel into multiple channels. The coarsest grain sizes (cobble) are deposited at the terminus of the single channel and in the distributary channels; anabranching channels contain sand‐size sediments. This study shows that, in a transition from single channel to anabranching, the channel dimensions decrease as the number of channels increases, resulting in a decrease in bed shear stress and the fining of bed material downstream.     

缝洞型油藏大尺度可视化水驱油物理模拟实验及机理

缝洞型碳酸盐岩油藏由于其缝洞组合关系的复杂性,使得储层流体的渗流规律有别于常规油藏。根据某油田奥陶系缝洞型碳酸盐岩的实际情况设计制作了大尺度可视化物理模型,并按照现场实际设计了二注三采的实验井网及注采方案,以应用物理模拟实验来研究缝洞型碳酸盐岩油藏的开发特征及油井生产规律。结合实验结果,利用J.Cruz-Hernández建立的数学模型对生产数据进行拟合,以研究缝洞型碳酸盐岩油藏中流体的渗流机理,J.Cruz-Hernández数学模型中的空间系数和流动速度对采收率影响较大,提出模型的改进应考虑重力分异作用、溶洞比例等影响因素。     

海底流体渗漏区的热流探测技术与方法

海底流体渗漏区往往伴随着丰富的海底资源,但具有区域较小、流体渗漏比较活跃、地质背景复杂等特点.为了更好地研究流体流动特征和富集资源等复杂问题,需要精细测量海底流体渗漏区的热流.通过总结各种热流探针的技术特点及海底流体渗漏区的热流特征,并结合最新的海洋地质调查发现,由机械手操控的热流探针,可以快速、准确地测量地质背景复杂的海底流体渗漏区的热状态.      

示踪试验在探查灰岩含水层突水通道中应用

为了查明新庄孜煤矿63301工作面1组煤层底板灰岩含水层的突水通道,分别从太原组灰岩、奥陶系灰岩含水层投放NaCl示踪剂,寒武系灰岩含水层观测孔投放KI示踪剂,在工作面突水点间隔采集样品。经测试分析其浓度随时间变化关系曲线发现:底板太原组灰岩含水层中存在多条小通道和一条大通道,奥陶系灰岩含水层中存在多条小通道,而寒武系灰岩含水层存在2条通道,在不同灰岩含水层通道中,水流速度存在较大差异性,反映了其岩溶裂隙发育非均匀性特点。此外,利用本次突水资料,计算灰岩含水层的参数。为工作面底板太原组灰岩水害治理提供依据。      

30°分水角明渠水流紊动特性试验研究

灌区分水口多为直角,附近普遍存在冲刷、淤积及结构破坏现象。天然河流分、汇流多为锐角,为探索非直角分水口区域明渠水流紊动特性,以30°分水角明渠为研究对象,采用声学多普勒点式流速仪测量各断面上的三维瞬时流速,分析典型断面上的时均流速、环流强度、紊动强度、紊动能和床面剪切应力分布规律。试验结果显示：分水口处水流横向流速沿渠宽方向变化较大,易形成环流,导致局部冲淤现象明显;靠近口门侧水流紊动强度变化剧烈且分布无规律,最大值出现在口门上唇断面;分水口处下层水体紊动能明显大于上层水体,紊动能较大值多集中在口门附近;口门断面下唇附近床面剪切应力较大,易出现冲刷侵蚀,破坏渠道稳定;与直角分水口相比,分水角为30°时,泥沙颗粒不易进入侧渠道,水流对渠底及边壁侵蚀速率较低。研究结果可为灌区渠系设计及运行提供参考。     

明渠恒定紊流的总流控制方程与能量损失特性

为进一步深化对明渠总流控制方程的认识,探究总流能量损失的构成与分布特点,通过理论分析,在黏性流体力学理论的框架下构建了描述明渠恒定紊流总体特性的总流积分模型与总流微分模型的控制方程,其模型参数能直接通过流动统计特征量在过流断面上的分布来获取,由此实现明渠流的流场特性描述与总流描述的统一,并得到了总流能量损失的显示表达式。同时指出,总流能量损失由黏性耗散与维持紊动两部分构成,在矩形明渠恒定均匀流紊流的分类能量损失构成中,壁面上以黏性耗散部分为主,维持紊动部分的能量损失密度随着离开壁面距离的增加而快速增加,且随着雷诺数的增大,其增加速度也越快。     

明渠恒定紊流的总流控制方程与能量损失特性

为进一步深化对明渠总流控制方程的认识,探究总流能量损失的构成与分布特点,通过理论分析,在黏性流体力学理论的框架下构建了描述明渠恒定紊流总体特性的总流积分模型与总流微分模型的控制方程,其模型参数能直接通过流动统计特征量在过流断面上的分布来获取,由此实现明渠流的流场特性描述与总流描述的统一,并得到了总流能量损失的显示表达式。同时指出,总流能量损失由黏性耗散与维持紊动两部分构成,在矩形明渠恒定均匀流紊流的分类能量损失构成中,壁面上以黏性耗散部分为主,维持紊动部分的能量损失密度随着离开壁面距离的增加而快速增加,且随着雷诺数的增大,其增加速度也越快。     

Estimation of lahar and lahar-runout flow hydrograph on natural beds

A numerical model is used to simulate channelized lahars, flowing with a near-constant sediment concentration and volume. Water and debris are usually mobilized in short times and the peak discharge of lahars may reach many thousands of cubic meters per second in a few minutes. A relation for the energy dissipation term must be provided in the model, which in turn depends on debris flow rheology and shape and status of channel bed. A discussion of the form of this term is performed through the simulation of some historical (among the well-documented) lahars and lahar-runout flows with concentrations ranging in a wide spectrum up to 70 percent by volume and irregularly shaped sand and coarser particles dispersed in a fluid matrix of water and fine material. As concentration increases and turbulence decreases, the dissipative term, which, in the first case, is proportional to the square average flow velocity (Manning or Chezy formulation) is well described by a linear dependence on flow velocity, as expected in the laminar case. The numerical reproduction of the examined historical cases suggests that the model can be used for hazard assessment, if some hypotheses are made about lahar hydrograph at the source, its volume, and the shape of the dissipative term.     

The motion of sediment-water mixtures during intense bedload transport: computer simulations

A new model, which couples fluid and particle dynamics, has been developed to study the motion of the sediment-water mixture during intense bedload transport, including the velocity profiles of both sediment and water, the roughness length of an upper plane bed and the thickness of moving sediment layers. Standard mixing length theory is used to model the motion of water above the boundary between the overlying water and the sediment-water mixture. The turbulent flow within the moving sediment layers is described by a shear stress model, in which the effective viscosity of the flowing water is proportional to the velocity difference between the fluid and the sediment. The particle dynamics method, in which the equations of motion of each of many particles are solved directly, is applied to model the movement of sediment particles. The particle-fluid interaction is expressed by a velocity-squared fluid drag force exerted on each sediment particle. Both computer simulation results and theoretical analysis have shown that the velocities of both sediment and fluid during intense sediment transport decrease exponentially with depth in the top layers of a fast-moving sediment—water mixture. The thickness of the moving sediment layers, obtained from the computer simulation results, is proportional to the shear stress, which agrees with previous experimental observations.     

浅水流动阻力特性的试验研究

为研究浅水流动的阻力特性,专门设计了一个试验模型,其规模较大,测量设施齐全。通过多种流量和加糙条件的试验得出:加糙密度将直接影响水流的阻力,小流量时的阻力系数变化明显,随流量的加大,加糙的影响逐渐减弱;阻力系数随相对粗糙度的加大而增加,不受流量变化的影响;表面流速分布均匀,其平均值与断面平均流速相关性良好。     

http://www.sciencedirect.com/science/article/pii/S1674987111000764

The Jinding Zn-Pb deposit has been generally considered to have formed from circulating basinal fluids in a relatively passive way,with fluid flow being controlled by structures and sedimentary facies,similar to many other sediments-hosted base metal deposits.However,several recent studies have revealed the presence of sand injection structures,intrusive breccias,and hydraulic fractures in the open pit of the Jinding deposit and suggested that the deposit was formed from explosive release of overpres-sured fluids.This study reports new observations of fluid overpressure-related structures from underground workings(Paomaping and Fengzishan).which show clearer crosscutting relationships than in the open pit.The observed structures include:I) sand(±rock fragment) dikes injecting into fractures in solidified rocks:2) sand(±rock fragment) bodies intruding into unconsolidated or semi-consolidated sediments;3) disintegrated semi-consolidated sand bodies;and 4) veins and breccias formed from hydraulic fracturing of solidified rocks followed by cementation of hydrothermal minerals.The development of ore minerals(sphalerite) in the cement of the various clastic injection and hydraulic fractures indicate that these structures were formed at the same time as mineralization.The development of hydraulic fractures and breccias with random orientation indicates small differential stress during mineralization,which is different from the stress field with strong horizontal shortening prior to mineralization. Fluid flow velocity may have been up to more than 11 m/s based on calculations from the size of the fragments in the clastic dikes.The clastic injection and hydraulic fracturing structures are interpreted to have formed from explosive release of overpressured fluids,which may have been related to either magmatic intrusions at depth or seismic activities that episodically tapped an overpressured fluid reservoir.Because the clastic injection and hydraulic structures are genetically linked with the mineralizing fluid source,they can be used as a guide for mineral exploration.     

植被发育斜坡土体大孔隙三维重构模型渗流场的LBM数值模拟

以云南昭通头寨植被发育斜坡土体为研究对象,结合CT扫描试验与数字图像处理技术实现了其三维大孔隙空间结构模型的重构。然后基于格子Boltzmann方法（LBM）的D3Q19模型,将固体土骨架视为渗流场边界,并设置反弹格式的边界条件模拟土骨架与水分间的相互作用,从细观角度研究了水在植被发育斜坡土体大孔隙中的渗流特性。研究结果表明：土体渗流过程中,在大孔隙纵向连通的通道内形成了优先流,且通道中的流速远超其他部位;而在封闭或者连通性差的大孔隙中导水率极小,流速几乎为0。土体中大孔隙通道内的流速由孔壁至通道中心逐渐增大,且流速与至通道中心的距离近似呈二次抛物线的分布特征。沿深度方向土体切片大孔隙率大的截面平均渗流流速Uz也较大,从整体变化趋势来看,截面大孔隙率与平均渗流流速有相似的变化规律,这充分说明植被发育斜坡土体内的大孔隙分布对土壤沿深度方向的渗流特性有重要影响。     

Lateral accretion in a deep-marine channel complex: implications for channellized flow processes in turbidity currents

Some deep-marine channels show striking similarities to fluvial channels, despite major differences in the properties of the flows that they conduct. Some field observations from deep-marine channel deposits within a Late Cretaceous palaeo-canyon in the Rosario Formation of Baja California, Mexico, that bear on these comparisons have been reported. These channel deposits contain erosively based lateral accretion sets. These sets are generally a few metres thick and resemble fluvial point bar deposits. Sediment movement and deposition within these accretion deposits was induced by turbidity currents several to many times thicker than the depth of the channel, moving at several metres per second. The inclined sets define laterally migrating and sinuous channels locally at a high angle to the confining canyon. The instantaneous channel widths varied from 6 to 39 m, the depths from 2·5 to 4 m and the sinuosities from 1·3 to 3·1. Palaeocurrent data, taken mostly from clast imbrication in conglomerates, indicates current modes along the channel thalweg, but with other directions representing either secondary flow (oriented primarily up the point bar) or over-passing canyon-confined flow. It is suggested that, at times, the lower part of the turbidity currents flowing down the channels behaved similarly to within-bank fluvial currents, with a cross-channel component of flow towards the cut bank, and return flow at the bed sweeping up the point bar. At other times, this secondary circulation may be absent or reversed, which may be related to changes in flow thickness, coupling with the overriding flow and possible flow separation.     

Characteristics of modern carbonate contourite drifts

Carbonate environments inhabit the realm of the surface, intermediate and deep currents of the ocean circulation where they produce and continuously deliver material which is potentially deposited into contourite drifts. In the tropical realm, fine‐grained particles produced in shallow water and transported off‐bank by tidal, wind‐driven, and cascading density currents are a major source for transport and deposition by currents. Sediment production is especially high during interglacial times when sea level is high and is greatly reduced during glacial times of sea‐level lowstands. Reduced sedimentation on carbonate contourite drifts leads to early marine cementation and hardened surfaces, which are often reworked when current strength increases. As a result, reworked lithoclasts are a common component in carbonate drifts. In areas of temperate and cool water carbonates, currents are able to flow across carbonate producing areas and incorporate sediment directly to the current. The entrained skeletal carbonate particles have variable bulk density and shapes that lower the prediction of transport rates in energy‐based transport models, as well as prediction of current velocity based on grain size. All types of contourite drifts known in clastic environments are found in carbonate environments, but three additional drift types occur in carbonates because of local sources and current flow diversion in the complicated topography inherent to carbonate systems. The periplatform drift is a carbonate‐specific plastered drift that is nearly exclusively made of periplatform ooze. Its geometry is built by the interaction of along‐slope currents and downslope currents, which deliver sediment from the adjacent shallow‐water carbonate realm to the contour current via a line source. Because the periplatform drift is plastered on the slopes of the platforms it is also subject to mass gravity flow and large slope failures. At platform edges, a special type of patch drift develops. These hemiconal platform‐edge drifts also contain exclusively periplatform ooze but their geometry is controlled by the current around the corner of the platform. At the north‐western end of Little and Great Bahama Bank are platform‐edge drifts that are over 100 km long and up to 600 m thick. A special type of channel‐related drift forms when passages between carbonate buildups or channels within a platform open into deeper water. A current flowing in these channels will entrain material shed from the sediment producing areas. At the channel mouth, the sediment‐charged current deposits its sediment load into the deeper basin. With continuous flow, a submarine delta drift is built that progrades into the deep water. The strongly focused current forming the delta drift, is able to rework coarse skeletal grains and clasts, making this type of carbonate drift the coarsest drift type.     

梯形断面明渠中纵向离散系数研究

基于最大信息熵原理,提出了一种确定梯形断面纵向流速分布的方法,研究了梯形断面明渠中流动横向不均匀和垂向不均匀对纵向离散的影响,建立了一个针对梯形断面明渠流动的纵向离散系数计算公式。公式将纵向离散系数与反映断面流速分布不均匀和壁面影响的参数建立联系,机理更加清楚,预测结果与其他学者相应的实验结果吻合良好,该方法理论推导过程严密,不依赖于特定的试验结果或实际测量资料,为梯形断面明渠的污染物混合输移过程中参数确定提供了有效的方法和途径。