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1.
以床面瞬时剪应力作为泥沙起动及输运的水动力机制,建立了结构物周围复杂流场下床面局部冲刷的数学模型。并应用大涡模拟方法对后台阶下游三维湍流流动进行数值模拟,得到台阶下游床面瞬时剪应力的分布规律。为了确定床面瞬时剪应力与泥沙上扬通量的关系,先应用数学模型对不同模型参数下,冲刷开始后5分钟时台阶下游床面形状进行试算。通过试算与实验结果的比较,确定床面瞬时剪应力与泥沙上扬通量关系中需要的模型参数。进一步对冲刷开始后30分钟内台阶下游床面演化规律进行模拟,模拟结果与实验结果相符合。 相似文献
2.
The instantaneous turbulent velocity field created by the breaking of spilling regular waves on a plane slope was measured in a plane running parallel to the slope using particle image velocimetry. The measurement plane was located at a height of about 1 mm above the bed. The measurement area encompassed the region where the large eddies generated at incipient wave breaking impinged on the bottom inside the surf zone. A total of 30 trials were conducted under identical experimental conditions. In each trial, six consecutive wave cycles were recorded. The measured velocity fields were separated into a mean flow and a turbulence component by ensemble averaging. The instantaneous turbulent velocity fields were analyzed to determine the occurrence frequency, location, geometry and evolution of the large eddies, and their contributions to instantaneous shear stresses, turbulent kinetic energy and turbulence energy fluxes. The motion of single glass spheres along the bed was also investigated. The two-phase flow measurements showed that the velocity and displacement of large solid particles on a smooth bed were significantly affected by the magnitude and direction of turbulence velocities. Overall, this study has examined the kinematic and dynamic properties of large eddies impinging on the bed and the interaction of these large-scale turbulent flow structures with the mean flow. The study has also highlighted the important role of large eddies in sediment transport. 相似文献
3.
Incipient Motion of Sediment Under Currents 总被引：4，自引：3，他引：4
DOU Guoren 《中国海洋工程》2000,14(4):391-406
Summarized in the paper are the author's studies on incipient motion of sediment in recent 40years.In addition to the forces of gravity,drag and lift,the cohesive force and the additional static pres-sure are important for fine particles.The relations between three stages of incipient motion are defined byuse of instantaneous velocity.Formulas for initial velocity and critical shear stress are given and overallverified by the author's and others'experimental data. 相似文献
4.
模型变率对斜坡上泥沙起动相似影响研究 总被引：1，自引：0，他引：1
本文在前人的理论分析及水槽试验工作的基础上，分析了上举力系数对边坡泥沙起动影响，研究了常用模型沙（木屑、煤屑、电木粉），以平床泥沙起动相似设计，变态模型变率对边坡及水流纵向坡面上泥沙起动相似偏离影响 相似文献
5.
Tide-driven bed load transport is an important portion of the net annual sediment transport rate in many shoreface and shelf environments. However, bed load transport under waves cannot be measured in the field and bed load transport by currents without waves is barely measurable, even in spring tidal conditions. There is, consequently, a strong lack of field data and validated models. The present field site was on the shoreface and inner shelf at 2 to 8.5 km offshore the central Dutch coast (far outside the surfzone), where tidal currents flow parallel to the coast. Bed load transports were carefully measured with a calibrated sampler in spring tidal conditions without waves at a water depth of 13–18 m with fine and medium sands. The near-bed flow was measured over nearly a year and used for integration to annual transport rates. An empirical bed load model was derived, which predicts bed load transports that are a factor of > 5 smaller than predicted by existing models. However, they agree with laboratory data of sand and gravel transport in currents near incipient motion. The damped transport rates may have been caused by cohesion of sediment or turbulence damping due to mud or biological activity. The annual bed load transport rate was calculated using a probability density function (pdf) derived from the near-bed current and orbital velocity data which represented the current and wave climate well when compared to 30 years of data from a nearby wave station. The effect of wave stirring was included in the transport calculations. The net bed load transport rate is a few m2/year. This is much less than predicted in an earlier model study, which is partly due to different bed load models but also due to the difference in velocity pdf. The annual transport rate is very sensitive to the probability of the largest current velocities. 相似文献
6.
A new instrumented tripod, the GEOPROBE system, has been constructed and used to collect time-series data on physical and geological parameters that are important in bottom sediment dynamics on continental shelves. Simultaneous in situ digital recording of pressure, temperature, light scattering, and light transmission, in combination with current velocity profiles measured with a near-bottom vertical array of electromagnetic current meters, is used to correlate bottom shear generated by a variety of oceanic processes (waves, tides, mean flow, etc.) with incipient movement and resuspension of bottom sediment. A bottom camera system that is activated when current speeds exceed preset threshold values provides a unique method to identify initial sediment motion and bed form development.
Data from a twenty day deployment of the GEOPROBE system in Norton Sound, Alaska, during the period September 24 – October 14, 1976 show that threshold conditions for sediment movement are commonly exceeded, even in calm weather periods, due to the additive effects of tidal currents, mean circulation, and surface waves. 相似文献
7.
各向同性均匀湍流近壁结构及泥沙起动 总被引：3，自引：1，他引：3
采用振动格栅产生的各向同性均匀湍流研究了泥沙的起动。应用PTV技术和局部光学放大技术对湍流场进行了测量，发现近壁面湍流动能的指数分布规律。分析了泥沙在均匀同性湍流中的起动机制，并用概率统计理论对泥沙的起动进行了定量的描述。通过测定泥沙起动事件的概率，得到了同种泥沙在不同流动参数下起动的临界湍流动能。 相似文献
8.
Sediment incipient motion is a fundamental issue in sediment transport theory and engineering practice. Silt has transitional behavior between cohesive and non-cohesive sediment and its incipient motion is still poorly understood. This study aims to find an expression for incipient motion from silt to sand from a unified perspective and analysis. From the analysis of forces, using the derivation method for the Shields curve, an expression for sediment incipient motion is proposed for both silt and sand under conditions of combined waves and currents. The differences and similarities in the sediment motion threshold were analyzed under the effects of waves and currents, and fine and coarse sediment. The Shields number was revised by introducing the cohesive force and additional static water pressure, which indicates that this study could be seen as an extension of the Shields curve method for silt. A number of experimental datasets as well as field data were used to verify the formula. The effect of bulk density on fine sediment was discussed and tested using experimental data. 相似文献
9.
Prediction of threshold conditions and incipient motion is the essential issue for the study of sediment transport. This work compares existing empirical threshold curves proposed for Shields diagram, a method based on the concept of probability of sediment movement, and an empirical method based on movability number. These methods are used to predict the incipient motion conditions for experimental runs taken from various studies. Most of the experimental data, used in this work, have not been used before in derivation of alternative formulations for Shields diagram and other methods. The empirical threshold curves based on the Shields entrainment function was the least successful at predicting the measured incipient motion conditions, while the use of the movability number gives good predictions of critical shear velocity compared with experimental data. 相似文献
10.
崎头洋附近群岛水域泥沙特征及悬沙来源分析 总被引：1，自引：0，他引：1
依据实测水文泥沙资料分析舟山群岛南部崎头洋附近海域的悬沙与底沙特征,建立该水域二维潮流数学模型,同时根据底沙及悬沙的粒径分布和潮流的运动路径对该海域的悬沙来源做进一步分析.指出崎头洋附近群岛海域的悬沙除部分来源于潮汐动力作用下的底沙起动再悬浮之外,主要是杭州湾附近灰鳖洋水域的泥沙在潮汐动力作用下经群岛西北方向水道进入群岛内部水域的,进而在潮动力作用下经由东南方向水道进入东海及象山港附近水域. 相似文献
11.
从管道尾流区泥沙颗粒的受力平衡着手,探讨了管道冲刷的临界压力差。在此基础上,提出了一种防止或减少管道冲刷的新方法,即在管道下方安装柔性阻流器。通过物理模型试验,研究了单向恒定流及规则波作用下,柔性阻流器对管道两侧床面压力、冲刷深度的影响。试验结果表明:柔性阻流器的安装能明显减小管道两侧床面的压力差。在单向恒定流作用下,管道的冲刷深度随阻流器长度的增加而减小;当阻流器达到临界长度时,冲刷消失。规则波时,因阻流器的扰动作用,冲刷深度有所增加;但随阻流器长度的增加,其扰动作用减小,对床面的防护作用增加,因而冲刷深度减小。 相似文献
12.
《Marine Geology》2005,216(3):169-189
Simultaneous high frequency field measurements of water depth, flow velocity and suspended sediment concentration were made at three fixed locations across the high tide swash and inner surf zones of a dissipative beach. The dominant period of the swash motion was 30–50 s and the results are representative of infragravity swash motion. Suspended sediment concentrations, loads and transport rates in the swash zone were almost one order of magnitude greater than in the inner surf zone. The vertical velocity gradient near the bed and the resulting bed shear stress at the start of the uprush was significantly larger than that at the end of the backwash, despite similar flow velocities. This suggests that the bed friction during the uprush was approximately twice that during the backwash.The suspended sediment profile in the swash zone can be described reasonably well by an exponential shape with a mixing length scale of 0.02–0.03 m. The suspended sediment transport flux measured in the swash zone was related to the bed shear stress through the Shields parameter. If the bed shear stress is derived from the vertical velocity gradient, the proportionality coefficient between shear stress and sediment transport rate is similar for the uprush and the backwash. If the bed shear stress is estimated using the free-stream flow velocity and a constant friction factor, the proportionality factor for the uprush is approximately twice that of the backwash. It is suggested that the uprush is a more efficient transporter of sediment than the backwash, because the larger friction factor during the uprush causes larger bed shear stresses for a given free-stream velocity. This increased transport competency of the uprush is necessary for maintaining the beach, otherwise the comparable strength and greater duration of the backwash would progressively remove sediment from the beach. 相似文献
13.
Rheological Properties and Incipient Motion of Cohesive Sediment in the Haihe Estuary of China 总被引：2，自引：1，他引：2
The Haihe cohesive sediment, which is typical in China, is studied systematically for its basic physical and incipientmotion properties. Following the requirements of dredging works in the Haihe Estuary, cohesive sediment samples weretaken from three locations. Laboratory experiments were conducted to determine the rheological properties of these sam-ples and to examine the incipient motion of the cohesive sediment. It is found that the cohesive sediment has an obviousyield stress τb, which increases with the mud density in a manner of an exponential function, and so does the viscosityparameter η. The cohesive sediment behaves like a Bingham fluid when its density is below 1.38 ～ 1.40 g/cm3, andwhen denser than these values, it may become a power-law fluid. The incipient motion experiment also revealed that theincipient velocity of the cohesive sediment increases with the density in an exponential manner. Therefore, the incipientmotion is primarily related to the density, which is different from the case for non-cohesive sediment in which the incipi-ent motion is correlated with the diameter of sand particles instead. The incipient motion occurs in two different ways de-pending on the concentration of mud in the bottom. For sufficiently fine particles and a concentration lower than1.20 g/cm3, the cohesive sediment appears as fluidized mud, and the incipient motion is in the form of instability of aninternal wave. For a higher concentration, the cohesive sediment appears as general quasi-solid-mud, and the incipientmotion can be described by a series of extended Shields curves each with a different porosity for newly deposited alluvial mud. 相似文献
14.
Numerical Simulations of the Flow and Sediment Transport Regimes Surrounding a Short Cylinder 总被引：1，自引：0，他引：1
Hatton K.A. Foster D.L. Traykovski P.. Smith H.D. 《Oceanic Engineering, IEEE Journal of》2007,32(1):249-259
The 3-D flow field and bed stress surrounding a short cylinder in response to combined wave and mean-flow forcing events is examined. Model simulations are performed with a 3-D nonhydrostatic computational fluid dynamics model, FLOW-3D. The model is forced with a range of characteristic tidal and wave velocities as observed in 12-15 m of water at the Martha's Vineyard Coastal Observatory (MVCO, Edgartown, MA). The 2.4-m-long and 0.5-m diameter cylinder is buried 10% of the diameter on a flat, fixed bed. Regions of incipient motion are identified through local estimates of the Shields parameter exceeding the critical value. Potential areas of sediment deposition are identified with local estimates of the Rouse parameter exceeding ten. The model predictions of sediment response are in general in agreement with field observations of seabed morphology obtained over a one-week period during the 2003-2004 MVCO mine burial experiment. Both observations and simulations show potential transport occurring at the ends of the mine in wave-dominated events. Mean flows greater than 10 cm/s lead to the formation of larger scour pits upstream of the cylinder. Deposition in both cases tends to occur along the sides, near the center of mass of the mine. However, the fixed-bed assumption prohibits the prediction of full perimeter scour as is observed in nature. Predicted scour and burial regimes for a range of wave and mean-flow combinations are established. 相似文献
15.
16.
Results are reported herein of an open channel flow laboratory based study of the development of ripples on a fine silica sand bed, and under non-uniform turbulent subcritical flow conditions. The hydraulic model used included a diverging channel, which resulted in a variation of hydraulic and sediment transport parameters along the channel. Sediment supply limitation occurred during experimentation, impacting bed form development. The overall aim of this study was to improve the understanding and modelling capability of the development of bed forms under limited sediment supply and non-uniform flow conditions. In particular, the applicability of an existing empirical model capable of predicting ripple development was tested for the conditions of this study, using measured ripple dimensions. The ripple height and length results were extracted from detailed bed profile records, obtained using an acoustic Doppler probe traversed longitudinally over the sediment bed, at various experimentation time intervals. It was found that the non-uniform flow conditions affected the development rate of the bed forms, while sediment supply limitation impacted their steady state dimensions. The measured steady state ripple dimensions were lower, on average, than the corresponding equilibrium dimensions predicted using existing empirical equations. Non-uniform flow also caused the simultaneous occurrence of bed forms at different stages of development along the hydraulic model, where 3D and 2D ripples and incipient bed forms were recorded. Such a scenario can occur in estuarine and coastal flows, due to changing hydraulic conditions and/or a limitation of sediment supply. The ripple development model tested was verified for the conditions of this study, with its accuracy being shown to depend on an accurate determination of steady state parameters. 相似文献
17.
An optical instrument which detects the light attenuation in sediment-laden flows has been developed to measure the instantaneous and time-average suspended concentration of cohesive sediments. In a flume test with natural muds, the device showed that the instantaneous sediment concentration oscillated with the waves but that the amplitude decreased rapidly with the distance above the mud bed. Strong sediment motion was found near the benthic boundary layer. This indicates a weak vertical turbulent diffusion in the upper portion of the water column in a pure wave erosion process. It is concluded that the probe design can be easily modified and the apparatus can be deployed for both laboratory and field measurements 相似文献
18.
新沂河海口控制工程闸下河道冲刷试验研究 总被引：2，自引：0，他引：2
新沂河海口控制工程深泓闸闸下泓道长约１．３ｋｍ,河床由淤积历时较长的淤泥质粘性土组成。试验中首先取原状土土样,通过水槽试验确定起动流速,据此选择模型沙和进行模型设计。试验表明,下游河道冲刷主要由过闸水流及河口跌水造成,其中,前者形成的冲刷坑为设计的控制条件。根据试验结果认为,对于新沂河海口控制工程这样流量大、河口潮位低的闸下泓道,冲刷是不可避免的,建议放弃海漫防冲的方法,改设板桩截墙。试验中采用的某些观点和方法,对于淤泥质粘土海岸地区河口段闸下冲刷研究具有一定的参考价值。 相似文献
19.
Direct bed shear stress measurements in bore-driven swash 总被引：1，自引：0，他引：1
Direct measurements of bed shear in the swash zone are presented. The data were obtained using a shear plate in medium and large-scale laboratory bore-driven swash and cover a wide range of bed roughness. Data were obtained across the full width of the swash zone and are contrasted with data from the inner surf zone. Estimates of the flow velocities through the full swash cycle were obtained through numerical modelling and calibrated against measured velocity data. The measured stresses and calculated flow velocities were subsequently used to back-calculate instantaneous local skin friction coefficients using the quadratic drag law. The data show rapid temporal variation of the bed shear stress through the leading edge of the uprush, which is typically two–four times greater than the backwash shear stresses at corresponding flow velocity. The measurements indicate strong temporal variation in the skin friction coefficient, particularly in the backwash. The general behaviour of the skin friction coefficient with Reynolds number is consistent with classical theory for certain stages of the swash cycle. A spatial variation in skin friction coefficient is also identified, which is greatest across the surf-swash boundary and likely related to variations in local turbulent intensities. Skin friction coefficients during the uprush are approximately twice those in the backwash at corresponding Reynolds number and cross-shore location. It is suggested that this is a result of the no-slip condition at the tip leading to a continually developing leading edge and boundary layer, into which high velocity fluid and momentum are constantly injected from the flow behind and above the tip region. Finally, the measured stress data are used to determine the asymmetry and cross-shore variation in potential sediment transport predicted by three forms of sediment transport formulae. 相似文献
20.
使用波潮仪、流速计以及浊度计现场测量记录海堤前的波高、潮高、流速和浊度值,与室内测定的浊度和水中悬浮颗粒物含量对应值比较,分析计算浊度与现场测量波高、潮高、流速水动力因素间的相关性,得出堤前泥沙起动的决定性水动力因素。在悬移质运移为主情况下,由于海堤的阻挡,波浪破碎产生的旋涡和紊动是泥沙起动的主要动力因素,泥沙起动量与波高值呈现较高相关性;海堤前复杂水动力条件中,波浪破碎前的流速值与悬移质泥沙起动量相关系数较小,启动流速的主要组成部分是破波产生的旋涡和紊动的流速。 相似文献