Research on Measurement of Bed Shear Stress Under Wave-Current Interaction

The movement of sediment in estuary and on coast is directly restricted by the bed shear stress. Therefore, the research on the basic problem of sediment movement by the bed shear stress is an important way to research the theory of sediment movement. However, there is not a measuring and computing method to measure the bed shear stress under a complicated dynamic effect like wave and current. This paper describes the measurement and test research on the bed shear stress in a long launder of direct current by the new instrument named thermal shearometer based on micro-nanotechnology. As shown by the research results, the thermal shearometer has a high response frequency and strong stability. The measured results can reflect the basic change of the bed shear stress under wave and wave-current effect, and confirm that the method of measuring bed shear stress under wave-current effect with thermal shearometer is feasible. Meanwhile, a preliminary method to compute the shear stress compounded by wave-current is put forward according to the tested and measured results, and then a reference for further study on the basic theory of sediment movement under a complicated dynamic effect is provided.

Experimental Study on the Bed Shear Stress Under Breaking Waves

The object of present study is to investigate the bed shear stress on a slope under regular breaking waves by a novel instrument named Micro-Electro-Mechanical System （MEMS） flexible hot-film shear stress sensor. The sensors were calibrated before application, and then a wave flume experiment was conducted to study the bed shear stress for the case of regular waves spilling and plunging on a 1 ： 15 smooth PVC slope. The experiment shows that the sensor is feasible for the measurement of the bed shear stress under breaking waves. For regular incident waves, the bed shear stress is mainly periodic in both outside and inside the breaking point. The fluctuations of the bed shear stress increase significantly after waves breaking due to the turbulence and vortexes generated by breaking waves. For plunging breaker, the extreme value of the mean maximum bed shear stress appears after the plunging point, and the more violent the wave breaks, the more dramatic increase of the maximum bed shear stress will occur. For spilling breaker, the increase of the maximum bed shear stress along the slope is gradual compared with the plunging breaker. At last, an empirical equation about the relationship between the maximum bed shear stress and the surf similarity parameter is given, which can be used to estimate the maximum bed shear stress under breaking waves in practice.

Estimation of Bed Shear Stresses in the Pearl River Estuary

Mean and fluctuating velocities were measured by use of a pulse coherent acoustic Doppler profiler （PC-ADP） and an acoustic Doppler velocimeter in the tidal bottom boundary layer of the Pearl River Estuary. The bed shear stresses were estimated by four different methods： log profile （LP）, eddy correlation （EC）, turbulent kinetic energy （TKE）, and inertial dissipation （ID）. The results show that （a） all four methods for estimating bed stresses have advantages and disadvantages, and they should be applied simultaneously to obtain reliable frictional velocity and to identify potential sources of errors; （b） the LP method was found to be the most suitable to estimate the bed stresses in non-stratified, quasi-steady, and homogeneous flows; and （c） in the estuary where the semi-diurnal tidal current is dominant, bed shear stresses exhibit a strong quarter-diurnal variation.

An experimental study of the incipient bed shear stress partition in mobile bed channels filled with emergent rigid vegetation

This paper investigates the bed shear stress based on the condition of the incipient motion of sediment in a uniform-flow flume covered with emergent rigid vegetation,which is represented by arrays of circular cylinders arranged in a regular pattern.A total of 148 tests are performed to observe the influence of the vegetation density,bed slope,flow depth and sediment size on the bed shear stress.The tests reveal that when the sediment is in incipient motion,the resistances acting on the flow passing the rigid vegetation contain the vegetation resistance and the bed shear stress.This shear stress could be divided into two parts:the grain shear stress and the shear stress caused by sand dunes,which are the deformed bedform with the sediment incipient motion.An empirical relationship between the shear stress of the sand dune and vegetation density,the Froude number,the apparent vegetation layer velocity is developed.

A simple method for estimating bed shear stress in smooth and vegetated compound channels

Instead of a large number of measurements of the streamwise velocity, a simple method is proposed to estimate the bed shear stress in smooth and vegetated compound channels, based on the Darcy-Weisbach equation. This method contains a dimension- less parameter Ai, to represent the relationship between the bed shear stress and the velocity close to the channel bed （Ub）, which is determined in each divided domain. This method is verified in two smooth compound channels with different geometries, and in one compound channel with emergent floodplain vegetation. The comparison of predicted and measured bed shear stresses indicates the good predictive capability of this method, particularly in the mixing region. This method is further discussed for a channel with submerged vegetation. Once the values of Ai in the main channel and the floodplain are determined, this method is a practical tool

Characterizing Clustering in Boulder Bed Channels and the Impact on Shear Stress Equations

Boulders and cobbles are often used in stream restoration projects to increase flow resistance and enhance channel stability and habitat diversity. Particle size metrics determined from the particle distribution are often used as a proxy for shear stress in field equations. Clustering of large particles has been thought to contribute to shear stress, but the effect of clustering is not accounted for in equations that use a representative particle size, such as the D84. In this paper, clustering is defined using the upper tail (≥84%) in a variable called Topsum. The number of clusters, average size of clusters, and shear stress are evaluated using the proposed definition of cluster. Findings suggest that the upper tail represents the roughness height better than the commonly used proxy of D84 for boulder bed streams (streams which have a D84 particle 0.05 - 0.15 meters).

Numerical Study of Flow Features Around Submerged Circular and Square Piles at Flat and Scoured Beds Using OpenFOAM

Elucidating the flow features around piles in local scouring processes is crucial for studies of local scouring mechanisms and scour depth estimates.This study details the flow turbulence characteristics of two submerged piles that are determined by solving the Navier-Stokes equations with the improved delayed detached eddy simulation model.This model is verified by comparing experimental and numerical results for hydrodynamic parameters with the literature for both square-crossing piles(SCPs)and circular-crossing piles(CCPs).Original topographies of flat and scoured beds(i.e.,the initial and equilibrium scouring stages)are based on experimental results obtained by the authors in the present paper.SCP and CCP flow features in the scouring process are discussed.The results indicate that during the scouring process,the time-averaged drag coefficient and root mean square(rms)of the lift coefficient increase linearly in the CCP test,while the rms of the lift coefficient in the SCP test decreases linearly.Moreover,the minimum pressure coefficient is always located in the upstream corners in the SCP case but moves from 72.5°to 79.5°when the scour hole is completely developed in the CCP case.Downward flow behind the pile,which is generated by separated boundary layers above the top face of the pile,can reach the sand bed and turn the separated shear layers into patches of small vortices in the near-wake regions.Thus,the high shear stress zones are mainly at the scour edges under scoured-bed conditions.

Shear Stress Variation at Scour Hole of Circular Pier

The causes of local scour are generally categorized into flow condition, structure, and riverbed material. A three-dimensional vortex flow generated with the influence of the structure is the main factors of the flow conditions, and the size of the particles is assumed to be the main factor of the riverbed case. Various studies about pier local scour have been carried out by researchers since the 1960s, and a large number of experimental formulas have been suggested. Difficulties were encountered by these past studies, however, in terms of considering the influence of various riverbed materials and scour changes (floods, etc.) on time, with the condition of maximum scour depth. In the case of Korea, especially, scour influenced by various riverbed materials and the frequency of floods have been determined to be very important factors. Therefore, the ultimate purpose of this study on pier scour is to suggest the scour examination method that could consider various riverbed materials and the frequency of floods. In this study, the periodic changes in local scour based on the differences in the diameters of four types of bed materials, and on the hydraulic condition of the initial scour, were determined and compared with those in former studies. Using the results of the comparison, this study aims to determine the changes in the shear-stress around piers for various bed materials through the effect of time on scour depth (S, Smax), the shear-stress around piers, and the particles’ critical shear stress (τc).

洋口港潮滩沉积过程研究

为了研究江苏典型淤积潮滩泥沙沉积特征,于2021年7月在洋口港开展断面调查。研究结果表明,该区域潮流为正规半日潮,涨潮流向为西南向、落潮流向为东北向。观测期间,波高随风场的增强明显变大,潮流大小主要随着潮周期变化;增强的波浪有助于水体扰动和泥沙悬浮;底部切应力主要受潮流作用,受波浪作用较小;波流联合切应力大部分时间都小于临界侵蚀应力,水体中的悬沙多倾向于沉降,滩面易淤积;悬沙总体上沿岸向南和平行向岸输送。

Study on the double-logarithmic profile of tidal flow velocity in the near-bed layers

Tidal current velocity profile in the near-bed layers has been widely studied. The results showed that velocity profile in the near-bed layer obviously departure from the traditional logarithmic profile, due to the acceleration or deceleration. Although the logarithmic linear profile can reduce the rate of deviation from this, only it is a lower-order approximate solution. In this paper, considering the unsteady and non-linear features of tidal motion, the double logarithmic profile near-bed layers in estuarine and coastal waters is established on the assumption that the turbulent shear stress along the water depth was parabolic distribution, and on the basis of Prandtl＇s mixing length theory and yon Karman＇s self-similar theory. Having been verified the data observed at the West Solent in the south of England, and comparison of the logarithmic linear profile, it found that the double logarithmic profile is more precious than the latter. At last, the discussed results showed that： （1） The parabolic distribution of the tidal shear stresses verified good by the field data and experimental data, can be better reflected the basic features of the tidal shear stress deviating from linear distribution that is downward when to accelerate, upward when to decelerate. （2） The traditional logarithmic velocity profile is the zero-order approximation solution of the double logarithmic profile, the logarithmic linear profile is the first order, and the logarithmic parabolic profile is the second order. （3） Ignoring the conditions of diffusion and convection in the tida movement, the double logarithmic profile can reflect the tidal properties of acceleration or deceleration, so that the calculation of the friction velocity and roughness length are more reasonable. When the acceleration or the deceleration is about zero, the double logarithmic profile becomes the logarithmic profile.

Experimental and Three-Dimensional Numerical Simulation of Phenomena Induced by Submerged Oblique Jet Scouring

Scouring experiments were conducted using a three-dimensional laser scanning technology for angles of the jet spanning the interval from 0°to 30°,and the characteristics of the scour hole in equilibrium conditions were investigated accordingly.The results indicate that the optimal scouring effects occur when the jet angle is in the ranges between 15°and 20°.Moreover,the dimensionless profiles of the scour hole exhibit a high degree of similarity at different jet angles.Numerical simulations conducted using the Flow-3D software to investigate the bed shear stress along the jet impingement surface have shown that this stress is influenced by both the resultant force and the jet impingement surface area.It reaches its maximum value when the jet is vertical,decreases rapidly as the jet starts to tilt,then increases slightly,and decreases again significantly when the angle exceeds 20°.

灌门水道水轮机阵列布置对潮流运动特性的影响

潮流能水轮机机组的运行会改变潮流运动结构,引起局部区域的水动力特性发生变化,进而影响机组的获能效率。对此,以舟山群岛灌门水道为例,基于Delft 3D软件建立三维水动力学模型,研究机组阵列交错布置对运动潮流的影响。通过网格嵌套分别模拟舟山群岛和灌门水道的潮流运动特征,采用多孔盘法模拟水轮机阵列对潮流空间结构的作用。研究表明:多排机组阵列运行之后,水轮机及下游潮流流速会显著下降,并引起垂向上转轮上、下边缘区流速增加;水轮机迎流方向水位上升,涨、落急时刻最大壅水差值达10cm;水轮机底部及机组之间的床面切应力增大,尾流影响区切应力减小;尾流叠加,床面切应力进一步减小,落急时刻尾流区的床面切应力减小量可达54.03%,阵列两侧床面切应力增大量可达177.93%。在未来的潮流能开发选址中,应全面评估布置水轮机后可能引起的潮流运动特性变化,优化阵列布置,降低对环境的影响。

透水丁坝对不同曲率弯道内三维水流结构影响数值模拟研究

透水丁坝相较于传统丁坝,具有更加卓越的稳定性,安全性,可靠性,因此在绿色航道推广中得到重视,研究的目的在于对比不同曲率河道内三维水流结构变化,进而为不同曲率河道治理提供依据。研究采用数值模拟与实测相结合的方式,对3种湍流模型进行比选和验证,Standard k-ε,RNG k-ε,Realizable k-ε三种湍流模型内岸模拟流速和实测流速相关性系数均低于外岸,RNG k-ε湍流模型实测流速和模拟流速相关性系数最高,最终采用RNG k-ε模型对90°和180°弯道模拟,并对三维流线,流速,床面剪切,水面线分析。结果显示丁坝下游回流区涡轴线受垂向和径向速度梯度影响出现不同程度倾斜,水流撞击2号丁坝在丁坝之间产生两个相反方向的涡,这将对透水丁坝群结构稳定形成威胁但同时也会防止大量泥沙在透水丁坝之间淤积,保证透水丁坝在使用过程中不至于丧失透水性。河道曲率增大导致强剪切层向内岸迁移,180°弯道内出现两个强剪切层,这将导致河床向阶梯型演化,对于曲率大而且狭窄的山区河道在布置透水丁坝后要对内岸侵蚀效果评估,防止强剪切层迁移对内岸构成威胁。水面线呈现内低外高的现象,且在x/L=0.48~0.5的范围里内外岸水面线差值最小,水面出现凹陷。

扰流板对海上风电单桩基础冲刷特性影响研究

在单独恒定流条件下,以某一海上风电大直径单桩基础为研究对象,提出一种扰流板防冲刷结构,建立三维数值模型。基于数值模拟,研究扰流板结构对桩基础周围局部冲刷的影响机制,探讨扰流板长度L和高度H_(2)对局部马蹄涡位置和强度的影响,分析床面剪切应力受扰流板几何的影响规律。研究发现,扰流板结构能有效地减弱局部马蹄涡强度和减小桩周海床床面剪切应力;局部马蹄涡受扰流板几何的影响呈现3种流动机制;当L/H2=1时,扰流板具有较好的冲刷防护特性。总的来说,扰流板防冲刷结构对单桩基础局部冲刷具有较好的防护效果。

卵石河床清水冲刷粗化层破坏临界条件试验研究

针对山区卵石河道普遍存在的床面粗化层现象,进行了水流强度连续小幅增大的清水冲刷粗化层破坏试验研究,旨在探讨粗化层破坏与水流强度之间的临界关系。试验结果表明,水流强度的增大并不一定能破坏床面粗化层,只有当水流强度增大到一定程度时,粗化层才会破坏。对于本系列试验所用泥沙,只有床面切应力大于初始粗化床面切应力的1.2倍,粗化层才会破坏。粗化层床面结构对粗化床面切应力有着重要的影响。

风暴过程中潮滩悬沙浓度和悬沙输运的变化及其动力机制——以长江三角洲南汇潮滩为例

悬沙浓度是淤泥质海岸重要的环境指标。为探讨潮滩悬沙浓度和悬沙输运对风暴事件的响应过程及其动力机制,于2014年9月"凤凰"台风过境前、中、后在长江三角洲南汇潮滩进行了现场观测,获得同步高分辨率的水深、波高、近底流速和浊度剖面时间序列(9个潮周期)。结果表明,风暴中平均和最大波高、波-流联合底床剪切应力、悬沙浓度和悬沙输运率可比平静天气高数倍;风暴期间高潮位低流速阶段悬沙沉降导致近底发育数十厘米厚的浮泥层(悬沙浓度大于10g/L)。研究认为风暴事件中淤泥质海岸悬沙浓度和悬沙输运的剧烈变化其根本动力机制是风暴把巨大能量传递给近岸水体,进而显著增大波-流联合底床剪切应力,导致细颗粒泥沙再悬浮。

明渠非恒定流流速分布及推移质运动研究进展

明渠非恒定流流速分布及非恒定流作用下推移质运动分别是水力学和泥沙运动力学研究中的重要问题之一。近几十年来,国内外许多学者在这些领域做过很多探索,取得了一定进展,也存在较多问题。在总结相关研究成果的基础上,简述了明渠非恒定流试验方法、流速分布、床面切应力、非恒定流输沙与水流运动不同步现象、产生原因、以及非恒定流推移质输沙公式等方面的成果,指出了现有研究中存在的不足,提出了需要进一步研究的关键问题:应用先进的量测仪器,研究非恒定流作用下非均匀沙运动输移规律。

波、流共同作用下的泥沙起动

对波、流共存时的床面剪切力及摩阻系数等作了探讨,根据泥沙起动试验,建立了泥沙起动Shields曲线,导出了波、流共同作用下层流和紊流时的泥沙起动公式和起动波高、起动水深。

渤海海峡潮流底应力与沉积物分布的关系

用数值模型方法计算了渤海海峡区潮流引起的最大和平均底应力 ,以及相应的活动层厚度的空间分布 ,并根据沉积物参数计算了沉积物临界起动切应力。计算结果的分析表明 :(1)最大潮流底应力和沉积物临界起动切应力的关系与底质分布之间存在着明显的对应 ,最大潮流底应力大于临界起动切应力的地方 ,分布着粗颗粒沉积物 ,若前者小于后者 ,对应的是细颗粒沉积物 ,若两者近于相等 ,则对应着较细颗粒的沉积物 ;(2 )在山东半岛北部近岸和渤海海峡的东北角 ,最大潮流底应力小于或接近于临界起动切应力的量值 ,而对应的沉积物是粉砂和砂 ,这一例外可能是由于残留沉积的出露 ,这一假说需要根据全新世海面上升历史、古水动力环境模拟、渤海海峡沉积环境演化等方面的研究来验证 ;(3)活动层厚度的分布规律与海底沉积物的分布存在着相关性 :即活动层厚度越大 ,对应的海底沉积物越粗 ;活动层厚度越小 。

波浪作用下床面切应力测量初探

在前人研究基础上,进行了波浪水槽实验。采用新型热敏传感器直接测量波浪作用下床面切应力分布情况。通过分析输出电压与理论切应力之间的关系,得出传感器电压值与理论计算切应力均随时间而周期性变化,充分验证了热敏传感器在测量床面切应力时的可行性,为进一步直接测量床面切应力奠定了基础。