A study of the effect of local scour on the flow field near the spur dike

The flow field near a spur dike such as down flow and horseshoe vortex system(HVS)are susceptible to the topographic changes in the local scouring process,resulting in variation of the sediment transport with time.In this study,large eddy simulations with fixed-bed at different scouring stages were conducted to investigate the changes in flow field.The results imply that the bed deformation leads to an increase in flow rate per unit area,which represent the capability of sediment transportation by water,in the scour hole.Moreover,the intensity of turbulent kinetic energy and bimodal motion near the sand bed induced by the HVS were also varied.However,the peak moments between the two sediment transport mechanisms were different.Hence,understanding the complex feedback mechanism between topography and flow field is essential for the local scour problem.

Experiments and numerical simulations on transport of dissolved pollutants around spur dike

The flow field around a spur dike has three-dimensional characteristics. In order to analyze the influence of the flow field on pollutant transport, based on a compressive volume of fluid （VOF） scheme, the three-dimensional transient compressive pollutant transport model （CPTM） and the cubic equation （CE） bounded differencing scheme were developed. For the calibration and validation of CPTM, laboratory experiments were carried out in a flume with a non-submerged spur dike. The spur dike was angled at 60°, 90°, and 120° from the upstream direction. The simulation results agreed with the experimental results. The simulations and experiments showed that the distribution of pollutant concentration was determined by circumfluence and the main flow. Concentration decay in the circumfluenee zone was slower than that in the main flow. Downstream of the spur dike, the concentration fluctuation became intensive with the increase of spur dike angle.

Influence of Spur Dike on Hydrodynamic Exchange Between Channel and Shoal of Generalization Estuary in Physical Model Test

Widely applied in maintaining estuarial waterway depth, the spur dike has played an important role in currents and sediment exchange between channel and shoal and sediment back-silting in the channel. Through establishing a generalized physical model at a bifurcated estuary and conducting current tests under the joint action of runoff and tide, the influence of the spur dike length on current exchange between channel and shoal is analyzed. Results show that when the spur dike length reaches a certain value, the direction of the flow velocity shear front between the channel and shoal will change. The longer the spur dike, the larger the transverse fluctuating velocity at the peak of flood in the channel shoal exchange area, while the transport of the transverse hydrodynamics is obvious in the process of flood. There is an optimum length of spur dike when the shear stress in the channel and the longitudinal velocity in flood and ebb reach the maximum, and the flow velocity will decrease when the spur dike length is smaller or larger than the optimum. For a certain length of spur dike, the larger the channel shoal elevation difference, the larger the peak longitudinal flow velocity in the middle of the navigation channel in flood and ebb. However, the transverse flow velocity will first decrease and then increase. The transverse transportation is obvious when the channel shoal elevation difference increases.

Numerical investigation of flow pattern and components of three-dimensional velocity around a submerged T-shaped spur dike in a 90° bend

Spur dike is one of the river training structures. This work presented a numerical simulation of flow field and three-dimensional velocity around a T-shaped spur dike located in bend using SSIIM model. The main objective of this work is to investigate the three-dimensional velocities and streamlines at transverse and longitudinal sections and plan views around the T-shaped spur dike in different submergence ratios(0, 5%, 15%, 25% and 50%). It is concluded that by increasing the submergence ratio from 5% to 50%, the maximum of scour is reduced; the maximum of longitudinal velocity increases by 7.7% and occurs at the water surface in spur dike axis. Near the bed, the maximum of vertical velocity occurs at the end of spur wing. By analyzing the streamlines at transverse sections, the followings were deduced for different submergence ratios: different dimensions and different positions of vortices around the spur dike.

Variation of hydraulic parameters with different wing of a T-shape spur dike in bend channels

In the present research,by using a numerical model,some analyses were performed on flows around a T-shape spur dike and a support structure located at its upstream under different wing to length ratios of T-shape spur dike in the order of 0.25,0.50,0.75 and 1.00.In order to verify numerical model,physical model data were used in presence of a single T shape spur dike.Results from numerical model are desirably in agreement with those of physical one because the regression between both data is 0.86 up to 0.92.In this research,all hydraulic parameters of flows,streamlines and dimensions of flow separation zones were studied in order to select the most practical model.Increased W/L results in 7%–12%increase in the length of flow separation zone and in 2%increase in the width of this zone compared to W/L=0.25.

长江上游水井湾航道整治数值模拟

水井湾滩段是长江上游弯道分汊河道,具有急、浅、险等多种碍航特性。洪中枯流量的差异引起主支汊分流比与分沙比的变化,河床不稳定。采用数值模拟方法,结合丁顺坝平面初步布置方案,研究丁顺坝坝头不同偏角布置下水井湾航道的通航水流条件及汊道分流比变化规律,对比分析各布置方案的计算结果得到平面布置最优方案。研究表明:通航水流条件与丁顺坝坝头偏角不成正向变动关系,坝头偏角存在合适区间;分汊主河道分流比与流量成反向变动关系,坝头偏角对河道分流比影响较小。研究结果可为同类型河流航道整治方案提供理论依据。

北江丁坝群对鲫鱼栖息地适宜度的影响

为了探究北江丁坝群对鱼类栖息地的影响,以北江干流英德段为研究区域,采用非结构化网格有限体积法(FVCOM),计算了不同流量条件下鲫鱼栖息地加权可利用面积(WUA),得到鱼类最适生态流量值。基于鱼类最适生态流量值开展了丁坝群对流速、水深、鱼类栖息地适宜性指数和水力生境多样性变化的影响。结果表明:当上游为最适生态流量350 m 3/s时,丁坝群使得WUA由7.142 km 2降低至5.692 km 2;丁坝群会显著改变河道中心和近岸的流速,导致流速适宜度指数下降,进而降低整个研究区域的栖息地适宜度指数;丁坝群的水位壅高作用有限,最大壅高水深为0.38 m,对水深适宜度指数分布影响较小;在丁坝群作用下,栖息地可接受性高的面积占比由52.151%降至32.271%,降低了鱼类栖息地质量;丁坝群造成水力生境多样性辛普森指数由0.547降至0.529,使得研究区域内的水力生境多样性略微减少。研究成果可为北江鱼类栖息地保护和生态恢复提供参考。

透水丁坝孔型对坝体附近水流特性影响研究

为深入探讨透水丁坝透水孔型对其附近水流特性的影响,通过室内高精度水槽试验,采用三维声学多普勒流速仪对矩形、方圆形和椭圆形三种透水孔型下丁坝附近水流流速进行测量,获得不同透水孔型影响下的丁坝附近水流流速和紊动强度分布特征。结果表明,坝后水流流速波动和紊动强度明显大于坝前,透水丁坝对近坝区水流影响较大,距丁坝越远,流速均匀性越好,紊动强度越小。透水丁坝的缓流效果与透水孔型密切相关,矩形透水孔的坝后缓流率为35.4%,缓流效果较好,方圆形透水孔次之,椭圆形最小,研究结果可为丁坝的透水孔型选择提供指导和参考。

开孔率与布置间距对透空型双丁坝水动力与冲淤特性的影响

采用三维水动力和泥沙数值模型,模拟分析了不同开孔率和不同布置间距的透空型双丁坝局部三维水动力特性和冲淤特性以及对水生生物生境的影响。结果表明:透空型双丁坝周围流场结构丰富,形成了上丁坝坝内环流区、坝间主副环流区以及下丁坝坝后缓流区,有利于有机质富集;开孔率15%时透空型双丁坝坝间主环流区和坝间涡量最大,坝间涡量以及紊动能随布置间距的增大而增大,控制开孔率大小并适当增大布置间距有利于底栖生物索饵洄游;最大切应力与上游切应力的比值随着开孔率和布置间距的增大逐渐降低,适当增大布置间距并选择较大的开孔率可减轻下泄水流对坝后河床的冲刷;坝内及坝后淤积范围和高度随开孔率增大而减小,随布置间距增大而增大,实际工程中适当提高开孔率和布置间距有利于透空丁坝减速促淤,为底栖生物提供丰富的饵料。

北江丁坝群水动力特性数值模拟及其对通航安全的影响研究

为了深入探究北江丁坝群对通航安全的影响,研究采用非结构化网格有限体积法计算模型(Finite Volume Coastal Ocean Model,FVCOM),以英德段23 km的北江干流河段为研究区域,构建了三维水动力模型,并通过实测数据验证了模型的有效性。基于此,预设了当恒定来流量为420 m^(3)/s情况下,模拟了有、无丁坝2种不同情境下的水动力特性,重点关注和分析研究区域的急弯段。计算结果表明,丁坝使得近岸水流流速降低了44.6%~90.3%,航道水深增大了0~0.2 m,主流流速增大了66.6%~121.8%。此外,丁坝集中了横向主流,游摆程度最大降低了41.6%,摆幅最大降低了50.2%,对提高航道通航能力起到了正面效应。然而,丁坝的布置也增加了一些负面效应的风险,如单侧长丁坝导致对岸近岸流速增大了73.3%~129.0%,航道水平主流线曲折程度由784.2 m降低到774.5 m,水流动力轴线弯曲半径减小了14.0%~36.7%,对通航安全存在一定的风险。成果不仅揭示了丁坝在特定河段水动力特性中的复杂作用,也为天然航道中丁坝对通航安全的分析提供了重要的参考依据。

弗劳德数影响下的沙质河床丁坝群局部冲刷数值模拟

为分析弗劳德数对沙质河床丁坝群局部冲刷的影响,基于MIKE21对三种泥沙粒径沙质河床在不同弗劳德数下的丁坝群局部冲刷进行数值模拟,总结了冲刷范围及特征断面的变化规律。结果表明,局部冲刷除Ⅱ号坝头产生淤积外其余丁坝坝头均产生局部冲刷;同一弗劳德数下,泥沙粒径越小,冲刷范围及断面变化越大,高弗劳德数时更为明显,而同一粒径下,弗劳德数越大,冲刷范围及断面变化会越大,在泥沙粒径较小时更为明显;在泥沙粒径较小、弗劳德数较大时,冲刷范围几乎影响整个丁坝群河道且首坝附近河床整体为先冲后淤。

微芯桩监测技术在黄河丁坝根石走失预警中的应用

黄河下游丁坝根石走失问题频发,传统监测方法成效有限。为提高根石走失预警的准确性和科学性,创新性地将微芯桩监测设备应用在黄河神堤控导工程13#、14#、15#、20#坝,监测根石走失情况以及坝体倾斜角度、振动冲击合加速度等变化情况。微芯桩监测系统利用物联网一体化感知技术,通过感知终端实现对水下根石运行状态的24 h全天候监测。应用情况表明,丁坝基础石料在水流或者外力作用下产生滑移,使丁坝发生轻微的位移、倾斜或振动时,微芯桩能灵敏地捕捉到这些指标信息,监测精度较高。

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。

Purification efficiency of ecological spur dikes for river pollutants in different geometric arrangements:Experiments and numerical modeling

The traditional ecological spur dike is improved,by using the biological fillers such as the plant materials,combined with microorganisms,to obtain a larger water purification capacity.Most of the researches on spur dikes focused on the hydrodynamic force and the migration and the diffusion of the pollutants,but not so much on the effects of the spur dike materials and the arrangement parameters on the water purification capacity.In this paper the biological zeolite is used to make ecological spur dikes.The laboratory flume experiment shows that compared with the natural zeolite spur dike,the purification effect of the biological zeolite spur dike is significantly improved,and the average removal rate of the NH3-N is kept at 6.8%after 24 h compared with 3.5%of the natural zeolite spur dike.The NH3-N removal rates of a single spur dike,the counterpart spur dikes and the staggered spur dikes decrease with the increase of the inlet flow,and the staggered spur dikes have the best purification effect.Secondly,a numerical model based on the MIKE21 is verified by the experimental results and applied for the Tangjiali Section of the Xutangqiao River in Xinwu District,Wuxi City,Jiangsu Province.The orthogonal test shows that the length of the spur dike has the most important effect on the interception of pollutants and the purification,followed by the angle of the spur dike and the spacing of the spur dikes has the least important effect.The geometric arrangement parameters of the ecological spur dike group with the highest efficiency on the pollutant interception and the purification are:60°(angle),6.7 m(length)and 20 m(spacing).

桥台的冲刷机理和冲刷深度

根据水工模型试验观测和大量试验数据及图象资料，应用流体力学原理对桥台及丁坝绕流的流场和旋涡体系进行了分析，阐明了桥台及丁坝冲刷的旋涡机理，发现并论证了决定冲刷的关键因素是天然河槽水力因素（弗汝德数Ｆｒ）和路堤及丁坝阻水因素（阻水面积Ａｚ或阻水长度ＬＤ）。根据本文和近年来国外发表的长历时平衡冲刷数据，应用图解和回归分析，建立了桥台及丁坝平衡冲刷深度公式。公式简明易用，并得到汛后工程实测资料验证。

非淹没丁坝坝头冲刷深度的计算

基于丁坝坝头水流结构 ,提出了坝头绕流挤压流动模式 ;由此根据有效局部水流连续原理 ,建立了丁坝坝头冲刷深度计算公式 ,该式涉及因素较全面 ,公式建立过程中所使用的数学模式清晰 ,物理意义明确 .经野外及室内资料验证表明 ,计算与实测结果较为吻合 。

溢流丁坝附近自由水面的实验研究与数值模拟

在整治河道中,丁坝的存在使得丁坝附近自由水面及流场的三维特性非常突出,对这种三维特性非常突出的自由水面和流场的模拟在计算流体力学的应用中一直是个难点。作者对溢流丁坝附近的自由水面进行了实验研究,应用VOF方法和标准的k～ε模型耦合求解对丁坝附近自由水面的模拟,模拟结果与实测值吻合较好,说明VOF方法可以在丁坝附近自由水面的计算中应用推广。同时,还对溢流丁坝附近的流场进行了预测。

线性与非线性紊流模型及其在丁坝绕流中的应用

本文采用线性与非线性三维紊流数值模型对丁坝绕流进行了仿真分析。采用的线形紊流模型为Zhu Shih修正的k ε模型,非线性紊流模型为Shih等提出的k ε模型。计算分析了单一丁坝和丁坝群两种不同布置方式下绕流流场的紊动特征,并与实验结果进行了比较。结果表明,非线性模型对平均流场的描述要优于线性模型。文章对非线性模型能提高模拟精度的原因进行了分析。

丁坝对弯道水流影响的分析

采用三维声学多普勒流速仪对弯道内有、无丁坝两种工况下的水流特性进行了系统的试验研究,总结了弯道水流水面横比降、横向环流、流速重分布等特性,重点分析了丁坝对弯道水流流态的影响,进一步证实了丁坝的护岸机理,为相关的工程实践提供理论依据。

非恒定流作用下丁坝局部冲刷研究

丁坝水毁现象普遍存在且受多种因素影响,目前关于丁坝水毁的研究成果多是在恒定流条件下得到的,没能很好地解决丁坝水毁的实际问题。因此,通过水槽概化模型试验,在天然河流日均流量过程作用下,分别对比了相同重现期时年最大洪峰流量和有效洪水周期遭遇、年最大洪峰流量和半月最大洪水总量遭遇丁坝局部冲刷的程度,指出洪峰流量对冲刷深度起主导作用,洪水总量对坝头冲刷坑范围起主导作用,有效洪水周期对冲刷坑范围及冲刷深度均起主导作用;并对非恒定流作用下坝头冲刷坑的发展过程进行分析,认为随着冲刷坑下游沙垄形成与消亡的变化,冲刷坑范围并非随时间延长而增大。