共查询到19条相似文献,搜索用时 906 毫秒
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综合糙率是采用曼宁公式确定河道水位和流量关系的关键参数。在河道冰封期,冰盖的出现增加了流动的阻力,明流条件下确定的综合糙率不再适用,需要重新估算。基于Einstein阻力划分过流断面的原理,冰盖下矩形河道的过水断面可划分为冰盖区、河床区和边壁区。根据总流的连续性方程,在确定各分区糙率系数、水力半径和断面面积的基础上,提出了冰盖下矩形河道综合糙率的计算公式。采用已有的试验水槽测量数据和天然河道实测资料对公式进行了验证,结果表明:公式计算的综合糙率与实测值吻合较好,与Einstein公式和Sabaneev公式相比,计算精度更高;对于冰封水流,宽浅河道采用分区水深代替水力半径进行简化计算的条件有别于明渠水流,在宽深比大于20时,计算结果才满足精度要求。 相似文献
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传统的谢才-曼宁公式只能计算断面平均流速,不能计算垂线平均流速。但垂线平均流速的研究对于解决断面流速分布以及对水力学理论研究等方面都有很重要的意义。作者通过对均质边壁矩形、对称三角形断面的垂线平均流速与断面平均流速之间关系的分析研究和水力学试验,建立了用谢才-曼宁公式计算上述断面的垂线平均流速公式。进而通过拟手段求出均质边壁不规则断面的垂线平均流速。经用实验资料验证表明,计算与实验点据拟合良好。 相似文献
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为探究边壁双曲率对其水动力特性的影响,应用RNG k-ε紊流模型和部分面积体积障碍模拟技术(FAVOR),针对圆管非满流建立三维数值模型,通过物理模型试验结果验证该数值模型的可行性。针对底坡和充满度不同组合条件进行数值试验,分析断面流速、壁面切应力和雷诺切应力分布规律等。研究结果表明:圆管非满流断面上不同垂线的流速分布具有较好的相似性,据此提出了垂线流速分布的抛物线公式;针对不同充满度条件提出了壁面切应力沿湿周分布的统一表达式;雷诺切应力沿垂线服从线性分布规律,充满度越大且距中垂线越远,沿垂线的变化梯度越小;当充满度大于0.5时,由于受二次流影响断面中垂线上雷诺切应力出现负值。 相似文献
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为探究边壁双曲率对其水动力特性的影响,应用RNG k-ε紊流模型和部分面积体积障碍模拟技术(FAVOR),针对圆管非满流建立三维数值模型,通过物理模型试验结果验证该数值模型的可行性。针对底坡和充满度不同组合条件进行数值试验,分析断面流速、壁面切应力和雷诺切应力分布规律等。研究结果表明:圆管非满流断面上不同垂线的流速分布具有较好的相似性,据此提出了垂线流速分布的抛物线公式;针对不同充满度条件提出了壁面切应力沿湿周分布的统一表达式;雷诺切应力沿垂线服从线性分布规律,充满度越大且距中垂线越远,沿垂线的变化梯度越小;当充满度大于0.5时,由于受二次流影响断面中垂线上雷诺切应力出现负值。 相似文献
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为揭示散粒粗糙床面的当量糙率尺度特征,采用4种散粒粗糙床面开展了8组粒子图像测速法(PIV)水槽明渠紊流试验,探讨了理论床面位置、当量糙率尺度和阻力系数的沿程分布规律,分析了当量糙率尺度与阻力系数的内在关系。结果表明:(1)散粒粗糙床面明渠水流特性沿程可分为颗粒区和粒间区,各区的理论床面位置参数ξ、相对当量糙率尺度ks0和阻力系数f具有显著差异,颗粒区0. 65≤ξ≤0. 85,ks0波动明显,f较大;粒间区ξ=1,ks0变化相对平缓,f较小。(2) ks0沿程平均值与颗粒直径d成正比,与排列间距Lx及水流雷诺数Re成反比,在d、Lx及Re相同时,梅花形排列的ks0大于矩形排列。(3) f随ks0呈递增变化,变化特性与相对光滑度h/d密切相关。研究成果可为散粒粗糙床面沿程阻力系数确定提供科学参考。 相似文献
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深入分析横断面流速分布特性,提出横断面垂线平均流速分布模型及计算公式。该模型利用大断面、河道水位及断面上一条垂线的平均流速,即可较准确地计算出断面上任意一条垂线的平均流速,从而计算出断面流量。这种方法缩短了测流历时,在保证测流精度的同时,为防汛调度决策延长了预见期。讨论了该方法的应用条件、误差来源和误差控制方法。为明渠实时在线监测提供了一种新的方法,为明渠流量计研发奠定了理论基础。 相似文献
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通过多组次水槽试验,应用声速多普勒测速仪研究入汇角为90°时,不同汇流比水流条件下支流入汇区域及其附近的水面比降变化和时均流速分布特征。试验表明,入汇口上游的干支流水面坡降取决于干支流相互顶托影响,而入汇口处及其下游合流掺混区水面坡降随干流流量增大而增大,随汇流比增大而有所减小。不同位置垂线时均流速分布主要受主支汇流比和水流流态的双重影响,随着汇流比的增大,入汇口上游断面时均流速逐渐减小,而入汇口下游断面产生了明显的高、低流速带,形成回流分离区。 相似文献
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河道中冰盖显著改变了水流流动结构.采用k-ε紊流模型建立了冰盖下水流流动垂向二维数值模型;根据量纲分析理论提出了流速分布规律的影响因素;针对各种因素的不同组合进行了数值计算,并对其流动特性如最大流速点位置、冰区及床面区平均流速等进行了分析研究;对冬季封冻河道的二点测流法精度进行了理论分析.研究结果表明,冰盖下水流的纵向流速在流动核心区并不遵循对数分布规律,同时揭示了冰盖底部与河床的相对粗糙比、河床相对粗糙度及雷诺数对流速分布规律的影响. 相似文献
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Piles socketed into rock are increasingly used to support loads from large-span bridges and heavy buildings. Peak side resistance is typically related to unconfined compressive strength, sidewall roughness and rock mass quality. This paper presents the results of tests on piles socketed in a weak, artificial rock made of sand, cement, gypsum powder and water. The test results are compared with methods of estimation in which the roughness of the pile–rock interface is modeled explicitly by assuming sinusoidal undulations along the interface. The testing program includes 10 model piles. Some of these piles have nonzero base resistance; others are unsupported at the base. The results indicate that both the degree of roughness of the socket sidewall and the base stiffness are of major importance to the load response of rock-socketed piles. The ultimate unit side resistance was observed to increase substantially with both increasing sidewall roughness and increasing base stiffness, but there is an upper limit to socket roughness beyond which very little increase in side resistance can be obtained. Most of the available correlations used to predict the ultimate side resistance of rock-socketed piles produced conservative estimates for the test piles in this study. 相似文献
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以水力半径分割法的思想为出发点,建立了新的明渠阻力分割计算关系式,在此基础上分析研究明渠中处于淹没状态下床面柔性植物对水流阻力的影响。采用多普勒流速仪测量边壁附近的流速,用间接法中的流速测量法测量边界上的切应力,用棕榈毛作为植物的模拟材料。以植物粗糙度表示植物阻力,用植物高度、密度的函数表示床面粗糙度,在植物高度和密度变化的情况下,进行水槽试验。经过对试验数据分析,发现植物粗糙度随植物高度、密度的增加而增加,且植物粗糙度越大时,在数值上越接近植物高度。拟合粗糙度与植物高度和密度的经验关系式,进而得出了植物阻力的计算式。 相似文献
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ABSTRACT Laboratory observations regarding the limit conditions for particle entrainment into suspension are presented. A high‐speed video system was used to investigate conditions for the entrainment of sediment particles and glass beads lying over a smooth boundary as well as over a rough bed. The results extend experimental conditions of previous studies towards finer particle sizes. A criterion for the limit of entrainment into suspension is proposed which is a function of the ratio between the flow shear velocity and particle settling velocity. Observations indicate that particles totally immersed within the viscous sublayer can be entrained into suspension by the flow, which contradicts the conclusions of previous researchers. A theoretical analysis of the entrainment process within the viscous sublayer, based on force–balance considerations, is used to show that this phenomenon is related to turbulent flow events of high instantaneous values of the Reynolds stress, in agreement with previous observations. In the case of experiments with a rough bed, a hiding effect was observed, which tends to preclude the entrainment of particles finer than the roughness elements. This implies that, as the ratio between particle and roughness element sizes becomes smaller, progressively higher bed shear stresses are required to entrain particles into suspension. On the other hand, an overexposure effect was also observed, which indicates that a particle moving on a smooth bed is more prone to be entrained than the same particle moving on a bed formed by identical particles. 相似文献
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Heterogeneous coarse grained channels are often characterized by local transitions in bed surface roughness. Distinct spatial zones in terms of grain size have been reported, for example sand ribbons and bedload sheets. The transition from areas of finer to coarser grained surface sediment is often abrupt. However, the effects of these transitions on the shape of the velocity profile and associated shear velocity and roughness length estimates have not been investigated in detail in coarse grained channels. This paper therefore examines the combined effects of a sudden change in surface roughness and of superimposed scales of resistancé on the structure of the turbulent boundary layer. Measurements along roughness transitions from smooth to rough beds were conducted in a flume using artificial roughness features and in a natural gravel bed river. Immediately at the transition from a zone of close packed roughness to a rougher section dominated by obstacles superimposed on the more or less uniform roughness surface, boundary shear stress and roughness length increase considerably. Downstream from this transition, velocity profiles become concave upwards. Downstream and upstream sections show significant differences in terms of near bed velocities (deceleration downstream of the transition), velocity gradient and turbulence intensity of the streamwise velocity component. Comparing the mean velocity profiles corresponding to these two different roughness surfaces gives some indication of the proportion of total shear velocity (or shear stress) associated with the pressure drag produced by large and isolated obstacles. 相似文献
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Prashanth Reddy Hanmaiahgari Nooka Raju Gompa Debasish Pal Jaan Hui Pu 《Natural Hazards》2018,91(3):1075-1096
The present study attempts to predict the reservoir sedimentation in 32 km region of the Tenryu River between the Hiraoka and Sakuma Dams in Japan. For numerical simulations of the reservoir sedimentation, the one-dimensional model of the Hydrologic Engineering Centre-River Analysis System (HEC-RAS) is used together with the inclusion of channel geometry, bed gradation curve, Exner-5 bed sorting mechanisms, fall velocity of the particle, and flow and sediment boundary conditions pertaining to modeling region. The modeling region of the Tenryu River is divided into 48 river stations with 47 reaches in the numerical simulations. The numerical model is calibrated using the available data for 48 years from 1957 to 2004. The formulae of sediment transport function, Manning’s roughness coefficient, computational increment and fall velocity have been identified for getting the best estimation of the Sakuma Dam reservoir sedimentation. Combination of obtained sensitive parameters and erodible limits of 2 m gave the best comparison with the measured bed profile. The computed results follow the trend of measured data with a small underestimation. Although Manning’s roughness coefficient has an effect on the sedimentation, no direct relation is found between the Manning’s roughness coefficient and reservoir sedimentation. It is found that the temperature of water has no effect on the reservoir sedimentation. 相似文献
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The effect of aspect ratio on the mechanical behaviour and micromechanics of two different assemblies during drained triaxial shearing are reported in this paper. Discrete element simulations are done on two different sets of assemblies—first assembly consists of particles with aspect ratio 1.0 and second assembly consists of particles with aspect ratio 1.5. A log normal distribution of particle size is adopted for both the samples. The constitutive behaviour of the assemblies and the evolution of the microstructure of the samples under shearing are closely examined and is related to the aspect ratio of the particles constituting the assembly. The spherical harmonic distributions of contact forces and contact normals along with 3-D histograms are plotted to give quantitative information of the variation of these parameters as the loading progresses. The results indicate that as the aspect ratio increases, there is an increase in the maximum deviatoric stress at the macroscopic level. At the microscopic level, the values of the anisotropic coefficients which are representative of the microparameters also show an increase in the magnitude for the assembly with higher aspect ratio particles. 相似文献
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