摘要:

This study describes a two-dimensional numerical model simulating flushing of sediments from water reservoirs. The numerical model solves the depth-averaged Navier-Stokes equations on a two-dimensional grid. A zeroequation turbulence model is used. The resulting flow field is extrapolated to three dimensions, and the convectiondiffusion equation for the sediment concentration is solved. A formula for the bed concentration is used as boundary condition, resulting in a calculation of bed material load. Continuity for the cells closest to the bed is used to find the bed changes. The pressure field is used to calculate the location of the water surface. The grid is adaptive in the vertical direction, and changes according to the calculated water and bed levels. A porosity model is used to simulate the process when the water surface drops under the bed level at some locations of the geometry. The results from the numerical model were compared with data from physical model studies. The main features of the erosion pattern were reproduced, and the deviation between the calculated and measured scour volume was small. This showed that most of the simplifications made in the numerical model were reasonable. There were deviations between measured and calculated bed levels in a bend. This was most likely due to secondary currents that were not possible to model using a two-dimensional approach.

ISSN:0022-1686    

DOI:10.1080/00221689909498529

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

River bed scour is studied in the paper. Two types of scour are identified, namely local scour and fluvial bed scour. Fluvial bed scour, which occurs if the flow carries sediment load less than its capacity, can be further classified according to the causes as flood scour, downstream reservoir scour, hyperconcentrated flow scour and channelized river scour. The rate of scour is a key problem for understanding of scour process. More than 100 experiments with clear water flow and 33 experiments with sediment-laden flows were conducted to investigate the laws of scour rate. A scour rate formula, the first of its kind, is proposed in the paper, which is proved valid for unsteady flows. In sediment-laden flow the scour rate reduces in an exponential law following increase in the ratio of incoming rate of sediment load to the difference between the capacity and load of the flow. River bed inertia is a new concept of morphological dynamics which represents an important property of the river bed. The inertia of various bed compositions are calculated and the results illustrated that the inertia increases with sorting coefficient of bed materials. By employing the bed inertia and the Exner equation the sediment transport rate in the scour process is theoretically solved. The scour rate under various flow and incoming sediment load conditions can also be calculated from bed inertia. Calculated scour rates for natural rivers agree well with the measured data. A method for calculating maximum scoured depth of river bed during flood is suggested for hydraulic engineers to design safety depth of hydraulic works and pipelines across rivers.

ISSN:0022-1686    

DOI:10.1080/00221689909498530

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Experiments on initiation of motion and rate of bed load transport of different fractions in case of nonuniform sediments are reported. Analysis of these data as well as those from earlier studies by other investigators has shown some limitations of the available relationships for critical tractive stress (CTS) of sediment mixtures. Further analysis of the data has resulted in a new relationship for CTS in terms of the size of the particular fraction in question (di), the geometric mean size (dg) and the geometric standard deviation (σg) of the mixture. An empirical relationship has been obtained for the critical tractive stress of the sizedσdefined asdgσg.

ISSN:0022-1686    

DOI:10.1080/00221689909498531

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Large-scale coherent vortices such as separated vortices and kolk-boils are often observed and generated behind crests of sand dunes in fluvial rivers. The kolk-boil vortex greatly affects transport of suspended sediment which develops up to the free surface. In the present study, both visualization by dye-injection technique and turbulence measurements with a laser Doppler anemometer were conducted in order that the elemental structure of vortices could be clarified. 3-D space-time structures of the coherent vortices are discussed by evaluating conventional correlation coefficients and also by using conditional sampling techniques. Instantaneous velocity-velocity combinations at different points were simultaneously measured by means of two sets of laser Doppler anemometers and an automatic traversing system. Refined physical models of coherent vortices are proposed in the present study.

ISSN:0022-1686    

DOI:10.1080/00221689909498532

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

In this study, existing computational hydraulic engines are used to generate numerical solutions that are treated as 'noiseless' field data. Artificial neural networks (ANNs) are then used to transform this data into what are, in effect, numerical schemes, and these are used in their turn to generate the partial differential equations that govern the observed phenomena. Since it is thereby shown that the trained ANNs can reinstate the governing partial differential equations, it is argued that they contain the same knowledge, or have the same semantic content, as these equations. Besides raising confidence in the capabilities of ANNs in a future generation of sub-symbolic engines, this study opens up another way to mine data for knowledge discovery. Although introduced here only for a limited range of flow problems, the methods advanced here appear to be quite generally applicable.

ISSN:0022-1686    

DOI:10.1080/00221689909498533

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

This technical note presents the equation for one-dimensional gradually-varied steady flow profiles with constant discharge, in a rectangular-section prismatic channel of constant roughness and small longitudinal slope.

ISSN:0022-1686    

DOI:10.1080/00221689909498534

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The phenomenon of fluid-dynamic feed-back, which leads to organised periodic vortex shedding and oscillation of the free shear layer below a submerged sluice gate, is investigated. The feedback mechanism occurs through the impingement of the vortices onto a second parallel sluice gate installed at some distance downstream. The pressure fluctuations at the point of impingement are fed back upstream along the shear layer to the sensitive region of flow separation near the lip of the sluice gate. This amplifies the instability and enhances the vortex formation at the source of instability. The process of vortex impingement, upstream propagation of the disturbances, and the incitement of the velocity field at the vortex source, eventually lead to organised periodic oscillation of the shear layer and the accompanying vortex formation process. Over the present tests ranges of 30 <Lc/θO< 170, and 2,000 <Reθ < 10,000, fluid-dynamic feedback was observed to occur within a Strouhal number range of 0.005 <Sθ< 0.025, whereReθ andSθare the Reynolds and Strouhal numbers defined in terms of the initial momentum layer θo, andLcthe impingement length. The present results agree well with those of previous studies, as well as that predicted from linear stability analysis.

ISSN:0022-1686    

DOI:10.1080/00221689909498535

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The objective of this paper is to model the behavior of secondary flows between two successive and reverse curves. The influence of different hydraulic and geometric parameters on the secondary current damping was also analysed. A three-dimensional finite element model has been developed based on Navier-Stokes equations assuming fluid incompressibility and hydrostatic pressure. An eddy viscosity formulation based on the mixing length concept has been introduced to correctly reproduce velocity profiles of turbulent flows. To validate the numerical model, velocity measurements have been conducted in the laboratory using two identical 90° curves separated by a straight reach. The predicted velocities have been compared with the experimental velocities measured at different channel sections. The validity of the numerical model formulation was confirmed by the laboratory work.

ISSN:0022-1686    

DOI:10.1080/00221689909498536

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

ISSN:0022-1686    

DOI:10.1080/00221689909498537

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

ISSN:0022-1686    

DOI:10.1080/00221689909498528

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor