摘要:

A simulation model for the formation and evolution of frazil in open channels is developed. In this model, the primary nucleation is assumed to be due to mass exchange of seeding crystals at the free surface. The model of frazil crystals growth is based on the rate of heat transfer between crystals and the ambient turbulent flow. Secondary nucleation and flocculation are simulated based on binary collisions of frazil particles. The model is validated with existing experimental data. It is capable of simulating the variation of water temperature during the frazil formation period and the evolution of size and concentration distributions of frazil in the flow. Effects of the surface heat exchange, the rate of seeding, and the flow condition are examined.

ISSN:0022-1686    

DOI:10.1080/00221689509498572

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A complete Reynolds-stress transport model of turbulence is used to predict the effects of asymmetry, surface roughness and side-wall inclination on the behaviour of turbulent flows in open, compound, channels. The turbulence model is one which was previously developed with reference to simple, rectangular, open channels and is used here without modification. Comparisons are made with mean-flow and turbulence data from several experiments and these show that the computed results correlate reasonably well with the measured behaviour. The simulations also reveal a hitherto unreported feature of the present flows, namely the noncoincidence at various locations within the flow of the points where the shear stresses and the relevant rates of strain are zero.

ISSN:0022-1686    

DOI:10.1080/00221689509498573

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A depth integrated numerical model has been refined to simulate the tidal flows in a model square harbour. The harbour entrance width is narrow and of similar dimensions to the mean water depth. The advective accelerations and the turbulence stress terms of the governing hydrodynamic equations are therefore significant in describing the complex flow patterns. A split-operator approach is employed in the model with the advective terms solved by the minimax-characteristics method. The turbulence stress terms have been examined in detail and three important components established, including: bottom generated turbulence, jet induced turbulence and momentum dispersion due to the vertical integration of the secondary motion arising from the horizontal curved flow. An order of magnitude estimation has shown that the momentum dispersion is of primary importance. A simple zero order turbulence model has been used to account for the three mechanisms, with a semi-slip boundary condition being included to cater for the wall shear. Convergence of the solution has been achieved by successively reducing the time step and grid size. The computed results have been shown to compare well with experimental measurements.

ISSN:0022-1686    

DOI:10.1080/00221689509498574

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

One-dimensional and two-dimensional Laser Doppler Velocimeter (LDV) were used respectively to measure velocity of smooth-wall open channel flows with channel bed slopes varying between 0.25% and 3%. Turbulent flows were all supercritical with Froude numbers varying from 1.5 to 3.5 and Reynolds number having a magnitude of 105. Using 1-D LDV the law of the wall and the law of the wake were examined and the von Karman constant K was verified to be 0.4 universally, while the integral constant A might change with distance from the side wall. The law of the wake exists clearly with a wake strength Π of about 0.3. Moreover, from the 2-D LDV measurements, the influence of channel slopes on the turbulent characteristics were fully investigated.

ISSN:0022-1686    

DOI:10.1080/00221689509498575

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A Lagrangian (random walk) sediment deposition model is used to investigate the competing effects of gravity and turbulence on deposition to a fully absorbing bed. This approach permits an analysis of the probability density function of deposition velocity as well as its mean. A distinct change in transport behavior is observed at a critical value ofw*c=ws/U*= 0.1, wherewsis the fall velocity in still fluid andU*is the shear velocity. Turbulence controls the transport of particles beloww*cwhich deposit faster than similar particles in still water. For particles abovew*c, the still water fall velocity is a good estimate of the mean deposition velocity. A discrepancy between the Lagrangian model results and experimental data (forw*<w*) may suggest that existing diffusion models are an incomplete representation of the physical system. An alternative conceptual model is suggested, based on coherent intermittent turbulence structures, which appears to explain the experimental results more effectively. Many natural and engineered sedimentation systems havew*<w*and are dominated by turbulent sediment transport. New sediment removal technologies are suggested based on turbulence enhanced deposition.

ISSN:0022-1686    

DOI:10.1080/00221689509498576

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Cavitation damage to spillway surfaces may be prevented with the use of aeration devices. These serve to introduce air into the layers close to the channel bottom in order to reduce cavitation erosion. Under some circumstances, the aerator can be drowned out, will cease to protect the spillway surface, and act potentially as a cavitation generator. This article analyses the conditions of filling. Then experimental data for ten aerator geometries are reviewed. Depending upon the aerator geometry, the cavity filling occurs when the Froude number is less than a critical value or when the ratio of the flow depth over the total offset is larger than a characteristic value.

ISSN:0022-1686    

DOI:10.1080/00221689509498577

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

This paper presents a critical analysis of the energy loss at drops. Using the results of an experimental study on three drops, several assumptions made by White and Gill in developing an equation for the energy loss at drops have been examined. This paper also presents some interesting measurements on the velocity distribution in the falling jet as well as in the sliding jet on the side of the pool. This work has also indicated that the loss at a drop is mainly due to the mixing of the jet with the pool behind the jet. Further, a method has been developed to predict the energy loss at a drop.

ISSN:0022-1686    

DOI:10.1080/00221689509498578

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A two-tube method is proposed to extend the Preston tube technique for determining the local shear stress in both rough and smooth surfaces. The analytical calibration functions of the Preston tube are derived based on the Rotta's model for the velocity profile on smooth and rough surfaces and a formula for the displacement factor of the Preston tube. The new method, unlike the conventional Preston tube method, is a combination of the measurement by two tubes of different diameters and iterative computation by the analytical calibration functions to determine the roughness parameter and local shear stress. The analytical calibration curves are compared with experimental data. The use of the two-tube method on the smooth surface is verified by experiment.

ISSN:0022-1686    

DOI:10.1080/00221689509498579

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The frequency down-shift of swash oscillations compared to incident waves is examined by laboratory experiments and numerical simulations in connection with the uprush-backwash interaction in the swash zone and the standing wave in the surf zone. The experimental and numerical results show that the interaction between uprush and backwash actually generates swash oscillations with the frequency lower than that of incident waves when the power of short waves or bores near the shoreline is large. When the power of the bores is weak, a group-induced standing wave is dominant in the swash oscillations.

ISSN:0022-1686    

DOI:10.1080/00221689509498580

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

ISSN:0022-1686    

DOI:10.1080/00221689509498581

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor