The following are abstracts from the Journal of Hydraulic Research, 40, Issue 1
An experimental study of Roman dropshaft hydraulics
H. Chanson
In Roman aqueducts, series of vertical dropshafts were used to dissipate the kinetic energy of the flow: i.e., the dropshaft cascades. A re-analysis of Roman dropshaft hydraulics is conducted with physical model tests. Three basic flow patterns are observed. The results demonstrate that the vertical dropshafts could be very efficient energy dissipators and re-oxygenation structures, under appropriate flow conditions. The optimum operation of Roman dropshaft is discussed and an analytical model is developed to predict these conditions. Further the performances of aqueduct dropshafts are compared with modern dropshaft designs, and the operation of dropshaft cascades is discussed. Additional material is available upon request.
Flow pattern in the scour hole around a cylinder
W. H. Graf and I. Istiarto
Local scour around a bridge pier (cylinder) is a problem of much concern to hydraulic engineers. This is a complex phenomenon resulting from the interaction of the three-dimensional turbulent flow field around the cylinder and the mobile channel bed (see Fig. 1). In the vicinity of the cylinder, scouring is the consequence. In this paper, the three-dimensional flow field in an established (equilibrium) scour hole is experimentally investigated. An acoustic-Doppler velocity profiler (ADVP) was used to measure instantaneously the three components of the velocities in the vertical symmetry (stagnation) plane of the flow before and after the cylinder. The vorticity of the flow field was calculated. Results of the study show, that a vortex-system is established in the front and a trailing wake-vortex system of strong turbulence is formed in the rear of the cylinder.
Experiments on incipient channelization of submarine fans
J. Imran, G. Parker and P. Harff
Experiments on the formation of channelized submarine fans by the passage of successive turbidity currents are presented. Channels similar to subaerial rivers are found in many submarine fans. The inception of channels on submarine fans, which are essentially depositional environments, can be explained in terms of the lateral distribution of the rates of deposition and erosion of suspended sediment associated with a passing turbidity current. The experiments reported here supplement the findings of a numerical model developed earlier to study the inception of channel-levee systems on submarine fans. The experimental setup is a simple replication of a surface at the base of a continental slope receiving a turbidity current from a canyon. The turbidity current was generated by releasing a sediment-water mixture of low sediment concentration from an overhead tank into a quiescent water body held in a rectangular tank. The bottom slope of the tank was varied between 3 and 10%. Two sediment sizes, with geometric mean sizes of 71 m and 110 m were used. The bed profile was measured after the completion of each run. A 2-D depth-averaged numerical model of turbidity current was then run with the input conditions from the experiment. The resulting bed profiles were compared with the measured data. Generally good agreement between the experimental and numerical findings is observed.
Undular bores and secondary waves-Experiments and hybrid finite-volume modelling
S. Soares Frazão and Y. Zech
Secondary free-surface undulations (Favre waves), appearing for example after the opening of a sluice gate or at the head of a bore, cannot be reproduced by numerical models based on the hydrostatic pressure assumption. The Boussinesq equations take into account the extra pressure gradients but are difficult to integrate due to the high-order derivative terms. The paper describes the physics of wave initiation and proposes a demonstration of the Boussinesq equation based on relatively wider assumptions than usually adopted. A linear stability analysis is developed in finite-difference frame to highlight some potential source of numerical instabilities. These conclusions are transposed in a new hybrid finite-volume / finite-difference scheme, which reveals a better accuracy in period and amplitude when evaluated against experiments.
A transient shear stress model for the analysis of laminar water-hammer problems
R. A. Prado and A. E. Larreteguy
A transient shear stress model for the solution of water-hammer problems for laminar flow in pipes is presented. The model is based on the polynomial expansion of the radial profiles of axial velocities, and the solution of the resulting set of equations by the method of characteristics. This approach, as compared to the usual quasi-steady model (which can be regarded as a particular case of the new method), allows for a better representation of the shear stress at the wall during the pressure transients. The present model can be included with only minor modifications into any existing code for solving water-hammer problems that uses the characteristics method and the quasi-steady model. The test of the new model against experimental results of Holmboe and Rouleau, given in [8], and mathematical models and numerical simulations of other authors [5] [8] show that it is less cpu and memory demanding, and is able of obtaining comparable results.
Leak detection in pipes by frequency response method using a step excitation
W. Mpesha, M. Hanif Chaudhry and S. L. Gassman
This paper presents a new procedure utilizing transient state pressures to detect leakage in piping systems. Transient flow, produced by opening or closing a valve, is analyzed in the time domain by the method of characteristics and the results are transformed into the frequency domain by the fast Fourier transform. This method is used to develop a frequency response diagram at the valve end. The frequency response diagram of a system with leaks has additional resonant pressure amplitude peaks (herein called the secondary pressure amplitude peaks) that are lower than the resonant pressure amplitude peaks for the system if there were no leaks (herein called primary amplitude peaks). The location of a leak is determined from frequencies of the primary and secondary pressure amplitude peaks and the leak discharge is determined from the maximum and minimum discharge amplitudes. This method is applicable for practical values of the friction factor over the range 0.01 to 0.025 and can be used to detect leaks in real-life pipe systems conveying different types of fluids, such as water and petroleum. It can be used directly by comparing the frequency response diagram of a modelled system without leaks to the frequency response diagram developed by gradually opening or closing a valve at the downstream end of a pipe and taking measurements of pressure head and discharge at only one location.
Unsteady two-dimensional orifice flow: a large-size experimental investigation
H. Chanson, Shin-ichi Aoki and M. Maruyama
Orifice flows were used as water clocks since the Antiquity up to the 16th century. Today orifices and nozzles are used for measuring discharges. Most works were conducted with steady flow conditions and there is little information on the unsteady flow pattern. In this study, the writers describe an experimental investigation of an unsteady orifice flow discharging vertically. The study was conducted in a large-size facility with a rectangular orifice (0.75-m by 0.07-m) discharging up to 1.2 m3 in about 10 seconds. The study presents new information on the unsteady flow patterns, the discharge capacity and the velocity field in the reservoir. The results are compared with ‘classical’ orifice flow results.
Numerical simulation of the draft tube and tailwater flow interaction
W. Yuan and R. Schilling
This work focuses on the numerical simulation of the interaction between the draft tube and tailwater flowing lowhead hydro power plants. An approach to the solution of incompressible free surface flow problems is developed. The flow field and the free surface location are calculated by coupling the free surface kinematic and dynamic conditions with the equations of motion for the bulk flow. A pressure correction method is applied to the flow calculation in a moving grid system. TheRNG k-å model is used for turbulence modelling. In order to reduce the computational time, the PVM(Parallel Virtual Machine) subroutines are implemented.
Air demand behind high head gates during emergency closure
I. Aydin
Pressure drop and consecutive air demand behind high head gates during emergency closure is studied by physical and mathematical models. Measurements are done on hydraulic model of a leaf gate installed in the intake structure of a penstock. Local loss coefficients are determined as functions of Reynolds number and gate openings from measurements of discharge and piezo-metric levels at static positions of the gate. A mathematical model for the unsteady flow due to closing gate is formed by applying the integral continuity and energy equations on control volumes upstream and downstream of the gate. Dimensionless numbers relevant to the problem are obtained by dimensional analysis of the governing equations. Time-wise variations of air discharge in the ventilation shaft and pressure behind the gate are obtained from numerical solution of the model equations. The relative air demand is computed over substantial ranges of dimensionless parameters and some design considerations are discussed.
On the incipient aerated flow in chutes and spillways
A. Moñino Ferrando and J. Riera Rico Natural self-aeration of water flows in open channels protects surfaces in contact with the flow from cavitation damage if enough air content is reached (Falvey [6] [7], Peterka [10], Russel & Sheenan [11]), although it could lead to an increase in both flow depth and velocity. Also, self-aerated flow enhance the process of atmospheric gases exchange within the fluid, improving water quality downstream of hydraulic structures (Chanson [5]). So it is of great interest to evaluate accurately the critical point where air entrainment begins, that is, the location of the inception point. The note first shows a review on some methods to evaluate the inception point location. Second, a brief explanation on calculation development is made and an expression is obtained which enables to compute the point of incipient self-aeration as a function of unit discharge, bottom slope of the chute and uniform surface roughness in a more simple and accurate way. Finally, comparison with results provided by other methods is made and conclusions are obtained.
Use of resistance coefficients derived from single planes to estimate time of concentration of two-plane systems
T. S. W. Wong
By coupling the Darcy-Weisbach friction formula with the kinematic wave time of concentration formula, a kinematic-Darcy-Weisbach time of concentration formula for a series of planes is derived. The formula is applicable to a cascade of planes, to planes of different roughnesses, to planes of different flow regimes, to planes of different soil types and infiltration rates resulting in different net rainfall intensities, to planes subject to different rainfall intensities, and to planes with a combination of all these variables. By applying the observed times of concentration to the formula for single planes, the Darcy-Weisbach resistance coefficient for a concrete surface and for an artificial grass surface are derived. Based on the derived resistance coefficients, they indicate that the flow regime on concrete was transitional and the flow regime on artificial grass was laminar. Further, by applying the derived resistance coefficients to two-plane systems (i.e. planes with combinations of concrete surface and artificial grass surface in series), the comparisons of the estimated times of concentration from the formula with the observed values show very good agreement. In one particular two-plane system, even with the occurrence of kinematic shocks, the agreement is still good. This study shows that the resistance coefficients that are derived from uniform planes can be applied to two-plane systems.
The following abstracts are from 40, Issue 2.
Wave effects on blockwork structures: model tests
G. Muller, P. Hull, W. Allsop, T. Bruce, M. Cooker and L. Franco
Up to the middle of this century many coastal structures were built from blockwork, using either natural stone blocks or concrete blocks. Those blockwork structures subjected to breaking wave impacts often show a particular damage type, whereby individual blocks are shifted out of their position and moved into the sea. Engineers have suspected for a long time that wave impact pressures can travel into the water filled cracks and joints of such structures, building up pressures inside of the structure and thus destroying the structure from within. In order to verify the damage mechanism, and to investigate the characteristics of impact induced pressure pulses, model tests on the propagation of wave impact pressures into water filled cracks were conducted. It was found that impact generated pressure pulses can enter water filled cracks and that they exhibit wave-like characteristics such as finite propagation speed, reflection, superposition and attenuation. Changes in cross section were found not to affect pressure pulse magnitude or duration. The possibility of wave impact pressures to damage or destroy cracked or fissured structures from within the structure could thus be verified.
Bed load motion and grain sorting in a meandering stream
P. Y. Julien and D. J. Anthony
A three-dimensional moment analysis defines both particle mobility and the average orientation angle of moving bedload particles in meander bends. Accordingly, under identical bed shear stress and near-bed streamline orientation angle on a side slope, bedload particles of different sizes move in different directions. This sorting mechanism has been verified in the sharp meander bends of Fall River, Colorado. The extensive field data set includes near-bed sediment transport measurements by size fractions from 0.125 to 32 mm using a Helley-Smith sampler. The field measurements in two meander bends corroborate the theoretical model: particles finer than d50 preferentially move up the point bar and particles coarser than d50 move toward the thalweg. The measured deviation angle between the mean trajectory of 0.125 mm and 32 mm particles reaches 20 degrees near the bend apex.
Modelling of three-dimensional velocity field in open channel flows
W. Czernuszenko and A. Rylov, Dr.
A comparatively simple model for calculation of the three-dimensional, stationary velocity field is presented. The model is able to calculate the streamwise velocity distribution as well as the secondary flow in a cross-section of regular channel. The Reynolds equations are closed by a new anisotropic turbulence model which consists of two sub-models: one for the shear stresses and the other for normal stresses. The numerical solution of the parabolic approximation of the model equations gives reasonably good secondary flow patterns as well as the longitudinal velocity distribution in the channel cross-section.
Modelling of supercritical flow conditions revisited; NewC Scheme
V. Kutija and C. J. M. Hewet
A hydrodynamic numerical model for one-dimensional free-surface flows,named ‘NewC’, is presented. NewC is a finite difference scheme which has a major advantage over schemes used currently in engineering applications in that, while the algorithmic structure is of the sub-critical-flow-type, it is capable of modelling subcritical, supercritical and transcritical flow conditions without requiring any changes to the governing equations. The scheme is shown to be unconditionally stable for a range of Courant numbers even for Froude numbers greater than or equal to one. The computational effort expended compares favourably with the finite difference schemes used currently. The NewC scheme has the additional advantage that it is relatively straightforward to incorporate into algorithms for the solution of flows in free-surface networks.
Hydraulic geometry of straight alluvial channels and the principle of least action
H. Qing Huang, G. C. Nanson and S. D. Fagan Natural rivers exhibit regular hydraulic geometry relationships for which no widely accepted explanation has been given. This paper applies the physical principle of least action to the determination of stable alluvial-channel form. For steady, uniform alluvial-channel flow, both theoretical inferences and a case study show that least action occurs when the criteria of minimum potential energy and MFE (Maximum Flow Efficiency, defined here as the maximum sediment transporting capacity per unit available stream power) are satisfied. The consistency between bankfull hydraulic geometry relationships of natural channels and those of maximally efficient or ‘least action’ channels identified in this study demonstrates that alluvial channels commonly adjust to a maximally efficient section. Support for the use of the extremal hypotheses of maximum sediment transporting capacity and minimum stream power is provided by illustrating that they are essentially expressions of, and hence subsumed by, the more general principle of MFE.
Simulation of flow around piers
K. H. M. Ali and O. Karim
FLUENT CFD was used to predict the three-dimensional flow field around a circular cylinder. Solutions were obtained for rigid beds and for scour holes of different sizes resulting from different time-durations. The numerical results were used to obtain the variation of bed shear-stress around the cylinder. These results were used in the sediment continuity equation to obtain an expression for the variation of scour depth with time. The asymptotic scour depth was found to depend on three dimensionless numbers: the pile number, the sediment size number and the duration time number. The theoretical relationship was calibrated using various laboratory and field results.
Hydrodynamic pressures acting on rigid gravity dams during earthquakes
Y. -S. Cho and P. L.-F. Liu
A boundary integral equation model is developed based on the analytical integrals for three-dimensional potential problems. All necessary integrals are first converted into line integrals around a target element and then integrated analytically. The developed model is applied to a practical problem concerning computation of the hydrodynamic pressure acting on a dam face of a dam-reservoir system during earthquakes. The obtained numerical solutions are compared with available two-dimensional experimental data and analytical solutions. A very good agreement is observed. The model is then used to investigate three-dimensional effects of a complex dam-reservoir system.
Developing generic hydrodynamic models using artificial neural networks
Y. B. Dibike
Possibilities for the development of a new modelling paradigm, namely allowing models to ‘construct themselves’ by learning from existing numerical-hydraulic models, was investigated by extending previous works to encompass schemes that can be applied over arbitrary bathymetries with variable distances and time steps. For the simplest possible cases of one and two dimensional flow problems considered in this study, the relatively elementary technology of artificial neural network was found to provide acceptable results. Moreover, it was demonstrated that the well-trained networks could be substituted in place of the finite difference schemes in the hydrodynamic model formulation and could perform like numerical operators. This new paradigm is intended in future to supplement, and even in some instances to replace the current one.
A self-adaptive boundary search genetic algorithm and its application to water distribution systems
Z. Y. Wu and A. R. Simpson
The success of the application of genetic algorithms (GA) or evolutionary optimization methods to the design and rehabilitation of water distribution systems has been shown to be an innovative approach for the water industry. The optimal design and rehabilitation of water distribution systems is a constrained non-linear optimization problem. Constraints (for example, the minimum pressure requirements) are generally handled within genetic algorithm optimization by introducing a penalty cost function. The optimal or near optimal solution is found when the pressures at some nodes are close to the minimum required pressure or at the boundary of critical constraints. This paper presents a new approach called the self-adaptive boundary search strategy for selection of penalty factor within genetic algorithm optimization. The approach co-evolves and self-adapts the penalty factor such that the genetic algorithm search is guided towards and preserved around constraint boundaries. Thus it reduces the amount of simulation computations within the GA search and enhances the efficacy at reaching the optimal or near optimal solution. To demonstrate its effectiveness, the self-adaptive boundary search strategy is applied to a case study of the optimization of a water distribution system in this paper. It has been shown that the boundary GA search strategy is effective at adapting the feasibility of GA populations for a wide range of penalty factors. As a consequence, the boundary GA has been able to successfully find the least cost solution in the case study more effectively than a GA without the boundary search strategy. Thus a reliable least cost solution is guaranteed for the GA optimization of a water distribution system.
Structure of the turbulent hydraulic jump in a trapezoidal channel
N. Afzal and A. Bushra
The axial flow structure of turbulent hydraulic jump has been analysed and the general equation valid for a channel of arbitrary cross section has been proposed. Based on the Reynolds equations of mean turbulent motion in two dimensional steady incompressible flow subjected to hydrostatic pressure distribution, the integral equations of depth averaged flow over a channel of arbitrary cross sectional area are obtained. An integral method has been developed where inertia, pressure gradient and depth averaged normal Reynolds stress play the dominant role. The closure model for variation of depth averaged normal Reynolds stress has been expressed as product of the constant eddy viscosity and the gradient of the depth averaged axial velocity with respect to axial distance. In the trapezoidal channel the closed form solution for the upper surface profile and axial length of the hydraulic jump have been obtained. The comparison of the theory with experimental data is remarkably good. The theory shows that for F1 larger than a fixed value, the surface profile approaches a limiting universal solution provided the variables are appropriately non-dimensionalized. Further, the present predictions on the roller length are also supported by experimental data in rectangular and triangular channels.
Design of minimum water-loss canal sections
P. K. Swamee, G. C. Mishra and B. R. Charar
The canal water losses constitute of seepage and evaporation losses. Where as seepage loss depends on the channel geometry, evaporation loss is proportional to the area of free surface. On account of complexities of analysis, the design of minimum water loss section has not been attempted as yet. In this investigation explicit equations for the design variables of minimum water loss sections for triangular, rectangular, and trapezoidal canals have been obtained using non-linear optimization technique. The proposed equations along with tabulated section shape parameters facilitate easy design of the minimum water loss section and computation of water loss from the section without going through the conventional and cumbersome trial and error method. A design example has been included to demonstrate the simplicity of the method.
