Journal of Hydraulic Engineering

Intense transient shallow flows over erodible bed imply the appearance of a changing horizontal density attributable to the presence of sediment particles in the water layer. The lack of consideration of the variability of the bulking density of the mixture is not admissible when modeling severe types of erosional flow such as the release of a dam break wave over a sedimentary bottom. Such events can lead to significant changes in the wave hydrodynamics, since the inertia of the flow can be larger and consequently its erosion/deposition capacity can be altered. From a numerical point of view a new complex erosion/deposition source term appears. For the integration of these source terms two strategies have been explored in this work: upwind and pointwise. Hence, this work is focused on the development and validation of a novel numerical scheme based on an approximate augmented Riemann solver, where the erosion/deposition rates play an important role in the variation of mixture density. Several analytical test cases have been derived in order to validate the computational tool. The numerical predictions have also been compared against experimental data.

It has been well established that leakage from pipes is more sensitive to changes in pressure than the square root relationship predicted by the orifice equation. The main reason for this is that leak areas are not static, but vary with changes in system pressure. While previous studies have shown that the leak area is a linear function of pressure for elastic materials, the effect of the viscoelastic behavior of plastic pipes on the pressure-leakage response is not yet well understood. In this study, finite element analysis was used to investigate the effect of viscoelastic behavior on round holes and longitudinal cracks in both high density polyethylene (HDPE) and polyvinylchloride (PVC) pipes. The standard differential equation for linear viscoelastic deformation was then calibrated to the finite element results, resulting in equations that accurately describe the viscoelastic variation in leak areas as functions of time. The results also showed that the time-dependent behaviors of both pipe materials are proportional to their elastic behavior, and stabilize after about 12 h. The total deformations of HDPE and PVC pipes were found to be approximately 220 and 122% of their respective elastic deformations.

In this article, a comprehensive review of existing methods is presented and computationally efficient sparse null space algorithms are proposed for the hydraulic analysis of water distribution networks. The linear systems at each iteration of the Newton method for nonlinear equations are solved using a null space algorithm. The sparsity structure of these linear equations, which arises from the sparse network connectivity, is exploited to reduce computations. A significant fraction of the total flops in the Newton method are spent in computing pipe head losses and matrix-matrix multiplications involving flows. Because most flows converge after a few iterations, a novel partial update of head losses and matrix products is used to further reduce computational complexity. Convergence analyses are also presented for the partial-update formulas. A new heuristic for reducing the number of pressure head computations of a null space method is proposed. These savings enable fast near-real-time control of large-scale water networks. It is often observed that the linear equations that arise in solving the hydraulic equations become ill-conditioned due to hydraulic solutions with very small and zero flows. The condition numbers of the Newton equations are bounded using a regularization technique with insignificant computational overheads. The convergence properties of all proposed algorithms are analyzed by posing them as an inexact-Newton method. Small-scale to large-scale models of operational water networks are used to evaluate the proposed algorithms.

The study of water hammer in air-water, two-phase flows in hydraulic structures such as pressurized pipelines and tunnels, siphons, culverts, and junctions is of great importance for design purposes. Water hammer if combined with a periodic slug flow would lead to severe periodic transient pressure fluctuations inside the conduit. Laboratory experiments have been conducted to investigate water-hammer pressure inside a horizontal rectangular conduit carrying a two-phase, air-water slug flow. Tests were performed in an experimental apparatus comprising a 6.8-m-long transparent pipeline 0.06 m wide and 0.1 m high. By rapidly closing a control gate at the end of the conduit, propagating pressure surges were generated. Transient pressure fluctuations were recorded by means of pressure transducers. Furthermore, a digital camera was used to document flow properties and air-bubble characteristics throughout the pipeline. Pressure measurements along the pipe indicated that two scenarios could be considered as (1) below the gas pocket (Type 1), and (2) below the liquid column (Type 2). Results demonstrated that at the downstream sections, pressure oscillations in Type 2 are sharp, quickly damping, and associated with great maximum pressures (up to 50 times the initial pressure). However, other sections were observed to have low-frequency pressure oscillations, which were damped gradually. Likewise, the latter has also been observed throughout the conduit in Type 1. Results confirmed that the transient wave speed is no longer constant as in the single-phase flow, and it varies along the pipeline depending on variations in local pressure and local void fraction. It was found that besides the control gate, damping time in Type 2 increases with the air/water rates ratio. However, this parameter for Type 1 remains relatively constant and is independent of the air/water rates ratio.

This paper investigates the source of the spurious numerical oscillations often observed in simulations using the well-known Preissmann slot method and proposes a nonoscillatory numerical fix that can efficiently suppress the numerical oscillations. The root of these oscillations is identified by comparing the orbits calculated by a first-order Godunov type model with an ideal numerical orbit in the phase plane. It is found that in the very thin layer in the vicinity of the conduit roof the numerical model has insufficient numerical viscosity to avoid the often-observed oscillations. In order to remove these spurious oscillations, an approximate Riemann solution is proposed that automatically enhances the numerical viscosity whenever the water level is in the vicinity of the conduit roof and the pressurization of the conduit is proximate. A comparison of results from the proposed model with both experimental data and analytical solutions show that it can provide nonoscillatory solutions over a wide range of the wave velocities ranging from 10 to $\mathrm{1,000}\mathrm{m}/\mathrm{s}$. Furthermore, the proposed model effectively controls data smearing.

A graph theory-based algorithm is demonstrated for optimal pump scheduling of two example application water networks. The hydraulic part of the problem is solved using a dedicated and efficient hydraulic solver. The pump scheduling part of the problem is solved using a skeletonized operational graph, representing only the basic logic operational relations existing in the network required for pump selection: the pumping units (with nominal operating costs), water tanks and clustered demand nodes. The hydraulic solver advances one time step at a time. After each time step advance, the nodes of the model are checked to see if satisfy minimum service pressure and minimum water tank level. For nodes not satisfying the service constraints, the Dijkstra’s shortest path algorithm is applied to the skeletonized graph to determine the optimal pumping unit to be activated and then updating the pumps operation pattern in the model. The hydraulic solver is then reinitialized to resolve and recheck the time steps one by one. The algorithm ends when the solver reaches the last time step with all nodes meeting service constraints. The algorithm returns an optimal minimal cost pump-scheduling pattern under greater-than constraints over the examined time period, such as (1) minimal consumer service pressure, and (2) water balance closure at the water tanks. The algorithm returns discrete pump operation scheduling with minimal pump switching and minimal water age in the tanks, demonstrating short solution times (28 s to schedule 11 pumps over a 168-hour period). The algorithm may be applicable for real-time pump scheduling. Future research may include water quality constraints and variable frequency drive pump scheduling. Pump selection is based on the assumption that the optimal pump operation order is not affected by changes in the network’s hydraulic conditions, such as water tank levels and location along the pump efficiency curve (assuming constant efficiency). If hydraulic conditions change the optimal activation order, then the pumps’ working points must by updated after each time step solution, which is not addressed in the current work.

The contraction scour resulting from a two-dimensional (2D) mobile bed numerical model is superimposed on the local scour measured on a large scale mobile bed physical model to evaluate the total scour at bridge piers. Compared with the empirical methods currently used in engineering practice, this procedure provides more accurate, specific, and not excessively conservative results. Moreover, the approach avoids the shortcomings of physically modeling extended river reaches. The results given by the suggested method are compared on a case study to the ones calculated with empirical formulas.

Most studies on scouring seem to have overlooked the effect of viscosity, the assumption being that the flow is free of viscous effects inside the scour hole because of the presence of highly turbulent flow structures such as down-flow, horseshoe vortex, and wake vortices irrespective of the approach flow regime. The present work reports the results of experiments conceived to revisit the effect of viscosity on scouring. The experiments were performed by changing the Reynolds-like numbers for two flow shallowness values and keeping constant the remaining dimensionless parameters that control the scour process. The values of the Reynolds number corresponded to transitional approach flows. The experimental results indicate that scouring does depend on viscosity in usual laboratory conditions, the viscous effect leading to scour depths in the safety side.

Experiments in a stilling basin model with air injected at its downstream end exhibited plunging flow resulting in a downward roller, return flow along the bottom, and a downstream near-bed flow after the plunging flow impingement point. Turbulence measurements showed that air injection reduces turbulence both at the exit of the stilling basin and near the downstream bed. Thus the effect of air injection as a modifier of the turbulence structure of the flow downstream of the stilling basin is postulated as a plausible explanation for the observed scour reduction.

The present study shows that the sediment pickup rate is better correlated with the densimetric Froude number than the Shields number. With laboratory data collected in open channel flows, an empirical formula for estimating the pickup rate is proposed. Because it does not involve the critical condition for the incipient sediment motion, the formula can be applied to sediment entrainment of different stages including weak sediment motion. In spite of the fact that the experiments were carried with a limited range of sediment particle diameters, ranging from 0.23 to 0.86 mm, the results obtained are also supported by several series of data reported in the literature.