Journal of Hydraulic Engineering

Multiple buoyant jets are found in the natural and artificial environment: thermal discharges from fossil and nuclear-fueled electricity generation, domestic and industrial wastewater discharges, brine disposal from desalination plants, and various heat sources in the built environment. An overview of theoretical and experimental modeling of multiple buoyant jets over the past three decades is presented. Basic measurements of the structure of buoyant jet flows, integral jet modeling and three-dimensional numerical solutions of the Reynolds-averaged equations are reviewed. A semianalytical model is proposed to predict the dynamic interaction of multiple buoyant jets in stagnant fluid. The unknown jet trajectories are obtained from an iterative solution of an integral jet model and the irrotational external flow. Predictions are in good agreement with experiments of clustered jet groups, turbulent plume pairs, alternating diffusers, and rosette buoyant jet groups; the approach can also be extended to multiple jets in cross-flow. The mixing of a rosette buoyant jet group in a cross-flow is reviewed. The use of jet theory in solving two unconventional urban environment problems are highlighted: (1) the unraveling of the cause of the severe acute respiratory syndrome (SARS) outbreak in Hong Kong in 2003; and (2) design of a complex river junction for flood control under tight space constraints. It is suggested that experiments will remain a source of new theoretical ideas and the need for a civil engineer to solve complex problems with tractable models and analytical clarity will prevail.

Laboratory experiments of sediment entrainment were carried out in a flume for different flow configurations with particle image velocimetry (PIV) used to measure the flow velocity. The experiments were done in a low submergence condition. Measurements revealed that the vertical distribution of streamwise flow velocity at the instant of entrainment is significantly different from the temporal mean flow profile. The role of turbulence for a given flow depth is found to decrease with an increase in exposure of the particle. An expression for threshold velocity is proposed that accurately represents the enhanced near-bed turbulence during entrainment. A Shields-type parameter, expressed in terms of critical near-bed flow velocity rather than shear velocity, is used to define the sediment transport threshold.

Gravel bed rivers commonly form an armored surface layer within which cluster bedforms develop. Clusters can form in isolation or develop cluster groups of increasing density. Flume experiments are presented in which a clustered, armored surface was formed on a gravel bed. Flows were measured over an area without a cluster and areas with an isolated cluster, a coupling of two clusters, and a group of three clusters. Turbulent flow parameters, Reynolds stresses, and turbulent kinetic energy were calculated from acoustic Doppler velocimeter measured flows and visualizations of the flow patterns around clusters. Flow separated over the cluster crest, creating an increase in the turbulent flow properties and a high-turbulence intensity region when flow reattached downstream. Turbulent kinetic energy increased around the coupled clusters, but around the grouped clusters, the flow field was distinguished by a high degree of uniformity. The net effect of increasing density of cluster grouping was an increase in the magnitude and variability of the turbulent flow field around coupled clusters, followed by an overall dampening of turbulent flows around grouped clusters in which the flow patterns generated interfered with one another, and the cluster forms sheltered the bed surface.

Flow within an alluvial channel bend is significantly affected by channel geometry, including curvature ratio (bend radius/channel width, $R/B$) and aspect ratio (channel width/flow depth, $B/H$). High curvature bends ($R/B\le 3$) can experience substantially more erosion than milder curvature bends. This study employs a three-dimensional Reynolds-Averaged Navier-Stokes (RANS) model to investigate the effects of curvature ratio and aspect ratio on bend flow with respect to a high curvature ($R/B=1.5$) base case in a 135° bend. Experimental data are used to validate the RANS model predictions for the high curvature base case with a flat bed (FB) and an equilibrium deformed bed (DB). Five curvature ratios (1.5, 3, 5, 8, and 10) and four aspect ratios (5.00, 6.67, 9.09, and 12.50) are investigated. Results show that a decrease in $R/B$ or $B/H$ for the FB cases results in a strong increase in total circulation of the regions associated with the primary cell of cross-stream circulation (${\mathrm{\Gamma }}_{+}$), an increase in maximum bed shear stress, and an increase in the contribution of the cross-stream component to the total magnitude of bed shear stress. The values of $R/B$ and $B/H$ also affect the structure of the cross-stream flow. The primary cell of cross-stream circulation splits into two clockwise-rotating cells at low $R/B$ values and the cell situated closer to the inner wall induces strong ejections of vorticity. At high $R/B$ values, a secondary counter-clockwise rotating cell forms at the outer bank. At lower $B/H$ values, the primary cell splits into two clockwise-rotating cells. This study shows that the position and size of regions of high bed shear stress, and thus the capacity of the flow to entrain sediment, depend strongly on bend curvature.

Investigations undertaken to understand the mechanics of the motion of large sandbags and to compute their trajectories are reported in this paper, along with the details of the experimental setup and procedures. The motion of sandbags is recorded from the side of a flume by a high-definition charge-coupled device (CCD) camera, and the digital particle tracking velocimetry (DPTV) technique is used to track the motion of the bags. An equation is developed for the normalized maximum horizontal settling distance from the experimental data. It is found that the particle velocity normal to the flow depends mainly on the characteristic diameter of the particle and the Froude number of the flow, whereas the particle velocity in the streamwise direction shows lower dependency on the Froude number. Analysis of the particle tumbling shows that the Magnus force may be neglected for the purpose of modeling the trajectories of sandbags in uniform flows. A model of particle motion is developed by solving the Lagrangian equation numerically. Two approaches to computing the trajectory of sandbags are investigated. The results show that the approach in which the drag coefficient is varied based on the orientation of a particle gives better results than if the drag coefficient is kept constant and is based on the broadside orientation of the particle. The results also show that the change in the drag coefficient may be as low as 28% to as high as 76% based on the orientation of the particle with respect to the flow.

Runoff from roadways on which road salt (NaCl) has been applied for driving safety in winter can form a saline water layer at the bottom of a lake, pond, reservoir, or river impoundment. Natural vertical mixing of lentic surface water bodies can be hindered by the presence of a benthic saline layer and thereby affect lake water quality and ecology. To study the formation and disappearance of the saline layer, temperature and specific conductance profiles were measured intermittently over two years (2007, 2008) and at high frequency during one year (2009) in an urban lake of the northern temperate region (Tanners Lake, Oakdale, Minnesota). Erosion of the saline layer in the spring occurred only in year 2007. In years 2008 and 2009, the saline layer persisted throughout the summer only to be removed during fall turnover when thermal stratification was at a minimum. In all three years, salinity dominated density stratification after ice-out, but was quickly overtaken by temperature stratification as the epilimnion warmed. The deterministic, unsteady dynamic one-dimensional (1D) lake temperature and dissolved oxygen (DO) model MINLAKE was modified by including vertical salinity gradients, and it was used to simulate summer stratification and mixing dynamics in Tanners Lake. The daily adjustment of the hypolimnetic eddy diffusion as a function of lake number was an important component in the developed model. This addition allowed mixing in the hypolimnion to be stronger in the fall and spring when the lake stratification was weaker than in the summer after thermal stratification formed. Model results of dissolved oxygen in the water column demonstrated that the saline benthic layer can prevent dissolved oxygen from reaching lake sediments. The adverse consequences of dissolved oxygen depletion on phosphorus recycling from the sediments, benthic microbial communities, and fish habitat are well known. Overall, the results show how salinity from road salt applications can influence water quality and natural mixing in urban lakes.

Deep storm-water storage tunnels may undergo pressurization during intense rain events. In the process, air pockets may become entrapped and pressurized, causing significant flow changes. Currently, the role of nearby surge relief structures is uncertain with respect to air-water interactions. This paper presents results from experimental and numerical investigations on air pocket entrapment and compression following reflection of inflow fronts on system boundaries. Steady flows were established in the pipe apparatus, with the upstream portion flowing in the pressurized regime while the downstream flowed in free surface conditions. Sudden flow obstruction caused by valve maneuvering at the downstream end generated unsteady conditions that were monitored by transducers and a MicroADV probe. Partial valve maneuver runs were performed, aiming to represent cases in which surge relief is provided during air pocket compression/expansion cycles. Whereas experiments performed without surge relief (complete valve obstruction) yielded an oscillatory pressure pattern upon air pocket entrapment, when relief was available, a single pressure pulse pattern was observed instead, and no subatmospheric pressures were recorded. A simple two-phase model that includes essential features of the problem was developed, and its pressure predictions compared well with the experimental data.

Emptying of an initially water-filled horizontal PVC pipeline driven by different upstream compressed air pressures and with different outflow restriction conditions, with motion of an air-water front through the pressurized pipeline, is investigated experimentally. Simple numerical modeling is used to interpret the results, especially the observed additional shortening of the moving full water column due to formation of a stratified water-air “tail.” Measured discharges, water-level changes, and pressure variations along the pipeline during emptying are compared using control volume (CV) model results. The CV model solutions for a nonstratified case are shown to be delayed as compared with the actual measured changes of flow rate, pressure, and water level. But by considering water-column mass loss due to the water-air tail and residual motion, the calibrated CV model yields solutions that are qualitatively in good agreement with the experimental results. A key interpretation is that the long air-cavity celerity is close to its critical value at the instant of minimum flow acceleration. The influences of driving pressure, inertia, and friction predominate, with the observed water hammer caused by the initiating downstream valve opening insignificantly influencing the water-air front propagation.

Among water hammer damping devices, air chambers are often used in pumping plants to reduce pressure surges to acceptable values. The chamber is more effective if a throttling device is used, resulting in a reduction of the required volume. Design charts for a simple and fast sizing of air volume and orifice diameter are available in the literature using the rigid column theory (incompressible flow) and the De Sparre rule. Nevertheless, in many cases the pressure pattern is far from being constant during the first quarter of the transient period and lower pressures can be attained; furthermore, the rigid column model is not able to reproduce elastic phenomena arising from throttling, and so water hammer equations should be used instead. Although orifice induced pressure waves are evident only in the first part of the transient, differences between air chamber pressure and pipe pressure can be quite significant. Negative pressure surge should be limited because column separation and cavitation could occur as a consequence of low pressures. Because the maximum down surge inferred from the design charts does not ensure safe design, a simplified approach was proposed in this paper to design throttled air chambers under the elastic hypothesis. The analysis showed small deviations between the minimum pressure and the design pressure, unlike the inelastic approach, which exhibits even very large differences with water hammer equations.

The local deterioration of the mechanical properties of steel-lined pressure shafts and tunnels induces a decrease of the celerity and an increase of the energy attenuation of water hammer waves. This deterioration may arise from the weakening of the backfill concrete and the surrounding rock mass, which provide the radial support of the steel liner. A new online monitoring system implemented and tested at the Grimsel II pumped-storage shaft in Switzerland is presented. The difficulties encountered during the in situ measurements are outlined. The new monitoring approach is based on acquiring and analyzing the dynamic pressure data generated by the normal operation of pumps and turbines. The wave celerity and the exponential attenuation coefficient of water hammer waves are estimated based on pressure records. Monitoring charts for these indicators are established using the statistical quality control method. The results show a stable monitoring scheme that needs longer acquisition data series to consolidate its control limits.

An experimental study of new proposed stepped-spillway configurations using sills or baffles on the step treads with sharp or rounded brinks is introduced to investigate the location of and the flow depth at inception point of air entrainment and the training wall height. The silled-shifted rounded spillway is recommended to be used in cavitation damages mitigation because it gives the smallest inception-point length among all spillways for large Froude numbers range. The baffled-edged spillway gives the largest inception-point depth among the spillways for large Froude numbers range and consequently, is not favourable. The baffled-shifted sharp spillway is the most efficient regarding the cost of construction because it gives the smallest training wall height among spillways for large Froude numbers range. The location and depth of inception point of air entrainment and the training wall height were expressed as functions of a so-called roughness Froude number in empirical equations to develop design guidance for the proposed spillway configurations.