Journal of Hydraulic Engineering

Comprehensive laboratory experiments on multiport diffusers for dense effluents such as brine into stationary receiving waters are reported. Tracer concentration fields were mapped in three dimensions by laser-induced fluorescence (3DLIF). The effect of port spacing is described by $s/d\mathsf{F}$ where $s$ is the port spacing, $d$ the nozzle diameter, and $\mathsf{F}$ the jet densimetric Froude number. For $\mathrm{s}/d\mathsf{F}>\sim 2$, the jets did not merge, and the results followed expected asymptotic solutions for single jets. For $s/d\mathsf{F}<2$ the jets merged, but did not follow expected asymptotic line source solutions. As the port spacing was reduced, the rise height and other geometrical variables decreased. The dilutions also decreased, but much more rapidly than predicted. These were caused by Coanda effects and merging. The Coanda effect caused an under pressure on the interior jet surfaces which caused them to curve more sharply inwards. This shortened their trajectories, reducing the external surface area available for entrainment. Jet merging restricted entrainment of clear water to the inner surfaces and exacerbated the Coanda effect. The implications for numerical modeling are discussed. Integral entrainment models that assume unrestricted entrainment water, neglect reentrainment, and neglect dynamic interaction (Coanda effect) between jets may significantly overestimate dilution. Insight into port spacing effects can be obtained from studies on positively buoyant jets. To prevent reduction in dilution attributable to restricted entrainment, it is recommended to maintain adequate port spacing so that $\mathrm{s}/d\mathsf{F}>\sim 2$.

A light attenuation sensor system (LASS) for measurements in waters is described in this paper. The LASS records irradiance at multiple levels in the water column to provide a measure of the diffuse light attenuation coefficient which is strongly affected by suspended sediment. Dimensional analysis and geometric optical theory are used to relate the irradiance attenuation to sediment properties through a dimensionless product. The latter is termed as a light attenuation number for suspended sediment in waters. The LASS and dimensional analysis results are validated in the laboratory using fluvial sediments collected from a third-order stream and monodisperse quartz sediment. The attenuation coefficient estimated with LASS data varied nonlinearly with total suspended sediment concentration due to particle shadowing and multiple scattering at large optical depths. The light attenuation coefficient for each sediment type is well described as a function of total suspended sediment concentration by empirical power law relationships, which provides confidence in the functioning of LASS. Light attenuation curves for different sediment types collapsed onto a single curve when replotted according to dimensionless numbers arising from the dimensional analysis, which confers confidence in the analysis for future research and application. A successful field demonstration of LASS in a small stream highlights its potential application in hydraulic and ecological research as well as future avenues of research to improve the sensor.

Numerical simulations are carried out to investigate the spreading of two-dimensional plane turbulent aerated and nonaerated jets in a tank filled with finite water depth. A multiphase model is applied to simulate the problem under investigation. The governing equations, their numerical scheme, and the boundary conditions are presented. Aerated and nonaerated turbulent jets are simulated for a range of the jet velocity and width at exit, the initial air content at exit, and the water depth in tank. The simulated results show that a self-similar Gaussian velocity distribution exists from the distance downstream being larger than five jet slot width for both the aerated and nonaerated jets. Good agreement between the simulated velocity profiles and available laboratory experiments is obtained. The simulated slope of the jet velocity decay along the jet center line is in good agreement with the experimental measurements. The effect of air content on pressure distribution and the maximum impinging hydrodynamic pressure at the tank bottom is discussed.

Three-dimensional Reynolds averaged Navier-Stokes modeling, validated against experimental data, is used to parameterize the flow features and time scales in idealized rectangular cavities for a wide range of width-to-length ratios, $0.4\le W/L\le 1.1$, and Reynolds number based on the depth, $\mathrm{5,000}\le {\mathsf{R}}_D\le \mathrm{20,300}$, representative of isolated dead zones in small natural streams. The flow features for this parameter range are similar to open cavity flows and consist of a mixing layer spanning the entire length of the dead zone together with a single main recirculation region. The Langmuir time scale (ratio of dead-zone volume to discharge) based on the assumption of a well-mixed dead zone is found to be a function of the mean rotation time scale (inverse of average rotation rate) within the dead zone, the momentum thickness of the upstream boundary layer, and the dead-zone width. The entrainment coefficient, used to relate the exchange velocity to the average free- stream velocity, is shown to be directly related to the upstream boundary layer momentum thickness nondimensionalized by the width of the dead zone. Using passive tracer to quantify the mean residence time showed that the dead zone can be characterized by two perfectly mixed regions including a core or secondary region around the center of the eddy and a surrounding primary region that interacts directly with the free-stream through the mixing layer. A two-region model is developed to obtain time scales associated with the primary and secondary regions within the dead zone using an optimization procedure based on the computational data. The time scale associated with the primary region is representative of the Langmuir time scale and is found to be a strong function of the aspect ratio $W/L$ and the Reynolds number. The secondary region time scale represents the long-time asymptotic behavior of the tracer concentration and is found to be a strong function of the dead- zone geometric parameters only.

Transients in pressurized polymeric pipes are analyzed by means of a two-dimensional (2D) Kelvin-Voigt viscoelastic model, calibrated by means of a microgenetic algorithm on the basis of pressure traces. The reliability of the proposed model is then tested by comparing numerical and experimental profiles of the axial component of the local velocity, the latter measured by means of an ultrasonic Doppler velocimeter. Differences between transients in viscoelastic and elastic pipes are pointed out by considering a 2D model in which an elastic behavior is assumed for pipe material. The comparison between the two 2D models allows attribution of the faster decay of pressure oscillations and velocity profiles to viscoelasticity because of the time-shift between pressure oscillation and retarded circumferential strain. The 2D analysis shows that the viscoelastic model generally presents flatter velocity profiles with respect to the elastic model.

Laboratory experiments were performed to study the effect of an axial-flow hydrokinetic turbine model on an erodible channel under both clear water and live-bed conditions. Clear water experiments were performed at two scales with a local bed shear stress just below the critical state. Live-bed experiments, performed at small scale, examined the interactions between relatively large-scale bedforms and the flow induced by an axial flow turbine. Spatiotemporal topographic measurements were obtained by sonar and by a state-of-the-art high-resolution scanning system integrated into an automated data acquisition carriage designed and fabricated at the St. Anthony Falls Laboratory at the Univ. of Minnesota. Results indicate that the presence of the turbine rotor increases the local shear stress resulting in accelerated and expanded scour development when compared with typical bridge pier scour mechanisms. The inferred key difference is the alteration of the flow patterns in the rotor wake leading to an accelerated flow region below the bottom tip. The footprint of the rotor is observed in the extension and scaling of the bed surface area impacted by the turbine and consistent with the near-wake region. Temporally averaged bed topography data from live-bed experiments indicate amplified scour depths in the turbine near-wake region as compared with the clear water results despite spatial patterns remaining qualitatively similar.

Entrapped air pockets are linked to various operational issues in closed pipe flows. Tracking of entrapped air pockets is a relevant task in numerical simulation of hydraulic systems subjected to air pocket entrapment. However, few studies have focused on the kinematics of discrete air pockets and the motion of these at various combinations of pipe flow velocities, air pocket volumes, and pipe slopes. Such information is relevant for the improvement of current numerical models for the simulation of transition between pressurized and free surface flows, and related air pocket entrapment. This work explores the link between pipe flow velocity, pipe slope, and entrapped air pocket celerities. Systematic experimental studies were performed and provided insights on the nature of entrapped air pocket motion in cases when buoyancy forces are either summed or opposed to flow drag. Results indicated that when flow drag and buoyancy forces are summed, the observed air pocket celerity can be approximated by the celerity in quiescent conditions plus the pipe velocity. This approximation fails in cases when drag force opposed buoyancy. This work also proposes a novel approach for the simulation of the air pocket motion as non-Boussinesq gravity currents. Modeling results compared favorably with experimental measurements in horizontal slope and pipe flow velocity cases. Such results are an indication of the feasibility of integrating this approach into more complex hydraulic models that are able to simulate flow regime transitions.

A total of 595 real-time kinematic global positioning system (RTK-GPS) water surface elevations (WSE) were measured using georectified and nongeorectified survey systems along a 21-km-long river reach to evaluate vertical coordinate accuracy of a nongeorectified RTK-GPS acoustic Doppler current profiler (ADCP) system. A georectified survey of a series of quality control benchmarks produced a vertical root-mean-square error (RMSE) of 0.033 m, which was considered globally accurate for river survey purposes. The WSE measured simultaneously at the same locations comparing the nongeorectified to the georectified system resulted in a $\mathrm{RMSE}=0.741$ m. Evaluation of intersetup precision of the nongeorectified WSE confirmed relative accuracy to each initialized arbitrary vertical datum but were found not to be globally accurate. Only 7% of the nongeorectified base station setups resulted in RMSE less than the combined manufacturer’s tolerances when compared to the georectified system at the same spatial locations. When vertical datum corrections were applied to the nongeorectified system from the globally accurate RTK-DGPS system, 80% of the base station setups demonstrated RMSE within the combined manufacturer’s tolerances. Datum corrections varied greatly, ranging from $-2.43$ to $+1.80\mathrm{m}$. This study demonstrates that when using nongeorectified RTK-GPS ADCP systems for purposes other than discharge measurement, such as in the extraction of bathymetry data for use in hydraulic assessments and models, great care must be exercised when verifying WSEs

Even though broad-crested side weirs have been subjected to numerous studies, guidance for discharge coefficient determination for broad-crested side weirs still has not been clearly given. Experimental research was carried out to describe the flow conditions in front of a trapezoidal broad-crested side weir at low Froude numbers in the approach channel for various combinations of inflow and overflow discharges. Three side weir variants with different oblique angles were studied. Flow characteristics at the side weir were analyzed and assessed based on the results obtained from extensive measurements performed on a hydraulic model. The empirical relation between the coefficient of obliqueness, the angle of side weir obliqueness, and Froude numbers upstream and downstream of the weir was derived.

This paper presents the results of clear-water scour experiments with a vertical-wall abutment. The upstream face of the latter was provided with roughening elements as devices to intercept the downflow responsible for the formation of the principal vortex. The time evolution of the scour depth at several locations around the abutment was measured for different hydrodynamic and geometric configurations. The countermeasure performance was quantified using an appropriate dimensionless framework, and a best arrangement of the roughening elements was proposed within the range of the experimental conditions investigated. The full database is provided as supplemental material.