Scale Effects in Flume Experiments on Flow around a Spur Dike in Flatbed Channel

This paper presents the findings from a series of flume experiments conducted to determine scale effects in small-scale models of flow around a single spur dike (wing-dam, groyne, or abutment) placed in a channel whose bed is fixed and flat. The flow features of primary interest are flow-thalweg alignment (line of maximum streamwise velocity) around a dike, and area extent of the flow-separation region (wake) immediately downstream of the dike. Those features are of practical concern in the deployment of dikes for various channel control purposes. The scale effects of concern herein are those attributable to use small length scales together with a bed-shear stress parameter (e.g., Shields parameter) as the primary criterion for dynamic similitude. Small-scale models, especially micromodels, often are used for investigating channel-control issues. Also, the shear-stress criterion is commonly used for models of flow in loose-bed channels, whereas Froude number commonly is the primary similitude criterion for models of fixed-bed open-channel flows. The experiments show that use of a shear-stress parameter as the primary criterion for dynamic similitude influences the flow thalweg and flow separation region at a dike. It does so by distorting pressure gradients around the model dike and by affecting turbulence generated by the dike. It also is shown that, for a range of small models, thalweg alignment and extent of separation region do not scale with model length scales. These findings are important for interpreting results from small hydraulic models, especially micromodels, of flow in loose-bed channels.     

Measured and Simulated Flow near a Submerged Spur Dike

To improve knowledge of the flow and scour processes associated with spur dikes, three-dimensional flow velocities were measured at 2,592 points using an acoustic Doppler velocimeter over a fixed flat bed with a trapezoidal shaped submerged spur dike in a laboratory flume. General velocity distribution and detailed near field flow structures were revealed by the measurements and numerical simulations performed using a free surface turbulent flow model with a k–ε closure scheme. The three-dimensional flow separation characterized in this study was found to yield forces on the bed that were significantly different from nonsubmerged vertical obstructions that have been measured in other studies. Values of bed shear stress derived from both measured and simulated values were similar but indicated that local scour would be initiated in one rather than in the two locations of initial local scour measured in previous experiments with a similar flow.     

Evaluation of Mean Velocity and Turbulence Measurements with ADCPs

To test the ability of acoustic Doppler current profilers (ADCPs) to measure turbulence, profiles measured with two pulse-to-pulse coherent ADCPs in a laboratory flume were compared to profiles measured with an acoustic Doppler velocimeter, and time series measured in the acoustic beam of the ADCPs were examined. A four-beam ADCP was used at a downstream station, while a three-beam ADCP was used at a downstream station and an upstream station. At the downstream station, where the turbulence intensity was low, both ADCPs reproduced the mean velocity profile well away from the flume boundaries; errors near the boundaries were due to transducer ringing, flow disturbance, and sidelobe interference. At the upstream station, where the turbulence intensity was higher, errors in the mean velocity were large. The four-beam ADCP measured the Reynolds stress profile accurately away from the bottom boundary, and these measurements can be used to estimate shear velocity. Estimates of Reynolds stress with a three-beam ADCP and turbulent kinetic energy with both ADCPs cannot be computed without further assumptions, and they are affected by flow inhomogeneity. Neither ADCP measured integral time scales to within 60%.     

Laboratory Measurements of Flow and Turbulence in Discontinuous Distributions of Ligulate Seagrass

Turbulent flow characteristics were investigated in laboratory flume studies of a ligulate plant canopy interrupted by a gap representing discontinuities observed in seagrass prairies. The reliability of velocity measurements obtained using an acoustic Doppler velocimeter within the canopy was shown using specifically designed experiments. In relatively fast flow (mean velocity 5.5?cm?s?1), the mean flow profile was logarithmic above the canopy, had an inflection point near its top, and uniformly low values within it. Within the gap, a recirculation cell formed. Reynolds stress maxima were approximately coincident with the mean flow inflection point. Quadrant analysis revealed an ejection-dominated upper layer, a sweep-dominated region around the top of the canopy and within the gap, and no dominant quadrant within the canopy. In slower flow (mean velocity 1.7?cm?s?1) the plants were quasiemergent and the flow fields more uniform. Sweeps similarly dominated the region near the top of the canopy and within the gap. In both flows, autocorrelation of longitudinal velocity fluctuations showed a Lagrangian time scale maximum at the downstream end of the gap.     

Sediment Exchange between a River and Its Groyne Fields: Mobile-Bed Experiment

Experiments have been carried out in a mobile-bed laboratory flume in order to study the sediment exchange process between the main channel and the groyne fields. The flume represented half the width of a schematized river reach with a series of groynes. The experiment was designed to represent typical dimensions of the Dutch River Waal at a geometrical scale of 1:100. The conditions were set to guarantee bed load as well as suspended load sediment transport. Conditions with submerged and emerged groynes were investigated. In addition to traditional measurements, viz., bed-level changes, suspended sediment concentrations, and flow velocities, bed-form propagation was measured in two dimensions using a the particle image velocimetry technique. The results were analyzed with focus on sediment exchange mechanisms and sediment transport patterns. The results demonstrate that under all flow conditions there is a net import of sediment into the groyne fields. The prevailing transport mechanisms vary with the flow stage: if the groynes are emerged it is mainly advection by the primary circulation cell, whereas if the groynes are submerged it is rather residual advection by large-scale coherent flow structures (in a straight reach). Additional entrainment of sediment by enhanced turbulence complicates the erosion/deposition patterns.     

Particle Image Velocimetry Measurements of the Mean Flow Characteristics in a Bubble Plume

A direct measurement method for the velocity field in multiphase flows using the particle image velocimetry (PIV) and particle tracking velocimetry (PTV) methods is developed to study the flow characteristics of an unbounded bubble plume in quiescent, unstratified ambient conditions. A single camera is used to obtain images containing both bubbles and fluid tracer particles. Using gray-scale thresholding, phase-separated images of the bubbles are produced, and bubble velocities are obtained from these images using the standard PTV method. Regular PIV is applied to the mixed fluid images, and bubble vectors are removed using a velocity threshold and vector median filter that is calibrated to the PTV result. From the separate velocity fields, the time-averaged flow characteristics of a bubble plume are studied. Gaussian velocity profiles match the entrained fluid velocity, and top-hat velocity profiles match the bubble velocity. Time-averaged values are also presented of velocity, plume width, entrained fluid volume flux, and void fraction as a function of height. From these data, the entrainment coefficient for the entrained ambient fluid is calculated and lies between 0.08 near the plume source and 0.05 in the upper reaches. The results for the entrainment coefficient, together with those from the literature, are correlated to a nondimensional velocity, given by the ratio of the bubble slip velocity us to a characteristic velocity in the plume (B/z)1/3, where B = kinematic buoyancy flux and z is the height above the source.     

Laboratory Experiments on Frazil-Size Characteristics in a Counterrotating Flume

In an attempt to increase the fundamental understanding of the formation and evolution of frazil ice, laboratory experiments have been conducted in a counterrotating flume at the Hydraulics Research and Testing Facility, University of Manitoba. A high-precision thermometer and recently improved digital image processing system were used to acquire data from these experiments, which will hopefully aid in the development of numerical models. The image processing system is described, and results from a series of experiments are presented. The effects of air temperature, water velocity, and bottom roughness on the formation of frazil ice are discussed. A lognormal distribution appears to provide a reasonable fit to the observed frazil particle size distributions.     

Effects of Groyne Layout on the Flow in Groyne Fields: Laboratory Experiments

This research is aimed at finding efficient alternative designs, in the physical, economical, and ecological sense, for the standard groynes as they are found in the large rivers of Europe. In order to test the effects of various groyne shapes on the flow in a groyne field, experiments were performed in a physical model of a schematized river reach, geometrically scaled 1:40. Four different types of schematized groynes were tested, all arranged in an array of five identical groyne fields, i.e., standard reference groynes, groynes with a head having a gentle slope and extending into the main channel, permeable groynes consisting of pile rows, and hybrid groynes consisting of a lowered impermeable groyne with a pile row on top. Flow velocities were measured using particle tracking velocimetry. The design of the experiment was such that the cross-sectional area blocked by the groyne was the same in all cases. Depending on the groyne head shape and the extent of submergence variations in the intensity of vortex shedding and recirculation in the groyne field were observed. The experimental data are used to understand the physical processes like vortex formation and detachment near the groyne head. It is demonstrated that the turbulence properties near and downstream of the groyne can be manipulated by changing the permeability and slope of the groyne head. It is also observed that for submerged conditions the flow becomes complex and locally dominated by three-dimensional effects, which will make it difficult to predict by applying depth average numerical models or by three-dimensional models with a coarse resolution in the vertical direction.     

Backwater Prediction due to the Blockage Caused by a Single, Submerged Spur Dike in an Open Channel

A method is proposed for predicting the backwater effect due to a single, submerged spur dike located within an open channel flow. A theoretical analysis based on the momentum principle relates the backwater effect to the drag force exerted by the spur dike on the flow. Experimental data obtained in laboratory flumes having subcritical flow conditions throughout the flow field have been used in developing predictive relationships for the spur dike drag coefficient, which is found to be strongly correlated to the blockage created by the spur dike within the flow cross section. The predictive relationships provide a means of obtaining a first-level estimate of the backwater effect due to a single, submerged spur dike in an open channel flow.     

Flow Field at a Vertical-Wall Abutment

Experiments were conducted to measure the three-dimensional turbulent flow field, using the acoustic Doppler velocimeter, at a short vertical-wall abutment (ratio of abutment length to approach flow depth less than unity) before and after the development of a scour hole under a clear water scour condition. In the upstream, the presentation of flow field through vectors at vertical sections shows a primary vortex associated with the downflow. In the downstream, the upward flow is comprised of with irregularities owing to the vortex shedding. The flow separation near the bed and within the scour hole is evident from the turbulent kinetic energy distribution. Using Reynolds stresses, the bed shear stresses are calculated.     

Mean Flow Field of a Nonbuoyant Rectangular Surface Jet

The flow field for a low Froude number nonbuoyant rectangular surface jet is studied using laser induced fluorescence and laser Doppler velocimetry. It is shown that a surface current develops in turbulent, but not in laminar jets. The existence of a self-similar regime is established. The lateral growth rate of the jet below the surface is about 4.8 times its vertical growth rate, while that of the surface current is 8.8 times. The vertical growth rate is about half that of a plane surface jet and less than one third that of a free axisymmetric jet. While the horizontal profiles of the streamwise velocity are similar to those observed in free, submerged, and wall jets, the vertical distributions are distinctly flatter.     

Near-Bed Turbulence Characteristics at the Entrainment Threshold of Sediment Beds

This experimental study is devoted to quantification of the near-bed turbulence characteristics at an entrainment threshold of noncohesive sediments. Near the bed, the departure in the distributions of the observed time-averaged streamwise velocity from the logarithmic law is more for immobile beds than for entrainment-threshold beds. In the Reynolds shear stress distributions, a damping that occurs near the bed for sediment entrainment is higher than that for immobile beds. Quadrant analysis reveals that in the near-bed flow zone, ejections and sweeps on immobile beds cancel each other, giving rise to the outward interactions, whereas sweeps are the dominant mechanism toward sediment entrainment. The bursting duration for entrainment-threshold beds is smaller than that for immobile beds. On the other hand, the bursting frequency for entrainment-threshold beds is larger than that for immobile beds. The third-order correlations indicate that during sediment entrainment, a streamwise acceleration associated with a downward flux and advection of streamwise Reynolds normal stress is prevalent. The streamwise and the downward vertical fluxes of turbulent kinetic energy (TKE) increase with sediment entrainment. The TKE budget proves that for sediment entrainment, the pressure energy diffusion changes drastically to a negative magnitude, indicating a gain in turbulence production.     

Subcritical Contraction for Improved Open-Channel Flow Measurement Accuracy with an Upward-Looking ADVM

Acoustic Doppler velocity meters (ADVMs) provide an alternative to more traditional flow measurement devices and procedures such as flumes, weirs, and stage rating for irrigation and drainage canals. However, the requirements for correct calibration are extensive and complex. A three-dimensional computational fluid dynamics (CFD) model was used to design a subcritical rapidly varied flow contraction that provides a consistent linear relationship between the upward-looking ADVM sample velocity and the cross-sectional average velocity in order to improve ADVM accuracy without the need for in situ calibration. CFD simulations validated the subcritical contraction in a rectangular and trapezoidal cross section by showing errors within +1.8 and ?2.2%. Physical testing of the subcritical contraction coupled with an upward-looking ADVM in a large rectangular flume provided laboratory validation with measurement errors within ±4% without calibration.     

Laboratory Test and Particle Flow Simulation of Silos Problem with Nonhomogeneous Materials

Underground mine caving may induce collapse of the ground during ore caving, particularly when there is a loose material on top of the mine. In the present paper, a laboratory test is performed to model this mining problem which is followed by a particle flow simulation to simulate the interaction and movement between the two materials. For this nonhomogeneous problem, an interesting columniform channel for the loose material is formed during the drawing in both the experiment tests and the numerical modeling. The formation of this channel greatly affects the efficiency of ore drawing and the stability of the mine caving. The numerical modeling can be used for the optimization of ore drawing, ore drawing management and the prediction of ore loss and dilution indexes. The importance of the moisture content on ore drawing process and the efficiency of ore drawing are also investigated by both experimental and numerical tests, and useful results are found from the present study.     

Laboratory Measurements of Flow through Culverts

Laboratory apparatus to simulate flow through culverts has been used to collect discharge and water level measurements. Two different shapes of culvert barrels, namely square and circular, were tested. The measurements presented in this note are intended to provide useful information regarding the variety of flow regimes (including overtopping) through culverts, and the transitions from one flow regime to another. It is known that modeling the culvert flow regimes and capturing the transitions among these regimes numerically is a challenging task. To that effect, the laboratory measurements presented herein can provide a testing and validation data set for numerical modeling of hydraulic structures such as culverts.     

Flow Structure at Different Stages in a Meander-Bend with Bendway Weirs

Streambank erosion is an important management issue, particularly for meandering rivers. Recently, bendway weirs have become popular control measures for bank erosion along small meandering streams in the agricultural Midwest. Although these structures have successfully mitigated bank erosion in some cases, there is evidence that the weirs do not always perform as anticipated. Scientific understanding of how bendway weirs influence flow dynamics, streambank erosion, and aquatic habitat is limited. Current design criteria are based primarily on expert judgment rather than a formalized technical design procedure. At field-scale studies, the present paper represents a first step toward an integrated geomorphological and engineering evaluation of the performance of bendway weirs in rivers. To accomplish this initial phase, three-dimensional (3D) velocity data were collected on Sugar Creek at Brookside Farm, Ill., and 3D numerical simulations for low-flow conditions were performed to validate the computational fluid dynamic model. Overall results show good agreement between measured and simulated data for streamwise velocities and turbulence kinetic energy. The model is less accurate at predicting the velocity and turbulence kinetic energy in the shear layer immediately downstream from the weir tips. Based on the validation for low-flow condition, 3D simulations were carried out for medium and high flows where the bendway weirs are completely submerged. These simulations indicate that 3D patterns of flow, especially flow near the outer bank, change dramatically with changes in flow stage. Flow patterns at high-flow condition indicate that bank retreat over the tops of weirs is associated with locally high-shear stresses, thus producing a “shelf” along the base of the outer bank as observed in the field.     

Assessment of Rheological Characteristics of a Natural Bingham-Plastic Mixture in Turbulent Pipe Flow

Estimating rheological parameters of a non-Newtonian fluid is performed with rheometers, but experiments are limited to fine sediments, in the absence of appreciable sedimentation. An approach based on pipe flow measurements may be more flexible and convenient. The aim of this paper is to experimentally verify the latter approach in order to assess the rheology of natural mixtures of heavy materials with high tendency toward sedimentation.     

Coupled Effects of Small-Scale Turbulence and Phytoplankton Biomass in a Small Stratified Lake

Recent laboratory studies demonstrated that small-scale fluid motion mediates phytoplankton physiological responses. We have investigated to what extent the laboratory studies are consistent with field measurements in a small stratified lake. We propose the rate of energy dissipation and corresponding Kolmogorov velocity are important scaling variables that describe the enhanced algal growth and the uptake of nutrients in a moving fluid under laboratory and field conditions. The ratio of nutrient flux to an alga in a moving fluid versus the nutrient flux in a stagnant fluid (Sherwood number) is quantified by the ratio of advective nutrient transport to molecular diffusion of a nutrient (Péclet number, PeK). The advective transport of nutrients is described by the layer-averaged Kolmogorov velocity (K). An enhanced algal growth due to fluid motion is proposed over the Péclet number range 6.7>PeK>1.3, with the maximal growth at PeK = 2.9. Field measurements recorded by a microstructure profiler demonstrated encouraging agreement between laboratory and field findings. The current mechanistic models of phytoplankton population dynamics could consider the proposed Péclet number with redefined characteristic velocity scale (K) in the formulation of subgrid scale closure fluxes on nutrient uptake and growth rate. Furthermore, the laboratory and field results presented in this study are intended to motivate researchers to question the validity of standard laboratory biotoxicity protocols and to modify existing procedures in the examination of effluent toxicity in the environment by including the fluid motion.     

Electrochemical Reduction of 2,4-Dinitrotoluene in a Continuous Flow Laboratory Scale Reactor

An electrochemical laboratory scale reactor was used to treat 2,4-dinitrotoluene (DNT). Experiments were conducted by using a graphite carbon cylinder impregnated with glassy carbon (zero porosity) as the cathode and a platinum wire as the anode. All experiments were conducted under anoxic conditions. Initially, experiments simulating batch conditions were conducted to obtain the optimum operating conditions for the reactor. During this batch-mode study, the effect of various parameters such as applied current, electrolyte concentration, and type of electrolyte on the reduction of DNT were evaluated. Results showed that the rates of DNT reduction increased with an increase in current or concentration of electrolyte. Based on the results obtained from the batch simulation experiments, continuous flow experiments were conducted at three different currents and one electrolyte concentration. The ionic strength of the feed solution was maintained at 0.027 M. A current of 200 mA (current density 0.088 mA/cm2) provided a stable reduction of DNT at the 80% level for a period of 14 days after which reactor cleaning was necessary for removal of suspended solids that were formed within the reactor. End products determined for the experiments showed 80–100% molar balance closure.     

Automated Sediment Erosion Testing System Using Digital Imaging

Measurement of vertical profiles of the critical shear stress, τc, and the erosion rate, E, from the same undisturbed sediment core is crucial for modeling the resuspension of fine-grained natural sediments. The automated sediment erosion testing system (ASETS) was developed to determine profiles of τc and E with centimeter spatial (vertical) resolution in an undisturbed (Shelby tube) sediment core, whose surface was eroded by steady turbulent flow through a flume. The unique feature of ASETS is that it is a real-time imaging method that accurately determines the position of the core surface during erosion for both calculating the vertical profile of E and controlling a motor-driver system that automatically pushes up the core to maintain its surface flush with the flume bottom. Undisturbed, field cores were tested over a range of flow (average bed shear stress, τb) conditions. The amount of eroded sediment from both optical backscattering measurements and the imaging method were in good agreement, which validated ASETS. Measured vertical profiles of τc and E were similar to those reported in literature. E correlated well with (τb?τc)2, which agrees with previous results in literature.