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.     

Characteristics of Horseshoe Vortex in Developing Scour Holes at Piers

The outcome of an experimental study on the turbulent horseshoe vortex flow within the developing (intermediate stages and equilibrium) scour holes at cylindrical piers measured by an acoustic Doppler velocimeter (ADV) are presented. Since the primary objective was to analyze the evolution of the turbulent flow characteristics of a horseshoe vortex within a developing scour hole, the flow zone downstream of the pier was beyond the scope of the investigation. Experiments were conducted for the approaching flow having undisturbed flow depth ( = 0.25?m) greater than twice the pier diameter and the depth-averaged approaching flow velocity ( = 0.357?m/s) about 95% of the critical velocity of the uniform bed sand that had a median diameter of 0.81?mm. The flow measurements by the ADV were taken within the intermediate (having depths of 0.25, 0.5, and 0.75 times the equilibrium scour depth) and equilibrium scour holes (frozen by spraying glue) at a circular pier of diameter 0.12?m. In order to have a comparative study, the ADV measurements within an equilibrium scour hole at a square pier (side facing the approaching flow) of sides equaling the diameter of the circular pier were also taken. The contours of the time-averaged velocities, turbulence intensities, and Reynolds stresses at different azimuthal planes (0, 45, and 90°) are presented. Vector plots of the flow field at azimuthal planes reveal the evolution of the characteristics of the horseshoe vortex flow associated with a downflow from intermediate stages to equilibrium condition of scour holes. The bed-shear stresses are determined from the Reynolds stress distributions. The flow characteristics of the horseshoe vortex are discussed from the point of view of the similarity with the velocity and turbulence characteristic scales. The imperative observation is that the flow and turbulence intensities in the horseshoe vortex flow in a developing scour hole are reasonably similar.     

Dynamic Soil–Structure Interaction of Bridge Substructure Subject to Vessel Impact

The paper describes the in situ investigation, site stratigraphy, field monitoring, data reduction, and subsequent time-domain analysis of soil–structure interaction from a full scale vessel impact loading of a bridge pier at the St. George Island Causeway. The in situ investigation included standard penetration testing, electric cone, dilatometer, and pressuremeter testing to identify soil stratigraphy, engineering properties (strength and moduli), and axial and lateral static pile resistance (T–z, and P–y). Field instrumentation included soil total stress and pore pressure gauges in front of and behind the pile cap, a fully instrumented pile (strain gauges along length), dynamic load cells to monitor barge impact loads, and accelerometers to monitor pier accelerations, velocities, and displacements. Analyses of the field data reveal significant dynamic forces within the soil–structure system as a result of the duration and magnitude of the loading. Inertia from the piers, cap, and piles provide significant resistance in the early portion of the impact. However, postpeak inertia (i.e., pier deceleration) resulted in maximum deformations of the pier. Soil damping provided most of the resisting force at the peak barge loading, whereas static soil resistance dominated at the peak lateral displacement. Time-domain finite element analysis of an impact event employing viscous soil dashpots, nonlinear P–y and T–z springs with nonlinear beam, and shell elements for the pier, cap, and piles resulted in reasonable load displacement predictions.     

Turbulence Characteristics of Open-Channel Flow with Bed Suction

The influence of bed suction on the characteristics of turbulent open channel flow is studied in a laboratory flume using a two-component laser Doppler velocimeter. The experimental results show how bed suction significantly affects the mean flow properties, turbulence levels, and Reynolds stress distributions. The data reveal the presence of a more negative vertical (downward) velocity. The results also show how the horizontal and vertical turbulence intensities and Reynolds shear stresses respond to suction. All these properties are found to reduce with increasing relative suctions: decreasing more rapidly around the bed region than that near the free surface. In the downstream direction, the flow structure in the suction zone undergoes a process of rapid readjustment within a transitional region. Beyond this region, the turbulence flow structures asymptotes toward an “equilibrium” region.     

Turbulence Characteristics in Flows Subjected to Boundary Injection and Suction

The influence of seepage (lateral flow) on the turbulence characteristics in free-surface flows over an immobile rough boundary is investigated. Steady flows having zero-pressure gradient over an immobile rough boundary created by uniform gravels of 4.1 mm in size were simulated experimentally with injection (upward seepage) and suction (downward seepage) applied through the boundary. A Vectrino (acoustic Doppler velocimeter) was used to measure the instantaneous velocities, which are analyzed to explore second- and third-order correlations, turbulent kinetic energy, turbulent energy budget, and conditional Reynolds shear stresses. It is observed that the second-order correlations decrease in presence of injection and increase in suction. The turbulent diffusivity and mixing length increase in presence of injection and decrease in suction. The third-order correlations suggest that the ejections are prevalent over the entire flow depth. The near-boundary flow is significantly influenced by the existence of upward seepage, which is manifested by a reduction in streamwise flux and the vertical advection of streamwise Reynolds normal stress. In addition, the upward flux and the streamwise advection of vertical Reynolds normal stress are also affected. The streamwise flux of turbulent kinetic is found to migrate upstream, while the vertical flux of turbulent kinetic energy is transported upward. The fluxes increase in presence of injection and decrease in suction. Energy budget evidences a lag between the turbulent dissipation and production and an opposing trend in the turbulent and pressure energy diffusions. A quadrant analysis for the conditional Reynolds shear stresses reveals that the ejection and sweep events are the primary contributions toward the total Reynolds shear stress production, with ejections dominating over the entire flow depth. The effect of seepage is shown to affect the magnitude of such events. However, in case of sweeps, this phenomenon is the opposite. The mean-time of occurrence of ejections and that of sweeps in suction are more persistent than those in no-seepage and injection.     

Numerical Study of the Transition Regime between the Skimming and Wake Interference Flows in a Water Flume by Using the Lattice-Model Approach

A numerical study to describe the transition regime between the skimming and wake interference flows due to the influence of an idealized bed roughness in a water flume was carried out here using the lattice model approach. The model reproduced the skimming, transition, and wake interference regimes for different aspect ratios that determine the bed roughness geometry. The simulated turbulent structures were visualized by drawing the trajectories of a large number of passive tracer particles released in the computational domain, and the results agreed with those reported by the research works. The dimensionless streamwise and vertical turbulent intensities were calculated at five test sections. The results obtained supported the visualized flow patterns permitting us to detect the presence of a shear layer developed at the top of the roughness element, whose strength varied according to the flow regime simulated.     

Turbulence Characteristics in Gradual Channel Transition

A field study was conducted to determine the effects of a channel transition on turbulence characteristics. Detailed three-dimensional (3D) flow measurements were collected at a cross section that is located downstream of a gradual channel expansion. These measurements were obtained via an acoustic doppler velocimeter and include the 3D velocity field, the mean local velocities, the turbulent intensities, the frictional characteristics of the flow, the secondary velocity along the transverse plane, and the instantaneous shear stress components in the streamwise and transverse directions. Analysis of the 3D flow data indicates that the turbulent flow on the outer bank of the channel is anisotropic. Such anisotropy of turbulence, which is attributed to the gradual expansion in the channel and bed roughness, yields the development of a secondary flow of Prandtl’s second kind as reported in 1952. In particular, it was found that turbulent intensities in the vertical and transverse directions on the outer bank section are different in magnitude creating turbulence anisotropy in the cross-sectional plane and secondary flows of the second kind. Turbulent intensities increase toward the free surface indicating the transfer of a higher-momentum flux from the channel bed to the free surface, which contradicts common wisdom. Results for the normalized stress components in the streamwise and transverse direction show similar behavior to the intensities. Moreover, the nonlinear distribution of stresses is indicative of the oscillatory nature of the flow induced by the secondary flows of Prandtl’s second kind. A similar behavior was found for flows in straight rectangular channels over different roughness. Finally, a comparison between the secondary current velocity with the mainstream velocity indicates that secondary flow of Prandtl’s second kind is present within the right half of the measured cross section.     

Mean Flow and Turbulence around a Laboratory Spur Dike

The three-dimensional turbulent flow field around a spur dike in a plane fixed-bed laboratory open channel was studied experimentally using a microacoustic Doppler velocimeter. Mean and turbulence characteristics in all three spatial directions were evaluated at upstream and downstream cross sections near the dike. Results showed that the primary flow separated in both lateral and vertical directions. Two counter-rotating flow circulations, consisting of the lateral and vertical velocity components, originated at the dike section. Downstream of the dike, the circulation in the flow-separation zone is stronger than the one in the contracted primary flow zone. The maximum bed-shear stresses estimated using Reynolds stresses is about three times as large as the mean bed-shear stress of incoming flow.     

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.     

Fluidization in an Anaerobic EGSB Reactor: Analysis of Primary Wakes and Modeling of Sludge Blanket

Fluidization of biogranules in an anaerobic expanded granular sludge bed (EGSB) reactor is stochastic in nature and it is a function of the size distribution and the frequency of generation of flow-through gas bubbles in the reactor. Other factors that contribute to the distribution of granules along the height of the reactor are the settling characteristics of granules and the fluid velocity. A simulation was conducted in a test column to obtain a relationship between the flow-through gas and granules at different heights along the column. This relationship was combined with the pattern of gas flow through an identical EGSB reactor to create a model to predict the concentration of granules at different heights along the reactor. The model can predict well the stochastic nature of the axial distribution of granules but underestimates the number of granules at different heights. The reasons for such deviations are explained. The pattern of granule shedding from the primary wake associated with spherical cap bubbles and terminal velocities of bubbles have also been studied and modeled to estimate the maximum height of ascent of granules under isolated spherical cap bubbles. The results of this model agreed well with the experimental observations.     

Control of Scour at Vertical Circular Piles under Waves and Current

An experimental study on the control of scour at vertical circular piles under monochromatic waves and a steady current is presented. The experiments on wave and steady currents were carried out under live-bed and clear-water regimes, respectively. In waves, splitter plate attached to the pile along the vertical plane of symmetry and threaded pile (helical wires or cables wrapped spirally on the pile to form threads) were found to be effective to reduce the scour depth. For the Keulegan–Carpenter numbers 6–100, the vortex shedding is the main mechanism of scour under waves. The splitter plate and threaded pile disrupt the vortex shedding. The average reduction of the scour depth by the splitter plate was 61.6%. For threaded piles, different combinations of cable and pile sizes were tested, and the best combination was found for a cable–pile diameter ratio equaling 0.75, in which average scour depth reduction was 51.1%. The average reductions of scour depths for other cable–pile diameter ratios of 0.33 and 0.5 were 43.2 and 48.1%, respectively. On the other hand, in a steady current, the threaded pile proved to be effective to control scour depth to a great extent. Cables wrapped spirally forming threads on the pile help to weaken the downflow and horseshoe vortex, which are the principal agents of scour under a steady current. The experimental results showed that the scour depth consistently decreases with an increase in cable diameter and the number of threads, and with a decrease in thread angle. The maximum reduction of scour depth observed was 46.3% by using a triple threaded pile having a thread angle of 15° and a cable–pile diameter ratio of 0.1. The proposed methods of controlling scour are easy to install and are economical.     

Measurements of Water Surface Profile and Velocity Field at a Circular Pier

Flow around a 60-mm-diameter pier on a smooth bed was measured in an open-channel flume. By varying the approach flow velocity and water depth, a wide range of subcritical flow conditions was produced. Water surface elevation was measured at 0, 90, and 180° to the approach flow direction near the surface of the pier; and three-dimensional velocity vector field around the pier was measured in two horizontal planes, one close to the bed and the other near the free surface. The velocity field measurements were obtained using a stereoscopic particle image velocimetry system. It was found that the change in water surface elevation around the pier was related to the Froude number and relative water depth. However, no direct relationship between the Froude number and the measured velocity fields was found. The approach flow conditions affected the pier flow field mainly behind the pier; the flow pattern was related to the pier Reynolds number. It was also found that the direction and magnitude of the ensemble-averaged velocity field was more dependent on the pier Reynolds number near the bed.     

Use of Sacrificial Piles as Pier Scour Countermeasures

Laboratory studies on the use of sacrificial piles as a pier scour countermeasure are reported. Sacrificial piles, which previous studies have shown to result in significant scour depth reductions under clear-water conditions, were found to be rather less effective under live-bed conditions due to the passage of bed forms. Sacrificial piles are not recommended unless the flow remains aligned and the flow intensity is relatively small.     

Vertical Dispersion of Fine and Coarse Sediments in Turbulent Open-Channel Flows

Through using a kinetic model for particles in turbulent solid–liquid flows, underlying mechanisms of sediment vertical dispersion as well as sediment diffusion coefficient are investigated. Four hydrodynamic mechanisms, namely gravitational settling, turbulent diffusion, effect of lift force, and that of sediment stress gradient, coexist in two-dimensional (2D) uniform and steady open-channel flows. The sediment diffusion coefficient consists of two independent components: one accounts for the advective transport of sediment probability density distribution function due to sediment velocity fluctuations, and the other results from sediment–eddy interactions. Predictions of the kinetic model are in good agreement with experimental data of 2D open-channel flows. In such flows, it is shown that: (1) the parameter γ (i.e., the inverse of the turbulent Schmidt number) may be greater than unity and increases toward the bed, being close to unity for fine sediments and considerably large for coarse ones; (2) effects of lift force and sediment stress gradient become significant and need to be considered below the 0.1 flow depth; and (3) large errors may arise from the traditional advection–diffusion equation when it is applied to flows with coarse sediments and/or high concentrations.     

Influence Zone for End Bearing of Piles in Sand

An attempt is made to establish an analytically based estimate of the influence zone surrounding the tip of a loaded pile in sand. In the framework of the cavity expansion theory and a confined local failure mechanism, explicit expressions are derived in which the sizes of the upward and downward influence zones are properly linked with the angle of shearing resistance, the stiffness, the volumetric strain, and the mean effective stress of the sand at the pile tip. Based on a series of parametric analyses, the mean range of the influence zone is suggested. For piles in clean sand, the influence zone above the pile tip is between 1.5 and 2.5D and the zone below the tip ranges from 3.5 to 5.5D, where D is pile diameter. For piles in more compressible silty sand, the influence zone extends between 0.5 and 1.5D above the pile tip and between 1.5 and 3D below the tip. Because of its analytical nature, the present study may provide a meaningful insight into the current empirical interpretations.     

Two-Phase Analysis of Vertical Sediment-Laden Jets

In this study, we investigated a vertical dilute sediment-laden jet both experimentally and theoretically. First, an instantaneous whole-field velocimetry tool, particle image velocimetry, was applied to measure the sediment and fluid mean and fluctuating velocities of a downward sediment-laden jet at the same time. Subsequently, an analysis was performed based on two-phase conservation equations for both downward and upward jets. The analysis shows that the mean sediment velocity can be taken as the sum of fluid velocity and the settling velocity in both cases. For the downward jets, the decay rate of the centerline sediment concentration increases with the sediment settling velocity while decreases with the initial discharge velocity. The zone of flow establishment for the sediment velocity is found to be longer than that of the fluid. For the upward jets, the maximum rise of the sediment particles and their deposition distribution on the ground were derived theoretically. The predicted results compare well to the experimental data in the literature.     

Centrifuge Model Study of Laterally Loaded Pile Groups in Clay

A series of centrifuge model tests has been conducted to examine the behavior of laterally loaded pile groups in normally consolidated and overconsolidated kaolin clay. The pile groups have a symmetrical plan layout consisting of 2, 2×2, 2×3, 3×3, and 4×4 piles with a center-to-center spacing of three or five times the pile width. The piles are connected by a solid aluminum pile cap placed just above the ground level. The pile load test results are expressed in terms of lateral load–pile head displacement response of the pile group, load experienced by individual piles in the group, and bending moment profile along individual pile shafts. It is established that the pile group efficiency reduces significantly with increasing number of piles in a group. The tests also reveal the shadowing effect phenomenon in which the front piles experience larger load and bending moment than that of the trailing piles. The shadowing effect is most significant for the lead row piles and considerably less significant for subsequent rows of trailing piles. The approach adopted by many researchers of taking the average performance of piles in the same row is found to be inappropriate for the middle rows, of piles for large pile groups as the outer piles in the row carry significantly more load and experience considerably higher bending moment than those of the inner piles.     

Clearwater Local Scour at Complex Piers

A new methodology to predict local scour depth at a complex pier is presented herein that combines existing expressions for scouring respectively at uniform piers, caisson-founded piers, pile groups with debris rafts, and pile groups alone. The method recognises the relative scouring potentials of the components of complex piers and the transition of scouring processes occurring for varying pile-cap elevation. The validity of the method is confirmed herein using the present and also historical measurements of local scour at complex piers. The proposed methodology has the advantages of being conceptually consistent with observed scour behaviours, relatively simple to apply, applicable to wide ranges of flow and sediment conditions (through incorporation into a more general analysis framework), and applicable over the entire range of possible pile-cap elevations. For design purposes, the present method highlights respective pile-cap elevations that maximize (i.e., to be avoided over the pier life) and minimize local scour at complex piers. The present method reinforces that where the pile-cap elevation relative to the bed can vary with time at a bridge site, potential local-scour depths need to be assessed over the range of possible pile-cap elevations for the pier.     

Nonlinear Efficiency of Bored Pile Group under Lateral Loading

A 3×3 bored pile group consisting of nine cast-in-drilled-hole reinforced concrete shafts and a comparable single-shaft were subjected to reversed cyclic, lateral head loading to investigate group interaction effects across a wide range of lateral displacements. The piles had the same diameter of d = 0.61?m and similar soil conditions; however, various equipment constraints led to two differences: (1) a fixed head (zero rotation) boundary condition for the single pile versus minor pile cap rotation in the vertical plane for the group and (2) shaft longitudinal reinforcement ratios of 1.8% for the single pile and 1% for the group piles. To enable comparisons between the test results, a calibrated model of the single pile (1.8% reinforcement) was developed and used to simulate the response of a single shaft with 1% reinforcement. Additional simulations of the pile group were performed to evaluate the effects of cap rotation on group response. By comparing the simulated responses for common conditions, i.e., 1% reinforcing ratio and zero head rotation, group efficiencies were found to range from unity at lateral displacements <0.004×d to 0.8 at small displacements ～ 0.01–0.02×d and up to 0.9 at failure (displacements >0.04×d). Hence, we find that group efficiency depends on the level of nonlinearity in the foundation system. The general group efficiency, although not its displacement-dependence, is captured by p-multipliers in the literature for reinforced concrete, fixed-head piles.     

Characteristics of Shear Layer Structure in Skimming Flow over a Vertical Drop Pool

The characteristics of shear layer structure between the sliding jet and the pool for skimming flows over a vertical drop pool were investigated experimentally, using flow visualization technique and high speed particle image velocimetry. Four series of experiments having different end sill ratios (h/H = 0.12, 0.43, 0.71 and 1.0, where h=end sill height and H=drop height) with various approaching flow discharges were performed to measure the detailed quantitative velocity fields of the shear layer. The mean velocities and turbulence properties were obtained by ensemble averaging the repeated measurements. From the velocity profiles, it is found that the growth of the shear layer in the downward direction as the jet slides down the pool represents the momentum exchange. Analyzing the distribution of measured velocity, the similarity profile of the mean velocity at different cross sections along the shear layer was obtained. The proposed characteristic scales provided unique similarity profiles having promising regression coefficient. The selection of these characteristic scales is also discussed. Further, the spatial variations of mean velocity profiles, turbulence intensities, in-plane turbulent kinetic energy, and Reynolds shear stress were also elucidated in detail. The imperative observation is that the Reynolds shear stress dominates the major part along the shear layer as compared to the viscous shear stress. The study also provides an insight into the flow phenomena through the velocity and turbulent characteristics.