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The well-known Shields diagram is developed for unvegetated open channel flows to describe incipient sediment motion by means of critical bed shear stress. Due to difficulties in estimating the bed shear stress in vegetated flows, it is not clear whether the Shields diagram is applicable in the presence of vegetation. By applying the phenomenological theory of turbulence, a new method to evaluate the bed shear stress in vegetated flows is proposed in this study. With this method, the critical bed shear stress subject to vegetated flows is calculated using the published data. The result shows that the calculated critical shear stresses are consistent with the Shields diagram.

This study experimentally investigates the mean and turbulent flow fields of a propeller jet impinging on a vertical quay wall. Four impingement distances, namely, wall clearances (= longitudinal distance between the propeller and vertical wall), were used to examine their influence on the flow behaviors. In order to investigate the characteristics of the three-dimensional impinging jet, both the streamwise (jet central plane) and transverse (impingement plane) flow fields were measured for each impingement distance using the particle image velocimetry (PIV) technique. Based on the wall clearance, three regions are identified, namely free, impingement and wall jet regions. Additionally, the results show that the evolution of the impinging jet is governed by two mechanisms: jet diffusion and wall obstruction. Their relative importance under varying wall clearances was interpreted in terms of the mean flow patterns and energy dissipation. Moreover, the proper orthogonal decomposition (POD) method is employed to identify the dominant flow structures associated with these two mechanisms in terms of the POD modes and their corresponding dominant frequency, which in turn provides a quantitative means to single out the individual flow mechanism. Nanyang Technological University The authors acknowledge the financial support provided by Nanyang Technological University for this research.

Local scour in a sediment bed varies depending on the type, size and shape of hydraulic structures, and the properties of approach flow and sediment particles. Published studies are hitherto confined primarily to empirical experiments based on particular types of hydraulic structure and there is almost no comparison of scour amongst different types of hydraulic structures. This study aims to provide an attempt to unify different types of clear-water scour based on a new length scale, which is proposed to characterize the size of the large-scale flow structure that governs scouring processes. The new length scale serves as a generalized hydraulic radius, which is applicable for all types of clear-water scour related to junction flows at bridge piers and abutments, and wall jets including culverts. The analysis indicates that when normalized with the new length scale, the equilibrium scour depths can be described with a unified function of the densimetric Froude number, regardless of different scour types. In addition, this study also shows that the proposed function varies with the scour efficiency, in a fashion resembling the bedload function in the range of low to high transport regimes. This finding, which is founded on solid physical grounds, reveals that different kinds of local scour phenomena can be interpreted in unison rather than based on the individual type of hydraulic structures. Published version The authors would gratefully acknowl-edge the financial support from the National Natural Science Foundation of China (Grant No. 51979242).

Flow constriction caused by bridge abutment increases bed shear stress and thus enhances local scour. For scaling the maximum scour depth at the abutment, either abutment length or flow depth has been empirically used in previous studies. By performing a step-by-step analysis, this study proposes a new length scale, which is able to represent combined effects of abutment length, approach flow depth and channel width. Physically, the new length scale describes the maximum possible dimension of the associated vortex system (or large-scale turbulence). Six series of data compiled from the published literature were used in the analysis. The results indicate that the new length scale helps improve the agreement of predictions with the experimental data.

This study evaluates sediment gradation effects on clear-water scour at bridge piers. By considering the average erosion and deposition rates in a pier-scour hole, the normalized equilibriu...

In vegetated open channel flow, sediment transport is affected by both boundary shear and vegetation-induced turbulence. This study presents a theoretical derivation of the critical flow co...

This paper presents results obtained from a detailed analysis of the experimental data of three hydraulic jumps with low Froude numbers (1.5–1.9) in a narrow flume. With the increasing Frou...

The pickup probability of bedload sediment can be defined as the channel width fraction that is only associated with entrained particles. With this definition, the dimensionless bedload tra...

This paper presents a compilation of a large database consisting of 588 culvert scour experiments covering a wide combination of different culvert shapes, culvert outlet conditions, and sed...

This study presents measurements of the bed shear stress associated with the three-dimensional boundary layer induced by a wall-bounded swirling jet. The velocity profile within the viscous sublayer was measured in detail using a high-resolution particle image velocimetry system that enables direct evaluation of the bed shear stress from the velocity gradient at the wall. The three-dimensional flow field was reconstructed based on the measurements conducted in a series of intersecting streamwise and transverse planes. At each intersection, the resultant bed shear stress was calculated as the vector sum of its components. The results show that the planar distribution of the bed shear stress is highly asymmetrical about the propeller axis due to the swirling rotation, which furnishes a description of how the jet path migrates over the plane boundary under the swirling effect. Compared with the expected scour hole that the swirling jet could induce on an erodible bed, the results presented in this study shed light on a qualitative understanding of the distinctive jet impingement mechanisms associated with rigid and erodible bed boundaries. Moreover, a close examination of the mean velocity distributions of both individual and resultant velocities reveals a consistent discrepancy from the universal logarithmic law in the log layer, in which the velocity distribution is subjected to outer layer bulk flow properties, thereby highlighting the evident difference in boundary layer flow between an impinging jet and canonical wall-bounded flows.

Wei, M.X.; Chiew, Y.M., and Guan, D.W., 2018. Flow behavior of Propeller Jet around an Open-Type Quay. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 956–960. Coconut Creek (Florida), ISSN 0749-0208.The flow characteristics of a confined ship-propeller jet in the presence of an open-type quay slope with a rigid bed were experimentally studied using particle image velocimetry (PIV) technique. In the study, the toe clearance (Xt), namely the horizontal distance between the propeller and slope toe, was varied from 2Dp to 9Dp, where Dp = propeller diameter (= 7.5 cm). The propeller speed (n) and clearance height (Zb) were 500 rpm and 7.5 cm (= 1Dp), respectively. In each test, the field of view of the PIV measurement was focused on a rectangular region (∼30 cm × 20 cm) around the slope toe, which is the location of the greatest interest for engineering problems, such as scour damages in the case of an erodible seabed. The jet evolutions under different toe clearances are presented in the form of mean velocity and turbulence intensity from an overall perspective of the selected field of view. Due to the asymmetrical entrainment of the ambient fluid above and below the jet, observations reveal that the jet has a tendency to impinge onto the bed or quay slope. With increasing toe clearances, the jet undergoes significant diffusion before it reaches the slope. However, the turbulence intensity profiles show that the presence of slope causes a significant increase of turbulence in the near-bed region.

This paper aims to explore the combined main flow and turbulence influence on sediment pickup rates by investigating four purposely devised cases, namely, (1) unobstructed flow over a plane bed; (2) unobstructed flow over a rigid‐dune bed; (3) flow obstructed by a horizontal pipe; and (4) flow obstructed by a vertical pipe. The four cases were used to represent typical scenarios of mean flow combined with turbulence of varying intensities. By analyzing the dependence of sediment pickup rate on different combinations of the mean and turbulent flow parameters, a modified densimetric Froude number, incorporating both the bulk‐averaged velocity and maximum root‐mean‐square value of the fluctuating velocity, is found to be reasonably correlated with the measured pickup rates for all the four cases. Consequently, by relating the pickup rate with the dimensionless particle diameter and the modified densimetric Froude number, a generalized equation that predicts the pickup rates under both unobstructed as well as obstructed flow conditions is obtained. Published version

This paper presents an extensive review of the characteristics of a propeller jet-induced flow field and its impact on scour formation. As the driving force of scour, the jet flow behavior is first explored, including both the free and bounded jets under various boundary conditions. Accordingly, their resulting scour, namely, unconfined and confined scour, are expounded in terms of the temporal and spatial development of the scour depth. Hitherto, existing methods for predicting the scour depth are principally developed empirically, which exclude in-depth scrutiny into the mechanism governing the scouring process. With the detailed flow fields obtained in recent research, this paper aims to piece together a state-of-the-art understanding of the scouring mechanism governing confined propeller scour, that is, the combined effect of jet flow and junction flow on the scour hole development. By comparing the similarities and differences between the scouring mechanisms in open and closed quays, this paper provides a comprehensive insight into the scouring process associated with bounded propeller jets. Moreover, in terms of the near-bed flow variations during the scouring process, implications for sediment transport under highly turbulent flow are discussed. Finally, future research needs are identified. Published version

Although local scour around submarine pipelines has been extensively studied in the last few decades, understanding of the mechanism of local scour around pipelines is still in its infancy stage due to the complex nature of flow-pipeline-seabed interactions, especially when the pipeline is subjected to vibrations. This experimental study aims to obtain an improved perception of the scour mechanism around a pipeline subjected to vortex-induced vibrations. The experiments were conducted in a flow recirculation flume in clear-water scour conditions in which a circular cylinder with diameter (D) of 35 mm was used as the pipeline model. The initial gap (G0) between the underside of the pipeline and undisturbed flat-bed level was 0.45D. The time evolution of the pipeline motion and scour profile around the pipeline was measured using a high-speed camera and laser sources. The flow fields around the vibrating pipeline were obtained using the particle image velocimetry (PIV) technique and phase-average analysis. Based on the characteristics of the development of the scour hole and pipeline motions, three scour stages are identified. The downward motion of the vibrating pipeline and interactions between the counterclockwise vortex (downstream of the pipeline) that sheds from the lower shear layer of the pipeline and sediment bed are found to be the primary mechanisms that cause the formation of the scour holes beneath the pipeline. The interactive coupling effects among the vibrating pipeline, flow field, and scour process also are discussed in this study. Singapore Maritime Institute (SMI) The authors gratefully acknowledge the financial support provided by Singapore Maritime Institute (SMI) and EMAS AMC, under the SMI Deepwater Technology R&D Programme (Research Grant No. SMI-2014-OF-12). The first author also acknowledges support provided by the Young Scientists Fund of the National Natural Science Foundation of China (51709082) and the Fundamental Research Funds for the Central Universities (2018B13014).

An experimental method is presented in this paper to facilitate visualization of the detailed flow fields and determination of the near-boundary shear and normal stresses within an equilibrium scour hole induced by a vibrating pipeline. This method involves the implementation of a pipeline vibration system in a straight flume, a time-resolved particle image velocimetry (PIV) system for pipeline displacement tracking and flow fields measurements. The displacement time-series of the vibrating pipeline are obtained by using the cross-correlation algorithms. The steps for processing raw particle laden images obtained by using the time-resolved PIV are described. The detailed instantaneous flow fields around the vibrating pipeline at different vibrating phases are calculated by using a multiple-time-interval cross-correlation algorithm to avoid displacement bias error in the flow regions with a large velocity gradient. By applying the wavelet transform technique, the captured images that have the same vibrating phase are accurately cataloged before the phase-averaged velocity fields are obtained. The key advantages of the flow measurement technique described in this paper are that it has a very high temporal and spatial resolution and can be simultaneously used to obtain the pipeline dynamics, flow fields, and near-boundary flow stresses. By using this technique, more in-depth studies of the 2-dimensional flow field in a complex environment, such as that around a vibrating pipeline, can be conducted to better understand the associated sophisticated scour mechanism.

This paper presents an experimental study on the characteristics of the propeller-induced flow field and its associated scour hole around a closed type quay (with a vertical quay wall). An &ldquo, oblique particle image velocimetry&rdquo, (OPIV) technique, which allows a concurrent measurement of the velocity field and scour profile, was employed in measuring the streamwise flow field (jet central plane) and the longitudinal centerline scour profile. The asymptotic scour profiles obtained in this study were compared with that induced by an unconfined propeller jet in the absence of any berthing structure, which demonstrates the critical role of the presence of the quay wall as an obstacle in shaping the scour profile under the condition of different wall clearances (i.e., longitudinal distance between propeller and wall). Moreover, by comparing the vortical structure within the asymptotic scour hole around the vertical quay wall with its counterpart in the case of an open quay (with a slope quay wall), the paper examines the effect of quay types on the formation of the vortex system and how it determines the geometrical characteristic of the final scour profile. Furthermore, the temporal development of the mean vorticity field and the vortex system are discussed in terms of their implications on the evolution of the scour hole. In particular, comparison of the circulation development of the observed vortices allows a better understanding of the vortex scouring mechanism. Energy spectra analysis reveals that at the vortex centers, their energy spectra distributions consistently follow the &minus, 5/3 law throughout the entire scouring process. As the scour process evolves, the turbulent energy associated with the near-bed vortex, which is responsible for scouring, is gradually reduced, especially for the small-scale eddies, indicating a contribution of the dissipated turbulent energy in excavating the scour hole. Finally, a comparison of the near-bed flow characteristics of the average kinetic energy (AKE), turbulent kinetic energy (TKE), and Reynolds shear stress (RSS) are also discussed in terms of their implications for the scour hole development.

This study proposes a new length scale that measures the size of the wave-induced vortex street on one side of a pipeline. By conducting dimensional analysis, we show that the equilibrium scour depth under a pipeline, when normalized by the vortex street size, is generally dependent on the densimetric Froude number. However, by analyzing the published data, the dependence is found to be insignificant if the current velocity is smaller than the maximum orbital velocity of water particles at the bed. This result suggests that the normalized scour depth can be simply taken to be a constant for both wave-alone and wave-plus-current conditions.

This study presents results of the temporal development of propeller scour around a sloping bank. During the scouring process, the centerline scour profile along the propeller axis comprises a toe scour hole, a primary scour hole, or a combination of both. Their formation is dependent on the length of the toe clearances, which is defined as the longitudinal distance between the propeller face and toe of the slope. As a result, the overall maximum scour depth along the centerline profile may occur either in the primary or toe scour hole. The results show that the temporal development of the overall maximum scour depth for all the cases consistently exhibits an exponential form. An empirical method was obtained to estimate the maximum scour depth evolution throughout the scouring process.

Since local scour at bridge piers in rivers and estuaries is a major cause of bridge failure, estimation of the maximum local scour depth is of great importance to hydraulic and coastal engineers. Although numerous studies that focus on scour-depth prediction have been done and published, understanding of the flow and turbulence characteristics of the horseshoe vortex that drives the scour mechanism in a developing scour hole still is immature. This study aims to quantify the detailed turbulent flow field in a developing clear-water scour hole at a circular pier using Particle Image Velocimetry (PIV). The distributions of velocity fields, turbulence intensities, and Reynolds shear stresses of the horseshoe vortex that form in front of the pier at different scour stages (t = 0, 0.5, 1, 12, 24, and 48 h) are presented in this paper. During scour development, the horseshoe vortex system was found to evolve from one initially small vortex to three vortices. The strength and size of the main vortex are found to increase with increasing scour depth. The regions of both the maximum turbulence intensity and Reynolds shear stress are found to form at a location upstream of the main vortex, where the large turbulent eddies have the highest possibility of occurrence. Results from this study not only provide new insight into the complex flow-sediment interaction at bridge piers, but also provide valuable experimental databases for advanced numerical simulations. The authors would like to thank Professors Robert Ettema and Subhasish Dey for their helpful discussions. Special thanks to the Associate Editor Professor Charles Melching for his thoughtful help in proofreading the manuscript. Anonymous reviewers are also acknowledged for their constructive comments. This research was supported by the Young Scientists Fund of the National Natural Science Foundation of China (51709082) and the Fundamental Research Funds for the Central Universities (2018B13014).

By using the particle image velocimetry (PIV) technique, experimental results of the propeller jet flow field within a developing scour hole around an open quay are presented. The experiments were conducted with four toe clearances (defined as the longitudinal distance between the propeller face and toe of the quay slope), which account for three distinct scour profiles corresponding to the three toe clearance fields, namely, near, intermediate, and far clearance fields. To better understand the underlying scour mechanisms associated with each field, the mean flow fields at selected time intervals (t = 0, 0.5, 2, 12, and 48 h) are presented in vector and streamline plots. The data provide an improved understanding of the development of the main jet flow and vortex that formed in front of the quay slope. The importance of both the main jet flow and vortex in terms of their contribution in shaping the final scour profile in each field is discussed. Moreover, the turbulence intensities and near-bed Reynolds stress, which are analyzed for the intermediate field, exhibited a declining trend with the development of the scour hole.

The flow properties of a propeller jet in the presence of a plane bed boundary were investigated using the particle image velocimetry technique. Three clearance heights, Z b = 2D p, D p, and 0.5D p, where D p = propeller diameter, were used to examine boundary effects on the development of the jet. In each case, the mean flow properties and turbulence characteristics were measured in a larger field of view than those used in past studies. Both the streamwise and transverse flow fields were measured to obtain the three-dimensional characteristics of the propeller jet. Similar to a confined offset jet, the propeller jet also exhibits a wall attachment behavior when it is placed near a plane boundary. As a result, in contrast to its unconfined counterpart, the confined propeller jet features three regions, namely the free jet, impingement and wall jet regions. The study shows that the extent of each region varies under different clearance heights. The development of the mean flow and turbulence characteristics associated with varying clearance heights are compared to illustrate boundary effects in these regions. In the impingement region, the measured transverse flow fields provide new insights on the lateral motions induced by the impingement of the swirling jet. In the wall jet region, observations reveal that the jet behaves like a typical three-dimensional wall jet and its axial velocity profiles show good agreement with the classical wall jet similarity function.

This study experimentally investigates the temporal and spatial development of the scour hole induced by a confined propeller jet in the presence of an open type quay. The longitudinal dist...

The present frequency domain method of calculating wave power may not be accurate enough for calculating the incident wave power of a specific site, which is primary measurement for evaluating the efficiency of wave energy converters (WECs) and an alternative measure, the time domain method, is proposed. Three sites including two nearshore sites and one deepwater site at Chengshantou sea area were selected, and a sample wave parameters data set was obtained from wave model SWASH to demonstrate the application of these two methods. A comparison of the results of each method was also performed and two influential parameters used in calculation were analyzed. The results show that frequency domain method is very likely to overestimate the wave power at both deepwater and nearshore site. The time domain method proposed in this paper is believed to be more superior in calculating the incident wave power during a short term.

Vortex-induced vibration (VIV) has been extensively studied and the related findings presented in published literature. However, when the cylinder is placed near an erodible sand bed, interactions between the vibrating cylinder, flow field and local scour evidently will become very complex. The aim of this study is to provide an improved understanding of the interaction between the flow field, free vibrating cylinder and scour by using flow visualization and a new PIV measurement technique. The results show that the amplitude and frequency of the vibrating cylinder are closely related to the depth of the scour hole. Based on the qualitative observation and quantitative measurement of the flow field, vibrating cylinder and scour dimensions, three distinct scour stages are identified in this study. The characteristics of the turbulence intensity, formation and transmission of the vortices in each of these three scour stages are discussed in this paper.