In a natural gravel-bed stream, the bed that has an organized roughness structure created by the ... more In a natural gravel-bed stream, the bed that has an organized roughness structure created by the streamflow is called the water-worked gravel bed (WGB). Such a bed is entirely different from that created in a laboratory by depositing and spreading gravels in the experimental flume, called the screeded gravel bed (SGB). In this paper, a review on the state-of-the-art research on WGBs is presented, highlighting the role of water-work in determining the bed topographical structures and the turbulence characteristics in the flow. In doing so, various methods used to analyze the bed topographical structures are described. Besides, the effects of the water-work on the turbulent flow characteristics, such as streamwise velocity, Reynolds and form-induced stresses, conditional turbulent events and secondary currents in WGBs are discussed. Further, the results form WGBs and SGBs are compared critically. The comparative study infers that a WGB exhibits a higher roughness than an SGB. Conseque...
Gravel-bed roughness structures for an unworked bed and a water-worked bed are analysed at differ... more Gravel-bed roughness structures for an unworked bed and a water-worked bed are analysed at different topographic scales measuring the bed surfaces by a 3D terrestrial laser scanner in a laboratory flume. The study represents a novel contribution in assessing the role of water-work on a uniform gravel-bed, demonstrating the development of small-scale bedforms (i.e. clusters). To support these observations, an in-depth statistical analysis of the bed topographies before and after water-working was applied at the grain scale and the mesoscale (i.e. a scale larger than the biggest individual particles), revealing that this is an essential prerequisite to properly interpret the roughness structures thus formed. In addition, the results show that the unworked bed is characterized by a multiscaling behavioural feature that represents multifractality of the bed roughness structure. For the water-worked bed, a simple scaling is prevalent, which implies that at small spatial lags, a statistic...
Abstract The aim of this experimental study is to investigate the interaction between turbulent f... more Abstract The aim of this experimental study is to investigate the interaction between turbulent flow and a gravel-bed that mimics the actual roughness structures of a natural bed and its implication on sediment transport. In particular, the response of the Reynolds stresses and the role of intermittency to the bed roughness is the primary focus of the current study. To this end, the flow field, measured with an Acoustic–Doppler Velocimeter (ADV), is thoroughly examined, considering the conditional Reynolds shear stresses and the high-order structure functions of velocity. The study results show that the features and the roughness structure of the water-worked gravel-bed (WWGB) have a strong impact on bursting events. The reason for this is attributed to the flow layer above the crest in the WWGBs being primarily affected by sweep events rather than ejection events, although the latter events are prevalent in this flow layer in an immobile, man-made, gravel-bed. These coherent structures, which are primarily responsible for sediment transport, also are the key cause of the presence of an intermittency excess that breaks the Kolmogorov self-similarity hypothesis, leading to multi-fractal behavior of the velocity structure functions.
In overbank flow due to the interaction mechanism between the main channel and floodplain, the fl... more In overbank flow due to the interaction mechanism between the main channel and floodplain, the flow property of the compound sections gets affected. The complexity is more when the compound channels have non-prismatic floodplains. Additional complexity occurs during the interaction between the subsections as well as due to non-uniformity of flow through converging parts of the compound channel. For prediction of flow, calculation of energy loss parameters from section to section is an important task for river engineers. In this paper, an experimental investigation for the energy losses of converging compound channels for different flow depths along the converging path is performed. The loss of energy due to contraction and compound geometry for a compound channel is evaluated, and the dependency of energy loss for such channels is analyzed. A generalized multivariable regression model has been developed to predict the energy slope with high accuracy. Using the expression of the energy loss concept, the discharge capacity in the converging compound is found to provide good results as compared to other standard model exists in the literature.
Iranian Journal of Science and Technology, Transactions of Civil Engineering
Prediction of boundary shear force distributions in open channel flow is crucial in many critical... more Prediction of boundary shear force distributions in open channel flow is crucial in many critical engineering problems such as channel design, calculation of losses and sedimentation. During floods, part of the discharge of a river is carried by the simple main channel and the rest is carried by the floodplains. For such compound channels, the flow structure becomes complicated due to the transfer of momentum between the deep main channel and the adjoining floodplains. The complexity further increases when dealing with a compound channel with non-prismatic floodplains. Knowledge of momentum transfer at the different interfaces originating from the junction between the main channel and floodplain can be acquired from the distribution of boundary shear in the subsections. The calculation of boundary shear and depth average velocity in non-prismatic compound channel flow is more complex and simple conventional approaches cannot predict the boundary shear and depth average velocity with sufficient accuracy. Hence, in this area, an easily implementable technique, the Artificial Neural Network can be used for predicting the boundary shear and depth average velocity at different sections of a converging compound channel for different geometry and flow conditions. The model’s performance has lead satisfactory results. Statistical error analysis is also carried out to know the degree of accuracy of the model.
The flow pattern of a compound channel becomes complicated due to the transfer of momentum betwee... more The flow pattern of a compound channel becomes complicated due to the transfer of momentum between the main channel and the adjoining floodplains. Experiments are carried out to compute the velocity as well as boundary shear along the wetted perimeter of a straight compound channel to quantify the momentum transfer along the expected interfaces originating at the junction region between main channel and flood plain. This is helpful to evaluate the stage-discharge relationship for a compound channel accurately. Discharge calculation can be done by using various hydraulic models. But the traditional discharge prediction models such as SCM, DCM fail to give accurate discharge as they don’t consider the effect of momentum transfer. Therefore some new models are being developed which makes discharge prediction more accurate than the traditional method by considering the effect of momentum transfer. In this study the experimental data reported by other investigators as well as data from t...
AbstractThe hydrodynamic force on sediment particles is decomposed into two components, hydrodyna... more AbstractThe hydrodynamic force on sediment particles is decomposed into two components, hydrodynamic drag and lift, whose estimations are made semiempirically by the introduction of drag and lift c...
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences
Characteristics of turbulence anisotropy in flow over two-dimensional rigid dunes are analysed. T... more Characteristics of turbulence anisotropy in flow over two-dimensional rigid dunes are analysed. The Reynolds stress anisotropy is envisaged from the perspective of the stress ellipsoid shape. The spatial evolutions of the anisotropic invariant map (AIM), anisotropic invariant function, eigenvalues of the scaled Reynolds stress tensor and eccentricities of the stress ellipsoid are investigated at various streamwise distances along the vertical. The data plots reveal that the oblate spheroid axisymmetric turbulence appears near the top of the crest, whereas the prolate spheroid axisymmetric turbulence dominates near the free surface. At the dune trough, the axisymmetric contraction to the oblate spheroid diminishes, as the vertical distance below the crest increases. At the reattachment point and one-third of the stoss-side, the oblate spheroid axisymmetric turbulence formed below the crest appears to be more contracted, as the vertical distance increases. The AIMs suggest that the tu...
Turbulent flow over a water-worked gravel bed (WGB) was investigated using the double-averaging m... more Turbulent flow over a water-worked gravel bed (WGB) was investigated using the double-averaging methodology (DAM). The flow measurements were carried out by the particle image velocimetry (PIV) technique. The double-averaged (DA) turbulent characteristics (DA Turbulent kinetic energy (TKE) components, form-induced TKE components, DA TKE fluxes, form-induced TKE fluxes, DA TKE budget) were analyzed for the WGB. To understand the effect of changed bed topography on the turbulent characteristics, the flow measurements were carried out over a screeded gravel bed (SGB), keeping the flow Froude number same as in case of WGB. Owing to water work, the bed topography of WGB was dissimilar to that of SGB, resulting in higher roughness size for the former than that for the latter. Comparative study of the DA turbulent characteristics of both the beds infers that especially in the near-bed flow zone, the flow parameters of the WGB are attaining higher values than those of the SGB. However, they...
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences
In this paper, we explore the mechanics of bed particle saltation in turbulent wall-shear flow, a... more In this paper, we explore the mechanics of bed particle saltation in turbulent wall-shear flow, analysing the forces on a particle to perform saltation. The hydrodynamic drag encompasses the form drag and turbulent drag. The hydrodynamic lift comprises the Saffman lift, Magnus lift and turbulent lift. The subtle role of the Basset force in governing the particle trajectory is accounted for in the analysis. The bedload flux, emanating from the mathematical analysis of bed particle saltation, is determined. The results reveal that for the particle parameter range 20–100, the transport stage function equalling unity corroborates the threshold of bed particle saltation, where the saltation height and length are 1.3 and 9 times the particle size. For a given transport stage function, the relative saltation height and length decrease with an increase in particle parameter. For the particle parameter range 20–100, the relative saltation height and length increase with an increase in transp...
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences
This review article, dedicated to the bicentenary celebration of Sir George Gabriel Stokes' b... more This review article, dedicated to the bicentenary celebration of Sir George Gabriel Stokes' birthday, presents the state-of-the-science of terminal fall velocity, highlighting his rich legacy from the perspective of fluvial hydraulics. It summarizes the fluid drag on a particle and the current status of the drag coefficient from both the theoretical and empirical formulations, highlighting the three major realms—Stokesian, transitional and Newtonian realms. The force system that drives the particle motion falling through a fluid is described. The response of terminal fall velocity to key factors, which include particle shape, hindered settling and turbulence (nonlinear drag, vortex trapping, fast tracking and effects of loitering), is delineated. The article puts into focus the impact of terminal fall velocity on fluvial hydraulics, discussing the salient role that the terminal fall velocity plays in governing the hydrodynamics of the sediment threshold, bedload transport and su...
In a natural gravel-bed stream, the bed that has an organized roughness structure created by the ... more In a natural gravel-bed stream, the bed that has an organized roughness structure created by the streamflow is called the water-worked gravel bed (WGB). Such a bed is entirely different from that created in a laboratory by depositing and spreading gravels in the experimental flume, called the screeded gravel bed (SGB). In this paper, a review on the state-of-the-art research on WGBs is presented, highlighting the role of water-work in determining the bed topographical structures and the turbulence characteristics in the flow. In doing so, various methods used to analyze the bed topographical structures are described. Besides, the effects of the water-work on the turbulent flow characteristics, such as streamwise velocity, Reynolds and form-induced stresses, conditional turbulent events and secondary currents in WGBs are discussed. Further, the results form WGBs and SGBs are compared critically. The comparative study infers that a WGB exhibits a higher roughness than an SGB. Conseque...
Gravel-bed roughness structures for an unworked bed and a water-worked bed are analysed at differ... more Gravel-bed roughness structures for an unworked bed and a water-worked bed are analysed at different topographic scales measuring the bed surfaces by a 3D terrestrial laser scanner in a laboratory flume. The study represents a novel contribution in assessing the role of water-work on a uniform gravel-bed, demonstrating the development of small-scale bedforms (i.e. clusters). To support these observations, an in-depth statistical analysis of the bed topographies before and after water-working was applied at the grain scale and the mesoscale (i.e. a scale larger than the biggest individual particles), revealing that this is an essential prerequisite to properly interpret the roughness structures thus formed. In addition, the results show that the unworked bed is characterized by a multiscaling behavioural feature that represents multifractality of the bed roughness structure. For the water-worked bed, a simple scaling is prevalent, which implies that at small spatial lags, a statistic...
Abstract The aim of this experimental study is to investigate the interaction between turbulent f... more Abstract The aim of this experimental study is to investigate the interaction between turbulent flow and a gravel-bed that mimics the actual roughness structures of a natural bed and its implication on sediment transport. In particular, the response of the Reynolds stresses and the role of intermittency to the bed roughness is the primary focus of the current study. To this end, the flow field, measured with an Acoustic–Doppler Velocimeter (ADV), is thoroughly examined, considering the conditional Reynolds shear stresses and the high-order structure functions of velocity. The study results show that the features and the roughness structure of the water-worked gravel-bed (WWGB) have a strong impact on bursting events. The reason for this is attributed to the flow layer above the crest in the WWGBs being primarily affected by sweep events rather than ejection events, although the latter events are prevalent in this flow layer in an immobile, man-made, gravel-bed. These coherent structures, which are primarily responsible for sediment transport, also are the key cause of the presence of an intermittency excess that breaks the Kolmogorov self-similarity hypothesis, leading to multi-fractal behavior of the velocity structure functions.
In overbank flow due to the interaction mechanism between the main channel and floodplain, the fl... more In overbank flow due to the interaction mechanism between the main channel and floodplain, the flow property of the compound sections gets affected. The complexity is more when the compound channels have non-prismatic floodplains. Additional complexity occurs during the interaction between the subsections as well as due to non-uniformity of flow through converging parts of the compound channel. For prediction of flow, calculation of energy loss parameters from section to section is an important task for river engineers. In this paper, an experimental investigation for the energy losses of converging compound channels for different flow depths along the converging path is performed. The loss of energy due to contraction and compound geometry for a compound channel is evaluated, and the dependency of energy loss for such channels is analyzed. A generalized multivariable regression model has been developed to predict the energy slope with high accuracy. Using the expression of the energy loss concept, the discharge capacity in the converging compound is found to provide good results as compared to other standard model exists in the literature.
Iranian Journal of Science and Technology, Transactions of Civil Engineering
Prediction of boundary shear force distributions in open channel flow is crucial in many critical... more Prediction of boundary shear force distributions in open channel flow is crucial in many critical engineering problems such as channel design, calculation of losses and sedimentation. During floods, part of the discharge of a river is carried by the simple main channel and the rest is carried by the floodplains. For such compound channels, the flow structure becomes complicated due to the transfer of momentum between the deep main channel and the adjoining floodplains. The complexity further increases when dealing with a compound channel with non-prismatic floodplains. Knowledge of momentum transfer at the different interfaces originating from the junction between the main channel and floodplain can be acquired from the distribution of boundary shear in the subsections. The calculation of boundary shear and depth average velocity in non-prismatic compound channel flow is more complex and simple conventional approaches cannot predict the boundary shear and depth average velocity with sufficient accuracy. Hence, in this area, an easily implementable technique, the Artificial Neural Network can be used for predicting the boundary shear and depth average velocity at different sections of a converging compound channel for different geometry and flow conditions. The model’s performance has lead satisfactory results. Statistical error analysis is also carried out to know the degree of accuracy of the model.
The flow pattern of a compound channel becomes complicated due to the transfer of momentum betwee... more The flow pattern of a compound channel becomes complicated due to the transfer of momentum between the main channel and the adjoining floodplains. Experiments are carried out to compute the velocity as well as boundary shear along the wetted perimeter of a straight compound channel to quantify the momentum transfer along the expected interfaces originating at the junction region between main channel and flood plain. This is helpful to evaluate the stage-discharge relationship for a compound channel accurately. Discharge calculation can be done by using various hydraulic models. But the traditional discharge prediction models such as SCM, DCM fail to give accurate discharge as they don’t consider the effect of momentum transfer. Therefore some new models are being developed which makes discharge prediction more accurate than the traditional method by considering the effect of momentum transfer. In this study the experimental data reported by other investigators as well as data from t...
AbstractThe hydrodynamic force on sediment particles is decomposed into two components, hydrodyna... more AbstractThe hydrodynamic force on sediment particles is decomposed into two components, hydrodynamic drag and lift, whose estimations are made semiempirically by the introduction of drag and lift c...
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences
Characteristics of turbulence anisotropy in flow over two-dimensional rigid dunes are analysed. T... more Characteristics of turbulence anisotropy in flow over two-dimensional rigid dunes are analysed. The Reynolds stress anisotropy is envisaged from the perspective of the stress ellipsoid shape. The spatial evolutions of the anisotropic invariant map (AIM), anisotropic invariant function, eigenvalues of the scaled Reynolds stress tensor and eccentricities of the stress ellipsoid are investigated at various streamwise distances along the vertical. The data plots reveal that the oblate spheroid axisymmetric turbulence appears near the top of the crest, whereas the prolate spheroid axisymmetric turbulence dominates near the free surface. At the dune trough, the axisymmetric contraction to the oblate spheroid diminishes, as the vertical distance below the crest increases. At the reattachment point and one-third of the stoss-side, the oblate spheroid axisymmetric turbulence formed below the crest appears to be more contracted, as the vertical distance increases. The AIMs suggest that the tu...
Turbulent flow over a water-worked gravel bed (WGB) was investigated using the double-averaging m... more Turbulent flow over a water-worked gravel bed (WGB) was investigated using the double-averaging methodology (DAM). The flow measurements were carried out by the particle image velocimetry (PIV) technique. The double-averaged (DA) turbulent characteristics (DA Turbulent kinetic energy (TKE) components, form-induced TKE components, DA TKE fluxes, form-induced TKE fluxes, DA TKE budget) were analyzed for the WGB. To understand the effect of changed bed topography on the turbulent characteristics, the flow measurements were carried out over a screeded gravel bed (SGB), keeping the flow Froude number same as in case of WGB. Owing to water work, the bed topography of WGB was dissimilar to that of SGB, resulting in higher roughness size for the former than that for the latter. Comparative study of the DA turbulent characteristics of both the beds infers that especially in the near-bed flow zone, the flow parameters of the WGB are attaining higher values than those of the SGB. However, they...
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences
In this paper, we explore the mechanics of bed particle saltation in turbulent wall-shear flow, a... more In this paper, we explore the mechanics of bed particle saltation in turbulent wall-shear flow, analysing the forces on a particle to perform saltation. The hydrodynamic drag encompasses the form drag and turbulent drag. The hydrodynamic lift comprises the Saffman lift, Magnus lift and turbulent lift. The subtle role of the Basset force in governing the particle trajectory is accounted for in the analysis. The bedload flux, emanating from the mathematical analysis of bed particle saltation, is determined. The results reveal that for the particle parameter range 20–100, the transport stage function equalling unity corroborates the threshold of bed particle saltation, where the saltation height and length are 1.3 and 9 times the particle size. For a given transport stage function, the relative saltation height and length decrease with an increase in particle parameter. For the particle parameter range 20–100, the relative saltation height and length increase with an increase in transp...
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences
This review article, dedicated to the bicentenary celebration of Sir George Gabriel Stokes' b... more This review article, dedicated to the bicentenary celebration of Sir George Gabriel Stokes' birthday, presents the state-of-the-science of terminal fall velocity, highlighting his rich legacy from the perspective of fluvial hydraulics. It summarizes the fluid drag on a particle and the current status of the drag coefficient from both the theoretical and empirical formulations, highlighting the three major realms—Stokesian, transitional and Newtonian realms. The force system that drives the particle motion falling through a fluid is described. The response of terminal fall velocity to key factors, which include particle shape, hindered settling and turbulence (nonlinear drag, vortex trapping, fast tracking and effects of loitering), is delineated. The article puts into focus the impact of terminal fall velocity on fluvial hydraulics, discussing the salient role that the terminal fall velocity plays in governing the hydrodynamics of the sediment threshold, bedload transport and su...
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