Bedforms in a turbulent stream: ripples, chevrons and antidunes

Sedimentology, 2014

The development of bedforms under unidirectional, oscillatory and combined-flows results from temporal changes in sediment transport, flow and morphological response. In such flows, the bedform characteristics (for example, height, wavelength and shape) change over time, from their initiation to equilibrium with the imposed conditions, even if the flow conditions remain unchanged. These variations in bedform morphology during development are reflected in the sedimentary structures preserved in the rock record. Hence, understanding the time and morphological development in which bedforms evolve to an equilibrium stage is critical for informed reconstruction of the ancient sedimentary record. This article presents results from a laboratory flume study on bedform development and equilibrium development time conducted under purely unidirectional, purely oscillatory and combined-flow conditions, which aimed to test and extend an empirical model developed in past work solely for unidirectional ripples. The present results yield a unified model for bedform development and equilibrium under unidirectional, oscillatory and combined-flows. The experimental results show that the processes of bedform genesis and growth are common to all types of flows, and can be characterized into four stages: (i) incipient bedforms; (ii) growing bedforms; (iii) stabilizing bedforms; and (iv) fully developed bedforms. Furthermore, the development path of bedform; growth exhibits the same general trend for different flow types (for example, unidirectional, oscillatory and combined-flows), bedform size (for example, small versus large ripples), bedform shape (for example, symmetrical or rounded), bedform planform geometry (for example, two-dimensional versus three-dimensional), flow velocities and sediment grain sizes. The equilibrium time for a wide range of bed configurations was determined and found to be inversely proportional to the sediment transport flux occurring for that flow condition.

Water Resources Research, 2009

1] New PIV-based experiments show that the nascent seed waves from which both ripples and dunes develop are generated on planar mobile sediment beds in a two-stage process. The first stage comprises the motion of random sediment patches that reflect the passage of sediment-transport events caused by attached eddies. These eddy-transport events propagate at speeds that are proportional to their size and less than overhead eddy convection velocities, but potentially larger than local average fluid and sediment velocities. In the second stage, interactions of the moving patches result in a bed disturbance that exceeds a critical height and interrupts the bed-load layer. Quasi-regular seed waves are then generated successively downstream of this stabilised growing disturbance via a scour-deposition wave that arises from the requirement of sediment mass conservation and the sediment-transport and bed-stress distributions downstream of a bed perturbation. Seed waves are thereby of preferred lengths that scale with the grain size, i.e. length = O(130) grain diameters, agreeing with compiled measurements. This two-stage generation mechanism is valid for fully-turbulent hydraulically-smooth and rough-bed flows of small to large sediment transport rates. It is furthermore valid for laminar flows, although the critical disturbances leading to seed-wave generation arise through bed discontinuities, and not eddy-based sediment-transport events. The identified generation mechanism, which accounts for turbulence effects, explains the observed similar scaling of alluvial, closed-conduit and lightweight-sediment seed waves. The present measurements highlight further aspects of the flow dynamics preceding seed-wave generation, including: decreases in von Kármán's constant due to bed mobility, near-bed eddy convection speeds in excess of local double-averaged (in time and space) streamwise velocities, and the validity of the four-range spectral scaling model for open-channel flows proposed earlier by Nikora.

Earth Surface Processes and Landforms, 2020

Fluvial bedforms generate a turbulent wake that can impact suspended-sediment settling in the passing flow. This impact has implications for local suspended-sediment transport, bedform stability, and channel evolution; however, it is typically not well-considered in geomorphologic models. Our study uses a three-dimensional OpenFOAM hydrodynamic and particle-tracking model to investigate how turbulence generated from bedforms and the channel bed influences medium sand-sized particle settling, in terms of the distribution of suspended particles within the flow field and particle-settling velocities. The model resolved the effect of an engineered bedform, which altered the flow field in a manner similar to a natural dune. The modelling scenarios alternated bed morphology and the simulation of turbulence, using detached eddy simulation (DES), to differentiate the influence of bedform-generated turbulence relative to that of turbulence generated from the channel bed. The bedform generated a turbulent wake that was composed of eddies with significant anisotropic properties. The eddies and, to a lesser degree, turbulence arising from velocity shear at the bed substantially reduced settling velocities relative to the settling velocities predicted in the absence of turbulence. The eddies tended to advect sediment particles in their primary direction, diffuse particles throughout the flow column, and reduced settling likely due to production of a positively skewed vertical-velocity fluctuation distribution. Study results suggest that the bedform wake has a significant impact on particle-settling behaviour (up to a 50% reduction in settling velocity) at a scale capable of modulating local suspended transport rates and bedform dynamics.

Journal of Hydraulic Research, 2009

The objective of this research is to examine the turbulent flow characteristics over artificial waveform structures and to compare turbulence between two types of isolated geometries. Measurement of turbulent velocity components have been performed over the respective structures of equal crest height and length at the Fluvial Mechanics Laboratory (FML) of the Indian Statistical Institute, Calcutta. The velocity data were analyzed to determine the relative importance of mean flows, Reynolds stresses and the contributions of burst-sweep cycles. The difference in the sizes of the separation zone for these two cases makes significant differences in the mean flow and turbulence. The motivation of this study is to identify the spatial changes of flow and turbulent events over these structures, and to gain better insight in the flow physics, which are different conditions for the process of sediment transport. RÉSUMÉ L'objectif de cette recherche est d'examiner les caractéristiques d'un écoulement turbulent sur des structures artificielles ondulées et de comparer la turbulence de deux types de géométries isolées. Sur les structures respectives de taille de crête et de longueur égales, les composantes turbulentes de vitesse ont été mesurées au Laboratoire Fluvial de Mécanique (FML) de l'Institut Statistique Indien, à Calcutta. Les données de vitesse ont été analysées pour déterminer l'importance relative des écoulements moyens, les contraintes de Reynolds et les contributions des cycles de bouffées. La différence de taille des zones de séparation dans ces deux cas induit des différences importantes sur l'écoulement moyen et la turbulence. La motivation de cette étude est d'identifier, par une meilleure compréhension de la physique, les changements spatiaux, au-dessus de ces structures, de l'écoulement et de la turbulence, qui conditionnent les processus de transport de sédiment.

Detailed laser-Doppler velocity and Reynolds stress measurements over fixed two-dimensional bed forms are used to investigate the coupling between the mean flow and turbulence and to examine effects that play a role in producing the bed form instability and finite amplitude stability. The coupling between the mean flow and the turbulence is explored in both a spatially averaged sense, by determining the structure of spatially averaged velocity and Reynolds stress profiles, and a local sense, through computation of eddy viscosities and length scales. The measurements show that there is significant interaction between the internal boundary layer and the overlying wake turbulence produced by separation at the bed form crest. The interaction produces relatively low correlation coefficients in the internal boundary layer, which suggests that using local bottom stress to predict bed load flux may not only be erroneous, it may also disregard the essence of the bed form instability mechanism. The measurements also indicate that topographically induced acceleration over the bed form stoss slope has a more significant effect in damping the turbulence over bed forms than was previously supposed, which is hypothesized to play a role in the stabilization of fully developed bed forms. INTRODUCTION A comprehensive understanding of the fluid dynamics of flows over nonuniform boundaries is of fundamental importance for an extensive array of problems in science and engineering. This problem is examined herein from an experimental standpoint for bed geometries that are typical of shear-erodible sediment beds. Observations and theoretical considerations indicate that if a flat sediment bed is acted upon by a turbulent flow capable of producing significant movement of the sediment grains making up the bed, that bed will be unstable to perturbations and will evolve into a train of bed forms (i.e., ripples and/or dunes). These features typically have relatively gentle upstream (stoss) slopes and steep leeside slopes that produce flow separation. In this special case of the more general problem of turbulent flow over nonuniform beds the fully developed flow and bed characteristics result from complex interactions between the flow field, the bed geometry, and the transport of sediment over the bed. The bed forms are created and altered by the flow and, conversely, the flow is acted upon by the bed forms through the production of form drag and significant changes in the local mean flow and turbulence fields. The consequence of these strong interactions is that neither the local nor the spatially averaged flow can be predicted without precise knowledge of the bed geometry, which is in turn dependent on the detailed characteristics of the near-bed flow field. Current understanding of the flow-bed coupling is insufficient to produce physically based predictive models for bed form generation and stability under arbitrary flow conditions. This presents a substantial barrier to progress in several areas of geophysics and hydraulics related to problems of significant environmental concern. For example, a precise understanding of the erodible-bed problem described above is a central part of making accurate sediment flux predictions and for determining stage-discharge relations in many rivers. in addition, paleoflood estimates and pa!eoen-vironmental interpretation based on the geologic record are critically dependent on the relations between bed form geometry and flow characteristics. At present, empirical techniques are used to address these problems and, although these techniques often provide good results over the range of flow and bed form parameters for which they were developed , they typically yield poor results when extrapolated to situations other than those for which they are specifically calibrated. Because one of the primary goals in developing relations for roughness, sediment transport, and bed geometry is to apply those relations to extreme events or other situations where flow and sediment transport data are difficult or impossible to measure, these empirical methods are inadequate in many cases. Furthermore, these techniques are ultimately unsatisfying in that they often circumvent the development of fundamental understanding of the general problem. To develop predictive models for the flow field, sediment transport, and bed geometry, an understanding of the mechanics of the various interactions between the flow and the bed is required. The first element in a comprehensive investigation of the bed form problem is an examination of the fluid dynamics of flow over shapes typical of well-developed bed forms. However , because of the spatial variability in local flow structure, highly detailed measurements of both mean and turbulence quantities are required to describe a flow field of this nature with sufficient accuracy to draw conclusions about local and spatially averaged momentum balances. An experiment specifically designed to provide the necessary degree of accuracy is described herein, and the results of that experiment are discussed for the case of low Froude number flow over 3935 3936 NELSON ET AL.: FLOW AND TURBULENCE OVER BED FO • S two-dimensional bed forms. The principal goals of this investigation are (1) to acquire and use experimental information to develop qualitative insight into the fluid dynamics of these flows and (2) to provide a high-quality data set that can be used to test the applicability of various computational models for flow over bed forms, especially with regard to turbulence closures. In this paper, relatively well-known fluid mechanical concepts are used in conjunction with detailed laboratory measurements of mean velocity and turbulence fields to address the first of these two goals. The measurements and interpretation provide a clearer picture of the local and spatially averaged flow field over bed forms and have significant implications concerning the interaction between the flow and the erodible bed, especially in regard to the initial bed form instability and finite amplitude stability. BACKGROUND

Physics of Fluids, 2014

Absolute and convective instability of cylindrical Couette flow with axial and radial flows Absolute and convective instabilities in a one-dimensional Brusselator flow model River dunes and antidunes are induced by the morphological instability of streamsediment boundary. Such bedforms raise a number of subtle theoretical questions and are crucial for many engineering and environmental problems. Despite their importance, the absolute/convective nature of the instability has never been addressed. The present work fills this gap as we demonstrate, by the cusp map method, that dune instability is convective for all values of the physical control parameters, while the antidune instability exhibits both behaviors. These theoretical predictions explain some previous experimental and numerical observations and are important to correctly plan flume experiments, numerical simulations, paleo-hydraulic reconstructions, and river works. C 2014 AIP Publishing LLC. [http://dx.

Earth Surface Processes and Landforms, 2011

Laboratory observations and computational results for the response of bedform fields to rapid variations in discharge are compared and discussed. The simple case considered here begins with a relatively low discharge over a flat bed on which bedforms are initiated, followed by a short period with double the original discharge during which the morphology of the bedforms adjusts, followed in turn by a relatively long period of the original low discharge. For the grain size and hydraulic conditions selected, the Froude number remains subcritical during the experiment, and sediment moves predominantly as bedload. Observations show rapid development of quasi-two-dimensional bedforms during the initial period of low flow with increasing wavelength and height over the initial low-flow period. When the flow increases, the bedforms rapidly increase in wavelength and height, as expected from other empirical results. When the flow decreases back to the original discharge, the height of the bedforms decreases in response rapidly, but the wavelength decreases much more slowly. Computational results for the same conditions simulate the formation and initial growth of the bedforms fairly accurately, and also predict an increase in dimensions during the high-flow period. However, the computational model predicts a much slower rate of wavelength increase, and performs less accurately during the final low-flow period, where the wavelength remains essentially constant, rather than decreasing. In addition, the numerical results show less variability in bedform wavelength and height than the measured values. Based on observations, these discrepancies may result from the simplified model for sediment particle step lengths used in the computational approach. Assuming a constant value for the step length neglects the role of flow alterations in the bedload sediment-transport process, which appears to result in predicted bedform wavelength changes smaller than those observed.

The present paper describes some results of experimental work carried out in a rectilinear flume to investigate the interactions between flow turbulence and sediment motion. The analysis is conducted through the comparison between the periodicity of turbulent structures evolving along the channel and the wavelength of bed forms (alternate bars) forming on the bed. The occurrence of turbulent events (inward interaction, ejection, sweep and burst) has been verified through the conditioned quadrant analysis. The quantitative information about the spatial and temporal scale of the events has been obtained through the space-time correlations of the conditioned data. The analysis essentially highlights that the periodicity of evolution of the turbulent structures compares well with the wavelength of bars found on the bed.

The present paper explores the turbulent flow characteristics and overall drag over the trough region of a pair of adjacent 2-D forward-facing dune-shaped artificial structures with two different stoss-side slopes and aims to make a comparative study. Structures considered here were of equal base length (λ) with a common gentle slope of 6° at the downstream face. The stoss-side angles were respectively 50° and 90°. Experiments were conducted at the Fluvial Mechanics Laboratory (FML) of Indian Statistical Institute (ISI), Kolkata. The velocity data were collected using a 3-D Micro-Acoustic Doppler Velocimeter (ADV) at the flume centerline to analyze the mean flows, Reynolds stresses and the overall drag at a Reynolds number, Re_h ≈ 1.44 × 10^5. One-dimensional profiles of turbulent flow parameters demonstrate that the flow separation bubble and a thin perturbed shear layer between the negative velocity region and outer layer with high velocity are the main sources of turbulence production. A change in turbulence characteristics between the two crest positions is identified. A greater amount of flow resistance is observed in the case of the structure having higher stoss-side slope. The spectral analysis reveals that peak power spectral density generally occurs at 0.25–1 Hz for the stream-wise velocity component and at 0.9–3.0 Hz for the cross-stream and vertical velocity components. Power spectra showed better defined peaks near the shear layer in the separation cell where it reaches to maximum which is two to three times greater than its values in the upstream and further downstream region. Strouhal numbers are calculated using the frequencies of eddies and vortices with different sets of characteristic length and velocity scales; and compared with the previous results of other researchers.

Journal of Geophysical Research, 1970

A similarity in morphology and sequence of development seems to exist between some common sedimentary structures and flow configurations encountered in transition and generation of turbulence. The question arises whether such flow instability patterns determine the embryonic configurations of bed deformation, which subsequently develop and grow, thus triggering the appearance of finite size natural bed forms.