Koen Jacques Ferdinand Blanckaert

Secondary flow in sharp open-channel bends

Secondary currents are a characteristic feature of flow in open-channel bends. Besides the classical helical motion (centre-region cell), a weaker and smaller counter-rotating circulation cell (outer-bank cell) is often observed near the outer bank, which is believed to play an important role in bank erosion processes. The mechanisms underlying the circulation cells, especially the outer-bank cell, are still poorly understood, and their numerical simulation still poses problems, not least due to lack of detailed experimental data. The research reported herein provides detailed experimental data on both circulation cells in an open-channel bend such as found in nature. Furthermore, the underlying dynamics are investigated by simultaneously analysing the vorticity equation and the kinetic energy transfer between the mean flow and the turbulence. This shows that turbulence plays a minor role in the generation of the centre-region cell, which is mainly due to the centrifugal force. By accounting for the feedback between the downstream velocity profile and the centre-region cell, a strongly simplified vorticity balance is shown to yield accurate predictions of the velocities in the centre region. For strong curvatures, however, a fully threedimensional flow description is required. Due to the non-monotonic velocity profiles, the centrifugal force favours the outer-bank cell. Moreover, terms related to the anisotropy of the cross-stream turbulence, induced by boundary proximity, are of the same order of magnitude and mainly enhance the outer-bank cell. Both mechanisms strengthen each other. The occurrence of the outer-bank cell is shown to be not just due to flow instability, like in the case of curved laminar flow, but also to kinetic energy input from turbulence.

Topographic steering, flow recirculation, velocity redistribution, and bed topography in sharp meander bends

The bed topography and associated flow field are investigated in a laboratory configuration with parameters that are representative for sharp natural meander bends. Zones of inward mass transport are characterized by a quasi-linear transverse bed profile, whereas zones of outward mass transport, induced by pronounced curvature variations, are characterized by a quasi-horizontal shallow point bar at the inside of the bend, a deep pool at the outside, and an increase in overall cross-sectional area. These quasi-bilinear bed profiles can be attributed to the curvature-induced secondary flow that is confined to the pool. Topographic steering, mainly due to mass conservation, concentrates the major part of the discharge over the deepest zones of the bend. But the pattern of depth-averaged velocities, which is relevant with respect to the development of the bed topography, does not show maximum values over the deepest zones. A term-by-term analysis of the depth-averaged streamwise momentum equation reveals that the water surface gradient is the principal mechanism with respect to flow velocity redistribution, although inertia and secondary flow are also processes of dominant order of magnitude. A required condition for the occurrence of adverse pressure gradients and flow recirculation due to planform curvature variations is established. A different type of flow recirculation, due to a subtle feedback between the flow and the bed topography, occurs over the point bar. The neglect of the influence of vertical velocities impinging on the bed in models for sediment transport is identified as a major shortcoming in the modeling of the morphodynamics of meandering river channels.

Means of noise reduction in acoustic turbulence measurements

Although three-receiver acoustic Doppler velocimeters (ADV) can accurately measure the three-dimensional mean flowfield, their turbulence measurements suffer from parasitical noise contributions. By adding a fourth receiver and optimizing the transducer configuration, the turbulence results can be considerably improved. Redundant information is obtained for all velocity components, which theoretically allows to achieve noise-free turbulence measurements. Experiments show that the parasitical noise contribution is not completely eliminated but reduced by an order of magnitude. At the same time, the useful low-noise frequency range is extended by one order of magnitude. Furthermore, the noise levels of the different components can be directly estimated from the redundant information, which allows to (i) check the quality of the measurements and the system; (ii) estimate the accuracy of the turbulence measurements; and (iii) optimally choose the measuring frequency. Good turbulence results with a four-receiver ADV require a sufficiently high acoustic scattering level of the fluid.A simple, low cost and non-polluting technique to enhance the acoustic scattering level by generating micro hydrogen bubbles in the flow is presented and its efficiency is demonstrated. The principles presented have general validity for 3DADV instruments, but are illustrated and validated by means of measurements with an acoustic Doppler velocity profiler (ADVP) developed at the EPF Lausanne

Saturation of curvature-induced secondary flow, energy losses, and turbulence in sharp open-channel bends: Laboratory experiments, analysis, and modeling

The paper investigates the influence of relative bend curvature on secondary flow, energy losses, and turbulence in sharp open-channel bends. These processes are important in natural streams with respect to sediment transport, the bathymetry and planimetry, mixing and spreading of pollutants, heat, oxygen, nutrients and biological species, and the conveyance capacity. Laboratory experiments were carried out in a configuration with rectangular cross section, consisting of a 193° bend of constant radius of curvature, preceded and followed by straight reaches. This somewhat unnatural configuration allows investigating the adaptation of mean flow and turbulence to curvature changes in open-channel bends, without contamination by other effects such as a mobile bed topography. Experiments were carried out for three different values of the curvature ratio, defined as the ratio of centerline radius of curvature over flow depth, which is the principal curvature parameter for hydrodynamic processes. Commonly used so-called linear models predict secondary flow to increase linearly with the curvature ratio. The reported experiments show that the secondary flow hardly increases in the investigated very sharp bends when the curvature ratio is further increased. This phenomenon is called saturation. Similar saturation is observed for the energy losses and the turbulence. This paper focuses on the analysis and modeling of the saturation of energy losses and turbulence. Secondary flow is found to be the dominant contribution to the curvature-induced increase in turbulence production, which leads to increased energy losses. The curvature-induced turbulence is explained by the fact that the turbulence dissipation lags behind the turbulence production, in agreement with the concept of the turbulence energy cascade. A 1-D model is proposed for the curvature-induced energy losses and turbulence. It could extend 1-D or depth-averaged 2-D models that are commonly used in long-term (scale of a flood event to geological scales) or large-scale (scale of a river basin) investigations on flood propagation, hazard mapping, water quality modeling, and planimetric river evolution.

Flow and bathymetry in sharp open-channel bends: Experiments and predictions

This paper focuses on experiments and simulations conducted in very sharp open-channel bends with flat and equilibrium bathymetry, corresponding to the initial and final phases of the erosion and deposition processes, respectively. The study of flow in curved open bends is relevant for flow in natural river configurations, as most river reaches are not straight. The configuration considered in the present work was designed as a test case in which the role of the cross-sectional flow is more severe than in meandering natural river reaches (radius of curvature of the channel is close to the channel width) and, thus, can serve for validation of numerical models used to predict flow and sediment transport in river engineering applications. This paper presents detailed new experimental data on the equilibrium bathymetry as well as depth-averaged distributions, vertical profiles, and cross-sectional patterns of the streamwise velocity, the cross-stream circulation, streamwise vorticity, and the turbulent kinetic energy at the initial and final stages of the erosion and deposition processes. The numerical simulations are performed using a three-dimensional nonhydrostatic RANS model for flow, sediment transport, and bathymetry, which employs fine meshes, accounts for the effect of small bed forms, and avoids the use of the law of the wall. The model predicts, rather accurately, the distribution of the streamwise velocity, the cross-stream circulation, and the turbulent kinetic energy in the simulations conducted with a fixed (flat and deformed bed corresponding to equilibrium conditions) prescribed bathymetry. In the case of a simulation conducted with loose bed, the model predicts satisfactorily the main features of the bathymetry at equilibrium conditions, despite the fact that including the interaction between the flow and the bathymetry increases the overall uncertainty in the model predictions. Results indicate that both improvements in the level of turbulence modeling and in the modeling of the sediment transport would allow further improvement in the predictive capabilities of morphodynamic models.

Meander dynamics: A nonlinear model without curvature restrictions for flow in open?channel bends

Despite the rapid evolution of computational power, simulation of meander dynamics by means of reduced and computationally less expensive models remains practically relevant for investigation of large?scale and long?term processes, probabilistic predictions, or rapid assessments. Existing meander models are invariantly based on the assumptions of mild curvature and slow curvature variations and fail to explain processes in the high?curvature range. This article proposes a nonlinear model for meander hydrodynamics without curvature restrictions. It provides the distribution of the main flow, the magnitude of the secondary flow, the direction of the bed shear stress, and the curvature?induced additional energy losses. It encompasses existing mild curvature models, remains valid for straight flow, and agrees satisfactorily with experimental data from laboratory experiments under conditions that are more demanding than sharp natural river bends. The proposed model reveals the mechanisms that drive the velocity redistribution in meander bends and their dependence on the rivers roughness Cf, the flow depth H, the radius of curvature R, the width B, and bathymetric variations. It identifies Cf?1H/R as the major control parameter for meander hydrodynamics in general and the relative curvature R/B for sharp curvature effects. Both parameters are small in mildly curved bends but O(1) in sharply curved bends, resulting in significant differences in the flow dynamics. Streamwise curvature variations are negligible in mildly curved bends, but they are the major mechanisms for velocity redistribution in sharp bends. Nonlinear feedback between the main and secondary flow also plays a dominant role in sharp bends: it increases energy losses and reduces the secondary flow, the transverse bed slope, and the velocity redistribution.

Flow and sediment dynamics in channel confluences

Confluences with relatively low discharge and momentum flux ratios where a small steep tributary with a high supply of poorly sorted sediment joins a large, low-gradient main channel commonly occur in nature, but they have not yet been investigated. Measurements of the three-dimensional velocity field, turbulence, sediment transport, bed material grain size and morphology are reported in a laboratory setting that is representative of confluences on the Upper Rhone River, Switzerland. The difference between the low-flow depth in the steep tributary and the higher flow depth in the main channel creates a marked bed discordance in the tributary zone. Due to this bed discordance, the tributary flow penetrates into the main channel mainly in the upper part of the water column, whereas the main-channel flow is hardly hindered by the tributary in the lower part of the water column, giving rise to a two-layer flow structure in the confluence zone. In confluences with high supply of coarse sediment from the tributary, the development of a deposition bar downstream from the confluence reduces the flow area and causes flow acceleration that contributes to an increase in sediment transport capacity. The sediment supplied by the tributary is mainly sorted and transported on the face of the bar by the near-bed flow originating from the main channel. The sediment transport capacity is further increased by the three-dimensionality of the flow, which is characterized by maximum velocities occurring near the bed, and by a considerable increase in turbulent kinetic energy generated in the shear layer at the interface of the flows originating from the main channel and the tributary. A conceptual model is proposed for the hydro-morpho-sedimentary processes, and compared to existing conceptual models for confluences with different characteristics.

Hydrodynamic processes in sharp meander bends and their morphological implications

The migration rate of sharp meander bends exhibits large variance and indicates that some sharply curved bends tend to stabilize. These observations remain unexplained. This paper examines three hydrodynamic processes in sharp bends with fixed banks and discusses their morphological implications: secondary flow saturation, outer-banks cells, and inner-bank flow separation. Predictions from a reduced-order hydrodynamic model show that nonlinear hydrodynamic interactions limit the growth of the secondary flow. This process is called the saturation of the secondary flow. For outer-bank cells and inner-bank flow separation, the analysis relies on experimental findings from flume studies in channels with fixed and mobile beds. The experiments reveal that outer-bank cells exist near steep as well as shelving banks and amplify with increasing steepness and roughness of the outer bank, and especially with increasing curvature. The effects of flow separation at the inner bank are found to be strongly conditioned by flow-sediment interactions, which lead to an increased scour depth near the outer bank and increased velocities near the toe of that bank. Overall the results suggest that secondary flow saturation and outer-bank cells tend to inhibit meander migration, whereas inner-bank separation may enhance migration. The relative importance of these three hydrodynamic processes depends on hydraulic, geometric, and sedimentologic conditions, which is consistent with the large variance in observed migration rates. The results suggest that large shallow rivers have the most dynamic meandering behavior, while the occurrence of stabilized meanders seems to be favored in narrow rivers.

Analysis of the role of turbulence in curved open-channel flow at different water depths by means of experiments, LES and RANS

In order to unravel the main flow and secondary flow characteristics and the role of turbulence in a curved single-bend open-channel flow, large-eddy simulations (LES) and Reynolds-averaged numerical simulations (RANS) were carried out and compared with experiments of the flow through a strongly bent laboratory flume. Turbulence was found to play an important role with respect to processes that are important in natural rivers. The strength of the curvature-induced secondary flow in the core of the flow domain, which is the most typical feature of curved open-channel flow, depends on the turbulence. Turbulence is especially important in the flow regions near the banks. Only the LES model is able to resolve accurately the boundary layer detachment and the formation of an internal shear layer at the inner bank as well as the outer-bank cell of secondary flow, whereas the RANS model is unable to reproduce these processes. Turbulence also conditions the magnitude of the bed shear stress, as indicated by the co...

Large-eddy simulation of a mildly curved open-channel flow

After validation with experimental data, large-eddy simulation (LES) is used to study in detail the open-channel flow through a curved flume. Based on the LES results, the present paper addresses four issues. Firstly, features of the complex bicellular pattern of the secondary flow, occurring in curved open-channel flows, and its origin are investigated. Secondly, the turbulence characteristics of the flow are studied in detail, incorporating the anisotropy of the turbulence stresses, as well as the distribution of the kinetic energy and the turbulent kinetic energy. Moreover, the implications of the pattern of the production of turbulent kinetic energy is discussed within this context. Thirdly, the distribution of the wall shear stresses at the bottom and sidewalls is computed. Fourthly, the effects of changes in the subgrid-scale model and the boundary conditions are investigated. It turns out that the counter-rotating secondary flow cell near the outer bank is a result of the complex interaction between the spatial distribution of turbulence stresses and centrifugal effects. Moreover, it is found that this outer bank cell forms a region of a local increase of turbulent kinetic energy and of its production. Furthermore, it is shown that the bed shear stresses are amplified in the bend. The distribution of the wall shear stresses is deformed throughout the bend due to curvature. Finally, it is shown that changes in the subgrid-scale model, as well as changes in the boundary conditions, have no strong effect on the results.