The effect of migrating dune forms on the flow field of an alluvial river

The bed of an alluvial river is highly susceptible to changes during the course of its existence. Besides variations of the large scale topography and plan form of the river, smaller scale dune forms can be observed. These recurring dune forms migrate on top of the large scale topography and can yield local yet important variations in the flow field. In order to study the effect of migrating dune forms on the flow characteristics and consequently the erosive capacity of an alluvial river, an experiment with mobile bed has been carried out in a laboratory flume representing a sharp meander bend. In this experiment, changes to an initially flat, slightly sloped river bed under a steady flow and sediment discharge were observed until a recurring pattern of migrating dune forms could be seen on top of the characteristic pool-bar topography of meander bends. Once the dune forms were established, an Acoustic Doppler Velocity Profiler (ADVP) was placed in several positions alongside the river bend and used to measure the flow depth and flow characteristics under the influence of the passing dunes. Several times during the experiment, the topography was mapped using laser altimetry on a grid of large spatial resolution in order to isolate the dune forms from the large scale topography and determine the dune characteristics and the dune celerity. In this paper the large scale topography and dune characteristics will be shown and the effect of the migrating dune forms on the flow field and the erosive capacity will be discussed in detail

Flow and turbulence in sharp open-channel bends

The sustainable development of rivers requires a knowledge on the three-dimensional mean flow field and the turbulence in complex morphologies. In a future, the computational capacity will be sufficient to simulate numerically the fine details of the flow. Our physical knowledge, however, is at present insufficient: the overwhelming majority of experimental research concerns straight-uniform flow and even complex numerical models are based on straight-uniform-flow knowledge. A sound understanding of the relevant physical processes will always be essential in complicated problems such as the river management, which concern a variety of different fields, and this irrespective of the available computational capacity. This PhD investigates, mainly experimentally, the mean-flow field and the turbulence in open-channel bends; this situation is considered as a generic case for complex highly three-dimensional flow. The experimental investigation is rendered feasible by the availability of a powerful Acoustic Doppler Velocity Profiler (ADVP), developed in our laboratory. The principal objectives of this PhD are: To provide a high-quality data base on three-dimensional open-channel flow, including all three mean velocity components and all six Reynolds stresses on a fine grid. To document interesting features of the flow field and the turbulence, such as the multi-cellular pattern of secondary circulation, the curvature influence on the turbulence, etc. To gain insight in the relevant physical mechanisms and processes underlying these features. To apply the acquired knowledge in an engineering sense, mainly by evaluating, improving and developing numerical simulation techniques. First, a limited series of experiments was conducted in a small laboratory flume, with the aim of testing the feasibility of the project. Subsequently, extended series of experiments have been designed in a large and optimized laboratory flume. The small-flume experiments yielded results beyond all expectations and form the core of this dissertation. The large-flume experiment are intended to confirm those results and to investigate newly emerged questions. Only few large-flume results are included in this dissertation; more large-flume results will be reported in literature in the future.   The structure of this dissertation follows the above-mentioned objectives. In PART I "Instrumentation and experimental set-up", the experimental set-up, the ADVP and the measuring strategy are presented. Furthermore, a method is proposed to improve acoustic turbulence measurements. PART II "Experimental observations" provides high-quality data on the mean flow and the turbulence and documents the most interesting features: The downstream velocity increases in outward direction and its vertical profiles are flatter (increased/decreased velocities in the lower/upper part of the flow depth) than in straight flow. A relatively small and weak outer-bank cell of secondary circulation exists besides the classical center-region cell (helical motion). The turbulence activity is reduced in the outer half of the cross-section in the investigated bend, as compared to a straight-uniform flow. Linear models that are commonly used to account for the effect of the secondary circulation in depth-integrated flow models are inaccurate for moderately to strongly curved flows. PART III "Fundamental research" investigates the physical mechanisms and processes underlying these observations, mainly by making term-by-term evaluations of the relevant flow equations (momentum, vorticity, turbulent kinetic energy) and by considering the instantaneous flow behavior. The distribution of the downstream velocity is dominated by both cells of secondary circulation, whereby the outer-bank cell has a protective effect on the stability of the outer bank by keeping the core of maximum velocity at distance. The center-region cell is mainly generated by the vertical gradient of the centrifugal force, : the non-uniform outward centrifugal force and the nearly-uniform inward pressure gradient, due to the super-elevation of the water surface, are on the average in equilibrium; their local non-equilibrium, however, gives rise to the centerregion cell. There exists a strong negative feedback between the vertical profile of the downstream velocity, vs, and the center-region cell: the center-region cell flattens the vs-profiles, which on its turn leads to a reduction of and a weakening of the center-region cell. Linear models that are commonly used to account for the effect of the secondary circulation in depth-integrated flow models perform poorly because they neglect this feedback. Similar outer-bank cells exist in straight turbulent flow as well as in curved laminar flow. In straight turbulent flow, they are induced by the anisotropy of turbulence whereas they come into existence in curved laminar flow when the curvature exceeds a critical value: the vs-profiles flatten to such an extent that the gradient of the centrifugal force changes sign near the water surface,  < 0, provocating the generation of the outer-bank cell. In curved turbulent flow, both mechanisms have a comparable contribution to the generation of the outer-bank cell and strengthen each other, whence the outer-bank cell is stronger in a curved turbulent flow than in a curved laminar or a straight turbulent flow. The restitution of kinetic energy from the turbulence to the mean flow plays an important role in the generation of the outer-bank cell, and the deficiency of standard k-ε turbulence closures to accurately simulate them is due to their inherent incapability to account for such kinetic-energy restitution. The turbulence structure is fundamentally different than in a straight-uniform flow: for the same amount of turbulent kinetic energy, there is less shear in a curved flow. This change in turbulence structure is responsible for the observed reduced turbulence activity. An analysis of the instantaneous flow behavior suggests that the turbulence fluctuations can be decomposed into two fundamentally different parts: a wave-like oscillation of the pattern of circulation cells embedded in background turbulence. PART IV "Applied research" tries to apply the acquired knowledge in an engineering sense. It proposes a non-linear model to account for the effect of the secondary circulation in depth-integrated flow models, that simulates the negative feedback between the downstream velocity profile and the center-region cell. Contrary to the commonly used linear models, this non-linear model agrees well with experimental data for strongly curved flow from both the small and the large-flume experiments. The model depends on the curvature ratio, the friction factor and the spanwise distribution of the downstream velocity, which can all be incorporated in a newly defined bend parameter, that allows an objective definition of weak, moderate and strong curvature. The linear model is found as the asymptotic solution for vanishing curvature. An evaluation for natural rivers has shown that differences between the linear model and the non-linear model are relevant. Moreover, outer-bank cells have been successfully simulated by means of a non-linear k-ε turbulence closure. As mentioned before, the small-flume experiments yielded results beyond all expectations. As a side-effect, the analysis of the large-flume experiments could not be accomplished within this dissertation and the work that is presently in progress is briefly described in PART V "Work in progress". Finally, PART VI summarizes the main conclusions of this dissertation

The young Van Dyck’s fingerprint : a technical approach to assess the authenticity of a disputed painting

The painting Saint Jerome, part of the collection of the Maagdenhuis Museum (Antwerp, Belgium), is attributed to the young Anthony van Dyck (1613–1621) with reservations. The painting displays remarkable compositional and iconographic similarities with two early Van Dyck works (1618–1620) now in Museum Boijmans van Beuningen (Rotterdam) and Nationalmuseum (Stockholm). Despite these similarities, previous art historical research did not result in a clear attribution to this master. In this study, the work’s authenticity as a young Van Dyck painting was assessed from a technical perspective by employing a twofold approach. First, technical information on Van Dyck’s materials and techniques, here identified as his fingerprint, were defined based on a literature review. Second, the materials and techniques of the questioned Saint Jerome painting were characterized by using complementary imaging techniques: infrared reflectography, X-ray radiography and macro X-ray fluorescence scanning. The insights from this non-invasive research were supplemented with analysis of a limited number of cross-sections by means of field emission scanning electron microscopy coupled with energy dispersive X-ray spectroscopy. The results demonstrated that the questioned painting’s materials and techniques deviate from Van Dyck’s fingerprint, thus making the authorship of this master very unlikely

Experimental study on a widening tributary channel and its influence on the confluence morphology

River morphodynamics and sediment transportRiver morphology and morphodynamic

Validation of a non-linear reduced hydrodynamic model for curved open-channel flow

River morphodynamics and sediment transportRiver morphology and morphodynamic

Scalar dispersion in strongly curved open-channel flows

River hydrodynamicsTurbulent open channel flow and transport phenomen

The effect of migrating dune forms on the flow field of an alluviral river

The bed of an alluvial river is highly susceptible to changes during the course of its existence. Besides variations of the large scale topography and plan form of the river, smaller scale dune forms can be observed. These recurring dune forms migrate on top of the large scale topography and can yield local yet important variations in the flow field. In order to study the effect of migrating dune forms on the flow characteristics and consequently the erosive capacity of an alluvial river, an experiment with mobile bed has been carried out in a laboratory flume representing a sharp meander bend. In this experiment, changes to an initially flat, slightly sloped river bed under a steady flow and sediment discharge were observed until a recurring pattern of migrating dune forms could be seen on top of the characteristic pool-bar topography of meander bends. Once the dune forms were established, an Acoustic Doppler Velocity Profiler (ADVP) was placed in several positions alongside the river bend and used to measure the flow depth and flow characteristics under the influence of the passing dunes. Several times during the experiment, the topography was mapped using laser altimetry on a grid of large spatial resolution in order to isolate the dune forms from the large scale topography and determine the dune characteristics and the dune celerity. In this paper the large scale topography and dune characteristics will be shown and the effect of the migrating dune forms on the flow field and the erosive capacity will be discussed in detail

Flow patterns induced by a bubble screen in a sharply curved flume based on Acoustic Doppler Velocity Profiler measurements

Open-channel bends are characterized by a bed morphology where erosion occurs near the outer bank and a point bar develops at the inner bend. This morphology is induced by complex interactions between the streamwise flow, the curvature-induced secondary flow and the bed topography. Several techniques already exist to counteract the development of bend scour, which can endanger foundations of structures. However, existing techniques mostly imply substantial construction works in the river. Preliminary laboratory experiments have shown that a porous tube placed near the outer bank can generate a bubble screen that modifies the flow patterns and leads to a substantial reduction of the bend scour. A better understanding of the hydrodynamic mechanisms induced by the bubble screen and involved in the morphological development will allow determining the range of application of the bubble-screen technique. Experiments performed in a sharply curved open-channel bend under live-bed conditions show that the bubble screen is able to redistribute the velocity patterns in the bend. The bed morphology is then partially modified, specially at the bend exit. This paper illustrates Acoustic Doppler Velocity Profiler measurements of the bubble-induced secondary flow and its interaction with the channel base flow in the most influenced cross-section

Influencing bend morphodynamics by means of an air-bubble screen-Topography and velocity field

Interactions between curvature-induced secondary flow, streamwise flow and bed morphology in open-channel bends lead to the development of a typical bar-pool bed morphology, shaped by scour at the outer bank and deposition at the inner bank. This typical bar-pool bed morphology may endanger the stability of the outer bank or reduce the navigable width of the channel. Preliminary laboratory experiments in a sharply curved channel with a fixed horizontal bottom (Blanckaert et al. 2008) have shown that a bubble screen located near the outer bank can generate secondary flow with a sense of rotation opposite to the curvature-induced secondary flow. The reported study investigates the application of bubble screens in configurations with mobile-bed morphology under live-bed and clear-water scour conditions. Velocity measurements show that the bubble-induced secondary flow decreases the strength of the curvature-induced secondary flow, and shifts it in inwards direction. Maximum scour occurs where the curvature-induced and bubble-induced secondary flow cells meet. At this same location, the maximum streamwise velocities and maximum vertical velocities impinging on the bed occur, which indicates their importance with respect to the formation of bend scour. The application of the bubble screen resulted in a reduction of the maximum bend scour by about 50%. Moreover, the location of maximum scour is shifted away from the outer bank and does not endanger its stability anymore. These preliminary experiments show the potential of a bubble screen to influence and modify the bed morphology