A study of the spatio-temporal behaviour of bed load transport rate fluctuations

This thesis addresses the problem of a statistical description of the transport of sediment as bed load. It highlights the role of fluctuations arising during the transport process, and their impact on macroscopic averages. The results presented here are based on four experimental studies. Two of them were published recently by Böhm et al. [2004] and Roseberry et al. [2012] while the other two were carried out during the thesis. In particular, two high speed cameras were used to automatically reconstruct particle trajectories over a window of approximately 1mlength, continuously over a few minutes. This constitutes, at the time of writing, one of the largest sets of experimental particle trajectory data available. Based on these experiments, two probabilistic models are proposed. The first one offers a macroscopic picture of the fluctuations of particle activity (concentration of moving particles). Based on a model recently proposed by Ancey et al. [2008], it allows for the accurate prediction of the fluctuations observed in the bed load flux. A new formula for the probability density function of the volume-averaged bed load flux is derived and compared to experimental data as well as to existing theory. By slightly modifying the original model, it was also possible to derive the probability density function of the inter-arrival time of particles. The latter shows an unusual bimodal shape due to the effect of collective entrainment. This phenomenon is referred to as the “separation of time scales” [Heyman et al., 2013]. Although providing an accurate picture of the macroscopic fluctuations arising in bed load transport, the first model does not gather information about the spatial behaviour of the bed load flux fluctuations. To remedy this, a new probabilistic model, able to locally describe the transport process, is proposed. This model lies in-between a kinetic description of the transport process and the macroscopic model proposed by Ancey et al. [2008]. In this regard, only the particle positions are treated as random variables, while particle velocities are assumed to be close to the Maxwellian equilibrium distribution. By a careful analysis of first and second moments, in both spatial and temporal dimensions, I prove the occurrence of large correlated structures that strongly perturb the average equilibrium, while not fundamentally modifying it. Moreover, I show that the validity of Taylor’s frozen flow hypothesis for bed load transport is severely called into question by the experimental data, proving the peculiar behaviour of those structures during transport by the mean fluid flow. Finally, a discussion about scales of fluctuations is provided. It follows from experimental results, as well as from theoretical predictions, that local fluctuations may play an important role in both an experimental setup and natural rivers. Indeed, the saturation and the correlation lengths are often so large that fluctuations may interact in a non-trivial way with the boundaries of the system, precluding the use of macroscopic average equations. Finally, this thesis suggests that the inherent fluctuations of bed load transport rates may need to be taken into account in numerical simulations in order to accurately describe the transport process

Mixing as a correlated aggregation process

Mixing describes the process by which scalars, such as solute concentration or fluid temperature, evolve from an initial heterogeneous state to uniformity under the stirring action of a fluid flow. Mixing occurs initially through the formation of scalar lamellae as a result of fluid stretching and later by their coalescence due to molecular diffusion. Owing to the linearity of the advection-diffusion equation, scalar coalescence can be envisioned as an aggregation process. While random aggregation models have been shown to capture scalar mixing across a range of turbulent flows, we demonstrate here that they are not accurate for most chaotic flows. In particular, we show that the spatial distribution of the number of lamellae in aggregates is highly correlated with their elongation and is also influenced by the fractal geometry that arises from the chaotic flow. The presence of correlations makes mixing less efficient than a completely random aggregation process because lamellae with similar elongations and scalar levels tend to remain isolated from each other. Based on these observations, we propose a correlated aggregation framework that captures the asymptotic mixing dynamics of chaotic flows and predicts the evolution of the scalar pdf based on the flow stretching statistics. We show that correlated aggregation is uniquely determined by a single exponent which quantifies the effective number of random aggregation events, and is dependent on the fractal dimension of the flow. These findings expand aggregation theories to a larger class of systems, which have relevance to various fundamental and applied mixing problems

From standing to breaking antidunes, a Hopf bifurcation.

Antidunes are bed morphologies often observed in steep slope mountain flows but also in small streams flowing on a sand beach. Linear stability analysis of the shallow water equations (SWE), when coupled to a sediment transport equation, predicts the growth of selected wavelength when the Froude number exceeds unity. If the bedform amplitude grows sufficiently (without being stabilized by nonlinear effects (Colombini and Stocchino (2008)), hydraulic jump can form on the lee side while a transcritical point, situated roughly on the crest of the dune, connects the sub and supercritical states. This has been described in the literature as cyclic steps. Their stationary travelling wave solution has already been theoretically and experimentally investigated (Balmforth and Vakil (2012); Taki and Parker (2005); Sun and Parker (2005)). In this talk, we provide numerical and theoretical evidence that a time dependent quasi-periodic solution of cyclic steps also exists in the SWE+Exner equations for a certain choice of the parameters. We compare this quasi-periodic phenomenon to breaking antidunes often observed in natural rivers

Channel meta-stability: effects on bedload transport.

Large fluctuations in the sediment transport rate are observed in rivers, particularly in mountain streams at intermediate flow rates (Ancey, 2006). These fluctuations seem to be, to some degree, correlated with the evolution of morphologies in the stream. Today the central question remains how to understand and acco unt for the strong bedload variability. In this presentation, new experimental results give some ideas about the link between channel bed evolution and intermittency in bedload transport

Experiments of flow separation at the inner bank of open-channel bends

In spite of its hydraulic and morphologic importance, the conditions of occurrence of flow separation at the inner bank of open channel bends are still not known. In a series of 23 experiments in a laboratory open channel bend with flat sand bed, it was investigated how the process of inner-bank flow separation depends on two control parameters: the ratio of flow depth H to minimum radius of curvature R, and the Froude number Fr. The Froude numbers investigated were 0.1, 0.2, 0.3, 0.4 and 0.5 and the H/R values investigated were 0.038, 0.053, 0.077, 0.112 and 0.153. All flow conditions concerned strongly curved subcritical flow. Flow did separate from the inner bank in all experiments, but recirculation zones with reversed velocities did not develop. Surprisingly, the flow separation did not show any dependence on Fr. The point of flow separation moved farther downstream in the bend with increasing H/R

Secondary flow in sharp open-channel bends: experiments and modelling

The dependence of curvature-induced secondary flow on the curvature ratio H/R and the Froude number Fr was systematically investigated in a series of 18 experiments in a sharply-curved laboratory flume. The investigated flow depths were 0.9 m, 0.13 m, 0.19 m and 0.26 m, resulting in H/R values of 0.053, 0.077, 0.112 and 0.153, and the Froude numbers were 0.1, 0.2, 0.3, 0.4 and 0.5. The normalized magnitude of the secondary flow did not increase with H/R, as predicted by commonly used parameterizations for secondary flow, but remained quasi-constant. This confirms observations by Blanckaert (2009), who called this phenomenon the saturation of the secondary flow. The experiments did not reveal any dependence of the secondary flow on Fr. Predictions of the magnitude of the secondary flow with the nonlinear model of Blanckaert and de Vriend ((2003, 2010) agreed very well with the experimental data

Improving the translation environment for professional translators

When using computer-aided translation systems in a typical, professional translation workflow, there are several stages at which there is room for improvement. The SCATE (Smart Computer-Aided Translation Environment) project investigated several of these aspects, both from a human-computer interaction point of view, as well as from a purely technological side. This paper describes the SCATE research with respect to improved fuzzy matching, parallel treebanks, the integration of translation memories with machine translation, quality estimation, terminology extraction from comparable texts, the use of speech recognition in the translation process, and human computer interaction and interface design for the professional translation environment. For each of these topics, we describe the experiments we performed and the conclusions drawn, providing an overview of the highlights of the entire SCATE project

Smart Computer-Aided Translation Environment (SCATE): Highlights

We present the highlights of the now finished 4-year SCATE project. It was completed in February 2018 and funded by the Flemish Government IWT-SBO, project No. 130041