Bruno Simon - Academia.edu (original) (raw)

Papers by Bruno Simon

Physics of Fluids, Sep 30, 2023

The aim of this work is to provide convincing evidence on the turbulent processes induced by thre... more The aim of this work is to provide convincing evidence on the turbulent processes induced by three-dimensional (3D) bores, based on physical and computational fluid dynamics studies of undular tidal bores, a phenomenon very similar to a tsunami-like bore propagating inland along a river. The numerical study is performed by solving the Navier-Stokes equations with a large eddy simulation method in order to access the turbulent flow evolution during the bore passage. Two-and three-dimensional simulations are performed with and without turbulence before bore generations to inspect the effect of coherent structures on the bore propagation. A complex three-dimensional flow takes place during the bore passage. Beneath the undulation crests, a strong shear is observed near the channel bed. Moreover, ejection of turbulent structures occurs during the propagation of undular bores depending on the initial flow conditions. These simulations provide the first detailed three-dimensional data of undular bores intricate flow structure. The results showed that the propagation of the bore front drastically changes the properties of the water column. It is also highlighted that for an upstream current exceeding a threshold value, near-bed eddies are generated and ejected in the water column independently of the free surface characteristics. Our simulations improve the understanding of positive surges which could be extended to tsunami-like bores studies.

HAL (Le Centre pour la Communication Scientifique Directe), Aug 26, 2013

Un mascaret est un type particulier d'onde positive présent dans de nombreux estuaires. La simula... more Un mascaret est un type particulier d'onde positive présent dans de nombreux estuaires. La simulation numérique d'ondes positives ondulées permet de décrire en détail l'écoulement du phénomène. La résolution des équations de Navier-Stokes incluant un modèle de turbulence pour des écoulements diphasique donne accès à la description complète du phénomène. La simulation est initialisée à partir de données physiques simplifiées et les résultats sont comparés, avec succès, à des données de laboratoire. Les vitesses sous l'onde positive ondulée montrent des inversions et des recirculations jamais observées.

La modelisation de la dissipation d'energie due au deferlement en eau peu profonde dans un co... more La modelisation de la dissipation d'energie due au deferlement en eau peu profonde dans un code de vagues completement non-lineaire et dispersif est etudiee. Le modele de propagation des vagues est fonde sur la theorie potentielle et suppose au depart que la surface libre ne se retourne pas. L'inclusion de la dissipation par deferlement est toutefois possible en ajoutant un terme similaire a une pression dans la condition limite dynamique de surface libre. Deux criteres issus de la litterature sont testes pour determiner le debut de deferlement, l'un de type geometrique et l'autre de type energetique. Deux methodes sont testees afin de dissiper l'energie, l'une basee sur la similarite d'une vague deferlante avec un ressaut hydraulique deferlant et l'autre utilisant un terme dissipatif de type viscosite turbulente. Les simulations numeriques sont effectuees a l'aide de combinaisons des deux criteres et des deux methodes de dissipation. Leurs result...

Journal of Ocean Engineering and Marine Energy, 2019

The modeling of wave breaking dissipation in coastal areas is investigated with a fully nonlinear... more The modeling of wave breaking dissipation in coastal areas is investigated with a fully nonlinear and dispersive wave model. The wave propagation model is based on potential flow theory, which initially assumes nonoverturning waves. Including the impacts of wave breaking dissipation is however possible by implementing a wave breaking initiation criterion and dissipation mechanism. Three criteria from the literature, including a geometric, kinematic, and dynamic-type criterion, are tested to determine the optimal criterion predicting the onset of wave breaking. Three wave breaking energy dissipation methods are also tested: the first two are based on the analogy of a breaking wave with a hydraulic jump, and the third one applies an eddy viscosity dissipative term. Numerical simulations are performed using combinations of the three breaking onset criteria and three dissipation methods. The simulation results are compared to observations from four laboratory experiments of regular and irregular waves breaking over a submerged bar, irregular waves breaking on a beach, and irregular waves breaking over a submerged slope. The different breaking approaches provide similar results after proper calibration. The wave transformation observed in the experiments is reproduced well, with better results for the case of regular waves than irregular waves. Moreover, the wave statistics and wave spectra are predicted well in general, and in particular for regular waves. Some differences are observed for irregular wave cases, in particular in the low-frequency range. This is attributed to incomplete absorption of the long waves in the numerical model. Otherwise, the wave spectra in the range [0.5fp, 5fp] are reproduced well, before, inside, and after the breaking zone for the three irregular wave experiments.

Coastal Engineering, 2019

Two methods to treat wave breaking in the framework of the Hamiltonian formulation of free-surfac... more Two methods to treat wave breaking in the framework of the Hamiltonian formulation of free-surface potential flow are presented, tested, and validated. The first is an extension of Kennedy et al. (2000)'s eddyviscosity approach originally developed for Boussinesq-type wave models. In this approach, an extra term, constructed to conserve the horizontal momentum for waves propagating over a flat bottom, is added in the dynamic free-surface condition. In the second method, a pressure distribution is introduced at the free surface that dissipates wave energy by analogy to a hydraulic jump (Guignard and Grilli, 2001). The modified Hamiltonian systems are implemented using the Hamiltonian Coupled-Mode Theory, in which the velocity potential is represented by a rapidly convergent vertical series expansion. Wave energy dissipation and conservation of horizontal momentum are verified numerically. Comparisons with experimental measurements are presented for the propagation of a breaking dispersive shock wave following a dam break, and then incident regular waves breaking on a mildly sloping beach and over a submerged bar.

Coastal Engineering Proceedings, 2018

Nearshore wave modeling over spatial scales of several kilometers requires balancing the fine-sca... more Nearshore wave modeling over spatial scales of several kilometers requires balancing the fine-scale modeling of physical processes with the model’s accuracy and efficiency. In this work, a fully nonlinear potential flow model is proposed as a compromise between simplified linear, weakly nonlinear or weakly dispersive models and direct CFD approaches. The core of present approach is the use of a series representation for the velocity potential. This series contains prescribed vertical functions and allows the determination of the velocity potential in terms of unknown horizontal functions. The resulting dimensionally reduced model retains the structure of the Hamiltonian water wave system Zakharov (1968), Craig & Sulem (1993), avoiding the solution of the Laplace problem for the potential. Instead, a numerically convenient linear system of horizontal equations needs to be solved at each step in the temporal evolution. No simplifications concerning the deformation of the physical boun...

International Journal of Sediment Research, 2018

Tidal bores may appear in some estuaries when the tides quickly reach a high level. This phenomen... more Tidal bores may appear in some estuaries when the tides quickly reach a high level. This phenomenon is rare but has a strong impact during its short duration: i.e. the river bed is significantly eroded and sediments are then transported. In this paper, the trajectories of suspended particles induced by this flow are numerically studied. Four undular bores with Froude numbers between 1.1 and 1.2 are studied. Despite similar Froude numbers, various initial flow conditions were selected to produce or not an inversion of the flow direction during the bore passage. The particle trajectories associated with each distinct flow configuration are presented and analyzed. These trajectories, estimated by solving the Maxey-Riley equation, appear to be very different even though the Froude numbers of flows are similar. These observations are important because the Froude number is often used to characterize a tidal bore as it describes well the free surface, however, it cannot describe the sediment transport. Finally, Chen's model of wave-current interactions is adapted to fit the cases studied and is applied to the four bores simulated. The results highlight that this latter model can reproduce the observed trajectories and dissociate their different components. From this model, it is shown that the inertial and Basset history effects can be neglected compared to the gravity and flow entrainment effects due to the viscous drag when one wants to determine the long-term trajectories of suspended particles.

A sudden rise of the water depth in an open channel flow creates a surge propagating in the chann... more A sudden rise of the water depth in an open channel flow creates a surge propagating in the channel called a positive surge. Positive surges can be observed as natural phenomena when a spring tide enters a funnel-shaped estuary under appropriate tidal and bathymetric conditions: the process is called a tidal bore. In this study, the free surface profile and the unsteady turbulent motion of positive surges were studied physically in a relatively large facility under controlled conditions based upon a Froude similarity. The experiments were conducted in a 12 m long 0.5 m wide rectangular channel mostly covered by a rough fixed gravel bed. The metrology included a combination of acoustic Doppler velocimeters (ADVs) and acoustic displacement meters (ADMs). The initially steady flow conditions were controlled by the water discharge, and the steady flow properties were investigated thorougly using a Pitot tube and ADV. The positive surge was generated by the fast closure of a downstream gate and the bore propagated upstream against the initially steady flow. Both free-surface and velocity measurements were repeated a number of times to perform an ensemble average. The free surface characteristics were studied for both undular and breaking bores. The passage of the bore was associated with large free surface fluctuations, particularly in the case of breaking bores. The free surface properties were in agreement with earlier findings. The bore shape was closely linked with the Froude number and found to be independent of the distance travelled by the bore. High frequency (200 Hz) instantaneous velocity measurements were conducted 6.13 m upstream of the downstream gate. In the breaking bore, the data showed a transient longitudinal velocity reversal next to the bed beneath and immediately behind the bore front. In the undular bore, a deceleration of the streamwise velocity was observed without velocity reversal of the ensemble averaged velocity, but with transient negative values of the instantaneous longitudinal velocity. For both breaking and undular bores, the vertical velocity data trend followed closely the time derivative of the instantaneous water depth. Large velocity fluctuations were observed during the passage of the bore and in its wake. The turbulent integral length scales of the longitudinal and transverse velocity components were similar in magnitude in steady and unsteady flows, with slightly larger values in the unsteady flow on the upper measurements (z/d o = 0.63). The turbulent integral time scales were larger in the unsteady flow. Further velocity measurements were performed immediately upstream of the gate and provided some insights into the time-variation of the velocity field during the bore generation. The turbulent stresses were larger beneath the bore close to the bed. After the bore front passage, the strongest fluctuations in terms of turbulent stresses were observed on the mid-water column and under the wave crests. Overall this study covered a broad range of unsteady flow conditions which were thoroughly investigated.

près des parois et amplifier par les ondulations pourrait induire un affouillement du fond et des... more près des parois et amplifier par les ondulations pourrait induire un affouillement du fond et des berges lors du passage des ondes. Les résultats obtenus sont un pas vers une meilleure compréhension des processus prenant place durant les mascarets et pourrait, par exemple, être utilisé pour suivre le morphodynamisme des estuaires soumis au mascaret. Une meilleure compréhension des mascarets pourrait améliorer la gestion de l'environnent estuarien. Par exemple, les opérations de dragage et les constructions (digue, pont, ponton) pourraient être réalisées à des emplacements qui amélioreraient la sécurité pour le trafic fluvial tout en préservant le phénomène, contrairement au mascaret de la Seine qui a presque disparu après des travaux d'emménagement de l'estuaire. Le résultat obtenu sur la turbulence et le mélange observé pourraient être utilisés pour aider à la préservation d'espèces qui dépendent de la turbulence générée par le mascaret pour se nourrir et se reproduire.

34th IAHR World Congress, …, 2011

Estuarine, Coastal and Shelf Science, 2011

The study details new sediment concentration measurements associated with some turbulence charact... more The study details new sediment concentration measurements associated with some turbulence characterisation conducted at high frequency in the undular tidal bore of the Garonne River (France). Acoustic Doppler velocimetry was used, and the suspended sediment concentration was deduced from the acoustic backscatter intensity. The field data set demonstrated some unique flow features of the tidal bore including some large and rapid turbulent velocity fluctuations during and after the bore passage. Some unusually high suspended sediment concentration was observed about 100 s after the tidal bore front lasting for more than 10 minutes. It is thought that the tidal bore passage scoured the bed and convected upwards the bed material, reaching the free-surface after the bore passage. Behind the tidal bore, the net sediment flux magnitude was 30 times larger than the ebb tide net flux and directed upstream. A striking feature of the data set was the intense mixing and suspended sediment motion during the tidal bore and following flood tide. This feature has been rarely documented.

Coastal Engineering Journal, 2018

A tidal bore is a natural estuarine phenomenon forming a positive surge in a funnel shaped river ... more A tidal bore is a natural estuarine phenomenon forming a positive surge in a funnel shaped river mouth during the early flood tide under spring tide conditions and low freshwater levels. The bore propagates upstream into the lower estuarine zone and its passage may induce some enhanced turbulent mixing, with upstream advection of suspended material. Herein the flow field and turbulence characteristics of tidal bores were measured using both numerical (CFD) and physical modelling. This joint modelling approach, combined with some theoretical knowledge, led to some new understanding of turbulent velocity field, turbulent mixing process, Reynolds stress tensor, and tidal bore hydrodynamics. The numerical CFD tool Thétis was used herein. Thétis is a CFD model using the volume of fluid technique (VOF) to model the free-surface and Large Eddy Simulation technique (LES) for the turbulence modelling. Physical data sets were used to map the velocity and pressure field and resolve some unusual feature of the unsteady flow motion. A discussion will be provided to explain why a detailed validation process is crucial, involving a physical knowledge of the flow. Comparison of the numerical model results and experimental data over broad ranges of conditions for the same flow is mandatory. The validation process from 2D to 3D will be commented and difficulties will be highlighted.

Physics of Fluids, 2023