Space-time measurement of breaking wave kinematics and void fraction in the surf zone

Particle Image Velocimetry (PIV) measurements were performed in a wave tank under regular water waves propagating and breaking on a 1=15 sloping beach. To obtain more accurate measurements, the domain from the initiation of the breaking to the swash zone is splited in fourteen overlapping windows. The full field is reconstructed at each time step by gathering the fourteen synchronized PIV fields. We measured then the complete space-time evolution of velocity field over the whole surf zone. From these data the ensemble-period-average and phase-average components of the flow with their associated fluctuating parts were computed. Moreover using the light intensity of the PIV images and velocity measurements, we estimated the void fraction in each point of the surf zone and at each time step

Propriétés cinématiques des vagues déferlantes sur une plage inclinée / Breaking wave kinematics on sloping beaches

12 p.National audienceDes mesures de champs de vitesses utilisant la technique de vélocimétrie par image de particules ont été réalisées pour des vagues déferlantes sur une plage inclinée. Le problème dans ce type d'écoulement est que pour obtenir une définition suffisante en vitesse, la taille des images de mesure doit rester petite, alors que le phénomène observé prend place sur plusieurs longueurs d'onde. Pour obtenir une information pertinente, sur toute la zone de surf, le champ de mesure a été scindé en 13 parties de taille identique et les images ont été assemblées en synchronisant les données laser et les données de sondes à vagues. A partir de ces champs assemblés, il est possible d'obtenir des informations statistiques telles que les moyennes de phase et les fluctuations associées. Une attention toute particulière est donnée au calcul des gradients permettant d'obtenir les diverses grandeurs physiques liées aux fluctuations

Run Up of Surface and Internal Waves

The evolution of breaking waves propagating towards the shore and more specifically the run-up phase over the swash-zone for surface as well as for internal waves is considered. The study is based on a) laboratory run up experiments for surface waves ; b) laboratory stratified flow experiments and c) on field data describing the internal wave run up. The presentation is focused on the breaking and energy transfer mechanisms common to surface and internal waves in the swash-zone. The mathematical model taking into account turbulent mixing and dispersion effects is discussed

Air flow structures in the very close vicinity of wind generated water waves

International audienceIn the large wind-wave tank of IRPHE (40m long, 3m wide, 1m deep water and 1.7m high for the air, wind speed up to 14m/s) we conducted a set of experiments to measure the structure of the air flow in the very close vicinity of the water-surface above wind-generated waves. We estimated momentum transfers from air to water and we made direct measure-ments of form drag and viscous drag quantities for different conditions of wind and waves

Laboratory PIV Measurements of Wave Breaking on Beach

ISBN 1-880653-62-1Experiment were conducted in the ESIM wave tank in Marseilles in order to study the space/time evolution of the flow field under waves breaking on an 1/15 beach slope. Fluid particles velocities were measured at different depths, before, during and after the breaking event using the Particle Imaging Velocimetry (PIV) technique. The mean and fluctuant components of the flow were calculated

Description fine de la couche limite aérienne à proximité immédiate de l'interface air-mer

National audienceDes expériences ont été conduites dans la grande soufflerie air-eau du laboratoire IRPHE à Marseille, mettant en œuvre de nouvelles techniques de mesure des caractéristiques physiques de l'écoulement aérien dans la couche limite au dessus des vagues. D'une part un capteur de pression, monté sur un suiveur à vagues maintenant l'appareil à une altitude constante par rapport à la surface de l'eau, nous a permis d'effectuer des mesures de fluctuations de pression statique à proximité immédiate de l'interface, notamment dans le creux des vagues. D'autre part nous avons mesuré les gradients verticaux de la composante horizontale de la vitesse du vent dans la sous-couche visqueuse,à moins d'un millimètre de la surface grâce à un anémomètre à fil chaud monté sur un dispositif plongeur traversant l'interface air-eau. L'ensemble de ces mesures nous permet d'analyser la répartition des contraintes normales et tangentielles dans le bilan des flux de transfert de quantité de mouvement de l'air vers la mer. Les résultats présentent les évolutions de ces contributions en fonction de la vitesse du vent, de l'état de mer et de l'age des vagues. Par moyennage de phase, nous décrivons l'évolution de la structure locale de l'écoulement le long de la vague depuis la sous-couche visqueuse jusqu'aux limites de la couche limite turbulente à flux constant. Nous discutons de l'influence des termes induits par les mouvements de l'interface dans les processus d'échange énergétiques ocean-atmosphère

Numerical study of the hydrodynamics of regular waves breaking on a sloping beach

International audienceIn the last three decades, great improvements have been brought to the knowledge of the hydrodynamics and general processes occurring in the surf zone, widely affected by the breaking of the waves. Nevertheless, the turbulent flow structure is still very complicated to investigate. The aim of this work is to present and discuss the results obtained by simulating twodimensional breaking waves by solving the Navier-Stokes equations, in air and water, coupled with a dynamic subgrid scale turbulence model (Large Eddy Simulation, LES). First, the ability of the numerical tool to capture the crucial features of this complicated turbulent two-phase flow is demonstrated. Numerical results are compared with experimental observations provided by Kimmoun and Branger [1]. Spilling/plunging breaking regular waves are considered. Generally, there is good agreement and the model provides a precise and efficient tool for the simulation of the flow field and wave transformations in the nearshore

On the onset of surface wind drift at short fetches as observed in a wind wave flume

International audienceOcean surface drift is of great relevance to properly model wind waves and specially the early stages of surfacewaves development and ocean-atmosphere fluxes during incipient wind events and storms. In particular, wavemodels are not so accurate predicting wave behaviour at short fetches, where wind drift onset might be veryimportant. The onset of surface drift induced by wind and waves is being studied through detailed laboratorymeasurements in a large wind-wave flume. Wind stress over the water surface, waves and surface drift aremeasured in the 40m long wind-wave tank at IRPHE, Marseille. While momentum fluxes are estimated directlythrough the eddy correlation method in a station about the middle of the tank, they provide reference informationto the corresponding surface drift onset recorded at rather short non-dimensional fetches. At each experimentalrun very low wind was on (about 1m/s) for a certain period and suddenly it was constantly accelerated to reachabout 13 m/s (as well as 8 and 5 m/s during different runs) in about 15 sec to as long as 600 sec. The windwas kept constant at that high speed for 2 to 10 min, and then suddenly and constantly decelerate to 0. Surfacedrift values were up to 0.5 cm/s for the highest wind while very distinctive shear was detected in the upper 1.5cm. Rather linear variation of surface drift was observed with depth. Evolution of the surface drift velocity isanalysed and onset behaviour is addressed with particular emphasis in accelerated winds. This work representsa RugDiSMar Project (CONACYT 155793) contribution. The support from ANUIES-ECOS M09-U01 project,CONACYT-187112 Estancia Sabática, and Institute Carnot, is greatly acknowledged

Wind-forced modulations of gravity waves

International audienceThe effect of wind on modulational instability of gravity waves is presented. A forced nonlinear Schödinger equation that governs the evolution in deep water of weakly nonlinear packets of surface gravity waves under wind forcing is derived. Stokes waves are shown to grow exponentially following Miles' linear mechanism, while modulational instability becomes explosive. These analytical results are completed by numerical simulations and experiments showing the amplification of modulations by wind. Therefore, we suggest that wind boosts the formation of rogue waves

Experimental and numerical study on a model of offshore vertical axis wind turbine with pitching blades

International audienc