Damien Sous

Turbulent vortex dipoles in a shallow water layer

This paper describes an experimental study on turbulent dipolar vortices in a shallow water layer. Dipoles are generated by an impulsive horizontal jet, by which a localized three-dimensional turbulent flow region is created. Dipole emergence is only controlled by the confinement number C=Q/H2tinj whereas the jet Reynolds number Re=Q/v has no influence in the studied range 50u200a000 2, the flow becomes quasi-two-dimensional and a single vortex dipole emerges in most cases. By qualitative observations and application of particle image velocimetry, the main dipole features have been determined. The shallow water dipoles are characterized by the simultaneous presence of several scales of turbulence: A quasi-two-dimensional main flow at large scale and three-dimensional turbulent motions at small scale. A vertical circulation takes place in the dipole front. A theoretical model i...

Friction law and turbulent properties in a laboratory Ekman boundary layer

We use spin-up/spin-down laboratory experiments to study the neutrally stratified Ekman boundary layer. The experiments are performed in the 13 m diameter, 1 m deep Coriolis rotating tank of the LEGI in Grenoble, France. A global flow rotation is produced by an initial change in the tank rotation speed. It then slowly decays under the effect of Ekman friction, evolving from the turbulent state to the laminar state. It is checked that the Ekman layer itself remains in a quasi-steady state during this decay. The velocity is measured by Particle Imaging Velocimetry (PIV) at two scales: the global rotation in a horizontal plane, and the vertical profile inside the boundary layer, where the three velocity components are obtained by stereoscopic PIV. The friction law is obtained by relating the decay rate of the bulk velocity to the velocity itself. This method is justified by the fact that this bulk velocity is independent of height beyond the top of the boundary layer (a few cm), as expected from the Taylor-P...

122. A LARGE-SCALE LABORATORY EXPERIMENT OF RIP CURRENT CIRCULATIONS OVER A MOVEABLE BED: DRIFTER MEASUREMENTS

The present paper presents a laboratory experiment of rip current circulations over a moveable bed. The rip current characteristics over four distinct beach morphologies, exhibiting more or less developed nature-like bar-rip morphology, were investigated. For each video run, the same offshore shore-normal waves were generated by the wavemaker with the same mean water level in order to study the sensitivity of the rip current characteristics as a function of the beach morphology only. In each case, a 1-hour video run was used to track a large number (~30) of drifters released within the surf zone. Image coordinates were then rectified to still water level Cartesian coordinates to compute drifter velocities, mean characteristics and surf zone retention rates. Results show the presence of classic rip current patterns with counter-rotating cells and a relatively narrow offshore-directed jet with, for three of the situations, a reasonably symmetric shape. Non-surprisingly, it was found that rip current intensity increases with increasing relative depth of the rip channel. The wave-driven circulations were strongly unstable. Computed standard deviation in flow intensity and direction provides high resolution information on the spatial variability of the rip current instabilities with, for instance, highly-pulsating and weakly directionally variable offshore-directed flow in the rip channel. Conversely to what was previously hypothesized in the literature, there was hardly trace of vortices being shed offshore and drifters exiting the surf zone compartment were not systematically caught by a pulsating jet. The cause for drifter exiting the semi-enclosed surf zone compartment remains, however, elusive and deserves further investigations. The computed surf zone retention rates (~90%) were of the order of those previously observed in the field, with no clear relationship with the mean rip current velocity or relative depth of the rip channel. Further video-runs will have to be analyzed to explore potential explanations.

Dynamics of large-scale vortices in the near shore

In spite of the ubiquity of long shore flows, rip currents and also tidal jets in the near shore, large-scale vortices associated with these phenomena remain poorly understood. In particular, little is known about the effect of the vertical confinement on vortex dynamics. To understand this phenomenon we present in this paper a new laboratory experiment on pulsed jets (assimilated to a rip current) in a shallow water layer. In this study we analyze the evolution of three-dimensional turbulence generated by a pulsed jet in a homogeneous shallow water layer. We show that the jet evolution depends meanly on one dimensionless parameter c = (√Q/H²) tinj (where Qtinj the injected momentum flux, H the water depth and the injection duration). C characterizes the vertical confinement. When C is weak, the jet spreading is free. The generated turbulence remains fully three-dimensional. When C is large (C>2), we observe a damping of the vertical motion and the formation of a large horizontal dipolar structure. We also identify inside the global 2D dipolar structure, local three dimensional vortices. We have developed a theoretical model in good agreement with these measurements.