Olivier Eiff

Experimental investigation of the collapse of a turbulent wake in a stably stratified fluid

This paper is devoted to an experimental characterization of the collapse under buoyancy forces of the turbulent motions in the wake of a sphere in a linearly stable saline stratification. This phenomenon is part of the transition between two commonly recognized regions of the wake, the near wake and the far wake. It is shown here that the wake evolves in four distinct stages: the three-dimensional near wake, the collapse, a transition region, and finally the far wake. Hot-film measurements in the near wake and the region beyond show that initially the mean defect velocity decreases as in a nonstratified fluid, but then increases briefly before continuing to decrease again. The short period of mean defect-velocity increase or mean-flow acceleration characterizes the collapse and results in the higher levels of kinetic energy found in stratified wakes as compared to nonstratified wakes. The far-wake region of the flow was studied by a digital particle image velocimetry technique yielding decay laws for the...

On the density structure of far-wake vortices in a stratified fluid

An experimental investigation of the three-dimensional density structure of far-wake vortices generated by moving a sphere in a linear saline stratification has been carried out via conductivity measurements. These measurements were performed in the vortex cores along vertical and horizontal profiles to capture the three-dimensional nature of the vortices. We first present a simple model of an isolated vortex, the calculated internal density field of which has been confirmed by the conductivity measurements performed in both turbulent and laminar wakes. The time-dependent density structure is also described, as well as the decay of the density peak within the vortices. An identical density structure corresponding to an intensification of the background stratification has been identified for far-wake vortices originating from either laminar or turbulent near-wakes. This suggests that these vortices exhibit universal features of quasi two-component structures in stratified fluids, such as large-scale vortices commonly found in the ocean.

Lee-wave breaking over obstacles in stratified flow

Experimental results are presented on the lee-wave breaking process which occurs at low Froude numbers when uniform and strongly stratified flow approaches two-dimensional and quasi two-dimensional Gaussian-shaped obstacles. It was found that the lee-wave breaking process is essentially independent of the two-dimensional and the quasi two-dimensional shape of the obstacles. The attainment of the critical condition where the steepening wave becomes statically unstable does not mark a threshold to breakdown. Instead, the wave remains dynamically stable for several buoyancy periods, overturning into an “S”-shape with maximum overturning reaching about 55° past the vertical. It is observed that the primary instability forms a quasi two-dimensional spanwise vortex over the central portion of the obstacles and is mainly shear driven. The quasi two-dimensional spanwise vortex persists for a few buoyancy periods before undergoing a three-dimensional convective instability, similar to a Rayleigh–Taylor instability...

Physical modelling of water, fauna and flora: knowledge gaps, avenues for future research and infrastructural needs

Physical modelling is a key tool for generating understanding of the complex interactions between aquatic organisms and hydraulics, which is important for management of aquatic environments under environmental change and our ability to exploit ecosystem services. Many aspects of this field remain poorly understood and the use of physical models within eco-hydraulics requires advancement in methodological application and substantive understanding. This paper presents a review of the emergent themes from a workshop tasked with identifying the future infrastructure requirements of the next generation of eco-hydraulics researchers. The identified themes are: abiotic factors, adaptation, complexity and feedback, variation, and scale and scaling. The paper examines these themes and identifies how progress on each of them is key to existing and future efforts to progress our knowledge of eco-hydraulic interactions. Examples are drawn from studies on biofilms, plants, and sessile and mobile fauna in shallow water fluvial and marine environments. Examples of research gaps and directions for educational, infrastructural and technological advance are also presented.

Far-Wake of a Sphere in a Stably Stratified Fluid: Characterization of the Vortex Structures

This article discusses the structure of the far-wake of a towed sphere in a saline stratification. We compare very low Froude number experiments to existing results at higher Froude numbers and investigate the vertical structure of the far-wake in terms of their vorticity and density fields. We show that the vertical propagation of vorticity is viscously dominated and propose a simple three-dimensional model for the quasi-equilibrium of the structure in terms of the density field.

A three-dimensional experimental investigation of the structure of the spanwise vortex generated by a shallow vortex dipole

The three-dimensional dynamics of shallow vortex dipoles is investigated by means of an innovative three-dimensional, three-component (3D-3C) scanning PIV technique. In particular, the three-dimensional structure of a frontal spanwise vortex is characterized. The technique allows the computation of the three-dimensional pressure field and the planar (x, y) distribution of the wall shear stress, which are not available using standard 2D PIV measurements. The influence of such a complex vortex structure on mass transport is discussed in the context of the available pressure and wall shear stress fields.

On the existence and evolution of a spanwise vortex in laminar shallow water dipoles

The present work investigates the existence and evolution of a spanwise vortex at the front of shallow dipolar vortices. The vortex dipoles are experimentally generated using a double flap apparatus. Particle image velocimetry measurements are performed in a horizontal plane and in the vertical symmetry plane of the flow. The dynamics of such vortical structures is investigated through a parametric study in which both the Reynolds number Re=U0D0/ν∈[90,470] and the aspect ratio α=h/D0∈[0.075,0.7], associated with the shallowness of the flow, are varied, where U0 is the initial velocity of the vortex dipole, D0 is the initial diameter, h is the water depth, and ν is the kinematic viscosity of the fluid. The present experiments confirm the numerical results obtained in a companion paper by Duran-Matute et al. [Phys. Fluids 22, 116606 (2010)], namely that the flow remains quasi parallel with negligible vertical motions below a critical value of the parameter α2Re. By contrast, for large values of α2Re and α≲0...