Instabilities in the wake of a circular disk.

Flow past a disk was investigated experimentally in a low-velocity water channel in the range of intermediate Reynolds numbers. Systematic experiments with flow visualization and particle image velocimetry measurements are presented. Different disks with the aspect ratio χ = d/h varying from 1 to 24 were investigated.

Turbulent spots in channel flow: an experimental study: large-scale flow, inner structure and low-order model.

We present new experimental results on the development of turbulent spots in channel flow. The internal structure of a turbulent spot is measured, with Time Resolved Stereoscopic Particle Image Velocimetry. We report the observation of travelling-wave-like structures at the trailing edge of the turbulent spot.

Experimental scaling law for the subcritical transition to turbulence in plane Poiseuille flow.

We present an experimental study of the transition to turbulence in a plane Poiseuille flow. Using a well-controlled perturbation, we analyze the flow by using extensive particle image velocimetry and flow visualization (using laser-induced fluorescence) measurements, and use the deformation of the mean velocity profile as a criterion to characterize the state of the flow. From a large parametric study, four different states are defined, depending on the values of the Reynolds number and the amplitude of the perturbation.

Spatiotemporal spectral analysis of a forced cylinder wake.

The wake of a circular cylinder performing rotary oscillations is studied using hydrodynamic tunnel experiments at Re=100. Two-dimensional particle image velocimetry on the midplane perpendicular to the axis of a cylinder is used to characterize the spatial development of the flow and its stability properties. The lock-in phenomenon that determines the boundaries between regions of the forcing parameter space where the wake is globally unstable or convectively unstable [see Thiria and Wesfreid, J.

Self-sustaining process through streak generation in a flat-plate boundary layer.

The existence of a self-sustaining process between streamwise vortices and streaks has been suggested at moderate Reynolds numbers. Such a mechanism has never been demonstrated experimentally. Using small cylinders as vortex generators to create streamwise counterrotating vortices, we show, through the characterization of the spatial transient growth of the energy of the longitudinal and spanwise velocity perturbations, that such a mechanism exists above a given Reynolds number.

Oscillation-induced sand ripples in a circular geometry.

This study deals with the observation of sand ripples in a circular geometry under oscillatory flow. We characterize the observed patterns as a function of the excitation parameters. We report the time evolution of the corrugated front invading the flat bed.

Transition to a time-dependent state of fluid flow in the wake of a sphere.

In this paper, the results of laboratory investigation about the flow behind the sphere in the range of 150 View Article and Find Full Text PDF

Transitions in the wake of a flapping foil.

We study experimentally the vortex streets produced by a flapping foil in a hydrodynamic tunnel, using two-dimensional particle image velocimetry. An analysis in terms of a flapping frequency-amplitude phase space allows the identification of (i) the transition from the well-known Bénard-von Kármán (BvK) wake to the reverse BvK vortex street that characterizes propulsive wakes, and (ii) the symmetry breaking of this reverse BvK pattern giving rise to an asymmetric wake. We also show that the transition from a BvK wake to a reverse BvK wake precedes the actual drag-thrust transition and we discuss the significance of the present results in the analysis of flapping systems in nature.

Wavelength selection of rolling-grain ripples in the laboratory.

We have performed an experimental study, at very high resolution, of the wavelength selection and the evolution of rolling-grain ripples. A clear distinction is made between the flat sand bed instability and the ripple coarsening. The observation of the initial wavelength for the rolling-grain ripples is only possible close to the threshold for movement which imposes a constraint on the parameters.

Granular size segregation in underwater sand ripples.

We report an experimental study of a binary sand bed under an oscillating water flow. The formation and evolution of ripples is observed. The appearance of a granular segregation is shown to strongly depend on the sand bed preparation.

Time behavior of the secondary flow between time-periodically corotating cylinders: a two-frequency forcing case.

We consider the oscillatory flow between time-periodically corotating cylinders, in the case of a two-frequency forcing. The angular velocity Omega(t) of the cylinders is the sum of a low-frequency omega(1) oscillation plus a harmonic frequency omega(2) oscillation at a lower amplitude, Omega(t)=Omega(1) cos(omega(1)t)+Omega(2) cos(omega(2)t). The temporal behavior of the secondary flow is characterized by ultrasound Doppler velocimetry.

Dynamical evolution of sand ripples under water.

We have performed an experimental study on the evolution of sand ripples formed under the action of an oscillatory flow. An annular sand-water cell was used in order to investigate a wide range of parameters. The sand ripples follow an irreversible condensation mechanism from small to large wavelength until a final state is reached.

Farey sequences of spatiotemporal patterns in video feedback.

In this paper we present an experimental and theoretical description of the dynamic of spatial patterns obtained in a video feedback loop. A video camera monitors the screen to which it is connected and can turn around its optical axis at an angle alpha. Under certain conditions of brightness and magnification, this optoelectronic system produces spatiotemporal patterns in the form of spots located on a circle on the screen.