Stéphane Perrard

• PhD
• Researcher at ESPCI Paris

We study single bubble deformation statistics in an homogeneous and isotropic turbulent flow by means of direct numerical simulations. We consider bubbles at low Weber number ( $We <3$ ) that have not been broken. We show that we can reproduce bubble deformations with a linear dynamics for each spherical harmonic mode. Inferring the coefficients of...

We describe the rising trajectory of bubbles in isotropic turbulence and quantify the slowdown of the mean rise velocity of bubbles with sizes within the inertial subrange. We perform direct numerical simulations of bubbles, for a wide range of turbulence intensity, bubble inertia and deformability, with systematic comparison with the corresponding...

A sub-millimetric bouncing droplet can walk on the surface of the fluid due to the resonant interaction with its own wave field. The present presentation will concentrate on the experimental investigation of the wave coupling behavior of two droplets bouncing in two different cavities and their associated trajectory. The center distance (L) between...

We investigate bubble deformations in an homogeneous and isotropic turbulent flow by means of direct numerical simulations of a single bubble in turbulence. We examine interface deformations by decomposing the local radius into the spherical harmonics base. We show that the linear dynamics of each mode, (for low Weber number), can be modeled by a f...

A sub-millimetric bouncing droplet can walk on the surface of the fluid due to the resonant interaction with its own wave field. The present presentation will concentrate on the experimental investigation of the wave coupling behavior of two droplets bouncing in two different cavities and their associated trajectory. The center distance (L) between...

This paper is associated with a video winner of a 2022 American Physical Society's Division of Fluid Dynamics (DFD) Gallery of Fluid Motion Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2022.GFM.V0008

Breaking dynamics of bubbles in turbulence produce a wide range of bubble sizes, which mediates gas transfer, in particular, at the ocean/atmosphere interface. At the scales close to the stability limit of bubbles torn away by inertial forces, a typical geometry that induces bubble breakup is the uniaxial straining flow. In this configuration, the...

Turbulence is hard to control. Many experimental methods have been developed to generate this elusive state of matter, leading to fundamental insights into its statistical and structural features as well as its onset. In all cases, however, the material boundaries of the experimental apparatus pose a challenge for understanding what the turbulence...

Even in the absence of externally applied temperature gradients, spontaneously generated temperature fluctuations arise in turbulent flows. We experimentally study these fluctuations in a closed von Kármán swirling flow of air at Mach number of order 10 ⁻³ , whose boundaries are maintained at a constant temperature. We observe intermittent peaks of...

We investigate the statistics of phase fluctuations of an acoustic wave propagating through a turbulent flow in line of sight (LOS) configuration. Experiments are performed on a closed von Kármán swirling flow whose boundaries are maintained at a constant temperature. In particular, we analyze the root mean square (RMS) and the power spectrum densi...

We investigate the statistics of phase fluctuations of an acoustic wave propagating through a turbulent flow in line of sight (LOS) configuration. Experiments are performed on a closed von Karman swirling flow whose boundaries are maintained at a constant temperature. In particular, we analyze the root mean square (RMS) and the power spectrum densi...

We present experiments on large air cavities spanning a wide range of sizes relative to the Hinze scale $d_{H}$ , the scale at which turbulent stresses are balanced by surface tension, disintegrating in turbulence. For cavities with initial sizes $d_0$ much larger than $d_{H}$ (probing up to $d_0/d_{H} = 8.3$ ), the size distribution of bubbles sma...

Turbulence is hard to control. A plethora of experimental methods have been developed to generate this ephemeral state of matter, leading to fundamental insights into its statistical and structural features as well as its onset at ever higher Reynolds numbers. In all cases however, the central role played by the material boundaries of the apparatus...

The bubble size distribution below a breaking wave is of paramount interest when quantifying mass exchanges between the atmosphere and oceans. Mass fluxes at the interface are driven by bubbles that are small compared with the Hinze scale dh, the critical size below which bubbles are stable, even though individually these are negligible in volume....

Information storage is a key element of autonomous, out-of-equilibrium dynamics, especially for biological and synthetic active matter. In synthetic active matter however, the implementation of internal memory in self-propelled systems is often absent, limiting our understanding of memory-driven dynamics. Recently, a system comprised of a droplet g...

We present experiments on large air cavities which are much bigger than the Hinze scale $d_\mathrm{H}$, the scale at which turbulent stresses are balanced by surface tension, disintegrating in turbulence. For cavities with initial sizes $d_0$ much larger than $d_\mathrm{H}$, the size distribution of bubbles smaller than $d_\mathrm{H}$ follows $N(d)...

We investigate the spatiotemporal quantity of coherence for turbulent velocity fluctuations at spatial distances of the order or larger than the integral length scale l0. Using controlled laboratory experiments, an exponential decay as a function of distance is observed with a decay rate that depends on the flow properties. The same law is observed...

The bubble size distribution below a breaking wave is of paramount interest when quantifying mass exchanges between the atmosphere and oceans. Mass fluxes at the interface are driven by bubbles that are small compared to the Hinze scale $d_h$, the critical size below which bubbles are stable, even though individually these are negligible in volume....

In incompressible flows, intermittent temperature fluctuations are expected to be spontaneously generated by viscous dissipation. We experimentally study these fluctuations in a closed \vk swirling flow of air at Mach number of order $10^{-3}$ and whose boundaries are maintained at a constant temperature. We observe intermittent peaks of low temper...

Gravity–capillary waves propagating along extended liquid cylinders in the inviscid limit are studied in the context of experiments on sessile cylinders deposited upon superhydrophobic substrates, with tunable geometries. In Part 1 of this work (Pham et al., J. Fluid Mech., vol. 891, 2020, A5), we characterised the non-dispersive regime of the vari...

We investigate the modes of deformation of an initially spherical bubble immersed in a homogeneous and isotropic turbulent background flow. We perform direct numerical simulations of the two-phase incompressible Navier-Stokes equations, considering a low-density bubble in the high-density turbulent flow at various Weber numbers (the ratio of turbul...

We investigate the spatio-temporal quantity of coherence for turbulent velocity fluctuations at spatial distances of the order or larger than the integral length scale $l_{0}$. Using controlled laboratory experiments, an exponential decay as a function of distance is observed with a decay rate which depends on the flow properties. The same law is o...

We study bubble break-up in homogeneous and isotropic turbulence by direct numerical simulations of the two-phase incompressible Navier-Stokes equations. We create the turbulence by forcing in physical space and introduce the bubble once a statistically stationary state is reached. We perform a large ensemble of simulations to investigate the effec...

Information storage, for short "memory", is a key element of autonomous, out-of-equilibrium dynamics, in particular in biological entities. In synthetic active matter, however, the implementation of internal memory in agents is often limited or even absent. As a consequence, most of the investigations in the field of active matter had no choice but...

We investigate numerically the influence of a weak current on wind-generated surface deformations for wind velocity below the onset of regular waves. In that regime, the liquid surface is populated by small disorganised deformations elongated in the wind direction, referred to as wrinkles. These wrinkles are the superposition of incoherent wakes ge...

We investigate the modes of deformation of an initially spherical bubble immersed in an homogeneous and isotropic turbulent background flow. We perform direct numerical simulations of the two-phase Navier-Stokes equations and resolving the bubble deformation in an homogeneous and isotropic turbulent flow at various Weber number (the ratio of turbul...

We study the shape and the geometrical properties of sessile drops with translational invariance (namely ``liquid cylinders'') deposited upon a flat super-hydrophobic substrate. We account for the flattening effects of gravity on the shape of the drop using a pendulum rotation motion analogy. In the framework of the inviscid Saint-Venant equations,...

We investigate numerically the influence of currents on wind-generated surface deformations for wind velocity below the onset of regular waves. In that regime, the liquid surface is populated by small disorganised deformations elongated in the streamwise direction, referred to as \textit{wrinkles}. These wrinkles are the superposition of incoherent...

Although bubble pinch-off is an archetype of a dynamical system evolving toward a singularity, it has always been described in idealized theoretical and experimental conditions. Here, we consider bubble pinch-off in a turbulent flow representative of natural conditions in the presence of strong and random perturbations, combining laboratory experim...

We present a wave-memory-driven system that exhibits intermittent switching between two propulsion modes in free space. The model is based on a pointlike particle emitting periodically cylindrical standing waves. Submitted to a force related to the local wave-field gradient, the particle is propelled, while the wave field stores positional informat...

We investigate the effect of a light turbulent wind on a liquid surface, below the onset of wave generation. In that regime, the liquid surface is populated by small disorganised deformations elongated in the streamwise direction. Formally identified recently by Paquier et al. (Phys. Fluids, vol. 27, 2015, art. 122103), the deformations that occur...

We investigate the dynamics of a deterministic self-propelled particle endowed with coherent memory. We evidence experimentally and numerically that it exhibits several stable free states. The system is composed of a self-propelled drop bouncing on a vibrated liquid driven by the waves it emits at each bounce. This object possesses a propulsion mem...

We investigate the dynamics of a deterministic self-propelled particle endowed with coherent memory. We evidence experimentally and numerically that it exhibits several stable free states. The system is composed of a self-propelled drop bouncing on a vibrated liquid driven by the waves it emits at each bounce. This object possesses a propulsion mem...

A walker is the association of a sub-millimetric bouncing drop moving along with a co-evolving Faraday wave. When confined in a harmonic potential, its stable trajectories are periodic and quantised both in extension and mean angular momentum. In this article we present the rest of the story, specifically the chaotic paths. They are chaotic and sho...

We investigate the effect of a light turbulent wind on a liquid surface, below the onset of wave generation. In that regime, the liquid surface is populated by small disorganised deformations elongated in the streamwise direction. Formally identified recently by Paquier et al. (2015), the deformations that occur below the wave onset were named wrin...

A walker is the association of a sub-millimetric bouncing drop moving along with a co-evolving Faraday wave. When confined in a harmonic potential, its stable trajectories are periodic and quantised both in extension and mean angular momentum. In this article, we present the rest of the story, specifically the chaotic paths. They are chaotic and sh...

We present a wave-memory driven system that exhibits a macroscopic diffusive-like behavior emerging from a deterministic set of microscopic rules. This diffusive-like motion originates from a self-scattering process that the wave-particle coupling generates spontaneously. We show that the stochastic aspect of this self-scattering process derives fr...

Video illustrating the dynamics of the wave memory-driven particle. The memory parameter is Me=50 and a straight-line motion is observed. The wavelength is 4.75 mm long and the movie duration is equivalent to 10 s.

Video illustrating the dynamics of the wave memory-driven particle. The memory parameter is Me=1250 and a part of the run and tumble dynamics is shown. The wavelength is 4.75 mm long and the movie duration is equivalent to 30 s.

We present a wave-memory driven system that exhibits a macroscopic diffusive-like behavior emerging from a deterministic set of microscopic rules. This diffusive-like motion originates from a self-scattering process that the wave-particle coupling generates spontaneously. We show that the stochastic aspect of this self-scattering process derives fr...

We investigate the crossing of an energy barrier by a self-propelled particle described by a Rayleigh friction term. We reveal the existence of a sharp transition in the external force field whereby the amplitude dramatically increases. This corresponds to a saddle point transition in the velocity flow phase space, as would be expected for any type...

We investigate the crossing of an energy barrier by a self-propelled particle described by a Rayleigh friction term. We show that a sharp transition between low and large amplitude of the external force field occurs. It corresponds to a saddle point transition in the velocity flow phase space, and would therefore occur for any type of force field....

The back-reaction of a radiated wave on the emitting source is a general problem. In the most general case, back-reaction on moving wave sources depends on their whole history. Here we study a model system in which a pointlike source is piloted by its own memory-endowed wave field. Such a situation is implemented experimentally using a self-propell...

The investigation of dynamical systems has revealed a deep-rooted difference between waves and objects regarding temporal reversibility and particlelike objects. In nondissipative chaos, the dynamic of waves always remains time reversible, unlike that of particles. Here, we explore the dynamics of a wave- particle entity. It consists in a drop boun...

The transmission of information can couple two entities of very different nature, one of them serving as a memory for the other. Here we study the situation in which information is stored in a wave field and serves as a memory that pilots the dynamics of a particle. Such a system can be implemented by a bouncing drop generating surface waves sustai...

A bouncing drop and its associated accompanying wave forms a walker. Based on previous works, we show in this article that it is possible to formulate a simple theoretical framework for the walker dynamics. It relies on a time scale decomposition corresponding to the effects successively generated when the memory effects increase. While the short t...

We present the results of a theoretical investigation of the dynamics of a droplet walking on a vibrating fluid bath under the influence of a harmonic potential. The walking droplet's horizontal motion is described by an integro-differential trajectory equation, which is found to admit steady orbital solutions. Predictions for the dependence of the...

A water cylinder deposited on a heated channel levitates on its own generated vapor film owing to the Leidenfrost effect. This experimental setup permits the study of the one-dimensional propagation of surface waves in a free-to-move liquid system. We report the observation of gravity-capillary waves under a dramatic reduction of gravity (up to a f...

We experimentally investigate the dynamics of water cooled from below at 0^oC and heated from above. Taking advantage of the unusual property that water's density maximum is at about 4^oC, this set-up allows us to simulate in the laboratory a turbulent convective layer adjacent to a stably stratified layer, which is representative of atmospheric an...

A droplet bouncing on a vertically vibrated liquid bath can be self-propelled by the surface waves it generates. Theses Faraday waves are sustained by the vertical bath vibration for a memory time which can be tuned experimentally. The wave field thus contains in its interference pattern a memory of the past-trajectory. The resulting entity called...

A bouncing drop and its associated accompanying wave forms a walker. Based on previous works, we show in this article that it is possible to formulate a simple theoretical framework for the walker dynamics. It relies on a time scale decomposition corresponding to the effects successively generated when the memory effects increase. While the short t...

The transmission of information can couple two entities of very different nature, one of them serving as a memory for the other. Here we study the situation in which information is stored in a wave field and serves as a memory that pilots the dynamics of a particle. Such a system can be implemented by a bouncing drop generating surface waves sustai...

A bouncing droplet on a vibrated bath can couple to the waves it generates, so that it becomes a propagative walker. Its propulsion at constant velocity means that a balance exists between the permanent input of energy provided by the vibration and the dissipation. Here we seek a simple theoretical description of the resulting non-Hamiltonian dynam...

A growing number of dynamical situations involve the coupling of particles or singularities with physical waves. In principle these situations are very far from the wave particle duality at quantum scale where the wave is probabilistic by nature. Yet some dual characteristics were observed in a system where a macroscopic droplet is guided by a pilo...

Dans de nombreuses situations géophysiques et astrophysiques, une couche de fluide turbulent se situe au dessus ou en-dessous d'une zone stratifiée stable. C'est par exemple le cas des zones convective et radiative des étoiles. Alors que cette zone stratifiée a longtemps été assimilée à une zone immobile, il s'avère qu'elle est en fait le siège de...

This study is devoted to the experimental and numerical analysis of the excitation of gravity waves by turbulent convection. This situation is representative of many geophysical or astrophysical systems such as the convective bottom layer of the atmosphere that radiates internal waves in the stratosphere, or the interaction between the convective a...

Levitating a liquid over a vapor film was limited to droplets. Here we show that on curved substrates a larger quantity of fluid can be suspended. This opens a new possibility for exploring free-liquid-surface phenomena without any contact with a solid. In one of the simplest possible situations, a large fluid torus is levitated over a circular tro...

A walker is a classical self-propelled wave particle association moving on a fluid interface. Two walkers can interact via their waves and form orbiting bound states with quantized diameters. Here we probe the behavior of these bound states when setting the underlying bath in rotation. We show that the bound states are driven by the wave interactio...