arXiv subjects

F. Moisy

Publications and source records attributed to F. Moisy.

15 recordsLinked to original sources

arXiv2025

Preferential orientation of small floaters drifting in water waves

Elongated floaters drifting in propagating water waves slowly rotate towards a preferential orientation with respect to the direction of incidence. In this article, we study this phenomenon in the small-floater limit $k L_x < 1 $, with $k$ the wavenumber and $L_x$ the floater length. Experiments show that short and heavy floaters tend to align longitudinally, along the direction of wave propagation, whereas longer and lighter floaters align transversely, parallel to the wave crests and troughs. We show that this preferential orientation can be modeled using an inviscid Froude-Krylov model, ignoring diffraction effects. Asymptotic theory, in the double limit of small wave slope and small floater, suggests that preferential orientation is essentially controlled by the non-dimensional number $F = k L_x^2 / \overline{h}$, with $\overline{h}$ the equilibrium submersion depth. Theory predicts the longitudinal-transverse transition {for homogeneous parallelepipeds} at the critical value $F_c = 60$, in fair agreement with the experiments that locate $F_c = 50 \pm 15$. Using a simplified model for a thin floater, we elucidate the physical mechanisms that control the preferential orientation. The longitudinal equilibrium for $F F_c$ arises from the variation of the submersion depth along the long axis of the floaters, which significantly increases the torque in the trough positions, when the tips are more submersed.

physics.flu-dyn↗

arXiv2023

Wind-wave growth over a viscous liquid

Experimental and theoretical studies on wind-wave generation have focused primarily on the air-water interface, where viscous effects are small. Here we characterize the influence of the liquid viscosity on the growth of mechanically generated waves. In our experiment, wind is blowing over a layer of silicon oil, of viscosity 20 and 50 times that of water, and waves of small amplitude are excited by an immersed wave-maker. We measure the spatial evolution of the wave slope envelope using Free-Surface Synthetic Schlieren, a refraction-based optical method. Through spatiotemporal band-pass filtering of the surface slope, we selectively determine the spatial growth rate for each forcing frequency, even when the forced wave is damped and coexists with naturally amplified waves at other frequencies. Systematic measurements of the growth rate for various wind velocities and wave frequencies are obtained, enabling precise determination of the marginal stability curve and the onset of wave growth. We show that Miles' model, which is commonly applied to water waves, offers a reasonable description of the growth rate for more viscous liquids. We finally discuss the scaling of the growth rate of the most amplified wave and the critical friction velocity with the liquid viscosity.

physics.flu-dyn↗

arXiv2022

Direct measurement of the aerotactic response in a bacterial suspension

Aerotaxis is the ability of motile cells to navigate toward oxygen. A key question is the dependence of the aerotactic velocity with the local oxygen concentration c. Here we combine simultaneous bacteria tracking and local oxygen concentration measurements using Ruthenium encapsulated in micelles to characterize the aerotactic response of Burkholderia contaminans, a motile bacterium ubiquitous in the environment. In our experiments, an oxygen gradient is produced by the bacterial respiration in a sealed glass capillary permeable to oxygen at one end, producing a bacterial band traveling toward the oxygen source. We compute the aerotactic response χ(c) both at the population scale, from the drift velocity in the bacterial band, and at the bacterial scale, from the angular modulation of the run times. Both methods are consistent with a power-law χ\propto c^{-2}, in good agreement with existing models based on the biochemistry of bacterial membrane receptors.

physics.bio-ph↗

arXiv2020

Effect of a weak current on wind-generated waves in the wrinkle regime

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 generated by the pressure fluctuations traveling in the turbulent boundary layer in the air. In this work, we account for the effect of a weak sheared current in the liquid, either longitudinal or transverse, by introducing a modified Doppler-shifted dispersion relation to lowest order in viscosity and current in the spectral theory previously derived by Perrard et al (2019). This theory describes the simplified one-way problem of surface deformations excited by a prescribed turbulent forcing, thereby neglecting the retroaction of waves on turbulence in the air. The forcing is taken from a set of direct numerical simulations of a turbulent channel flow. We determine the wrinkle properties (size and amplitude) as a function of the liquid viscosity and current properties (surface velocity, thickness and orientation). We find significant modifications of the wrinkle geometry by the currents: the wrinkles are tilted for a transverse current, and show finer scales for a longitudinal current. However, their characteristic size is weakly affected, and their amplitude remains independent of the current. We discuss the implications of these results on the onset of regular waves at larger wind velocity. In this work, we introduce a spectral interpolation method to evaluate the surface deformation fields, based on a refined meshing close to the dispersion relation of the waves. This method, which can be extended to any dispersive system excited by a random forcing, strongly reduces the discretization effects at a low computational cost.

physics.flu-dyn↗

arXiv2020

Investigation of the early stage of reactive interdiffusion in the Cu-Al system by in-situ transmission electron microscopy

The early stage of the reactive interdiffusion in the Cu-Al system was investigated at 350 {\textdegree}C and 300 {\textdegree}C thanks to in-situ transmission electron microscopy. A special care was given to find conditions where the electron beam and the sample free surface do not affect significantly the reaction. A special emphasis was then given on the influence of grain boundaries that are fast diffusion paths, and on nanoscaled particles that may interact with the transformation front. It was found that there is a transient state followed by a steady state where the mean growth rates of intermetallic compounds follow a parabolic law indicating that the kinetics is diffusion controlled. Thanks to the in-situ observations at the nanoscale, it was also possible to track the local velocity of interfaces between the different phases. Strong fluctuations were exhibited within length scales smaller than 100 nm and they are partly attributed to interface pinning by nanoscaled particles. Last, considering thermodynamic and kinetic arguments, it is shown that it is mainly an indirect effect induced by a local change of solute fluxes and of concentration gradients.

cond-mat.mtrl-sci↗

arXiv2019

Wind-sustained viscous solitons

When wind blows at the surface of a liquid of sufficiently high viscosity, a wave packet of small amplitude is first generated, which sporadically forms large-amplitude fluid bumps that rapidly propagate downstream. These nonlinear structures, first observed by Francis [Philos. Mag. 42, 695 (1954)], have an almost vertical rear facing the wind and a weak slope at the front. We call them viscous solitons. We investigate their dynamics in a wind-tunnel experiment using silicon oil of kinematic viscosity 1000 mm^2 s^-1 by means of laser sheet profilometry and particle image velocimetry. We give evidence of their subcritical nature: they are emitted in a region of large shear stress but, once formed, they are sustained by the wind and propagate in a region of lower stress. Their propagation velocity is given by the balance between aerodynamic drag in the air and viscous drag in the liquid. The stable soliton branch of the subcritical bifurcation diagram is reconstructed from the measured soliton amplitude at various wind velocities and distances along the channel. At large wind velocity, the emission frequency of solitons increases, resulting in a long-range sheltering of downstream mature solitons by newly formed upstream solitons, which limits their course.

physics.flu-dyn↗

arXiv2018

Counter-rotation in an orbitally shaken glass of beer

Swirling a glass of wine induces a rotating gravity wave along with a mean flow rotating in the direction of the applied swirl. Surprisingly, when the liquid is covered by a floating cohesive material, for instance a thin layer of foam in a glass of beer, the mean rotation at the surface can reverse. This intriguing counter-rotation can also be observed with coffee cream, tea scum, cohesive powder, provided that the wave amplitude is small and the surface covering fraction is large. Here we show that the mechanism for counter-rotation is a fluid analog of the rolling without slipping motion of a planetary gear train: for sufficiently large density, the covered surface behaves as a rigid raft transported by the rotating sloshing wave, and friction with the near-wall low-velocity fluid produces a negative torque which can overcome the positive Stokes drift rotation induced by the wave.

physics.flu-dyn↗

arXiv2014

Mach-like capillary-gravity wakes

We determine experimentally the angle $α$ of maximum wave amplitude in the far-field wake behind a vertical surface-piercing cylinder translated at constant velocity $U$ for Bond numbers $\mathrm{Bo}_D = D / λ_c$ ranging between 0.1 and 4.2, where $D$ is the cylinder diameter and $λ_c$ the capillary length. In all cases the wake angle is found to follow a Mach-like law at large velocity, $α\sim U^{-1}$, but with different prefactors depending on the value of $\mathrm{Bo}_D$. For small $\mathrm{Bo}_D$ (large capillary effects), the wake angle approximately follows the law $α\simeq c_{\rm g,min} / U$, where $c_{\rm g,min}$ is the minimum group velocity of capillary-gravity waves. For larger $\mathrm{Bo}_D$ (weak capillary effects), we recover a law $α\sim \sqrt{gD}/U$ similar to that found for ship wakes at large velocity [Rabaud and Moisy, Phys. Rev. Lett. {\bf 110}, 214503 (2013)]. Using the general property of dispersive waves that the characteristic wavelength of the wavepacket emitted by a disturbance is of order of the disturbance size, we propose a simple model that describes the transition between these two Mach-like regimes as the Bond number is varied. We show that the new capillary law $α\simeq c_{\rm g,min} / U$ originates from the presence of a capillary cusp angle (distinct from the usual gravity cusp angle), along which the energy radiated by the disturbance accumulates for Bond numbers of order of unity. This model, complemented by numerical simulations of the surface elevation induced by a moving Gaussian pressure disturbance, is in qualitative agreement with experimental measurements.

physics.flu-dyn↗

arXiv2012

Inertial waves and modes excited by the libration of a rotating cube

We report experimental measurements of the flow in a cubic container submitted to a longitudinal libration, i.e. a rotation modulated in time. Velocity fields in a vertical and a horizontal plane are measured in the librating frame using a corotating particle image velocimetry system. When the libration frequency $σ_0$ is smaller than twice the mean rotation rate $Ω_0$, inertial waves can propagate in the interior of the fluid. At arbitrary excitation frequencies $σ_0<2Ω_0$, the oscillating flow shows two contributions: (i) a basic flow induced by the libration motion, and (ii) inertial wave beams propagating obliquely upward and downward from the horizontal edges of the cube. In addition to these two contributions, inertial modes may also be excited at some specific resonant frequencies. We characterize in particular the resonance of the mode of lowest order compatible with the symmetries of the forcing, noted [2,1,+]. By comparing the measured flow fields to the expected inviscid inertial modes computed numerically [L.R.M. Maas, Fluid Dyn. Res. \textbf{33}, 373 (2003)], we show that only a subset of inertial modes, matching the symmetries of the forcing, can be excited by the libration.

physics.flu-dyn↗

arXiv2012

Earth rotation prevents exact solid body rotation of fluids in the laboratory

We report direct evidence of a secondary flow excited by the Earth rotation in a water-filled spherical container spinning at constant rotation rate. This so-called {\it tilt-over flow} essentially consists in a rotation around an axis which is slightly tilted with respect to the rotation axis of the sphere. In the astrophysical context, it corresponds to the flow in the liquid cores of planets forced by precession of the planet rotation axis, and it has been proposed to contribute to the generation of planetary magnetic fields. We detect this weak secondary flow using a particle image velocimetry system mounted in the rotating frame. This secondary flow consists in a weak rotation, thousand times smaller than the sphere rotation, around a horizontal axis which is stationary in the laboratory frame. Its amplitude and orientation are in quantitative agreement with the theory of the tilt-over flow excited by precession. These results show that setting a fluid in a perfect solid body rotation in a laboratory experiment is impossible --- unless tilting the rotation axis of the experiment parallel to the Earth rotation axis.

physics.flu-dyn↗

arXiv2012

Cross-waves induced by the vertical oscillation of a fully immersed vertical plate

Capillary waves excited by the vertical oscillations of a thin elongated plate below an air-water interface are analyzed using time-resolved measurements of the surface topography. A parametric instability is observed above a well defined acceleration threshold, resulting in a so-called cross-wave, a staggered wave pattern localized near the wavemaker and oscillating at half the forcing frequency. This cross-wave, which is stationary along the wavemaker but propagative away from it, is described as the superposition of two almost anti-parallel propagating parametric waves making a small angle of the order of $20^\mathrm{o}$ with the wavemaker edge. This contrasts with the classical Faraday parametric waves, which are exactly stationnary because of the homogeneity of the forcing. Our observations suggest that the selection of the cross-wave angle results from a resonant mechanism between the two parametric waves and a characteristic length of the surface deformation above the wavemaker.

nlin.PS↗

arXiv2009

Anisotropy and cyclone-anticyclone asymmetry in decaying rotating turbulence

The effect of a background rotation on the decay of homogeneous turbulence produced by a grid is experimentally investigated. Experiments have been performed in a channel mounted in the large-scale 'Coriolis' rotating platform, and measurements have been carried out in the planes normal and parallel to the rotation axis using particle image velocimetry. After a short period of about 0.4 tank rotation where the energy decays as $t^{-6/5}$, as in classical isotropic turbulence, the energy follows a shallower decay law compatible with $t^{-3/5}$, as dimensionally expected for energy transfers governed by the linear timescale $Ω^{-1}$. The crossover occurs at a Rossby number $Ro \simeq 0.25$, without noticeable dependence with the grid Rossby number. After this transition, anisotropy develops in the form of vertical layers where the initial vertical velocity remains trapped. These layers of nearly constant vertical velocity become thinner as they are advected and stretched by the large-scale horizontal flow, producing significant horizontal gradient of vertical velocity which eventually become unstable. After the $Ro \simeq 0.25$ transition, the vertical vorticity field first develops a cyclone-anticyclone asymmetry, reproducing the growth law of the vorticity skewness, $S_ω(t) \simeq (Ωt)^{0.7}$, reported by Morize, Moisy & Rabaud [{\it Phys. Fluids} {\bf 17} (9), 095105 (2005)]. At larger time, however, the vorticity skewness decreases and eventually returns to zero. The present results indicate that the shear instability of the vertical layers contribute significantly to the re-symmetrisation of the vertical vorticity at large time, by re-injecting vorticity fluctuations of random sign at small scales. These results emphasize the importance of the initial conditions in the decay of rotating turbulence.

physics.flu-dyn↗

arXiv2008

On the decrease of intermittency in decaying rotating turbulence

The scaling of the longitudinal velocity structure functions, $S_q(r) = < | δu (r) |^q > \sim r^{ζ_q}$, is analyzed up to order $q=8$ in a decaying rotating turbulence experiment from a large Particle Image Velocimetry (PIV) dataset. The exponent of the second-order structure function, $ζ_2$, increases throughout the self-similar decay regime, up to the Ekman time scale. The normalized higher-order exponents, $ζ_q / ζ_2$, are close to those of the intermittent non-rotating case at small times, but show a marked departure at larger times, on a time scale $Ω^{-1}$ ($Ω$ is the rotation rate), although a strictly non-intermittent linear law $ζ_q / ζ_2 = q/2$ is not reached.

physics.flu-dyn↗

arXiv2000

Passive scalar intermittency in low temperature helium flows

We report new measurements of turbulent mixing of temperature fluctuations in a low temperature helium gas experiment, spanning a range of microscale Reynolds number, $R_λ$, from 100 to 650. The exponents $ξ_{n}$ of the temperature structure functions $<|θ(x+r)-θ(x)|^n > \sim r^{ξ_{n}}$ are shown to saturate to $ξ_{\infty} \simeq 1.45 \pm 0.1$ for the highest orders, $n \sim 10$. This saturation is a signature of statistics dominated by front-like structures, the cliffs. Statistics of the cliff characteristics are performed, particularly their width are shown to scale as the Kolmogorov length scale.

nlin.CD↗

arXiv1998

Kolmogorov equation in fully developed turbulence

The Kolmogorov equation with a forcing term is compared to experimental measurements, in low temperature helium gas, in a range of microscale Reynolds numbers $R_λ$ between 120 and 1200. We show that the relation is accurately verified by the experiment (i.e. within +/- 3 % relative error, over ranges of scales extending up to three decades). Two scales are extracted from the analysis, and revealed experimentally, one characterizing the external forcing, and the other, varying as $R_λ^{-3/5}$, defining the position of the maximum of the function $- S_{3}(r)/r$, and for which a physical interpretation is offered.

chao-dyn↗