Wake behind contaminated bubbles in a solid-body rotating flow

International audienc

Evidence for Forcing-Dependent Steady States in a Turbulent Swirling Flow

We study the influence on steady turbulent states of the forcing in a von Karman flow, at constant impeller speed, or at constant torque. We find that the different forcing conditions change the nature of the stability of the steady states and reveal dynamical regimes that bear similarities to low-dimensional systems. We suggest that this forcing dependence may be applicable to other turbulent systems

Three-dimensional reconstruction of particle holograms: a fast and accurate multiscale approach.

10 pagesInternational audienceIn-line digital holography is an imaging technique that is being increasingly used for studying three-dimensional flows. It has been previously shown that very accurate reconstructions of objects could be achieved with the use of an inverse problem framework. Such approaches, however, suffer from higher computational times compared to less accurate conventional reconstructions based on hologram backpropagation. To overcome this computational issue, we propose a coarse-to-fine multiscale approach to strongly reduce the algorithm complexity. We illustrate that an accuracy comparable to that of state-of-the-art methods can be reached while accelerating parameter-space scanning

Influence of Reynolds number and forcing type in a turbulent von Karman flow

International audienceWe present a detailed study of of a global bifurcation occuring in a turbulent von Karman swirling flow. In this system, the statistically steady states progressively display hysteretic behaviour when the Reynolds number is increased above the transition to turbulence. We examine in detail this hysteresis using asymmetric forcing conditions -- rotating the impellers at different speeds. For very high Reynolds numbers, we study the sensitivity of the hysteresis cycle -- using complementary Particle Image Velocimetry (PIV) and global mechanical measurements -- to the forcing nature, imposing either the torque or the speed of the impellers. New mean states, displaying multiple quasi- steady states and negative differential responses, are experimentally observed in torque control. A simple analogy with electrical circuits is performed to understand the link between multi-stability and negative responses. The system is compared to other, similar "bulk" systems, to understand some relevant ingredients of negative differential responses, and studied in the framework of thermodynamics of long-range interacting systems. The experimental results are eventually compared to the related problem of Rayleigh-Benard turbulence

Evaporating droplet hologram simulation for digital in-line holography setup with divergent beam

International audienceGeneralized Lorenz-Mie Theory (GLMT) for a multilayered sphere is used to simulate holograms produced by evaporating spherical droplets with refractive index gradient in the surrounding air/vapor mixture. Simulated holograms provide a physical interpretation of experimental holograms produced by evaporating Diethyl Ether droplets with diameter in the order of 50 μm and recorded in a digital in-line holography configuration with a divergent beam. Refractive index gradients in the surrounding medium lead to a modification of the center part of the droplet holograms, where the first fringe is unusually bright. GLMT simulations reproduce well this modification, assuming an exponential decay of the refractive index from the droplet surface to infinity. The diverging beam effect is also considered. In both evaporating and non evaporating cases, an equivalence is found between Gaussian beam and plane wave illuminations, simply based on a magnification ratio to be applied to the droplets' parameters

MESURES LAGRANGIENNES DE GOUTTES ÉVAPORANTES DANS UNE TURBULENCE HOMOGÈNE ISOTROPE PAR HOLOGRAPHIE NUMÉRIQUE

International audienceWe present an optical technique capable of measuring 3D trajectories and size evolution of a dilute flow of droplets dispersing in a high Reynolds number turbulence, from a Lagrangian point of view. The technique used is an in-line digital holographic set-up, with an original reconstruction algorithm based on an inverse-problem approach. The experiment has been performed with water and freon droplets in a locally well defined homogeneous, nearly isotropic, turbulence. This technique allows to visualise the thermal wakes behind the droplets which are tracked

Instrumentations optique lagragienne pour l'étude de la granulométrie d'un écoulement chargé en particules

Nous présentons 2 instrumentations optiques permettant de mesurer, de manière Lagrangienne, la granulométrie d'un écoulement chargé en particules ou en gouttelettes. Un montage d'holographie numérique en ligne est présenté pour l'étude de l'évaporation de gouttelettes dans écoulement turbulent. Un montage d'ombroscopie PTV est mis en place pour étudier la dissolution de particules solides dans un écoulement de type Von Karman. De plus, ces 2 techniques ont aussi l'avantage d'offrir une visualisation dynamique du champ thermique ou de concentration dans le sillage des objets étudiés

Testing an in-line digital holography 'inverse method' for the Lagrangian tracking of evaporating droplets in homogeneous nearly isotropic turbulence

International audienceAn in-line digital holography technique is tested, the objective being to measure Lagrangian three-dimensional (3D) trajectories and the size evolution of droplets evaporating in high-Reλ strong turbulence. The experiment is performed in homogeneous, nearly isotropic turbulence (50 × 50 × 50 mm3) created by the meeting of six synthetic jets. The holograms of droplets are recorded with a single high-speed camera at frame rates of 1-3 kHz. While hologram time series are generally processed using a classical approach based on the Fresnel transform, we follow an 'inverse problem' approach leading to improved size and 3D position accuracy and both in-field and out-of-field detection. The reconstruction method is validated with 60 μm diameter water droplets released from a piezoelectric injector 'on-demand' and which do not appreciably evaporate in the sample volume. Lagrangian statistics on 1000 reconstructed tracks are presented. Although improved, uncertainty on the depth positions remains higher, as expected in in-line digital holography. An additional filter is used to reduce the effect of this uncertainty when calculating the droplet velocities and accelerations along this direction. The diameters measured along the trajectories remain constant within ±1.6%, thus indicating that accuracy on size is high enough for evaporation studies. The method is then tested with R114 freon droplets at an early stage of evaporation. The striking feature is the presence on each hologram of a thermal wake image, aligned with the relative velocity fluctuations 'seen' by the droplets (visualization of the Lagrangian fluid motion about the droplet). Its orientation compares rather well with that calculated by using a dynamical equation for describing the droplet motion. A decrease of size due to evaporation is measured for the droplet that remains longest in the turbulence domain

Measuring ocean surface velocities with the KuROS and KaRADOC airborne near-nadir Doppler radars: a multi-scale analysis in preparation of the SKIM mission, Submitted to Ocean SCience, July 2019

Surface currents are poorly known over most of the oceans. Satellite-borne Doppler Waves and Current Scatterom-eters (DWCS) can be used to fill this observation gap. The Sea surface KInematics Multiscale (SKIM) proposal, is the first satellite concept built on a DWCS design at near-nadir angles, and now one of the two candidates to become the 9th mission of the European Space Agency Earth Explorer program. As part of the detailed design and feasibility studies (phase A) funded by ESA, airborne measurements were carried out with both a Ku-Band and a Ka-Band Doppler radars looking at the sea surface at 5 near nadir-incidence in a real-aperture mode, i.e. in a geometry and mode similar to that of SKIM. The airborne radar KuROS was deployed to provide simultaneous measurements of the radar backscatter and Doppler velocity, in a side-looking configuration , with an horizontal resolution of about 5 to 10 m along the line of sight and integrated in the perpendicular direction over the real-aperture 3-dB footprint diameter (about 580 m). The KaRADOC system has a much narrower beam, with a circular footprint only 45 m in diameter. 10 The experiment took place in November 2018 off the French Atlantic coast, with sea states representative of the open ocean and a well known tide-dominated current regime. The data set is analyzed to explore the contribution of non-geophysical velocities to the measurement and how the geophysical part of the measured velocity combines wave-resolved and wave-averaged scales. We find that the measured Doppler velocity contains a characteristic wave phase speed, called here C 0 that is analogous to the Bragg phase speed of coastal High Frequency radars that use a grazing measurement geometry, with little 15 variations ∆ C associated to changes in sea state. The Ka-band measurements at an incidence of 12 • are 10% lower than the theoretical estimate C 0 2.4 m/s for typical oceanic conditions defined by a wind speed of 7 m/s and a significant wave height of 2 m. For Ku-band the measured data is 1 https://doi. 30% lower than the theoretical estimate 2.8 m/s. ∆ C is of the order of 0.2 m/s for a 1 m change in wave height, and cannot be confused with a 1 m/s change in tidal current. The actual measurement of the current velocity from an aircraft at 4 to 18 • incidence angle is, however, made difficult by uncertainties on the measurement geometry, which are much reduced in satellite measurements

The VKS experiment: turbulent dynamical dynamos

International audienceThe VKS experiment studies dynamo action in the flow generated inside a cylinder filled with liquid sodium by the rotation of coaxial impellers (the von Karman geometry). We report observations related to the self-generation of a stationary dynamo when the flow forcing is symmetric, i.e. when the impellers rotate in opposite directions at equal angular velocities. The bifurcation is found to be supercritical, with a neutral mode whose geometry is predominantly axisymmetric. We then report the different dynamical dynamo regimes observed when the flow forcing is asymmetric, including magnetic field reversals. We finally show that these dynamics display characteristic features of low dimensional dynamical systems despite the high degree of turbulence in the flow