An instrumented tracer for Lagrangian measurements in Rayleigh-B\'enard convection

We have developed novel instrumentation for making Lagrangian measurements of temperature in diverse fluid flows. A small neutrally buoyant capsule is equipped with on-board electronics which measure temperature and transmit the data via a wireless radio frequency link to a desktop computer. The device has 80 dB dynamic range, resolving milli-Kelvin changes in temperature with up to 100 ms sampling time. The capabilities of these "smart particles" are demonstrated in turbulent thermal convection in water. We measure temperature variations as the particle is advected by the convective motion, and analyse its statistics. Additional use of cameras allow us to track the particle position and to report here the first direct measurement of Lagrangian heat flux transfer in Rayleigh-B{\'e}nard convection. The device shows promise for opening new research in a broad variety of fluid systems.Comment: 14 page

Tracking the dynamics of translation and absolute orientation of a sphere in a turbulent flow

We study the 6-dimensional dynamics -- position and orientation -- of a large sphere advected by a turbulent flow. The movement of the sphere is recorded with 2 high-speed cameras. Its orientation is tracked using a novel, efficient algorithm; it is based on the identification of possible orientation `candidates' at each time step, with the dynamics later obtained from maximization of a likelihood function. Analysis of the resulting linear and angular velocities and accelerations reveal a surprising intermittency for an object whose size lies in the integral range, close to the integral scale of the underlying turbulent flow

Rotational intermittency and turbulence induced lift experienced by large particles in a turbulent flow

The motion of a large, neutrally buoyant, particle, freely advected by a turbulent flow is determined experimentally. We demonstrate that both the translational and angular accelerations exhibit very wide probability distributions, a manifestation of intermittency. The orientation of the angular velocity with respect to the trajectory, as well as the translational acceleration conditioned on the spinning velocity provide evidence of a lift force acting on the particle.Comment: 4 page, 4 figure

Characterizing flows with an instrumented particle measuring Lagrangian accelerations

We present in this article a novel Lagrangian measurement technique: an instrumented particle which continuously transmits the force/acceleration acting on it as it is advected in a flow. We develop signal processing methods to extract information on the flow from the acceleration signal transmitted by the particle. Notably, we are able to characterize the force acting on the particle and to identify the presence of a permanent large-scale vortex structure. Our technique provides a fast, robust and efficient tool to characterize flows, and it is particularly suited to obtain Lagrangian statistics along long trajectories or in cases where optical measurement techniques are not or hardly applicable.Comment: submitted to New Journal of Physic

Comparison between rough and smooth plates within the same Rayleigh-Bénard cell

International audienceIn a Rayleigh-Bénard cell at high Rayleigh number, the bulk temperature is nearly uniform. The mean temperature gradient differs from zero only in the thin boundary layers close to the plates. Measuring this bulk temperature allows to separately determine the thermal impedance of each plate. In this work, the bottom plate is rough and the top plate is smooth; both interact with the same bulk flow. We compare them and address in particular the question whether the influence of roughness goes through a modification of the bulk flow

Instrumentation Lagrangienne en Turbulence (Mise en œuvre et Analyse)

Le but de ce travail de thèse est l'étude expérimentale de la turbulence du point de vu lagrangien. Pour ce faire, nous avons développé une nouvelle technique de mesure. Nous avons mis au point des sondes instrumentés, enfermées dans coquilles sphériques de 1.5-2 cm de diamètre. Elles comprennent une pile, un circuit électronique, des capteurs et une liaison radiofréquence pour la récupération de données. L'ensemble est de la même densité que le fluide environnant (eau). Le premier instrument mesure la température et a été utilisé dans la convection de Rayleigh-Bénard. Nous avons obtenu une vision inédite du rôle des panaches thermiques dans le transport de chaleur. Afin d'étudier les forces imposées sur un objet advecté par un fluide turbulent, nous avons développé un second instrument, mesurant l'accélération tridimensionnelle subie par la sonde. Des mesures ont été effectuées dans des écoulements de von Kármán ainsi que dans un écoulement de Couette sphérique de grande taille à l'université du Maryland. Ces études soulèvent la question de la dynamique à six degrés de liberté - rotation et translation - d'une sphère advectée dans un écoulement turbulent. Ces propriétés ont été étudiées par un suivi optique de position et d'orientation. La dynamique de translation est étonnamment proche de celle d'un traceur lagrangien et la dynamique angulaire présente une intermittence très importante.The aim of this PhD thesis is the experimental study of turbulence from a Lagrangian point of view. To do so, we have designed a new measurement based on the use of active instrumented probes. We have developed "instrumented particles" in the form of spherical shells with a diameter of 1.5-2 cm. They include a battery, an electronic circuit, sensors and a radio frequency transmitter as a data link to the lab. The whole system is neutrally buoyant with respect to the surrounding fluid (water). The first instrumented particle includes a temperature measurement and has been used in a Rayleigh-Bénard convection cell. It gives an original insight on the role of thermal plumes in the transport of heat. In view of studying forces on objects advected by turbulent flows, a second probe has been designed which includes a 3-dimensional measurement of the acceleration of the sphere. Measurements have been made in a von Kármán flow, and also in a very large spherical Couette flow at the University of Maryland. These studies have raised the question of the 6-dimensional dynamics - translation and rotation - of a sphere advected by a turbulent flow field. This issue has been addressed in detail from optical tracking of the sphere positions and orientations. The dynamics of translation is found to be surprisingly similar to that of Lagrangian tracer particles and the dynamics of orientation has revealed a surprisingly high level of intermittency.LYON-ENS Sciences (693872304) / SudocSudocFranceF

An instrumented tracer for Lagrangian measurements in Rayleigh-Bénard convection

14 pagesWe have developed novel instrumentation for making Lagrangian measurements of temperature in diverse fluid flows. A small neutrally buoyant capsule is equipped with on-board electronics which measure temperature and transmit the data via a wireless radio frequency link to a desktop computer. The device has 80 dB dynamic range, resolving milli-Kelvin changes in temperature with up to 100 ms sampling time. The capabilities of these "smart particles" are demonstrated in turbulent thermal convection in water. We measure temperature variations as the particle is advected by the convective motion, and analyse its statistics. Additional use of cameras allow us to track the particle position and to report here the first direct measurement of Lagrangian heat flux transfer in Rayleigh-Bénard convection. The device shows promise for opening new research in a broad variety of fluid systems

The Lagrangian Exploration Module : An Apparatus for the Study of Statistically Homogeneous and Isotropic Turbulence

International audienceWe present an apparatus that generates statistically homogeneous and isotropic turbulence with a mean flow that is less than 10% of the fluctuating velocity in a volume of the size of the integral length scale. The apparatus is shaped as an icosahedron where at each of the 12 vertices the flow is driven by independently controlled propellers. By adjusting the driving of the different propellers the isotropy and homogeneity of the flow can be tuned, while keeping the mean flow weak

Simultaneous temperature and velocity Lagrangian measurements in turbulent thermal convection

International audienceWe report joint Lagrangian velocity and temperature measurements in turbulent thermal convection. Measurements are performed using an improved version (extended autonomy) of the neutrally-buoyant instrumented particle [Shew et al., 2007] that was used by Gasteuil et al. [2007] to performed experiments in a parallelepipedic Rayleigh-Bénard cell. The temperature signal is obtained from a RF-transmitter. Simultaneously, we determine particle's position and velocity with one camera, which grants access to the Lagrangian heat flux. Due to the extended autonomy of the present particle, we obtain well converged temperature and velocity statistics, as well as pseudo-eulerian maps of velocity and heat flux. Present experimental results have also been compared with the results obtained by a corresponding campaign of Direct Numerical Simulations and Lagrangian Tracking of massless tracers. The comparison between experimental and numerical results shows the accuracy and reliability of our experimental measurements and points also out the finite-size effects of the particle. Finally, the analysis of Lagrangian velocity and temperature frequency spectra is shown and discussed. In particular, we observe that temperature spectra exhibit an anomalous f −2.5 frequency scaling, likely representing the ubiquitous passive and active scalar behavior of temperature