Disentangling inertial waves from eddy turbulence in a forced rotating turbulence experiment

We present a spatio-temporal analysis of a statistically stationary rotating turbulence experiment, aiming to extract a signature of inertial waves, and to determine the scales and frequencies at which they can be detected. The analysis uses two-point spatial correlations of the temporal Fourier transform of velocity fields obtained from time-resolved stereoscopic particle image velocimetry measurements in the rotating frame. We quantify the degree of anisotropy of turbulence as a function of frequency and spatial scale. We show that this space-time-dependent anisotropy is well described by the dispersion relation of linear inertial waves at large scale, while smaller scales are dominated by the sweeping of the waves by fluid motion at larger scales. This sweeping effect is mostly due to the low-frequency quasi-two-dimensional component of the turbulent flow, a prominent feature of our experiment which is not accounted for by wave turbulence theory. These results question the relevance of this theory for rotating turbulence at the moderate Rossby numbers accessible in laboratory experiments, which are relevant to most geophysical and astrophysical flows

Turbulent drag in a rotating frame

What is the turbulent drag force experienced by an object moving in a rotating fluid? This open and fundamental question can be addressed by measuring the torque needed to drive an impeller at constant angular velocity $\omega$ in a water tank mounted on a platform rotating at a rate $\Omega$. We report a dramatic reduction in drag as $\Omega$ increases, down to values as low as $12$\% of the non-rotating drag. At small Rossby number $Ro = \omega/\Omega$, the decrease in drag coefficient $K$ follows the approximate scaling law $K \sim Ro$, which is predicted in the framework of nonlinear inertial wave interactions and weak-turbulence theory. However, stereoscopic particle image velocimetry measurements indicate that this drag reduction rather originates from a weakening of the turbulence intensity in line with the two-dimensionalization of the large-scale flow.Comment: To appear in Journal of Fluid Mechanics Rapid

La planification de la gestion de crise par les autorités publiques à l'épreuve des événements Charlie Hebdo: Vers une gestion centralisée mais non-monopolistique ?

Le 7 janvier 2015, les frères Kouachi attaquent la rédaction du journal Charlie Hebdo, déclenchant une vague de réactions qualifiée d’historique. Nous étudions ici la gestion de ces événements par l’appareil étatique, notamment au sein des services ministériels, face à l’urgence de la situation et la pression médiatique et questionnons la pertinence de son organisation au regard de la multiplicité des acteurs en jeu

Tunable Narrowband Tm:YAP Laser using a Transversally Chirped Volume Bragg Grating

International audienceTunable long wave infrared (LWIR) coherent sources emitting between 8 and 12 μm can be used for versatile remote gas sensing. In ref [1] we implemented a parametric oscillator in which wavelength tunability was achieved by temperature tuning of the crystal indices of refraction. It is well known that a similar wavelength range can be addressed by tuning the pump wavelength rather than the crystal temperature [2]. In addition, pump tuning can be significantly faster than temperature tuning. In this context we also need a narrow band spectrum to control the spectral output of the optical parametric oscillator, which is based on Vernier spectral filtering in doubly-resonant cavities. Seger and his team exhibited single longitudinal mode operation in a microlaser tuned with transversally chirped volume Bragg grating (TC-VBG) near 1 μm [3]. We show how this concept can be applied to a different laser emitting at 2 μm

Forced response analysis of hydroelectric systems

At off-design operating points, Francis turbines develop cavitation vortex rope in the draft tube which may interact with the hydraulic system. Risk resonance assessment by means of eigenmodes computation of the system is usually performed. However, the system response to the excitation source induced by the cavitation vortex rope is not predicted in terms of amplitudes and phase. Only eigenmodes shapes with related frequencies and dampings can be predicted. Besides this modal analysis, the risk resonance assessment can be completed by a forced response analysis. This method allows identifying the contribution of each eigenmode into the system response which depends on the system boundary conditions and the excitation source location. In this paper, a forced response analysis of a Francis turbine hydroelectric power plant including hydraulic system, rotating train, electrical system and control devices is performed. First, the general methodology of the forced response analysis is presented and validated with time domain simulations. Then, analysis of electrical, hydraulic and hydroelectric systems are performed and compared to analyse the influence of control structures on pressure fluctuations induced by cavitation vortex rope

The Yin-Yang of the Green Fluorescent Protein:Impact on Saccharomyces cerevisiae stress resistance

International audienceAlthough fluorescent proteins are widely used as biomarkers (Yin), no study focuses on their influence on the microbial stress response. Here, the Green Fluorescent Protein (GFP) was fused to two proteins of interest in Saccharomyces cerevisiae. Pab1p and Sur7p, respectively involved in stress granules structure and in Can1 membrane domains. These were chosen since questions remain regarding the understanding of the behavior of S. cerevisiae facing different heat kinetics or oxidative stresses. The main results showed that Pab1p-GFP fluorescent mutant displayed a higher resistance than that of the wild type under a heat shock. Moreover, fluorescent mutants exposed to oxidative stresses displayed changes in the cultivability compared to the wild type strain. In silico approaches showed that the presence of the GFP did not influence the structure and so the functionality of the tagged proteins meaning that changes in yeast resistance were certainly related to GFP ROS-scavenging ability (Yang)