Decaying grid turbulence in a rotating stratified fluid

Rotating grid turbulence experiments have been carried out in a stably stratified fluid for relatively large Reynolds numbers (mesh Reynolds numbers up to 18000). Under the combined effects of rotation and stratification the flow degenerates into quasihorizontal motions. This regime is investigated using a scanning imaging velocimetry technique which provides time-resolved velocity fields in a volume. The most obvious effect of rotation is the inhibition of the kinetic energy decay, in agreement with the quasi-geostrophic model which predicts the absence of a direct energy cascade, as found in two-dimensional turbulence. In the regime of small Froude and Rossby numbers, the dynamics is found to be non-dissipative and associated with a symmetric and highly intermittent vertical vorticity field, that displays k(h)(-3) energy spectra. For higher Rossby numbers, fundamental differences with the quasi-geostrophic model are found. A significant decay of kinetic energy, which does not depend on the stratification, is observed. Moreover, in this regime, although both cyclones and anticyclones are initially produced, the intense vortices are only cyclones. For late times the flow consists of an assembly of coherent interacting Structures. Under the influence of both rotation and stratification, they take the form of lens-like eddies with aspect ratio proportional to f/N

La seconde vague féministe et la féminisation du Parti socialiste français et du Parti québécois

Le Parti socialiste français (PS) et le Parti québécois (PQ) ont inséré dans leurs statuts respectifs des résolutions qui ont marqué les débuts officiels de leur féminisation. Il s’agit pour le premier, de la résolution établissant un quota de 10 % de femmes dans les instances du parti et, pour le second, de la résolution préconisant l’inclusion de plusieurs réformes féministes dans son programme.Sans la seconde vague féministe, ces deux partis n’auraient probablement pas pris l’initiative de féminiser leurs instances. C’est le militantisme des années 1960 et 1970 qui a encouragé les femmes socialistes et péquistes à exiger que leur partis fassent enfin place aux femmes et aux questions qui les touchent. Les différentes stratégies que les femmes du PS et PQ ont utilisées pour faire adopter leurs résolutions et, plus particulièrement, la stratégie élitiste des militantes socialistes et celle, plus inclusive, de leurs consoeurs péquistes, expliquent en partie pourquoi les mesures de féminisation sont mieux perçues chez les membres du PQ que chez ceux du PS.This article examines how the resolutions that officially launched the feminization of the Parti socialiste français (PS) and the Parti québécois (PQ)—notably, the resolution establishing a quota of 10 % women in the PS bodies and the resolution recommending the inclusion of several feminist reforms into the PQ program—were inserted into the parties' by-laws.If it had not been for the second-wave women's movement, these two parties would not have taken steps to feminize their internal organs on their own. Indeed, it was the feminist agitation of the 1960s and 1970s that encouraged Socialist and Péquiste women to demand that their parties finally recognize women's presence and their concerns. The different strategies that PS and PQ women used to ensure the passage of their resolutions and, more specifically, the elitist strategy of PS women and the more inclusive strategy of their PQ counterparts, help to explain why pro-feminization measures are better perceived in the PQ than in the PS

Symmetry-breaking phase-transitions in highly concentrated semen

New experimental evidence of self-motion of a confined active suspension is presented. Depositing fresh semen sample in an annular shaped micro- fluidic chip leads to a spontaneous vortex state of the fluid at sufficiently large sperm concentration. The rotation occurs unpredictably clockwise or counterclockwise and is robust and stable. Furthermore, for highly active and concentrated semen, richer dynamics can occur such as self-sustained or damped rotation oscillations. Experimental results obtained with systematic dilution provide a clear evidence of a phase transition toward collective motion associated with local alignment of spermatozoa akin to the Vicsek model. A macroscopic theory based on previously derived Self-Organized Hydrodynamics (SOH) models is adapted to this context and provides predictions consistent with the observed stationary motion

Carvin – Les Frégates

Le projet de construction d’un lotissement sur la commune de Carvin à proximité du secteur dit de « Willerval » a motivé une prescription du service régional de l’archéologie. Située dans le nord-est du département du Pas-de-Calais, l’emprise des futurs travaux couvre une surface de 43 015 m2 soit un peu plus de 4 ha localisée dans la géographie urbaine au bout des rues « des Frégates ; des Albatros ; des Oiseaux ». Le territoire communal de Carvin fait l’objet d’une surveillance archéologiqu..

Electrohydrodynamic ionic wind, force field, and ionic mobility in a positive dc wire-to-cylinders corona discharge in air

Ionic wind refers to the acceleration of partially ionized air between two high-voltage electrodes. We study the momentum transfer from ions to air, resulting from ionic wind created by two asymmetric electrodes and producing a net thrust. This electrohydrodynamic (EHD) thrust, has already been measured in previous studies with digital scales. In this study, we provide more insights into the electrohydrodynamic momentum transfer for a wire-to-cylinder(s) positive dc corona discharge. We provide a simple and general theoretical derivation for EHD thrust, which is proportional to the current/mobility ratio and also to an effective distance integrated on the surface of the electrodes. By considering various electrode configurations, our investigation brings out the physical origin of previously obtained optimal configurations, associated with a better tradeoff between Coulomb forcing, friction occurring at the collector, and wake interactions. By measuring two-dimensional velocity fields using particle image velocimetry (PIV), we are able to evaluate the resulting local net force, including the pressure gradient. It is shown that the contribution of velocity fluctuations in the wake of the collecting electrode(s) must be taken into account to recover the net thrust. We confirm the proportionality between the EHD force and the current/mobility ratio experimentally, and evaluate the ion mobility from PIV measurements. A spectral analysis of the velocity fluctuations indicates a dominant frequency corresponding to a Strouhal number of 0.3 based on the ionic wind velocity and the collector size. Finally, the effective mobility of charge carriers is estimated by a PIV based method inside the drift region

Electrohydrodynamic thrust for in-atmosphere propulsion

The electrohydrodynamic thrust generated by wire–cylinder electrodes under high dc voltage is experimentally analyzed. Some recent experimental studies have shown that electrohydrodynamic thrusters produced by corona discharge and ionic wind are able to deliver high thrust-to-power ratio, which reopens prospects for electrohydrodynamic propulsion. From simple considerations based on ultralight aircraft mass, aerodynamics, battery mass, and experimental electrohydrodynamic thrust densities, their potential for applications is showcased. Furthermore, an experimental study is performed, for which the experimental observations are presented in terms of electric field and thrust density. This allows a simplified and synthetic presentation of propulsive properties. Various experimental biases have been identified and corrected. The measure of time-periodic oscillations of the airflow in the back of the thruster pinpoints a possible wake effect due to the impact of ionic wind on electrodes. The variations of the associated drag are studied when varying the position of the collecting electrodes. It is shown that aerodynamic losses can be significant in experimental electrohydrodynamic thrusters