Inertial effects on two-particle relative dispersion in turbulent flows

We report experimental results on the relative motion of pairs of solid spheric particles with initial separations in the inertial range of fully developed turbulence in water. The particle densities were in the range of $1 \lessapprox \rho_{p}/\rho_{f} \lessapprox 8$, \textit{i.e.}, from neutrally buoyant to highly inertial; and their sizes were of the Kolmogorov scale. For all particles, we observed a Batchelor like regime, in which particles separated ballistically. Similar to the Batchelor regime for tracers, this regime was observed in the early stages of the relative separation for times $t \lessapprox 0.1 t_0$ with $t_0$ determined by the turbulence energy dissipation rate and the initial separation between particle pairs. In this time interval heavier particles separated faster than fluid tracers. The second order Eulerian velocity structure functions was found to increase with density. In other words, both observations show that the relative velocity between inertial particles was larger than that between tracers. Based on the widely used, simplified equation of motion for inertial point-particles, we derived a model that shows an increase in relative velocity between inertial particles. In its scale dependence, however, it disagrees quantitatively with the experimental results. This we attribute to the preferential sampling of the flow field by inertial particles, which is not captured by the model.Comment: 6 pages, 5 figures, 2 tables, epl2.cls, submitted to EP

Els Molière de Joan Oliver

Joan Oliver admirava en la comèdia francesa clàssica l'elegància fluida del llenguatge, l'amenitat de l'anècdota i l'habilitat constructora de situacions i personatges, cosa que no era impediment, sinó al contrari, per aprofundir en el coneixement de la naturalesa humana. Oliver trobava en la comèdia francesa dels segles XVII i XVIII, i molt especialment en Molière, uns atractius dels quals mancava greument la tradició teatral catalana. Des d'aquesta perspectiva, l'escriptor va elaborar tres traduccions esplèndides, en vers, de tres comèdies de Molière, la primera de les quals va ser realitzada a l'exili de Santiago de Xile durant els anys quaranta del segle passat. Els epistolaris d'Oliver amb Xavier Benguerel i Josep Ferrater Mora ens permeten de resseguir l'esforç del traductor durant anys i el rigor i la sensibilitat que va esmerçar en les traduccions esmentades.In the French classical comedy, Joan Oliver admired the fluent elegance of language, the agreeableness of the anecdote and the constructive ability of situations and characters, which, far from being an obstacle, were useful to go deeper into the knowledge of human nature. In the French comedy of the XVII and XVIII centuries, and especially in Molière, Oliver could see some appeals which were seriously missing from the Catalan theatre tradition. From this perspective, the writer produced three magnificent translations, in verse, of three comedies written by Molière. The first of them was carried out during his exile in Santiago de Chile during the first forty years of the last century. The collected letters of Oliver with Xavier Benguerel and Josep Ferrater-Mora allow us to follow the effort the translator did for many years as well as the rigor and sensitivity he devoted to the aforementioned translations

Turbulence attenuation by large neutrally buoyant particles

Turbulence modulation by inertial-range-size, neutrally-buoyant particles is investigated experimentally in a von K\'arm\'an flow. Increasing the particle volume fraction $\Phi_\mathrm{v}$, maintaining constant impellers Reynolds number attenuates the fluid turbulence. The inertial-range energy transfer rate decreases as $\propto\Phi_\mathrm{v}^{2/3}$, suggesting that only particles located on a surface affect the flow. Small-scale turbulent properties, such as structure functions or acceleration distribution, are unchanged. Finally, measurements hint at the existence of a transition between two different regimes occurring when the average distance between large particles is of the order of the thickness of their boundary layers.Comment: 7 pages, 4 figure

Subtraction of temperature induced phase noise in the LISA frequency band

Temperature fluctuations are expected to be one of the limiting factors for gravitational wave detectors in the very low frequency range. Here we report the characterisation of this noise source in the LISA Pathfinder optical bench and propose a method to remove its contribution from the data. Our results show that temperature fluctuations are indeed limiting our measurement below one millihertz, and that their subtraction leads to a factor 5.6 (15 dB) reduction in the noise level at the lower end of the LISA measurement band 10^{-4} Hz, which increases to 20.2 (26 dB) at even lower frequencies, i.e., 1.5x10^{-5} Hz. The method presented here can be applied to the subtraction of other noise sources in gravitational wave detectors in the general situation where multiple sensors are used to characterise the noise source.Comment: 8 pages, 6 figure