La résilience académique des étudiants d'université

National audienceil s'agit dans cet article d'évoquer la résilience académique, notamment celles des étudiants d'université. Plusieurs vignettes cliniques servent d'illustration

Super Bound States in the Continuum on Photonic Flatbands: Concept, Experimental Realization, and Optical Trapping Demonstration

In this work, we theoretically propose and experimentally demonstrate the formation of a super bound state in a continuum (BIC) on a photonic crystal flat band. This unique state simultaneously exhibits an enhanced quality factor and near-zero group velocity across an extended region of the Brillouin zone. It is achieved at the topological transition when a symmetry-protected BIC pinned at $k=0$ merges with two Friedrich-Wintgen quasi-BICs, which arise from destructive interference between lossy photonic modes of opposite symmetries. As a proof-of-concept, we employ the super flat BIC to demonstrate three-dimensional optical trapping of individual particles. Our findings present a novel approach to engineering both the real and imaginary components of photonic states on a subwavelength scale for innovative optoelectronic devices

Effets du parcours d'adoption sur la parentalité des adultes adoptés

International audienc

Vortex Pair in ground vicinity : Optimal Perturbation and Optimal Control

The risk of wake vortex encounter is a major issue in aviation. This is notably true in the vicinity of airports during take-o? and landing. In order to reduce wake vortex encounters, conservative separation distances are applied. However, these distances impede on the increase of the frequency of arrivals and departures at airports by the steadily increasing tra?c. In the present work, an analysis of the control of the two-dimensional vortex dynamics close and at the ground as a mean to reduce the separation distance is carried out. The behavior of vortices in ground e?ect has been a much investigated subject, motivated by this aviation issue but also by wall bounded turbulence (Stephan et al. 2013). It has been demonstrated that vortices (Harvey et al. 1971) rebound at the ground instead of going sideways like in the inviscid situation (Lamb 1932). Vortex rebound causes vortices to stay longer above runways and increase the risk of encounters. Two control strategies are described in order to alleviate the aforementioned issue. The ?rst one is based on an optimal perturbation approach aiming at mitigating vortices by increasing perturbations growth. An analysis of the e?ect of the perturbation symmetry and horizon time, along with a physical mechanism of the transient growth processes is achieved. The second approach is based on the optimal control of the vortex position through the implementation of active blowing/suction at the ground. The maximization of the lateral position of the vortices is achieved with the idea of suppressing vortex rebound, and promoting an inviscid like kinematics of the vortices (the vortices move sideways out of the runway). The method achieves a 50% increase in the lateral position of the vortices. A physical analysis of the change induced by the control on the ?ow is detailed

Measures de vent 3D avec le lidar Doppler coherent Live à bord d'un avion

International audienceA three-dimensional (3D) wind profiling Lidar, based on the latest high power 1.5 µm fiber laser development at Onera, has been successfully flown on-board a SAFIRE (Service des Avions Français Instrumentés pour la Recherche en Environnement) ATR42 aircraft. The Lidar called LIVE (LIdar VEnt) is designed to measure wind profiles from the aircraft down to ground level, with a horizontal resolution of 3 km, a vertical resolution of 100 m and a designed accuracy on each three wind vector components better than 0.5 m.s −1. To achieve the required performance, LIVE Lidar emits 410 µJ laser pulses repeating at 14 KHz with a duration of 700 ns and uses a conical scanner of 30 • total opening angle and a full scan time of 17 s.Un lidar vent 3D, basé sur le dernier développement de laser à fibre de 1,5 µm à haute puissance de l’ONERA a été testé avec succès à bord d’un avion SAFIRE ATR42. Le lidar appelé LIVE est conçu pour mesurer les profils de vent de l’avion jusqu'au sol, avec une résolution horizontale de 3 km, une résolution verticale de 100 m et une précision calculée supérieure à 0,5 m / s pour chaque composante du vecteur du vent

Clear air turbulence detection and characterisation in the delicat airborne lidar project

We report on a development of a long-range airborne UV high spectral resolution lidar, intended for the detection and characterisation of clear air turbulence (CAT). The detection of turbulence is based on the measurement of density fluctuations associated with the movement of turbulent air masses. These density fluctuations are measured by the variations in the molecular backscatter coefficient which is determined from the lidar signal by spectrally separating it from the aerosol backscatter. After an introduction, we review the CAT detection principle and describe the lidar system design. We then present the expected performance of the system and give an overview on the planned measurement campaign