Scaling laws of velocity circulation in quantum turbulence
mardi 29 septembre 2020, 10h30, Remote
Juan-Ignacio POLANCO - OCA, France
Vortices are highly rotating regions of fluid flows. Common examples are tornadoes or the wake of a river flow behind a solid obstacle. A measure of the intensity of a vortex
is provided by the velocity circulation, which is defined as the integral of the fluid velocity along a closed path surrounding the vortex. More generally, the circulation measures the rotation of the flow within a chosen path. Vortices in viscous flows are regions of high vorticity. In an idealised description, vortices may be seen as lines in space with vanishing thickness. This precise limit is in fact observed in nature, concretely in quantum fluids such as low-temperature liquid helium or Bose–Einstein condensates, where vortex filaments are discrete lines, each carrying a fixed (or quantised) circulation. Quantum flows can achieve a turbulent state not unlike classical flows. Quantum turbulence is characterised by a chaotic tangle of mutually interacting vortex filaments. It has long been suggested that, at large scales, such dynamics displays strong similarities with classical turbulence, notably exhibiting Kolmogorov's K41 phenomenology predictions. Nevertheless, the collective dynamics of turbulent structures (such as vortex filaments) at these scales is not fully understood. Scale-dependent quantities, such as the velocity circulation, may provide a better understanding of the spatial arrangement of these structures. Inspired by a recent study in classical flows, we consider the scale dependence of circulation statistics in quantum turbulence. We show that circulation moments follow two distinctive power law scalings, at small and large scales. For loops smaller than the mean inter-vortex distance, circulation moments display extreme intermittent behaviour due to the quantisation of circulation, in stark contrast with classical flows. At larger scales, K41 predictions are recovered, up to intermittency deviations that are strikingly close to those observed in classical turbulence. To date, this is the most convincing evidence of intermittency in the large scales of quantum turbulence. Our results strongly reinforce the analogy between classical and quantum turbulence, putting forward the idea of using quantum flows as a tool to improve our understanding of classical turbulence.
mardi 6 octobre 2020, 10h30, Remote or in-person
Dylan KLOSTER - OCA, France
Seismic data: the more the better, and how to deal with it
jeudi 8 octobre 2020, 11h00, Géoazur - Salle de conférence - Bat. 4
Malgozata Chmiel (ETH)
Ambient noise seismology has revolutionized seismic imaging and monitoring of the Earth's subsurface. Rapid development of ambient noise seismology would not have been possible without recent advances in seismic acquisition that
increase spatial sampling of seismic arrays and duration of seismic operations. The growing volume of ambient noise recordings calls for novel approaches allowing time-efficient processing of large seismic datasets in an automated manner. In this seminar, I will present an overview of methods to image and monitor the Earth’s subsurface by using ambient seismic noise recorded on dense arrays. I will start with an introduction to ambient noise seismology. I will then talk about an automated approach for noise source localization and dominant-weak source separation by using matched-field processing. Next, I will show how we can use surface waves generated by noise sources to image the underlying Earth structure. I will present and compare both ambient noise surface wave tomography based on picking group-velocity dispersion curves and a new surface wave tomography approach (“Seismic Michelson interferometer”) based on seismic interferometry. The presented methods will be illustrated with a case study using seismic data recorded on dense arrays in passive and active configurations at the local (array aperture < 10 km) scale.
Formation and composition of giant planets
mardi 13 octobre 2020, 10h30, Remote Seminar
Bertram Bitsch (MPIA, Heidelberg)
The formation of the cores of giant planets was long though to happen through the accretion of planetesimals. However, the accretion of planetesimals renders core accretion times longer than the disc
life time in the outer regions of protoplanetary discs. Instead, the accretion of mm-cm size particles, so called pebbles, can accelerate the process tremendously. When the planet reaches a mass of around 10 Earth masses, gas accretion can start and the planet can become as gas giant. At the same time, the planet interacts with the protoplanetary disc and migrates through it. In this talk, I will first briefly review the concepts of planetesimal and pebble accretion as well as planet migration. I will then show how giant planets can form in the models and how their subsequent evolution can be constraint by the observed eccentricity distribution of giant planets and their relation to super-Earths. In addition, I will show how the heavy element content of giant planets is influenced by the accretion of pebbles, planetesimals and gas enriched from evaporating pebbles.
Vacances de la Toussaint – pas de séminaire
mardi 20 octobre 2020
Vacances de la Toussaint – pas de séminaire
mardi 27 octobre 2020
The challenge of robust orbital measurements of directly-imaged exoplanets and brown dwarfs
mardi 17 novembre 2020
Anne-Lise Maire, Université de Liège
Orbital monitoring of circumstellar systems is complementary to the spectrometry of the atmospheres as it allows for analyzing their architecture, dynamical state and evolution, and formation. It is required to derive
the orbital parameters and robust model-independent mass measurements. Measuring these parameters for imaged exoplanets and brown dwarfs can lead to degeneracies and biases because a small fraction of the orbit is typically sampled (<20%). I will discuss these issues and methods that I developed to mitigate them in my studies. Precise and robust measurements of the position of the companions over time are critical. I showed the good and stable astrometric calibration of VLT/SPHERE since it has been made available to the community in 2015. This has allowed for precise position measurements down to ~1–3 mas, whereas the precision reached in pre-SPHERE studies was ~10 mas. My work has served as a reference for various studies carried out in the guaranteed-time survey and in open-time programs: the discovery of a handful of exoplanets and brown dwarfs, the rejection of companion candidates as background objects, the measurement of the position and orbit of a few tens of (sub)stellar companions, the analysis of companion-disk dynamical interactions and of the motion of disk features. I will also discuss the lessons learned from the astrometric study of SPHERE for the development of the high-contrast imaging instruments on the ELT.
The X-ray Integral Field Unit on board the ESA’s Athena space X-ray observatory: current status and the initiative of reducing the travel carbon footprint of the project
mardi 24 novembre 2020, 10h30, Remote seminar: https://zoom.us/j/8463056090?pwd=WG1FYmk5bXg0KzJCNjk0NzIvbWtwdz09
Didier BARRET - IRAP, France