3 results
Search Results
Now showing 1 - 3 of 3
- ItemInnovative and automated method for vortex identification(Les Ulis : EDP Sciences, 2022) Canivete Cuissa, J. R.; Steiner, O.Context. As a universally accepted definition of a vortex has not yet been established, the community lacks an unambiguous and rigorous method for identifying vortices in fluid flows. Such a method would be useful for conducting robust statistical studies on vortices in highly dynamical and turbulent systems such as the solar atmosphere. Aims. We aim to develop an innovative and robust automated methodology for the identification of vortices based on local and global characteristics of the flow, while avoiding the use of a threshold that could potentially prevent the detection of weak vortices in the process. Methods. We present a new method that combines the rigor of mathematical criteria with the global perspective of morphological techniques. The core of the method consists of an estimation of the center of rotation for every point of the flow that presents some degree of curvature in its neighborhood. For this purpose, we employed the Rortex criterion and combined it with morphological considerations of the velocity field. We then identified coherent vortical structures based on clusters of estimated centers of rotation. Results. We demonstrate that the Rortex is a more reliable criterion than the swirling strength and the vorticity for the extraction of physical information from vortical flows, because it measures the rigid-body rotational part of the flow alone and is not biased by the presence of pure or intrinsic shears. We show that the method performs well in the context of a simplistic test case composed of two Lamb-Oseen vortices. We combined the proposed method with a state-of-the-art clustering algorithm to build an automated vortex identification algorithm. The algorithm was applied to an artificial flow composed of multiple Lamb- Oseen vortices, with a random noisy background, and to the turbulent flow of a simulated magneto-hydrodynamical Orszag-Tang vortex test. The results demonstrate the reliability and accuracy of the method. Conclusions. The present automated vortex identification method can be considered a new tool for the detection and study of vortices in dynamical and turbulent (magneto)hydrodynamical flows. By applying the implemented algorithm to numerical simulations and observational data, as well as comparing it to existing detection methods, we seek to successively improve the reliability of the detections and, ultimately, our knowledge on swirling motions in the solar, stellar, and planetary atmospheres.
- ItemAstraeus I: The interplay between galaxy formation and reionization(Oxford : Oxford Univ. Press, 2021) Hutter, Anne; Dayal, Pratika; Yepes, Gustavo; Gottlöber, Stefan; Legrand, Laurent; Ucci, GrazianoWe introduce a new self-consistent model of galaxy evolution and reionization, ASTRAEUS (seminumerical rAdiative tranSfer coupling of galaxy formaTion and Reionization in N-body dArk mattEr simUlationS), which couples a state-of-the-art N-body simulation with the semi-analytical galaxy evolution DELPHI and the seminumerical reionization scheme CIFOG. ASTRAEUS includes all the key processes of galaxy formation and evolution (including accretion, mergers, supernova, and radiative feedback) and follows the time and spatial evolution of the ionized regions in the intergalactic medium (IGM). Importantly, it explores different radiative feedback models that cover the physically plausible parameter space, ranging from a weak and delayed to a strong and immediate reduction of gas mass available for star formation. From our simulation suite that covers the different radiative feedback prescriptions and ionization topologies, we find that radiative feedback continuously reduces star formation in galaxies with Mh ≲ 109.5 M☉ upon local reionization; larger mass haloes are unaffected even for the strongest and immediate radiative feedback cases during reionization. For this reason, the ionization topologies of different radiative feedback scenarios differ only on scales smaller than 1–2 comoving Mpc, and significant deviations are found only when physical parameters (e.g. the escape fraction of ionizing photons) are altered based on galactic properties. Finally, we find that observables (the ultraviolet luminosity function, stellar mass function, reionization histories and ionization topologies) are hardly affected by the choice of the used stellar population synthesis models that model either single stars or binaries.
- ItemAstraeus - III. The environment and physical properties of reionization sources(Oxford : Oxford Univ. Press, 2021) Hutter, Anne; Dayal, Pratika; Legrand, Laurent; Gottlöber, Stefan; Yepes, GustavoIn this work, we use the ASTRAEUS (seminumerical rAdiative tranSfer coupling of galaxy formaTion and Reionization in Nbody dArk mattEr simUlationS) framework that couples galaxy formation and reionization in the first billion years. Exploring a number of models for reionization feedback and the escape fraction of ionizing radiation from the galactic environment (fesc), we quantify how the contribution of star-forming galaxies (with halo masses Mh > 108.2 M☉) to reionization depends on the radiative feedback model, fesc, and the environmental overdensity. Our key findings are: (i) for constant fesc models, intermediate-mass galaxies (with halo masses of Mh ≃ 109−11 M☉ and absolute UV magnitudes of MUV ∼ −15 to −20) in intermediate-density regions (with overdensity log10(1 + δ) ∼ 0−0.8 on a 2 comoving Mpc spatial scale) drive reionization; (ii) scenarios where fesc increases with decreasing halo mass shift, the galaxy population driving reionization to lower mass galaxies (Mh ≲ 109.5 M☉) with lower luminosities (MUV ≳ −16) and overdensities [log10(1 + δ) ∼ 0−0.5 on a 2 comoving Mpc spatial scale]; (iii) reionization imprints its topology on the ionizing emissivity of low-mass galaxies (Mh ≲ 109 M☉] through radiative feedback. Low-mass galaxies experience a stronger suppression of star formation by radiative feedback and show lower ionizing emissivities in overdense regions; (iv) a change in fesc with galaxy properties has the largest impact on the sources of reionization and their detectability, with the radiative feedback strength and environmental overdensity playing a sub-dominant role; (v) James Webb Space Telescope-surveys (with a limiting magnitude of MUV = −16) will be able to detect the galaxies providing ∼60−70 per cent (∼10 per cent) of reionization photons at z = 7 for constant fesc models (scenarios where fesc increases with decreasing halo mass).