On the miscible Rayleigh-Taylor instability: two and three dimensions

We investigate the miscible Rayleigh-Taylor (RT) instability in both 2 and 3 dimensions using direct numerical simulations, where the working fluid is assumed incompressible under the Boussinesq approximation. We first consider the case of randomly perturbed interfaces. With a variety of diagnostics, we develop a physical picture for the detailed temporal development of the mixed layer: We identify three distinct evolutionary phases in the development of the mixed layer, which can be related to detailed variations in the growth of the mixing zone. Our analysis provides an explanation for the observed differences between two and three-dimensional RT instability; the analysis also leads us to concentrate on the RT models which (1) work equally well for both laminar and turbulent flows, and (2) do not depend on turbulent scaling within the mixing layer between fluids. These candidate RT models are based on point sources within bubbles (or plumes) and interaction with each other (or the background flow). With this motivation, we examine the evolution of single plumes, and relate our numerical results (of single plumes) to a simple analytical model for plume evolution.Comment: 31 pages, 27 figures, to appear in November issue of JFM, 2001. For better figures: http://astro.uchicago.edu/~young/ps/jfmtry08.ps.

Enabling Massive Deep Neural Networks with the GraphBLAS

Deep Neural Networks (DNNs) have emerged as a core tool for machine learning. The computations performed during DNN training and inference are dominated by operations on the weight matrices describing the DNN. As DNNs incorporate more stages and more nodes per stage, these weight matrices may be required to be sparse because of memory limitations. The GraphBLAS.org math library standard was developed to provide high performance manipulation of sparse weight matrices and input/output vectors. For sufficiently sparse matrices, a sparse matrix library requires significantly less memory than the corresponding dense matrix implementation. This paper provides a brief description of the mathematics underlying the GraphBLAS. In addition, the equations of a typical DNN are rewritten in a form designed to use the GraphBLAS. An implementation of the DNN is given using a preliminary GraphBLAS C library. The performance of the GraphBLAS implementation is measured relative to a standard dense linear algebra library implementation. For various sizes of DNN weight matrices, it is shown that the GraphBLAS sparse implementation outperforms a BLAS dense implementation as the weight matrix becomes sparser.Comment: 10 pages, 7 figures, to appear in the 2017 IEEE High Performance Extreme Computing (HPEC) conferenc

Lavoro e impresa digitale tra norme nazionali ed economia transnazionale = Work and digital enterprise between national norms and transnational economy. WP C.S.D.L.E. “Massimo D’Antona”.IT – 405/2019

The present paper aims at giving a general overview of digital work in Italian and EU labour law. The first part of the research is dedicated to the issue of legal qualification in digital relationships. Specifically, it is assessed, on the one hand, if the traditional characteristics of the employment contract fit these relationships while, on the other hand, who the employer is in digital work, with special regard to platform work. Then, the supranational and transnational dimensions of digital work are explored, to see how EU and transnational labour law have regulated the phenomenon concerned. After that, digital work is analysed from a collective labour law perspective, to see which actions Trade Unions have taken. Finally, the present Italian legal framework of digital work is assessed and an inventory of general protections for digital workers is drawn

The Ryanair case in the Italian and European framework: who decides the rules of the game? WP CSDLE “Massimo D’Antona”.INT – 148/2019

The present article aims at analysing the Ryanair case in the Italian and European framework from a twofold point of view: labour law and industrial relations. Firstly, the working conditions in the airline company are described. It is well known that Ryanair requires an absolute secrecy about this. However, the employment dimensions reached by the company and the increasing union conflict have brought out recently documentation and evidences which let us to have a clear view of the current labour relationships in Ryanair. The intention here is therefore to provide an overview of the Ryanair’s working conditions, verifying if the airline company adopts a strategy of limitation of employment protections directed to reduce costs and increase productivity. In the light of this, it is necessary to understand if EU transnational labour law has sufficient instruments to face law and forum shopping or if, on the contrary, the socalled ‘EU social deficit’ has affected also this sector, giving life to gaps exploited by companies to subjugate labour relationships to the most advantageous legislation and competent jurisdiction for themselves. Taking into account all of this, secondly, an overview of transnational issues (mainly concerning employment contract law, social security law and competent jurisdiction) is provided, considering also the solutions offered by the EU Institutions, so far. Thirdly, the Ryanair approach to industrial relations is addressed, in order to understand the position of the Irish company in respect of Trade Unions. Even here, the transnational nature of labour relationships in Ryanair produces important implications both making more difficult building a collective interest among workers and facilitating the escape from any kind of confrontation with Trade Unions and the rejection of the collective phenomenon itself in its multifaced dimensions by the company. Finally, some conclusions are drawn, with the aim of pointing out the importance of the Ryanair case in the European and Italian legal and industrial relations framework

Simulations of Astrophysical Fluid Instabilities

We present direct numerical simulations of mixing at Rayleigh-Taylor unstable interfaces performed with the FLASH code, developed at the ASCI/Alliances Center for Astrophysical Thermonuclear Flashes at the University of Chicago. We present initial results of single-mode studies in two and three dimensions. Our results indicate that three-dimensional instabilities grow significantly faster than two-dimensional instabilities and that grid resolution can have a significant effect on instability growth rates. We also find that unphysical diffusive mixing occurs at the fluid interface, particularly in poorly resolved simulations.Comment: 3 pages, 1 figure. To appear in the proceedings of the 20th Texas Symposium on Relativistic Astrophysic

Optimal, scalable forward models for computing gravity anomalies

We describe three approaches for computing a gravity signal from a density anomaly. The first approach consists of the classical "summation" technique, whilst the remaining two methods solve the Poisson problem for the gravitational potential using either a Finite Element (FE) discretization employing a multilevel preconditioner, or a Green's function evaluated with the Fast Multipole Method (FMM). The methods utilizing the PDE formulation described here differ from previously published approaches used in gravity modeling in that they are optimal, implying that both the memory and computational time required scale linearly with respect to the number of unknowns in the potential field. Additionally, all of the implementations presented here are developed such that the computations can be performed in a massively parallel, distributed memory computing environment. Through numerical experiments, we compare the methods on the basis of their discretization error, CPU time and parallel scalability. We demonstrate the parallel scalability of all these techniques by running forward models with up to $10^8$ voxels on 1000's of cores.Comment: 38 pages, 13 figures; accepted by Geophysical Journal Internationa

Improved graph SLAM with Open-StreetMap priors

LAUREA MAGISTRALESimultaneous Localization And Mapping (SLAM) consiste nella costruzione simultanea di un modello dell’ambiente (la mappa) e nella stima della posa del robot che si muove al suo interno. Per ottenere una ricostruzione accurata della scena, nell’ultimo decennio sono stati proposti molti sistemi SLAM basati su LiDAR, che sfruttano dense nuvole di punti (point clouds) per tracciare il movimento del robot e costruire una mappa 3D. I sistemi esistenti, tuttavia, di solito non includono informazioni aggiuntive come l’utilizzo di mappe preesistenti (map priors), che possono rivelarsi utili nell’intero processo SLAM. Per questo motivo, miglioriamo un sistema esistente che include mappe 2D preesistenti per risolvere il problema SLAM. Le mappe corrispondono alle forme degli edifici di una città e sono fornite da OpenStreetMap (OSM). Elaborando i dati provenienti da un LiDAR, il robot percepisce gli edifici circostanti e cerca di abbinarli alla mappa precedente. In questo modo, il robot simultaneamente si localizzerà e correggerà le mappe mentre si muove. Per affrontare questo problema, viene utilizzato e migliorato un sistema SLAM basato su grafi esistente (baseline), come contributo principale di questa tesi. Il sistema è costituito da un grafo (grafo delle pose) i cui nodi corrispondono alle pose del robot e degli edifici e i cui archi rappresentano vincoli tra le pose. Per migliorare la baseline, abbiamo sviluppato diversi algoritmi e procedure. Inizialmente abbiamo implementato una procedura per scaricare e bufferizzare in modo asincrono le mappe per gestire il lungo tempo di risposta dei server di OSM. Per estrarre le caratteristiche rilevanti dalle point cloud del LiDAR, abbiamo implementato un algoritmo per estrarre le linee usando RANSAC. Inoltre presentiamo un algoritmo di corrispondenza delle point cloud (scan matching) basato su linee per estrarre i vincoli (map priors) dalla mappa di OSM insieme alla sua estensione per generare vincoli geometrici al fine di non far sovrapporre gli edifici nella mappa. Infine, sfruttando l’API g2o, abbiamo partizionato il grafo delle pose per bilanciare i vincoli sulla localizzazione e quelli estratti dalla mappa. Abbiamo confrontato il sistema sviluppato con la baseline utilizzando un dataset di KITTI, dimostrando che il nostro sistema ha notevolmente migliorato la baseline, riducendo l’errore di localizzazione del robot e generando mappe visivamente più coerenti.Simultaneous Localization And Mapping (SLAM) consists in the concurrent construction of a model of the environment (the map), and the estimation of the pose of the robot moving within it. To obtain accurate reconstruction of the scene, in the last decade many LiDAR-based SLAM systems have been proposed, exploiting dense point clouds to track the motion of the robot and build a 3D map. Existing systems, however, usually do not include additional information like prior maps, which may prove to be useful in the whole SLAM process. For this reason, we improve an existing system that includes 2D maps information (prior map) to solve the SLAM problem. The maps correspond to the shapes of the buildings of a city and are provided by OpenStreetMap (OSM). By processing data coming from an on-board LiDAR, the robot perceives the surrounding buildings and tries to match them with the prior map. This way, the robot will simultaneously localize itself and correct the maps while moving. To deal with this problem, an existing Graph-based SLAM system (baseline) is used and improved, as main contribution of this thesis. The system consists of a graph (pose graph) whose nodes correspond to the poses of the robot and the buildings (i.e. their centroids) and whose edges represent constraints between the poses. To improve the baseline system, we developed different algorithms and procedures. First we implemented a procedure to asynchronously download and buffer the maps to deal with the long OSM servers response time. In order to extract the relevant features from the LiDAR point clouds, we implemented an algorithm to extract line segments using RANSAC. Moreover, we present a line segments based scan matching algorithm to extract the map priors along with its extension to generate geometrical constraints in order to prevent the overlapping of the buildings within the map. Finally, leveraging the g2o API, we partitioned the pose graph in order to balance the localization and map constraints. We compared the developed system against the baseline using the KITTI dataset, showing that our system has greatly improved the baseline system, reducing the robot localization error and generating visually more consistent maps

Large-Scale Simulations of Clusters of Galaxies

We discuss some of the computational challenges encountered in simulating the evolution of clusters of galaxies. Eulerian adaptive mesh refinement (AMR) techniques can successfully address these challenges but are currently being used by only a few groups. We describe our publicly available AMR code, FLASH, which uses an object-oriented framework to manage its AMR library, physics modules, and automated verification. We outline the development of the FLASH framework to include collisionless particles, permitting it to be used for cluster simulation.Comment: 3 pages, 3 figures, to appear in Proceedings of the VII International Workshop on Advanced Computing and Analysis Techniques in Physics Research (ACAT 2000), Fermilab, Oct. 16-20, 200