Méthode d'analyse des écoulements 3D à surface libre

Free surface flows are extremely complex and appear in various cases such as chemistry, oceanography, thermodynamics and turbomachinery. Their complexity is due to the fact that they are composed of two distinct phases which are only separated by a thin interface through which energy and heat transfers can occur. There are different types of free surface flows, such as liquid-liquid, illustrated by the injection of an oil droplet in water, and liquid-gas, for example bubbles in boiling water. We have focused our study on free surface flows in the field of hydraulic turbomachinery, and more precisely in Pelton turbine buckets. The theoretical and experimental studies conducted up to now have considerably enlightened the physics linked to these applications. However, there are still many mysteries left to be explored and the time spent during the design phase could certainly be reduced. Unfortunately, the codes needed to numerically model these flows have only recently appeared on the market and their reliability is therefore constantly questioned. In this study, we propose the development of a free surface flow analysis method based on a test case which approaches the Pelton bucket : the deviation of a water jet by a plane plate. This case allowed us to understand the physics of the flow, especially by comparing numerical and experimental results. Finally, based on the previous study, the flows within non-rotating and rotating 3D buckets were simulated. In parallel, transient measurements of the flows brought new insight on the detailed behavior of the water in the buckets. These numerical and experimental results were then compared in order to offer a complete validation of the proposed method.

Navigating the validity tradeoffs of entrepreneurship research experiments: A systematic review and best-practice suggestions

Jérémy Binder

Experimental methods provide important advantages for advancing academic understanding of entrepreneurship. Yet, the complex and comingled relationships between some of entrepreneurship's key characteristics pose thorny methodological challenges to entrepreneurship researchers notably to negotiate important tradeoffs between the ideals of external and construct validity. To facilitate the sound mobilization of experimental methods in entrepreneurship research, we present an overview of critical validity challenges plaguing entrepreneurship research experiments and assess the validation practices mobilized in 144 studies using such methods. Building on these findings, we develop a practical guide of actionable validation strategies to help experimenters navigate the above tradeoffs and conduct entrepreneurship research experiments that are realistic, theoretically meaningful, and that help establish the causal effects of their focal variables. By doing so, we contribute a set of pragmatic means to support the mobilization of experimental methods for advancing entrepreneurship research. Executive summary: Experimental methods hold important advantages for advancing the theoretical and practical understanding of entrepreneurial action. The power of research experiments lies in their ability to yield convincing evidence supporting the causal effects of the factors they investigate. Unfortunately, entrepreneurship research experiments are often criticized for offering pale copies of both the realities they attempt to model and the theoretical constructs they aim to study. The entrepreneurial process and its related activities count many intertwined characteristics-such as radical uncertainty, temporal dynamics, high personal stakes and other constraints-that can prove difficult to integrate in experimental studies. Moreover, these characteristics' comingled relationships raise important tensions with experimental methods' core principle of surgically focusing on the causal effects of a few manipulated variables. As a result, entrepreneurship research must overcome a number of validity challenges and tradeoffs in order to successfully leverage experimental methods and offer findings that convincingly support the causality of their theoretical developments. Compounding these difficulties, the guidance typically offered in research methods textbooks tends to focus on generic issues that are altogether separate from the specific challenges of conducting valid entrepreneurship research experiments. Because of this, many research manuscripts mobilizing experimental methods arrive at the review process with important shortcomings. This threatens the field's knowledge accumulation and typically calls for authors to refine their work and collect additional data. To help researchers face these challenges, we offer a three-part compendium focused on bolstering the validity of entrepreneurship research experiments. First, we complement the generic observations of research methods monographs by presenting an overview of the validity tradeoffs inherent to mobilizing experimental methods in entrepreneurship research - and of strategies for addressing these. Second, and to demonstrate that overcoming these challenges is not a trivial task, we conduct a structured literature review of the external and construct validation strategies deployed in a comprehensive corpus of 144 relevant articles published in both entrepreneurship-focused journals and broader generalist journals from the applied behavioral and social sciences. Among the most positive elements emerging from our analyses, we note upward trends in the mobilization of pilot tests, in the conduct of multiple experiments within single papers as well as in the use of parallel studies using different data collection methods. We also observe increased efforts to explain the practical relevance of experimental findings. To our surprise, however, our analyses uncover downward trends in the mobilization of pre-tests to examine the research material's external validity and representativeness, as well as the continued publication of studies that do not report empirical evidence regarding their focal manipulations' construct validity. This is concerning. Such practices undermine these experiments' abilities to yield convincing evidence in support of their theorized causal effects - and for what these imply for fostering entrepreneurial action. Third, and in light of the results obtained, we develop a practical guide of actionable strategies for navigating the validity tradeoffs of entrepreneurship research experiments. Grouping these strategies together allows us to systematize recommendations typically found across various methods monographs, thereby offering an overall scheme to help entrepreneurship researchers navigate the validity tradeoffs inherent to conducting realistic and theoretically meaningful experiments. To make our recommendations as actionable as possible, we develop an extensive step-by-step guide of design and assessment strategies relevant to entrepreneurship research experiments. The guide spans the entire research process - from conceiving an experimental study to collecting, analyzing and reporting the results. The guide also includes a number of relevant exemplars from many different studies we analyzed. By discussing these entrepreneurship-specific validity tradeoffs and combining that with an overview of strategies for addressing them, our study offers an entrepreneurship-centered synthesis that equips entrepreneurship researchers with the necessary tools for conducting experimental research that advances understanding of the causal effects supporting entrepreneurial action and its related phenomena.

ELSEVIER SCIENCE BV2019

Perturbative Techniques in Hadron Collider Physics

Paolo Torrielli

High-energy particle physics is going through a crucial moment of its history, one in which it can finally aspire to give a precise answer to some of the fundamental questions it has been conceived for. On the one side, the theoretical picture describing the elementary strong and electroweak interactions below the TeV scale, the Standard Model, has been well consolidated over the decades by the observation and the precise characterization of its constituents. On the other hand, the enormous technological potentialities nowadays available, and the skills accumulated in decades of collider experiments with increasingly high complexity, render for the first time plausible the possibility of addressing complicated and conceptually deep questions like the ones at hand. The best incarnation of this high level of sophistication is the CERN Large Hadron Collider (LHC), the most powerful experimental apparatus ever built, which is designed to shed light on the true nature of fundamental interactions at energies never attained before, and which has already started to open a new era in physics with the recent discovery of the longed-for Higgs boson, a true milestone for the human knowledge as well as one of the most important discoveries in the modern epoch. The knowledge that has been and is going to be reached in these crucial years would of course not be conceivable without a deep interplay between the theoretical and the experimental efforts. In particular, on the theoretical side, not only there are wide groups of researchers devoted to building possible extensions to the Standard Model, which draws the guidelines of current and future experiments, but also there is a vast community whose research is rather aimed at the precise predictions of all the physical observables that could be measured at colliders, and at the systematic improvement of the approximations that currently constrain such predictions. On top of representing the state-of-the-art of the human understanding of the properties that regulate elementary-particle interactions and of the formalisms that describe them, the developments of this line of research have an immediate and significant impact on experiments. Firstly, these detailed calculations are the very theoretical predictions against which experimental data are compared, so they are crucial in establishing the validity or not of the theories according to which they are performed. Secondly, the signals one wants to extract from data at modern colliders are so tiny and difficult to single out that the experimental searches themselves need be supplemented by a detailed work of theoretical modelling and simulation. In this respect, high-precision computations play an essential role in all analysis strategies devised by experimental collaborations, and in many aspects of the detector calibration. It is clear that, for theoretical computations to be useful in experimental analyses and simulations, the predictions they yield should be reliable for all possible configurations of the particles to be detected. Thus the key feature for the present theoretical collider physics is not particularly the computation of observables with high precision only in a limited region of the phase space, but the capability of combining (‘matching’) in a consistent way different approaches, each of which is reliable in a particular kinematic regime. With this perspective, matching techniques represent one of the most promising and successful theoretical frameworks currently available, and are considered as eminently valuable tools both on the theoretical and on the experimental sides. Matched computations are based on a perturbation-theory approach for the description of configurations in which the scattering products are well separated and/or highly energetic: in particular the precision currently attained for all but a few of the relevant processes within the Standard Model is the next-to-leading order (NLO) in powers of the strong quantum-chromodynamics (QCD) coupling constant αS; for the description of configurations in which the particles outgoing the collisions are close to each other and/or have low energy, it can be shown that the perturbation-theory expansion breaks down, and then a complementary method, like the parton shower Monte Carlo (PSMC), has instead to be employed. The task of matching is precisely that of giving a prediction that interpolates between the two approaches in a smooth and theoretically-consistent way. This thesis is focused on MC@NLO, a high-energy physics formalism capable of matching computations performed at the NLO in QCD to PSMC generators, in such a way as to retain the virtues of both approaches while discarding their mutual deficiencies. In particular, the thesis reports on the work successfully achieved in extending MC@NLO from its original numerical implementation, tailored on the HERWIG PSMC, to the other main PSMC programs currently employed by experimental collaborations, PYTHIA and Herwig++, confirming the advocated universality of the method. Differences in the various realizations are explained in detail both at the formal level and through the simulation of various Standard-Model reactions. Moreover we describe how the MC@NLO framework has been developed so as to render its implementation automatic with respect to the physics process one is about to simulate: beyond yielding an enormous increase in its potential for present and future collider phenomenology, and upgrading the standard of precision for high-energy computations to the NLO+PSMC level, this development allows for the first time the application of the MC@NLO formalism to a huge number of relevant and highly complicated reactions, through an implementation which is also easily usable by people well-outside the community of experts in QCD calculations. As example of this new version, called aMC@NLO, recent results are presented for complex scattering processes, involving four or five final-state particles. Finally, possible extensions of the framework to theories beyond the Standard Model, like the supersymmetric version of QCD, are briefly introduced.

EPFL2012

Experimental studies of the equilibrium and out-of-equilibrium electronic structure of non-symmorphic topological materials

Gianmarco Gatti

Symmetry and topology are fundamental properties of nature. Mathematics provides us with a general framework to understand these concepts. On one side, symmetry describes the invariance properties of an object for specific transformations. On the other side, topology classifies objects under continuous deformations. Two objects with different topologies cannot be deformed one into each other without creating or annihilating a singularity, sometimes referred to as 'node'. These concepts gradually found applications in physics, namely in the description of the electronic properties of solids, which is the focus of this thesis. Symmetry and topology protect special nodes in the band structure of a crystal, where several states are degenerate in energy. Close to a node, the electron wave function obeys the Dirac or Weyl Hamiltonians, which were formally introduced to describe fermions in high-energy particle physics. These exotic fermions exhibit unique optical and transport properties, fully manifesting their quantum nature. Symmetry guides us in the search for which classes of materials may host topological nodes. Recently the attention of the scientific community has been attracted by crystals with non-symmorphic symmetries (screw axes and glide planes), as promising candidates for topological phases. This thesis focuses on two families of non-symmorphic crystals, whose topological properties have been investigated by combining conventional angle-resolved photoelectron spectroscopy (ARPES) with state-of-the-art spin-resolved (spARPES) and time-resolved ARPES (trARPES). The experimental results are supported by calculations carried out in collaboration with theory groups. ZrSiTe and ZrSiSe belong to the same class of non-trivial semimetals. They host Dirac electrons with large mobility. The results of my spARPES experiments clarify that spin-orbit interaction (SOI) not only removes the topological nodes in ZrSiTe, but also induces a 'hidden' spin polarization of its bulk electronic states, otherwise forbidden by the inversion symmetry of the lattice. Moreover, trARPES data provide evidence that the electron-electron interaction is only partially screened in the metallic state of ZrSiSe. As a consequence, the band velocity is enhanced, at odds with general expectations. More importantly, I show that this band renormalization can be controlled at the ultrafast time scale (namely fs scale, with 1 fs=10^{-15} s) via intense optical excitation, paving the way for engineering the band structure of Dirac semimetals. Tellurium is a chiral semiconductor, with a small and direct band gap. The low-symmetry of its lattice and the simple chemical composition make it the ideal case to study the interplay between symmetry and topology. With ARPES I determine several Weyl nodes in its electronic structure. By means of spARPES, I demonstrate that in their surrounding the spin exhibits a hedgehog configuration. This observation is new and it highlights the connection between spin-dependent and topology-related properties in Te. Finally, I illustrate promising preliminary results based on trARPES that explore the appealing possibility of optically controlling the topology of the electronic structure of Te upon excitation of coherent phonons.

EPFL2020