3,172 research outputs found
Using nonlinear optimization to understand coherent structures in turbulence and transition
No full textLo scopo di questa tesi è quello di investigare i principali meccanismi coinvolti in un flusso transizionale e/o turbolento. L'idea principale è quella di usare una tecnica di ottimizzazione non lineare per indagare l'origine e il ruolo delle strutture coerenti osservate in questi flussi. Questo metodo è stato utilizzato in tre diversi contesti.
Per prima cosa, partendo da un flusso laminare linearmente stabile, sono stati calcolati, tra tutti i disturbi in grado di innescare transizione alla turbolenza, quelli ottimali in termini di energia.
Una volta che la turbolenza è sviluppata, un ottimizzazione 3D non lineare basata sulla massimizzazione dell'energia cinetica turbolenta, è stata utilizzata per studiare le strutture coerenti che popolano i flussi turbolenti, e che sono responsabili della crescita (ottimale) e dello scambio dell'energia stessa.
Infine, la teoria dei sistemi dinamici è stata applicata alle equazioni dei fluidi. La crescita transitoria è stata utilizzata per studiare la geometria dello spazio delle fasi e per rivelare l'importanza della varietà stabile e instabile. Nello stesso framework, è stato utilizzato un algoritmo di minimizzazione non lineare per calcolare le connessioni eterocline tra soluzioni invarianti delle equazioni di Navier-Stokes.This thesis aims at unraveling the main mechanisms involved in
transitional and turbulent flows. The central idea is that of using a nonlinear
optimization technique to investigate the origin and role of coherent structures usually
observed in these flows. This method has been used in three different contexts. First, a
linearly stable laminar flow has been considered and the optimization has been used to
compute the most amplified perturbations among all disturbances able to trigger
transition to turbulence. Once turbulence is well established, a fully 3D nonlinear
optimization maximizing the turbulent kinetic energy is used to study coherent
structures populating turbulent shear flow as well as investigate the mechanisms
responsible for the energy (optimally) growth and exchange. Then, a dynamical system
approach is applied to fluid flow equations. The geometry of the state space is
investigated by using transient growth theory to reveal the importance of the stable and
unstable manifold. In the same framework, a nonlinear minimization algorithm is used
to compute heteroclinic connections among invariant solutions of the Navier-Stokes
equations.Cette thèse vise à démêler les principaux mécanismes impliqués dans les
écoulements transitoires et turbulents. L’idée centrale est d'utiliser une technique
d’optimisation non linéaire pour étudier l’origine et le rôle des structures cohérentes
habituellement observées dans ces écoulements. Cette méthode a été utilisée dans trois
contextes différents. Tout d’abord, un écoulement laminaire linéairement stable a été
considéré et l'optimisation a été utilisée pour calculer les perturbations les plus
amplifiées parmi toutes les perturbations capables de déclencher une transition vers la
turbulence. Une fois que la turbulence est bien établie, une optimisation non linéaire
entièrement 3D maximisant l'énergie cinétique turbulente est utilisée pour étudier les
structures cohérentes qui peuplent l’écoulement turbulent et les mécanismes
responsables de la croissance et de l’échange d’énergie (optimale) sont étudiés. Ensuite,
une approche de type système dynamique est appliquée aux équations du mouvement.
La géométrie de l’espace des phases est étudiée en utilisant la théorie de la croissance
transitoire pour évaluer l’importance des variétés stable et instable dans la dynamique.
Dans le même cadre, un algorithme de minimisation non linéaire est utilisé pour
calculer les connexions hétérocliniques parmi les solutions invariantes des équations de
Navier-Stokes
Optimal wave packets in a boundary layer and initial phases of a turbulent spot
No full textThe three-dimensional global optimal dynamics of a flat-plate boundary layer is
studied by means of an adjoint-based optimization in a spatial domain of long –
but finite – streamwise dimension. The localized optimal initial perturbation is
characterized by a pair of streamwise-modulated counter-rotating vortices, tilted
upstream, yielding at the optimal time elongated streaks of alternating sign in the
streamwise direction. This indicates that perturbations with non-zero streamwise
wavenumber have a role in the transient dynamics of a boundary layer. A scaling
law is provided, describing the variation of the streamwise modulation of the
optimal initial perturbation with respect to the streamwise domain length and to the
Reynolds number. For spanwise-extended domains, a near-optimal three-dimensional
perturbation is extracted during the optimization process; it is localized also in the
spanwise direction, resulting in a wave packet of elongated disturbances modulated in
the spanwise and streamwise directions. The nonlinear evolution of the optimal and
near-optimal perturbations is investigated by means of direct numerical simulations.
Both perturbations are found to induce transition at lower levels of the initial
energy than local optimal and suboptimal perturbations. Moreover, it is observed
that transition occurs in a well-defined region of the convected wave packet, close to
its centre, via a mechanism including at the same time oscillations of the streaks of
both quasi-sinuous and quasi-varicose nature. Hairpin vortices are observed before
transition; they have an active role in the breakdown of the streaks and result in a
turbulent spot which spreads out in the boundary layer
Hydrogen uptake and diffusion in Callovo-Oxfordian clay rock for nuclear waste disposal technology
No full textThe Callovo-Oxfordian clay-rich rock formation is currently considered as host rock barrier in the French geological repository facility for radioactive waste (Meuse/Haute-Marne). After the closure of the facility, hydrogen gas is expected to develop mainly from anaerobic corrosion of steel containers and other ironcontaining structures. Gas pressure build-up could impact the safety of the repository. It is therefore important to acquire in-depth knowledge on the interaction between hydrogen gas and surrounding clay rock in terms of uptake ability and diffusion. Hydrogen uptake capacity was evaluated on dried clay rock samples: (i) at 20K, to allow for hydrogen liquefaction and determine the maximal Hy uptake of the clay, and (ii) at typical pressure and temperature conditions expected to develop in the repository (up to 363 K and a hydrogen pressure of 40-60 bar). H-2 absorption on the dried raw Callovo-Oxfordian start to saturate at about 30-40 bar, and the average adsorption above 40 bar is about 0.1% in weight. Quasi-elastic neutron scattering spectroscopy was used to study the diffusion mechanism of hydrogen gas in the clay rock at the microscopic scale and to determine hydrogen self-diffusion coefficients in the dry samples in the temperature range 25-300 K. Neutron data suggested that hydrogen diffuses in the dry clay rock according to Fick's law. The findings reported in this work can help to better understand the behavior of H-2 in clay rock samples. (c) 2014 Elsevier Ltd. All rights reserved
Hydrogen adsorption and diffusion in synthetic Na-montmorillonites at high pressures and temperature
No full textSodium montmorillonite (Na-Mt) was synthesized with the aim to investigate the adsorption and diffusion of hydrogen gas in a model smectite at high pressures (up to 90 bar) and non-cryogenic temperature (363 K). Na-Mt samples were synthesized from hydrogels in mild conditions (493 K and autogenous pressure). Two further Na-Mt samples with different levels of structural iron were prepared to investigate the effect of iron on the textural and hydrogen adsorption properties. Structural and elemental analyses confirmed that well crystalline smectite samples were obtained according to the nominal chemical formulae. Nitrogen adsorption-desorption isotherms revealed that the synthesized materials have specific areas in the range 90-120 m(2)/g and are mainly mesoporous. High pressure volumetric measurements showed that hydrogen absorption at 363 K saturated between 40 and 60 bar, reaching 0.2 +/- 0.02 wt% (i.e. similar to 1.0 mmol/g) at the plateau. Quasielastic neutron scattering revealed ' that hydrogen diffuses inside the clay porous network according to the Fick's law (continuous diffusion), while jump diffusion cannot be excluded at distances lower than 6.3 angstrom, i.e. less than the one between two Na+ exchangeable ions. The hydrogen self-diffusion coefficients in the temperature range 25-300 K were determined to fall in the interval 0.1-1.0 10(-7) m(2) s(-1). The results are compared with H-2(g) adsorption and diffusion in other systems. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved
Perancangan Sistem Informasi Pengelola Barang/Inventaris Di Jc Komp
No full textInventory information system is a system used to enter inventory data into the database, so that there are no errors in input, output data, and reporting based on the desired data. based on surveys and interviews with jc comp personnel, information was obtained that the existing system in the jc comp warehouse section is still manual. therefore, the system that will be created by the author is the result of a replication of the existing system in the jc comp warehouse section. in addition to the process of input and output of goods, this information system is also equipped with features for creating data reports, input and output of goods, and searching for goods data by item name. with the inventory information system is expected to be useful for the warehouse parts jc comp. By implementing this system in the jc comp warehouse, it is hoped that it can reduce errors that may occur. this system is also expected to further speed up the process of input, output, and report generation, which in turn will help the jc comp warehouseSistem Informasi Persediaan Barang adalah sebuah sistem yang digunakan untuk memasukkan data-data persediaan barang ke dalam database, sehinggga tidak terjadi kesalahan dalam input, output data, dan pembuatan laporan berdasarkan data yang diinginkan. Berdasarkan survey dan wawancara dengan bagian personalia Jc Komp, didapatkan informasi bahwa sistem yang ada dibagian gudang Jc Komp masih manual. Oleh karena itu, sistem yang akan dibuat oleh penulis adalah hasil replikasi dari sistem yang telah ada dibagian gudang Jc Comp. Selain proses input dan output barang, pada sistem informasi ini juga dilengkapi fitur pembuatan laporan data, input, dan output barang, dan pencarian data barang berdasarkan nama barang. Dengan adanya Sistem Informasi persediaan barang ini diharapkan dapat bermanfaat bagi bagian gudang Jc Komp. Dengan diterapkannya sistem ini pada bagian gudang Jc Comp, maka diharapkan dapat mengurangi kesalahan-kesalahan yang mungkin terjadi. Sistem ini juga diharapkan dapat lebih mempercepat proses input, output, dan pembuatan laporan yang pada akhirnya dapat membantu bagian gudang Jc Komp
Amenable L-2-Theoretic Methods and Knot Concordance
No full textWe reveal new structures in the topological knot concordance group. As a key ingredient, we develop obstructions using L-2-theoretic methods for amenable groups in Strebel's class recently introduced by Orr and the author. Concerning (h)-solvable knots, which are defined in terms of certain Whitney towers of height h in bounding 4-manifolds, we show the following: for any n>1, there are (n)-solvable but non-(n. 5)-solvable (and therefore nonslice) knots, which are not detected by prior methods using Cochran-Orr-Teichner L-2-signature obstructions as well as Levine algebraic obstructions and Casson-Gordon invariants.X1197sciescopu
Dynamics of Network Formation Processes in the Co-Author Model
No full textThis article studies the dynamics in the formation processes of a mutual consent network in game theory setting: the Co-Author Model. In this article, a limited observation is applied and analytical results are derived. Then, 2 parameters are varied: the number of individuals in the network and the initial probability of the links in the network in its initial state. A simulation result shows a finding that is consistent with an analytical result for a state of equilibrium while it also shows different possible equilibria.Dynamics, Network, Game Theory, Model,Simulation, Equilibrium, Complexity
High-level polyomavirus JC viruria following long-term steroid therapy
No full textCASE REPORT JC virus is a highly seroprevalent ubiquitous polyomavirus which is acquired at an early age through respiratory or oral route, Thereafter JCV establishes persistent, but mainly asymptomatic, infections in various tissues, including the genitourinary tract and brain Corresponding author Cristina Costa, MD S.C.D.U. Virologia Azienda Ospedaliero-Universitaria San Giovanni Battista di Torino Via Santena, 9 -10126 Torino E-mail: [email protected] increasing with age, with adult prevalence rate often between 15% and 60
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