1,134 research outputs found
Model-based pre-ignition diagnostics in a race car application
Since 2014, Formula 1 engines have been turbocharged spark-ignited engines. In this scenario, the maximum engine power available in full-load conditions can be achieved only by optimizing combustion phasing within the cycle, i.e., by advancing the center of combustion until the limit established by the occurrence of abnormal combustion. High in-cylinder pressure peaks and the possible occurrence of knocking combustion significantly increase the heat transfer to the walls and might generate hot spots inside the combustion chamber. This work presents a methodology suitable to properly diagnose and control the occurrence of pre-ignition events that emanate from hot spots. The methodology is based on a control-oriented model of the ignition delay, which is compared to the actual ignition delay calculated from the real-time processing of the in-cylinder pressure trace. When the measured ignition delay becomes significantly smaller than that modeled, it means that ignition has been activated by a hot spot instead of the spark plug. In this case, the presented approach, implemented in the electronic control unit (ECU) that manages the whole hybrid power unit, detects a pre-ignition event and corrects the injection pattern to avoid the occurrence of further abnormal combustion
Canonical sampling through velocity-rescaling
We present a new molecular dynamics algorithm for sampling the canonical
distribution. In this approach the velocities of all the particles are rescaled
by a properly chosen random factor. The algorithm is formally justified and it
is shown that, in spite of its stochastic nature, a quantity can still be
defined that remains constant during the evolution. In numerical applications
this quantity can be used to measure the accuracy of the sampling. We
illustrate the properties of this new method on Lennard-Jones and TIP4P water
models in the solid and liquid phases. Its performance is excellent and largely
independent on the thermostat parameter also with regard to the dynamic
properties
Nuclear quantum effects in solids using a colored-noise thermostat
We present a method, based on a non-Markovian Langevin equation, to include
quantum corrections to the classical dynamics of ions in a quasi-harmonic
system. By properly fitting the correlation function of the noise, one can vary
the fluctuations in positions and momenta as a function of the vibrational
frequency, and fit them so as to reproduce the quantum-mechanical behavior,
with minimal a priori knowledge of the details of the system. We discuss the
application of the thermostat to diamond and to ice Ih. We find that results in
agreement with path-integral molecular dynamics can be obtained using only a
fraction of the computational effort.Comment: submitted for publicatio
Arboriculture Biologique : 11 annĂ©es d'expĂ©rimentation en vergers de pĂȘchers et pommiers
Un programme de recherche a Ă©tĂ© conduit en arboriculture biologique pendant 11 ans (1994-2004) Ă l'unitĂ© expĂ©rimentale INRA (Institut National de la Recherche Agronomique) de Gotheron dans le Sud-Est de la France, en partenariat avec AgribiodrĂŽme (F-26150 Die), asssociation de dĂ©veloppement de l'Agriculture Biologique (AB) en DrĂŽme, et le GRAB (Groupe de Recherche en Agriculture Biologique, F-84911 Avignon). Les objectifs Ă©taient : (i) acquĂ©rir des rĂ©fĂ©rences en arboriculture biologique ; (ii) identifier et analyser les verrous techniques de la production de pĂȘches et pommes biologiques, afin d'optimiser la conduite des vergers AB et/ou de proposer de nouvelles expĂ©rimentations ; (iii) Ă©valuer Ă long terme l'impact du mode de production pour le sol et la communautĂ© d'arthropodes du verger.
Le suivi longitudinal rĂ©alisĂ© dans les 3 vergers expĂ©rimentaux de pĂȘchers et pommiers a permis de souligner certaines contraintes en culture pĂ©renne AB, notamment pour la conduite du sol (entretien de la fertilitĂ©, enracinement limitĂ© par le travail du sol sur le rang pour limiter les adventices) et la protection du verger.
Probablement en rapport avec une fertilisation modĂ©rĂ©e et une conduite de la protection favorable Ă des rĂ©gulations naturelles, certains ravageurs ne se sont pas dĂ©veloppĂ©s : acariens phytophages, pucerons du pĂȘcher,... D'autres bio-agresseurs ont pu ĂȘtre contrĂŽlĂ©s par l'utilisation de mĂ©thodes biotechnique ou microbiologique : confusion sexuelle (tordeuse orientale), virus de la granulose (carpocapse),... En revanche, la maĂźtrise de certains bio-agresseurs a nĂ©cessitĂ© l'utilisation d'importantes quantitĂ©s de matiĂšres actives prĂ©sentant des effets non intentionnels (cuivre contre la cloque du pĂȘcher, soufre contre la tavelure du pommier) et/ou a Ă©tĂ© insuffisante (puceron cendrĂ© du pommier, maladies de conservation du pĂȘcher).
Le suivi de la teneur en nitrates de la solution du sol à différentes profondeurs a permis d'établir que le risque de lessivage, et donc de pollution par les nitrates, était nul pour notre stratégie de fertilisation et notre situation. Enfin, l'étude de la communauté des arthropodes dans le verger de pommiers AB (2001-2003) a mis en évidence une biomasse plus élevée et un cortÚge d'auxiliaires de structure différente (prédominance de prédateurs polyphages) par rapport à d'autres vergers conduits en conventionnel.
En raison d'une efficacité limitée et/ou d'effets non intentionnels de la lutte directe contre les bio-agresseurs en verger, il est donc important de concevoir et de gérer le verger AB afin de limiter le recours aux intrants phytosanitaires : variétés, distances de plantation, architecture de l'arbre, aménagement de l'environnement du verger, prophylaxie,..
Directional translocation resistance of Zika xrRNA
xrRNAs from flaviviruses survive in host cells because of their exceptional dichotomic response to the unfolding action of different enzymes. They can be unwound, and hence copied, by replicases, and yet can resist degradation by exonucleases. How the same stretch of xrRNA can encode such diverse responses is an open question. Here, by using atomistic models and translocation simulations, we uncover an elaborate and directional mechanism for how stress propagates when the two xrRNA ends, 5 \u2032 and 3 \u2032, are driven through a pore. Pulling the 3 \u2032 end, as done by replicases, elicits a progressive unfolding; pulling the 5 \u2032 end, as done by exonucleases, triggers a counterintuitive molecular tightening. Thus, in what appears to be a remarkable instance of intra-molecular tensegrity, the very pulling of the 5 \u2032 end is what boosts resistance to translocation and consequently to degradation. The uncovered mechanistic principle might be co-opted to design molecular meta-materials
Well-Tempered Metadynamics: A Smoothly Converging and Tunable Free-Energy Method
We present a method for determining the free energy dependence on a selected
number of collective variables using an adaptive bias. The formalism provides a
unified description which has metadynamics and canonical sampling as limiting
cases. Convergence and errors can be rigorously and easily controlled. The
parameters of the simulation can be tuned so as to focus the computational
effort only on the physically relevant regions of the order parameter space.
The algorithm is tested on the reconstruction of alanine dipeptide free energy
landscape
Accurate sampling using Langevin dynamics
We show how to derive a simple integrator for the Langevin equation and
illustrate how it is possible to check the accuracy of the obtained
distribution on the fly, using the concept of effective energy introduced in a
recent paper [J. Chem. Phys. 126, 014101 (2007)]. Our integrator leads to
correct sampling also in the difficult high-friction limit. We also show how
these ideas can be applied in practical simulations, using a Lennard-Jones
crystal as a paradigmatic case
Colored-noise thermostats \`a la carte
Recently, we have shown how a colored-noise Langevin equation can be used in
the context of molecular dynamics as a tool to obtain dynamical trajectories
whose properties are tailored to display desired sampling features. In the
present paper, after having reviewed some analytical results for the stochastic
differential equations forming the basis of our approach, we describe in detail
the implementation of the generalized Langevin equation thermostat and the
fitting procedure used to obtain optimal parameters. We discuss in detail the
simulation of nuclear quantum effects, and demonstrate that, by carefully
choosing parameters, one can successfully model strongly anharmonic solids such
as neon. For the reader's convenience, a library of thermostat parameters and
some demonstrative code can be downloaded from an on-line repository
Networking Hybridizations within the Semiosphere: a research trajectory for the Cultural Transposition of the Chinese Lesson Study within a Western context
Interchain interaction and Davydov splitting in polythiophene crystals: An ab initio approach
The crystal-induced energy splitting of the lowest excitonic state in polymer crystals, the so-called Davydov splitting Î, is calculated with a first-principles density-matrix scheme. We show that different crystalline arrangements lead to significant variations in Î, from below to above the thermal energy kBT at room temperature, with relevant implications on the luminescence efficiency. This is one more piece of evidence supporting the fact that control of interchain interactions and solid-state packing is essential for the design of efficient optical devices
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