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Search Results: 1 - 10 of 1403 matches for " Marcello Righi "
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A Gas-Kinetic Scheme For The Simulation Of Compressible Turbulent Flows
Marcello Righi
Physics , 2013, DOI: 10.1063/1.4902750
Abstract: The kinetic theory of gases has suggested the idea of viscosity to model the effect of thermal fluctuations on the resolved flow. Supported by the assumed analogy between molecules and the eddies in a turbulent flows, the idea of an eddy viscosity has been put forward in the pioneering work by Lord Kelvin and Osborne Reynolds. In over hundred years of turbulence modeling, the numerical schemes adopted to simulate turbulent flow - with the exception of the Lattice Boltzmann methods - have never exploited this analogy in any other way. In this work, a gas-kinetic scheme is modified to simulate turbulent flow; the turbulent relaxation time is deduced from assumed turbulent quantities. The new scheme does not adopt an eddy viscosity, yet it relies even more strongly on the analogy between thermal and turbulent fluctuations, as turbulence dynamics is mathematically modeled by the Boltzmann equation. In the gas-kinetic scheme, a measure of the degree of rarefaction is introduced, as the ratio between unresolved and resolved time scales of motion. At low rarefaction, the turbulent gas-kinetic scheme deviates negligibly from a conventional Navier-Stokes scheme. However, as the degree of rarefaction increases, the kinetic effects become evident. This phenomenon is evident in the mathematical description of the turbulent stress tensor and also in numerical experiments. This study does not propose an innovative turbulence model or technique. It addresses the fact that the traditional coupling numerical scheme and turbulence modeling might improve the physical consistence of numerical simulations.
A gas-kinetic scheme for the simulation of turbulent flows
Marcello Righi
Physics , 2012, DOI: 10.1063/1.4769575
Abstract: Numerical schemes derived from gas-kinetic theory can be applied to simulations in the hydrodynamics limit, in laminar and also turbulent regimes. In the latter case, the underlying Boltzmann equation describes a distribution of eddies, in line with the concept of eddy viscosity developed by Lord Kelvin and Osborne Reynolds at the end of the nineteenth century. These schemes are physically more consistent than schemes derived from the Navier-Stokes equations, which invariably assume infinite collisions between gas particles (or interactions between eddies) in the calculation of advective fluxes. In fact, in continuum regime too, the local Knudsen number can exceed the value 0.001 in shock layers, where gas-kinetic schemes outperform Navier-Stokes schemes, as is well known. Simulation of turbulent flows benefit from the application of gas-kinetic schemes, as the turbulent Knudsen number (the ratio between the eddies' mean free path and the mean flow scale) can locally reach values well in excess of 0.001, not only in shock layers. This study has investigated a few cases of shock - boundary layer interaction comparing a gas-kinetic scheme and a Navier-Stokes one, both with a standard k-\omega turbulence model. Whereas the results obtained from the Navier-Stokes scheme are affected by the limitations of eddy viscosity two-equation models, the gas-kinetic scheme has performed much better without making any further assumption on the turbulent structures.
A Modified Gas-Kinetic Scheme for Turbulent Flow
Marcello Righi
Physics , 2013,
Abstract: By analogy with the kinetic theory of gases, most turbulence modeling strate- gies rely on an eddy viscosity to model the unresolved turbulent fluctuations. How- ever, the ratio of unresolved to resolved scales - very much like a degree of rarefaction - is not taken into account by the popular conventional schemes, based on the Navier- Stokes equations. This paper investigates the simulation of turbulent flow with a gas- kinetic scheme. In so doing, the modeling of turbulence becomes part of the numerical scheme: the degree of rarefaction is automatically taken into account; the turbulent stress tensor is not constrained into a linear relation with the strain rate. Instead it is modeled on the basis of the analogy between particles and eddies, without any as- sumptions on the type of turbulence or flow class. The implementation of a turbulent gas-kinetic scheme into a finite-volume solver is put forward, with turbulent kinetic energy and dissipation supplied by an allied turbulence model. A number of flow cases, challenging for conventional RANS methods, are investigated; results show that the shock-turbulent boundary layer is captured in a much more convincing way by the gas-kinetic scheme.
Adi??es ao gênero Glossodrilus (Oligochaeta, Glossoscolecidae)
Righi, Gilberto;
Revista Brasileira de Zoologia , 1982, DOI: 10.1590/S0101-81751982000100009
Abstract: four new species of glossodrilus, g. mairaro, g. arapaco, g. oliveirae and g. tico, are described from the region between 3o - 5on and 58o - 62ow, including part of brazil, venezuela and guyana. a new occurrence is indicated for g. schubarti. the subgenus glossodrilus (tonperoge) is invalidated. a pragmatic system is presented to group the 31 species and subspecies of glossodrilus.
Sulla traduzione macedone dell’Evgenij Onegin
Roberto Righi
Studi Slavistici , 2006,
Abstract: The Translation of Evgenij Onegin into Macedonian The Author analyses G. Stalev ‘s translation of Pu kin’s Onegin into Macedonian. This work is of fundamental importance in the formation of modern literary Macedonian language and testifies to its poetic and expressive possibilities. The decision to maintain the iambic verse of the original, caused Stalev several difficulties, due mainly to the difference in accent in Russian and Macedonian. The Author describes the techniques Stalev used to overcome the challenges offered by the Russian poetic text and the strategies that allowed the Macedonian poet to make an excellent translation. Forty years on, it is still a first-class example of the integration of Pu kin’s masterpiece into the linguistic and literary context of Macedonian culture.
Enumeration of ad-nilpotent ideals of parabolic subalgebras for exceptional types
Celine Righi
Mathematics , 2008,
Abstract: Using GAP4, we determine the number of ad-nilpotent and abelian ideals of a parabolic subalgebra of a simple Lie algebra of exceptional types E, F or G.
Number of "udu" of a Dyck path and ad-nilpotent ideals of parabolic subalgebras of sl_{l+1}(C)
Celine Righi
Mathematics , 2008,
Abstract: For an ad-nilpotent ideal $i$ of a Borel subalgebra of $sl_{l+1}(C)$, we denote by $I_i$ the maximal subset $I$ of the set of simple roots such that $i$ is an ad-nilpotent ideal of the standard parabolic subalgebra $p_I$. We use the bijection given by G.E. Andrews, C. Krattenthaler, L. Orsina and P. Papi between the set of ad-nilpotent ideals of a Borel subalgebra in $sl_{l+1}(C)$ and the set of Dyck paths of length $2l+2$, to explicit a bijection between ad-nilpotent ideals $i$ of the Borel subalgebra such that the cardinality of $I_i$ is equal to $r$ and the Dyck paths of length $2l+2$ having $r$ occurence "udu". We obtain also a duality between antichains of cardinality $p$ and $l-p$ in the set of positive roots.
A Case-Control Study on Leisure Time Physical Activity (LTPA) during the Last Three Months of Pregnancy and Foetal Outcomes in Italy  [PDF]
Guglielmina Fantuzzi, Elena Righi, Gabriella Aggazzotti
Health (Health) , 2016, DOI: 10.4236/health.2016.82016
Abstract: The association between Leisure Time Physical Activity (LTPA) during pregnancy and foetal outcomes has been extensively investigated. However, epidemiological studies specifically referred to LPTA in the last months of pregnancy are scarce. We evaluated the association between LPTA and the risk of both preterm delivery and small for gestational age (SGA) during the last three months of pregnancy in Italy. A nationwide case-control study was performed in nine Italian cities. A total of 299 preterm delivery, 364 SGA and 855 controls were enrolled in the study. A self-administered questionnaire was used to assess socio-demographic variables, medical and reproductive history, life-style habits and LTPA referred to the last three months of pregnancy. Univariate and multivariate regression analyses were performed in order to estimate Odds ratios and 95% CI. LTPA during the last three months of pregnancy decreases the risk of preterm delivery (adjusted OR = 0.56; 95% CI 0.39 - 0.79). Among the different types of physical activity, walking, the most frequently referred activity, appears significantly protective against preterm delivery (adjusted OR = 0.53; 95% CI 0.36 - 0.81). Moreover, a small protective effect of walking was evidenced against SGA (adjusted OR = 0.72; 95% CI 0.51 - 1.00). In conclusion, a mild physical activity such as walking in the last three months of pregnancy seems to reduce the risk of preterm delivery and, at a lesser extent, of SGA, confirming the beneficial effects of physical activity along the whole pregnancy.
Oligochaeta, Naididae of the West Indies and adjacent regions
Righi, Gilberto;Hamoui, Viviane;
Papéis Avulsos de Zoologia (S?o Paulo) , 2002, DOI: 10.1590/S0031-10492002000600001
Abstract: a very large collection of naididae of the west indies (153 localities), suriname (15), venezuela (2) and florida (1) is studied. five new species nais barua, dero scalariformis, d. tuna, aulophorus kalina, a. barbatus are described. dero magna, d. trifida, aulophorus tridentatus, allonais japonica, pristina sima are redescribed. the 46 naididae species of the west indies are discussed by their taxonomy, distribution and habitat.
Entre o desejo e a necessidade de aprender línguas: algumas reflex?es sobre o que também está em jogo nesse processo
Andrade, Eliane Righi de;
Revista Brasileira de Linguística Aplicada , 2009, DOI: 10.1590/S1984-63982009000100014
Abstract: based on a discoursive perspective, this article intends to analyze some discoursive segments produced by undergraduate teachers in oral interviews, searching for identifying some foreign language representations of learning, which can appear in their speech. as a theoretical support, we are going to develop two concepts from psychoanalysis (need and desire), and also the idea of technologies of the self, developed by foucault in his work. considering the results of the analysis, we can conclude that learning a foreign language requires not only specific knowledge, such as vocabulary or grammar, but also involves unconscious processes which constitute the subject and his/her identity as a teacher. this identity, however, is conditioned by history and social meanings, which are produced by symbolic and imaginary relations.
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