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Search Results: 1 - 10 of 342148 matches for " S. Di Francesco "
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Ramond Sector Characters and N=2 Landau-Ginzburg Models
P. Di Francesco,S. Yankielowicz
Physics , 1993, DOI: 10.1016/0550-3213(93)90452-U
Abstract: We give a direct proof of the new "product" expression for the Ramond sector characters of N=2 minimal models recently suggested by E. Witten. Our construction allows us to generalize these expressions to the D and E series of N=2 minimal models, as well as to other N=2 Kazama--Suzuki coset models such as $SU(N)\times SO(2(N-1))/SU(N-1)\times U(1)$. We verify that these expressions indeed coincide with the corresponding Landau--Ginzburg "elliptic genus", a certain topologically invariant twisted path integral with the effective Landau--Ginzburg action, which we obtain by using Witten's method. We indicate how our approach may be used to construct (or rule out) possible Landau--Ginzburg potentials for describing other N=2 superconformal theories.
Does a prestellar core always become protostellar? Tracing the evolution of cores from the prestellar to protostellar phase
S. Anathpindika,James Di Francesco
Physics , 2013,
Abstract: Recently, a subset of starless cores whose thermal Jeans mass is apparently overwhelmed by the mass of the core has been identified, e.g., the core {\small L183}. In literature, massive cores such as this one are often referred to as "super-Jeans cores". As starless cores are perhaps on the cusp of forming stars, a study of their dynamics will improve our understanding of the transition from the prestellar to the protostellar phase. In the present work we use non-magnetic polytropes belonging originally to the family of the Isothermal sphere. For the purpose, perturbations were applied to individual polytropes, first by replacing the isothermal gas with a gas that was cold near the centre of the polytrope and relatively warm in the outer regions, and second, through a slight compression of the polytrope by raising the external confining pressure. Using this latter configuration we identify thermodynamic conditions under which a core is likely to remain starless. In fact, we also argue that the attribute "super-Jeans" is subjective and that these cores do not formally violate the Jeans stability criterion. On the basis of our test results we suggest that gas temperature in a star-forming cloud is crucial towards the formation and evolution of a core. Simulations in this work were performed using the particle-based Smoothed Particle Hydrodynamics algorithm. However, to establish numerical convergence of the results we suggest similar tests with a grid-scheme, such as the Adaptive mesh refinement.
CFD modelling approach for dam break flow studies
C. Biscarini, S. Di Francesco,P. Manciola
Hydrology and Earth System Sciences (HESS) & Discussions (HESSD) , 2010,
Abstract: This paper presents numerical simulations of free surface flows induced by a dam break comparing the shallow water approach to fully three-dimensional simulations. The latter are based on the solution of the complete set of Reynolds-Averaged Navier-Stokes (RANS) equations coupled to the Volume of Fluid (VOF) method. The methods assessment and comparison are carried out on a dam break over a flat bed without friction, a dam break over a triangular bottom sill and a dam break flow over a 90° bend. Experimental and numerical literature data are compared to present results. The results demonstrate that the shallow water approach, even if able to sufficiently reproduce the main aspects of the fluid flows, loses some three-dimensional phenomena, due to the incorrect shallow water idealization that neglects the three-dimensional aspects related to the gravity force.
CFD modelling approach for dam break flow studies
C. Biscarini,S. Di Francesco,P. Manciola
Hydrology and Earth System Sciences Discussions , 2009,
Abstract: This paper presents numerical simulations of free surface flows induced by a dam break comparing the shallow water approach to fully three-dimensional simulations. The latter are based on the solution of the complete set of Reynolds-Averaged Navier-Stokes (RANS) equations coupled to the Volume of Fluid (VOF) method. The methods assessment and comparison are carried out on a dam break over a flat bed without friction and a dam break over a triangular bottom sill. Experimental and numerical literature data are compared to present results. The results demonstrate that the shallow water approach loses some three-dimensional phenomena, which may have a great impact when evaluating the downstream wave propagation. In particular, water wave celerity and water depth profiles could be underestimated due to the incorrect shallow water idealization that neglects the three-dimensional aspects due to the gravity force, especially during the first time steps of the motion.
Elliptic Genera and the Landau-Ginzburg Approach to N=2 Orbifolds
P. Di Francesco,O. Aharony,S. Yankielowicz
Physics , 1993, DOI: 10.1016/0550-3213(94)90463-4
Abstract: We compute the elliptic genera of orbifolds associated with $N=2$ super--conformal theories which admit a Landau-Ginzburg description. The identification of the elliptic genera of the macroscopic Landau-Ginzburg orbifolds with those of the corresponding microscopic $N=2$ orbifolds further supports the conjectured identification of these theories. For $SU(N)$ Kazama-Suzuki models the orbifolds are associated with certain $\IZ_p$ subgroups of the various coset factors. Based on our approach we also conjecture the existence of "$E$-type" variants of these theories, their elliptic genera and the corresponding Landau-Ginzburg potentials.
Folding of the Triangular Lattice with Quenched Random Bending Rigidity
P. Di Francesco,E. Guitter,S. Mori
Physics , 1996, DOI: 10.1103/PhysRevE.55.237
Abstract: We study the problem of folding of the regular triangular lattice in the presence of a quenched random bending rigidity + or - K and a magnetic field h (conjugate to the local normal vectors to the triangles). The randomness in the bending energy can be understood as arising from a prior marking of the lattice with quenched creases on which folds are favored. We consider three types of quenched randomness: (1) a ``physical'' randomness where the creases arise from some prior random folding; (2) a Mattis-like randomness where creases are domain walls of some quenched spin system; (3) an Edwards-Anderson-like randomness where the bending energy is + or - K at random independently on each bond. The corresponding (K,h) phase diagrams are determined in the hexagon approximation of the cluster variation method. Depending on the type of randomness, the system shows essentially different behaviors.
Influence of temperature and time during ovary transportation on in vitro embryo production efficiency in the buffalo species (Bubalus bubalis)
S. Di Francesco,L. Boccia,R. Di Palo,G. Esposito
Italian Journal of Animal Science , 2010, DOI: 10.4081/ijas.2007.s2.755
Abstract: The aim of this study was to evaluate the influence of temperature during ovary collection/transportation and that of the time interval between ovary collection and processing in the laboratory, on in vitro embryo production efficiency in buffalo species. A retrospective analysis of data collected over the last 4 years in our lab was carried out on 3461 oocytes, recovered over 120 replicates. No differences in oocytes developmental competence were observed in relation to the time interval between ovary collection and processing in the laboratory (3-6 h). On the contrary, a correlation was found between oocyte developmental competence, evaluated in terms of cleavage and blastocyst rate, and temperature during ovary collection/transportation. In particular, lowering temperature during ovary transportation significantly improved both cleavage (74.6 % vs 65.8 and 63.1 % for temperature ranges of 25-30°C, 30-34°C and 34-37°C, respectively ; P<0.05) and blastocyst rates (30.9 % vs 21.8 and 22.7 % for temperature ranges of 25-30°C, 30-34°C and 34-37°C, respectively ; P<0.05). It was concluded that extending the time interval between ovary collection and processing to 6 h is not detrimental for buffalo oocyte developmental competence. Furthermore, lowering temperature during ovary collection and transportation increases the in vitro embryo production efficiency in buffalo species.
Generalization of Wave Motions and Application to Porous Media  [PDF]
Romolo Di Francesco
Geomaterials (GM) , 2013, DOI: 10.4236/gm.2013.33014
Abstract: The examination of wave motions is traditionally based on the differential equation of D’Alambert, the solution of which describes the motion along a single dimension, while its bidimensional extension takes on the concept of plane waves. Considering these elements and/or limits, the research is divided into two parts: in the first are written the differential equations relating on the conditions two/three-dimensional for which the exact solutions are found; in the second the concepts are extended to the analysis of the propagation of wave motions in porous media both artificial and natural. In the end the work is completed by a series of tests, which show the high reliability of the physical-mathematical models proposed.

Exact Solution of Terzaghi’s Consolidation Equation and Extension to Two/Three-Dimensional Cases  [PDF]
Romolo Di Francesco
Applied Mathematics (AM) , 2013, DOI: 10.4236/am.2013.44099
Abstract:

The differential equation by Terzaghi and Fr?hlich, better known as Terzaghi’s one-dimensional consolidation equation, simulates the visco-elastic behaviour of soils depending on the loads applied as it happens, for example, when foundations are laid and start carrying the weight of the structure. Its application is traditionally based on Taylor’s solution that approximates experimental results by introducing non-dimensional variables that, however, contradict the actual behaviour of soils. The proposal of this research is an exact solution consisting in a non-linear equation that can be considered correct as it meets both mathematical and experimental requirements. The solution proposed is extended to include differential equations relating to two/three dimensional consolidation by adopting a transversally isotropic model more consistent with the inner structure of soils.

The Initial Conditions of Clustered Star Formation. II. N2H+ Observations of the Ophiuchus B Core
R. K. Friesen,J. Di Francesco,Y. Shimajiri,S. Takakuwa
Physics , 2009, DOI: 10.1088/0004-637X/708/2/1002
Abstract: We present a Nobeyama 45 m Radio Telescope map and Australia Telescope Compact Array pointed observations of N2H+ 1-0 emission towards the clustered, low mass star forming Oph B Core within the Ophiuchus molecular cloud. We compare these data with previously published results of high resolution NH3 (1,1) and (2,2) observations in Oph B. We use 3D Clumpfind to identify emission features in the single-dish N2H+ map, and find that the N2H+ `clumps' match well similar features previously identified in NH3 (1,1) emission, but are frequently offset to clumps identified at similar resolution in 850 micron continuum emission. Wide line widths in the Oph B2 sub-Core indicate non-thermal motions dominate the Core kinematics, and remain transonic at densities n ~ 3 x 10^5 cm^-3 with large scatter and no trend with N(H2). Non-thermal motions in Oph B1 and B3 are subsonic with little variation, but also show no trend with H2 column density. Over all Oph B, non-thermal N2H+ line widths are substantially narrower than those traced by NH3, making it unlikely NH3 and N2H+ trace the same material, but the v_LSR of both species agree well. We find evidence for accretion in Oph B1 from the surrounding ambient gas. The NH3/N2H+ abundance ratio is larger towards starless Oph B1 than towards protostellar Oph B2, similar to recent observational results in other star-forming regions. Small-scale structure is found in the ATCA N2H+ 1-0 emission, where emission peaks are again offset from continuum emission. In particular, the ~1 M_Sun B2-MM8 clump is associated with a N2H+ emission minimum and surrounded by a broken ring-like N2H+ emission structure, suggestive of N2H+ depletion. We find a strong general trend of decreasing N2H+ abundance with increasing N(H2) in Oph B which matches that found for NH3.
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