Comparison between different methods of estimating of the relaxation times in the FPU model

After a brief review of the Fermi-Pasta-Ulam (FPU) conservative system of N nonlinearly coupled oscillators, this paper addresses two problems: first, comparing two indicators for the equipartition, showing that the results are essentially identical; second, finding a method that allows fast integration to reach the long integration times required in this area. In particular this work proposes a symplectic algorithm based on the Fast Fourier Transform

Transient growth in the flow past a three-dimensional smooth roughness element

This work provides a global optimization analysis, looking for perturbations inducing the largest energy growth at a finite time in a boundary-layer flow in the presence of smooth three-dimensional roughness elements. Amplification mechanisms are described which can bypass the asymptotical growth of Tollmien–Schlichting waves. Smooth axisymmetric roughness elements of different height have been studied, at different Reynolds numbers. The results show that even very small roughness elements, inducing only a weak deformation of the base flow, can localize the optimal disturbance characterizing the Blasius boundary-layer flow. Moreover, for large enough bump heights and Reynolds numbers, a strong amplification mechanism has been recovered, inducing an increase of several orders of magnitude of the energy gain with respect to the Blasius case. In particular, the highest value of the energy gain is obtained for an initial varicose perturbation, differently to what found for a streaky parallel flow. Optimal varicose perturbations grow very rapidly by transporting the strong wall-normal shear of the base flow, which is localized in the wake of the bump. Such optimal disturbances are found to lead to transition for initial energies and amplitudes considerably smaller than sinuous optimal ones, inducing hairpin vortices downstream of the roughness element

Come parlano gli italiani

In molti paesi del mondo la realtà plurilingue delle società è stata a lungo soffocata o ignorata. Dalla seconda metà del Novecento in molti, forse in tutti i paesi, un intreccio di fenomeni sociopolitici ed economici e il progresso degli studi linguistici hanno portato alla ribalta le effettive realtà plurilingui. L'Italia è, dai secoli del Medio evo, un paese accentuatamente plurilingue. I deficit di istruzione hanno a lungo reso difficile la convergenza verso un idioma unico, cioè verso il fiorentino considerato dal ceto colto, fin dal Cinquecento, come la lingua nazionale..La convergenza si è finalmente realizzata, sotto la spinta di fattori demografici, economici e, in parte, educativi soltanto in anni recenti. L'adozione dell'italiano coinvolge ormai oltre il 95% della popolazione, che, tuttavia, al 60% conserva l'uso di uno dei molti dialetti caratteristici del Paese o di una delle 14 lingue di minoranza. L'uso della lingua comune è tuttora segnato sia da tratti oggettivamente contrastanti o sovrabbondanti nella morfologia e nel lessico sia da notevoli variazioni regionali sia, infine, da difficoltà che i locutori incontrano nel maneggiare una lingua di antica tradizione e non semplice.In many countries around the world, multilingual reality has long been suppressed or ignored. From the second half of the twentieth century in many if not in all countries, an interweaving of socio-political and economic phenomena and the progress of language studies have brought multilingual realities into the limelight. From the centuries of the Middle Ages, Italy has been an emphatically multilingual country. The deficits in education have long hindered the convergence towards a single language, that is to say towards the cautious Florentine from the cultured class, at the end of the Sixteenth century, as the national language. The convergence itself was finally achieved only in recent years under the impetus of demographic, economic and, in part, educational factors. Now, more than 90% of the population have adopted Italian, although 60% of them preserve the use of one of the many characteristic dialects of the country or one of the 14 minority languages. The use of the common tongue is still marked both by objectively clashing or overabundant features in the morphology and vocabulary, by considerable regional variations and, finally, by the difficulty that speakers must face in dealing with an ancient, traditional and far from easy language

An efficient flamelet progress-variable method for modeling non-premixed flames in weak electric fields

Combustion stabilization and enhancement of the flammability limits are mandatory objectives to improve nowadays combustion chambers. At this purpose, the use of an electric field in the flame region provides a solution which is, at the same time, easy to implement and effective to modify the flame structure. The present work describes an efficient flamelet progress-variable approach developed to model the fluid dynamics of flames immersed in an electric field. The main feature of this model is that it can use complex ionization mechanisms without increasing the computational cost of the simulation. The model is based on the assumption that the combustion process is not directly influenced by the electric field and has been tested using two chemi-ionization mechanisms of different complexity in order to examine its behavior with and without the presence of heavy anions in the mixture. Using a one- and two-dimensional numerical test cases, the present approach has been able to reproduce all the major aspects encountered when a flame is subject to an imposed electric field and the main effects of the different chemical mechanisms. Moreover, the proposed model is shown to produce a large reduction in the computational cost, being able to shorten the time needed to perform a simulation up to 40 times.Comment: 26 pages, 13 figures, paper accepted for publication on Computers and Fluid

Il fisico del neutrino

In memory of the famous physicist Bruno Pontecorvo, whereof is just occurred the centenary anniversary of the birth and the twentieth anniversary of the death, this paper traces the scientific and human adventure of the italian scientist, from the pioneering studies of nuclear physics together with his master Enrico Fermi, to the excellent results in particle physics, in particular his research and insights about the neutrino. As much attention is paid to the human experience, his relationship with politics and the choice to embrace the Soviet socialist model. A short portrait to commemorate a scientist symbol of the twentieth century.Comment: in Italia