Dynamical system analysis of ignition phenomena using the tangential stretching rate concept

We analyze ignition phenomena by resorting to the stretching rate concept formerly introduced in the study of dynamical systems. We construct a Tangential Stretching Rate (TSR) parameter by combining the concepts of stretching rate with the decomposition of the local tangent space in eigen-modes. The main feature of the TSR is its ability to identify unambiguously the most energetic scale at a given space location and time instant. The TSR depends only on the local composition of the mixture, its temperature and pressure. As such, it can be readily computed during the post processing of computed reactive flow fields, both for spatially homogeneous and in-homogenous systems. Because of the additive nature of the TSR, we defined a normalized participation index measuring the relative contribution of each mode to the TSR. This participation index to the TSR can be combined with the mode amplitude participation Index of a reaction to a mode – as defined in the Computational Singular Perturbation (CSP) method – to obtain a direct link between a reaction and TSR. The reactions having both a large participation index to the TSR and a large CSP mode amplitude participation index are those contributing the most to both the explosive and relaxation regimes of a reactive system. This information can be used for both diagnostics and for the simplification of kinetic mechanisms. We verified the properties of the TSR with reference to three nonlinear planar models (one for isothermal branched-chain reactions, one for a non-isothermal, one-step system, and for non-isothermal branched-chain reactions), to one planar linear model (to discuss issues associated with non-normality), and to test problems involving hydro-carbon oxidation kinetics. We demonstrated that the reciprocal of the TSR parameter is the proper characteristic chemical time scale in problems involving multi-step chemical kinetic mechanisms, because (i) it is the most relevant time scale during both the explosive and relaxation regimes and (ii) it is intrinsic to the kinetics, that is, it can be identified without the need of any ad hoc assumption

Entropy production and the g-scheme

Spatially homogeneous batch reactor systems are characterized by the simultaneous presence of a wide range of time scales. When the dynamics of such reactive systems develop very-slow and very-fast time scales separated by a range of active time scales, with large gaps in the fast/active and slow/active time scales, then it is possible to achieve multi-scale adaptive model reduction along-with the integration of the governing ordinary differential equations using the G-Scheme framework. The G- Scheme assumes that the dynamics is decomposed into active, slow, fast, and when applicable, invariant subspaces. We computed the contribution to entropy production by the four subspaces, with reference to a constant volume, adiabatic reactor. The numerical experiments indicate that the contributions of the fast and slow subspaces are much smaller than that of the active subspace

Entropy production at all scales

Spatially homogeneous systems are characterized by the simultaneous presence of a wide range of time scales. When the dynamics of such reactive systems develop very-slow and very-fast time scales separated by a range of active time scales, with large gaps in the fast/active and slow/active time scales, then it is possible to achieve multi-scale adaptive model reduction along-with the integration of the governing ordinary differential equations using the G-Scheme framework. The G-Scheme assumes that the dynamics is decomposed into active, slow, fast, and when applicable, invariant subspaces. We derive the expressions that express the direct link between time scales and entropy production by resorting to the estimates provided by the G-Scheme. With reference to a constant volume, adiabatic batch reactor, we compute the contribution to entropy production by the four subspaces. The numerical experiments show that, as indicated by the theoretical derivation, the contribution to entropy production of the fast subspace is of the same magnitude of the error threshold chosen for the numerical integration, and that the contribution of the slow subspace is generally much smaller than that of the active subspace

Numerical investigation of unsteady laminar methane/LOx flamelet at supercritical pressures

High-pressure combustion devices, such as liquid rocket engines, are usually characterized by transcritical and supercritical operating conditions. Propellants injected in the combustion chamber experience extremely high den- sity gradients and real fluid effects. In the present study, real fluid effects on flame structure are investigated in the framework of unsteady laminar flamelet equations, a well established representation and diagnostic tool for non pre- mixed combustion transient phenomena. Real fluid thermodynamic properties are taken into account by means of a computationally efficient cubic equation of state written in a general and comprehensive three-parameter fashion. High-pressure conditions for unsteady flame structure calculations and analysis are chosen as a representative range of a methane/liquid-oxygen rocket engine operating conditions. Particular focus is posed on the constant pressure specific heat behavior at low temperature, which influences the time evolution of the flame structure. Moreover time accurate integration of flamelet equations represent the very first building block of a conditional moment closure for supercritical turbulent combustion

Transhumance heritage in the bioregional perspective

The worldwide dynamics of the last thirty years are outlining opaque scenarios and forcing contemporaries to search for a catharsis whose urgency can no longer be postponed: cultures, economies and ecological balances in their current state are in danger of collapsing at an unprecedented rate. Nevertheless, it is possible to draw from the past references capable of updating consolidated paradigms, rediscovering approaches oriented towards the long term aimed at maintaining the conditions that have allowed progress and well-being for a large part of our species. The millenary practice of transhumance, today relegated to marginal contexts, is one of these: the extensive breeding of livestock, conducted on foot over long distances in search of pastures, is an exemplary adaptation to the scarcity of resources provided by the natural succession of seasons. In contrast, the problems arising from industrialised forms of animal husbandry are highlighted by numerous studies. The regeneration of the vast material and immaterial heritage inherited from transhumance, on the other hand, can provide answers to environmental, ethical and social issues that afflict every continent today: an ante litteram model of sustainability that finds in bioregional planning a concrete prospect of re-actualisation

The River Contract of the Tiber from Castel Giubileo to the Foce: An Innovative Practice for a Relationship between Tiber and Rome

The River Contract is a little-known practice and an unconventional planning procedure but an opportunity in many Italian Regions to overcome the problems of coherence between levels of Government and to promote participation. The River Contract is a voluntary agreement of strategic and negotiated planning, aiming the protection, proper management of water resources, recovery, and enhancement of the river territories, together with flood protection, contributing to the local development of such areas. In 2017, in Rome, started the procedure for the River Contract by Agenda Tevere Onlus. This paper illustrates a practice-oriented research built by multidisciplinary approach and a method able to mix cultural and participatory knowledge. Because the Tiber is the river of Rome, the research aims to raise the possibility of enhancing the strategic and fundamental role of the Tiber in relation to the City through four key issues: the urban landscape, because the Tiber is intrinsically related with the city; the public space, because the Tiber must be a good of the city; the sustainable use, because the Tiber must be for the city; the active participation, because the interventions on the Tiber must be shared in the city by those who live and work in Rome

Space-time adaptive reduction of unsteady flamalets

The Wavelet Adaptive Multiresolution Representation (WAMR) code and the G-Scheme framework are used for the numerical time integration of the flamelet model. The steep gradients are efficiently captured by the WAMR algorithm with an a-priori defined accuracy and an associated large reduction of the number of degrees of freedom (DOFs). A further opportunity to reduce the complexity of the problem is represented by the G-Scheme, to achieve multi-scale adaptive model reduction along-with the time integration of the differential equations

Criteri e metodi per la pianificazione intercomunale nell'area metropolitana romana

Il contributo si colloca all’interno del dibattito scientifico sui temi della dimensione metropolitana della città contemporanea e dei modelli di governo per la pianificazione territoriale di area vasta, con particolare riferimento alle esperienze in cui sono in corso sperimentazioni dal basso di associazionismo intercomunale. In particolare, il contributo ha l’obiettivo di presentare i risultati di una ricerca applicata sul tema della dimensione intercomunale, quale riferimento territoriale minimo per l’esercizio delle complesse funzioni di tutela e valorizzazione dell’ambiente e del paesaggio, a partire dall’ipotesi che si debba procedere, ai fini della individuazione di criteri e metodi per la definizione degli ambiti ottimali della cooperazione intercomunale, secondo modelli di aggregazione “non congiunturali” a geometria variabile. A tal fine la ricerca individua nei territori del Lazio, con particolare riferimento all’area metropolitana romana, il campo di sperimentazione progettuale, attraverso la tecnologia SIT/GIS, di possibili criteri per la perimetrazione degli ambiti territoriali della cooperazione, assumendo “il criterio ambientale e paesistico” per l’individuazione di aree dai caratteri intrinsecamente coerenti ed estensioni territoriali utili a definire una proposta di scenari alternativi di riassetto amministrativo

Evaporation and clustering of ammonia droplets in a hot environment

Recent developments in the transition to zero-carbon fuels show that ammonia is a valid candidate for combustion. However, liquid ammonia combustion is difficult to stabilize due to a large latent heat of evaporation, which generates a strong cooling effect that adversely affects the flame stabilization and combustion efficiency. In addition, the slow burning rate of ammonia enhances the undesired production of NOx and N2O. To increase the flame speed, ammonia must be blended with a gaseous fuel having a high burning rate. In this context, a deeper understanding of the droplet dynamics is required to optimize the combustor design. To provide reliable physical insights into diluted ammonia sprays blended with gaseous methane, direct numerical simulations are employed. Three numerical experiments were performed with cold, standard, and hot ambient in nonreactive conditions. The droplet radius and velocity distribution, as well as the mass and heat coupling source terms are compared to study the effects on the evaporation. Since the cooling effect is stronger than the heat convection between the droplet and the environment in each case, ammonia droplets do not experience boiling. On the other hand, the entrainment of dry air into the ammonia-methane mixture moves the saturation level beyond 100% and droplets condense. The aforementioned phenomena are found to strongly affect the droplet evolution. Finally, a three-dimensional Voronoi analysis is performed to characterize the dispersive or clustering behavior of droplets by means of the definition of a clustering index

Criteri e metodi per la pianificazione intercomunale nell’area metropolitana romana

Il contributo si colloca all’interno del dibattito scientifico sui temi della dimensione metropolitana della città contemporanea e dei modelli di governo per la pianificazione territoriale di area vasta, con particolare riferimento alle esperienze in cui sono in corso sperimentazioni dal basso di associazionismo intercomunale. In particolare, il contributo ha l’obiettivo di presentare i risultati di una ricerca applicata sul tema della dimensione intercomunale, quale riferimento territoriale minimo per l’esercizio delle complesse funzioni di tutela e valorizzazione dell’ambiente e del paesaggio, a partire dall’ipotesi che si debba procedere, ai fini della individuazione di criteri e metodi per la definizione degli ambiti ottimali della cooperazione intercomunale, secondo modelli di aggregazione “non congiunturali” a geometria variabile. A tal fine la ricerca individua nei territori del Lazio, con particolare riferimento all’area metropolitana romana, il campo di sperimentazione progettuale, attraverso la tecnologia SIT/GIS, di possibili criteri per la perimetrazione degli ambiti territoriali della cooperazione, assumendo “il criterio ambientale e paesistico” per l’individuazione di aree dai caratteri intrinsecamente coerenti ed estensioni territoriali utili a definire una proposta di scenari alternativi di riassetto amministrativo.The contribution is part of the scientific debate on issues related to the metropolitan dimension of the contemporary city and government models for large-scale territorial planning, with particular reference to on-going experiences characterized by bottom-up experiments of inter-municipal associations. In particular, the contribution has the aims of presenting the results of applied research on the theme of the inter-municipal dimension, intended as a minimum territorial reference for the organization of the complex functions of protection and enhancement of the environment and landscape. This, starting from the hypothesis that it is necessary to proceed according to "non-cyclical" aggregation models with variable geometry, in order to identify criteria and methods for defining the optimal areas of inter-municipal cooperation. In this regard, the research identifies the field of design experimentation in the territories of Lazio, with particular reference to the Roman metropolitan area. Through the SIT / GIS technology, possible criteria for the identification of the territorial areas of the cooperation have been outlined, assuming "the environmental and landscape criterion" for the identification of areas with intrinsically coherent characters and territorial extensions useful for defining a proposal of different scenarios related to an alternative administrative reorganization