1,721,125 research outputs found
Edizione Nazionale delle Opere di Aldo Moro. Sezione I. Scritti e discorsi. Vol. 5: Appendici - Sezione I. Tomo 1: Diario di Aldo Moro (1953 - 1954)
No full textIl tomo raccoglie l’edizione critica dei diari scritti da Aldo Moro tra il 1953 ed il 1954
Edizione Nazionale delle Opere di Aldo Moro, Sezione I, Scritti e Discorsi, Vol. 5, Appendici - Sezione I, a cura di Federico Perini, Tomo 2, Quel che Moro mi disse il 18 febbraio Eugenio Scalfari. La Repubblica, 14 ottobre 1978
No full textIl tomo raccoglie l'intervista rilasciata da Aldo Moro a Eugenio Scalfari il 18 febbraio 1978
Edizione Nazionale delle Opere di Aldo Moro. Sezione I. Scritti e discorsi. Vol. 5: Appendici - Sezione I. Tomo 3: Il memoriale di Aldo Moro nell'edizione critica coordinata da Michele di Sivo, Roma, 2019
No full textIl tomo raccoglie la digitalizzazione dell’edizione critica del Memoriale di Aldo Moro curata da Michele di Sivo (2019)
Numerical studies on semi-implicit and implicit methods for reaction-diffusion equations
No full textIn this report Rosenbrock, extended and generalized trapezoidal formulae are considered. Numerical studies on these methods have been developed on a linear and a nonlinear reaction diffusion convection equation
A study of direct and Krylov iterative sparse solver techniques to approach linear scaling of the integration of chemical kinetics with detailed combustion mechanisms
No full textThe integration of the stiff ODE systems associated with chemical kinetics is the most computationally demanding task in most practical combustion simulations. The introduction of detailed reaction mechanisms in multi-dimensional simulations is limited by unfavorable scaling of the stiff ODE solution methods with the mechanism’s size. In this paper, we compare the efficiency and the appropriateness of direct and Krylov subspace sparse iterative solvers to speed-up the integration of combustion chemistry ODEs, with focus on their incorporation into multi-dimensional CFD codes through operator splitting. A suitable preconditioner formulation was addressed by using a general-purpose incomplete LU factorization method for the chemistry Jacobians, and optimizing its parameters using ignition delay simulations for practical fuels. All the calculations were run using a same efficient framework: SpeedCHEM, a recently developed library for gas-mixture kinetics that incorporates a sparse analytical approach for the ODE system functions. The solution was integrated through direct and Krylov subspace iteration implementations with different backward differentiation formula integrators for stiff ODE systems: LSODE, VODE,
DASSL. Both ignition delay calculations, involving reaction mechanisms that ranged from 29 to 7171 species, and multi-dimensional internal combustion engine simulations with the KIVA code were used as test cases. All solvers showed similar robustness, and no integration failures were observed when using ILUT-preconditioned Krylov enabled integrators. We found that both solver approaches, coupled with efficient function evaluation numerics, were capable of scaling computational time requirements approximately
linearly with the number of species. This allows up to three orders of magnitude speed-ups in comparison with the traditional dense solution approach. The direct solvers outperformed Krylov subspace solvers at mechanism sizes smaller than about 1000 species, while the Krylov approach allowed more than 40% speed-up over the direct solver when using the largest reaction mechanism with 7171 species
An analytical Jacobian approach to sparse reaction kinetics for computationally efficient combustion modelling with large reaction mechanisms
No full textThis study presents an analytical Jacobian formulation for detailed gas-phase reaction kinetics, suitable for accurate and computationally efficient combustion simulations using either skeletal or detailed reaction mechanisms. A general chemical kinetics initial value problemin constant volume environments is considered, where the gas-phase mixture thermodynamic properties are polynomial functions of temperature according to the JANAF standard. Three different reaction behaviours are accounted for, including modified Arrhenius kinetic law reactions, third-body collisions, and pressure dependent reactions in Lindemann’s or Troe’s kinetic law forms. The integration of the chemistry ODE system is carried out using a software package specifically developed in Fortran language, and the solution compared to a reference chemical kinetics library. Two analytical Jacobian formulations, an exact one and a sparser, approximate one are proposed, and compared to numerical Jacobians computed by finite differences internally generated by a variety of commonly used stiff ordinary differential equations (ODE) solvers. The results show significant reductions in total computational times for the chemistry ranging from factors of 2 to more than two orders of magnitude for 29 species, 56 reactions to 2878 species, 8555 reactions, respectively. Finally, the code has been coupled to an engine combustion simulation software, where at each timestep the chemistry ODE system is integrated in each cell of the computational grid, allowing 77% faster computations with a 160 species combustion mechanism
A quasi-dimensional combustion model for performance and emissions of SI engines running on hydrogen–methane blends
No full textThe development of a predictive two-zone, quasi-dimensional model for the simulation of the combustion process in spark ignited engines fueled with hydrogen, methane, or hydrogen–methane blends is presented. The code is based on a general-purpose thermodynamic framework for the simulation of the power cycle of internal combustion engines. Quasi-dimensional modelling describes the flame front development assuming a simplified spherical geometry, as well as infinitesimal thickness. The flame front subdivides the in-cylinder volume into a zone of unburned mixture, and a second zone of burned gases. As far as the combustion process is concerned, attention is paid to the description of the physical and chemical phenomena controlling the flame development and the formation of combustion products. First of all, an empirical correlation has been defined for estimating the laminar burning velocity. The equation, tailored for arbitrary fuel blendings and equivalence ratios, has been validated against detailed experimental data. Furthermore, the influence of turbulence on flame evolution has been implemented according to a fractal-based model. Then, a physical and chemical computing environment for evaluating both gaseous mixtures’ thermodynamic properties, and equilibrium species concentrations of combustion products has been developed and coupled to the code. The validation has been performed by comparing numerical pressure traces against literature experimental data, on a standard CFR single-cylinder engine. A unique set-up of the model parameters has been obtained, suitable for both pure hydrogen and pure methane fuelings; finally, the predictive capabilities of the model have been applied to analyze different fuel blends and equivalence ratios: the comparison against experimental pollutant emissions (NO and CO) shows a reasonable accuracy
Development and calibration of an enhanced quasi-dimensional combustion model for HSDI diesel engines
No full textThe paper describes the development and validation of a quasi-dimensional combustion model, applicable to any type of high-speed direct-injection (HSDI) diesel engines. In this model, the fundamental in-cylinder processes are taken into account, including turbulence, fuel injection, spray dynamics, ignition, and combustion. In comparison with similar models presented in the literature, a more physical description of average in-cylinder turbulence properties and their interaction with spray dynamics is introduced, as well as a detailed modelling of fuel jet wall impingement. Some experimental measures available in the literature and three-dimensional computational fluid dynamics simulations were considered to calibrate the modelling parameters. These improved sub-models make results accuracy less dependent on the calibration carried out on each engine, so that the same parameter setting can be successfully applied to different combustion chamber configurations. The model was first applied to a small HSDI turbocharged diesel engine. The specific calibration was supported by both experimental and simulation results, the latest being obtained from the three-dimensional computational fluid dynamics analyses. Then, a different diesel engine was simulated, adopting the same set-up of the model parameters. For both engines, the comparison between experiments and simulation showed a very good agreement in terms of in-cylinder pressures and heat release rates, as well as of average in-cylinder turbulence properties. It is worth mentioning that the two engines had a quite different unit displacement, i.e. 312 and 697 cm3, respectively. As a conclusion, this model was demonstrated to be a reliable tool for addressing the optimization of the main engine design parameters, such as injection rates and timings, combustion chamber base geometry, and so fort
Origin of link gain fluctuations in analog radio over single-mode fiber systems
Full text linkIn analog Radio over Fiber systems based on single mode fibers, the possible presence of unexpected fluctuating behaviors of the received radiofrequency signal in front of monotonic environmental temperature variations, is analyzed experimentally and simulated theoretically. The possibility to understand all the aspects of this behavior allows to keep it in the right account, particularly when these systems have to be realized with stringent requirements in terms of cost and performance
An Analysis on Time Scale Separation for Engine Simulations with Detailed Chemistry
No full textThe simulation of combustion chemistry in internal combustion engines is challenging due to the need to include detailed reaction mechanisms to describe the engine physics. Computational times needed for coupling full chemistry to CFD simulations are still too computationally demanding, even when distributed computer systems are exploited. For these reasons the present paper proposes a time scale separation approach for the integration of the chemistry differential equations and applies it in an engine CFD code. The time scale separation is achieved through the estimation of a characteristic time for each of the species and the introduction of a sampling timestep, wherein the chemistry is subcycled during the overall integration. This allows explicit integration of the system to be carried out, and the step size is governed by tolerance requirements. During the subcycles each of the species is only integrated up to its own characteristic timescale, thus reducing the computational effort needed by the solver. The present ODE solver was first validated using constant pressure batch reactor simulations with two different reaction mechanisms. Then the solver was coupled with the KIVA-4 code, and validated using HCCI and DI diesel combustion cases. Performance is compared with the commonly used DVODE chemistry solver and the results show that significant reductions in the total computational time with comparable accuracy are obtained with the new solution methodology
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