42 research outputs found
Development of a CFD methodology for fuel-air mixing and combustion modeling of GDI Engines
Simulation of GDI engines represents a very challenging task for CFD modeling. In particular, many sub-models are involved since the evolution of the fuel spray and liquid film formation should be modeled.
Furthermore, it is necessary to account for both the influence of mixture and flow conditions close to the spark plug to correctly predict the flame propagation process. In this work, the authors developed a CFD methodology to study the air-fuel mixing and combustion processes in direct-injection, spark-ignition engines. A set of sub-models was developed to describe injection, atomization, breakup and wall impingement for sprays emerging from multi-hole atomizers. Furthermore, the complete evolution of
the liquid fuel film was described by solving its mass, energy and momentum equations on the cylinderw wall boundaries. To model combustion, the Extended Coherent Flamelet Model (ECFM) was used in combination with a Lagrangian ignition model, describing the evolution of the flame kernel and
accounting for both for flow, mixture composition and properties of the electrical circuit. The proposed approach has been implemented into the Lib-ICE code, which is based on the OpenFOAMR technology.
In this paper, examples of application are provided, including the simulation of the fuel-air mixing process in a real GDI engine and the prediction of the premixed turbulent combustion process in a constant-volume vessel for different operating conditions
Ion dynamics and coherent structure formation following laser pulse self-channeling
The propagation of a superintense laser pulse in an underdense, inhomogeneous
plasma has been studied numerically by two-dimensional particle-in-cell
simulations on a time scale extending up to several picoseconds. The effects of
the ion dynamics following the charge-displacement self-channeling of the laser
pulse have been addressed. Radial ion acceleration leads to the ``breaking'' of
the plasma channel walls, causing an inversion of the radial space-charge field
and the filamentation of the laser pulse. At later times a number of
long-lived, quasi-periodic field structures are observed and their dynamics is
characterized with high resolution. Inside the plasma channel, a pattern of
electric and magnetic fields resembling both soliton- and vortex-like
structures is observed.Comment: 10 pages, 5 figures (visit http://www.df.unipi.it/~macchi to download
a high-resolution version), to appear in Plasma Physics and Controlled Fusion
(Dec. 2007), special issue containing invited papers from the 34th EPS
Conference on Plasma Physics (Warsaw, July 2007
Surface Oscillations in Overdense Plasmas Irradiated by Ultrashort Laser Pulses
The generation of electron surface oscillations in overdense plasmas
irradiated at normal incidence by an intense laser pulse is investigated.
Two-dimensional (2D) particle-in-cell simulations show a transition from a
planar, electrostatic oscillation at , with the laser
frequency, to a 2D electromagnetic oscillation at frequency and
wavevector . A new electron parametric instability, involving the
decay of a 1D electrostatic oscillation into two surface waves, is introduced
to explain the basic features of the 2D oscillations. This effect leads to the
rippling of the plasma surface within a few laser cycles, and is likely to have
a strong impact on laser interaction with solid targets.Comment: 9 pages (LaTeX, Revtex4), 4 GIF color figures, accepted for
publication in Phys. Rev. Let
C in intense femtosecond laser pulses: nonlinear dipole response and ionization
We study the interaction of strong femtosecond laser pulses with the C
molecule employing time-dependent density functional theory with the ionic
background treated in a jellium approximation. The laser intensities considered
are below the threshold of strong fragmentation but too high for perturbative
treatments such as linear response. The nonlinear response of the model to
excitations by short pulses of frequencies up to 45eV is presented and analyzed
with the help of Kohn-Sham orbital resolved dipole spectra. In femtosecond
laser pulses of 800nm wavelength ionization is found to occur multiphoton-like
rather than via excitation of a ``giant'' resonance.Comment: 14 pages, including 1 table, 5 figure
What do cMOOC participants talk about in Social Media? A Topic Analysis of Discourse in a cMOOC
Creating meaning from a wide variety of available information and being able to choose what to learn are highly relevant skills for learning in a connectivist setting. In this work, various approaches have been utilized to gain insights into learning processes occurring within a network of learners and understand the factors that shape learners ’ interests and the topics to which learners devote a significant attention. This study combines different methods to develop a scalable analytic approach for a comprehensive analysis of learners ’ discourse in a connectivist massive open online course (cMOOC). By linking techniques for semantic annotation and graph analysis with a qualitative analysis of learner-generated discourse, we examined how social media platforms (blogs, Twitter, and Facebook) and course recommendations influence content creation and topics discusse
A local glucose-and oxygen concentration-based insulin secretion model for pancreatic islets
<p>Abstract</p> <p>Background</p> <p>Because insulin is the main regulator of glucose homeostasis, quantitative models describing the dynamics of glucose-induced insulin secretion are of obvious interest. Here, a computational model is introduced that focuses not on organism-level concentrations, but on the quantitative modeling of local, cellular-level glucose-insulin dynamics by incorporating the detailed spatial distribution of the concentrations of interest within isolated avascular pancreatic islets.</p> <p>Methods</p> <p>All nutrient consumption and hormone release rates were assumed to follow Hill-type sigmoid dependences on local concentrations. Insulin secretion rates depend on both the glucose concentration and its time-gradient, resulting in second-and first-phase responses, respectively. Since hypoxia may also be an important limiting factor in avascular islets, oxygen and cell viability considerations were also built in by incorporating and extending our previous islet cell oxygen consumption model. A finite element method (FEM) framework is used to combine reactive rates with mass transport by convection and diffusion as well as fluid-mechanics.</p> <p>Results</p> <p>The model was calibrated using experimental results from dynamic glucose-stimulated insulin release (GSIR) perifusion studies with isolated islets. Further optimization is still needed, but calculated insulin responses to stepwise increments in the incoming glucose concentration are in good agreement with existing experimental insulin release data characterizing glucose and oxygen dependence. The model makes possible the detailed description of the intraislet spatial distributions of insulin, glucose, and oxygen levels. In agreement with recent observations, modeling also suggests that smaller islets perform better when transplanted and/or encapsulated.</p> <p>Conclusions</p> <p>An insulin secretion model was implemented by coupling local consumption and release rates to calculations of the spatial distributions of all species of interest. The resulting glucose-insulin control system fits in the general framework of a sigmoid proportional-integral-derivative controller, a generalized PID controller, more suitable for biological systems, which are always nonlinear due to the maximum response being limited. Because of the general framework of the implementation, simulations can be carried out for arbitrary geometries including cultured, perifused, transplanted, and encapsulated islets.</p
1D simulation of a turbocharged Diesel engine with comparison of short and long EGR route solutions
This paper describes a detailed analysis of the unsteady flows in the intake and exhaust systems of a modern four-cylinder, turbocharged Diesel engine with different EGR circuits, by means of a research 1D thermo-fluid dynamic tool and detailed experimental data.Firstly, a detailed simulation has been carried out to predict the pressure pulses, average pressures and temperatures in several cross-sections of the pipe systems for different speeds, loads and EGR rates, considering the complex geometry of the air filter, the intake manifold, the intercooler, the EGR-cooler and the exhaust manifold. An extensive set of measured data has been used to validate the model and refine the 1D schematic for the simulation. Secondly, the model has been used to study the different response of the two main lay-outs of external EGR systems: the high pressure and the low pressure circuits, also known as short and long EGR routes respectively