•The influence of the principal pilot injection parameters is discussed for low-temperature combustion systems.•Swirl-sweep and dwell-time sweep results are combined to analyze soot emissions.•The ...pilot injection effects are investigated in injection profiles featuring rate-shaped main injections.
The potential of pilot injection has been assessed on a low-temperature combustion diesel engine for automotive applications, which was characterized by a reduced compression-ratio, high EGR rates and postponed main injection timings. Dwell time sweeps have been carried out for pilot injections with distinct energizing times under different representative steady-state working conditions of the medium load and speed area of the New European Driving Cycle. The results of in-cylinder analyses of the pressure, heat-release rate, temperature and emissions are presented.
Combustion noise has been shown to decrease significantly when the pilot injected mass increases, while it is scarcely affected by the dwell time between the pilot and main injections. The HC, CO and fuel consumption trends, with respect to both the pilot injection dwell time and mass, are in line with those of conventional combustion systems, and in particular decreasing trends occur as the pilot injection energizing time is increased. Furthermore, a reduced sensitivity of NOx emissions to both dwell time and pilot injected mass has been found, compared to conventional combustion systems. Finally, it has been observed that soot emissions diminish as the energizing time is shortened, and their dependence on dwell time is influenced to a great extent by the presence of local zones with reduced air-to-fuel ratios within the cylinder. A combined analysis of the results of swirl sweeps and dwell time sweeps is here proposed as a methodology for the detection of any possible interference between pilot combustion burned gases and the main injected fuel.
The effect of pilot injection on engine performance and emissions has also been assessed in the presence of rate-shaped main injections. These main injection profiles have been implemented with solenoid injectors by designing the injection fusion between a pre injection shot, which is added after the pilot injection, and the main injection. This innovative strategy shows benefits, with respect to combustion noise, although it still results in a reduced impact on NOx emissions. Furthermore, the brake specific fuel consumption and soot levels generally become worse than in the case of the simple pilot–main injection schedules. The injection fusion strategy has a significant impact on the soot versus dwell time dependence, which is influenced by the interference between the main injection and pilot combustion.
Graphene is ideally suited for optoelectronics. It offers absorption at telecom wavelengths, high-frequency operation and CMOS-compatibility. We show how high speed optoelectronic mixing can be ...achieved with high frequency (~20 GHz bandwidth) graphene field effect transistors (GFETs). These devices mix an electrical signal injected into the GFET gate and a modulated optical signal onto a single layer graphene (SLG) channel. The photodetection mechanism and the resulting photocurrent sign depend on the SLG Fermi level (E
). At low E
(<130 meV), a positive photocurrent is generated, while at large E
(>130 meV), a negative photobolometric current appears. This allows our devices to operate up to at least 67 GHz. Our results pave the way for GFETs optoelectronic mixers for mm-wave applications, such as telecommunications and radio/light detection and ranging (RADAR/LIDARs.).
Spontaneous Raman spectroscopy is a powerful characterization tool for graphene research. Its extension to the coherent regime, despite the large nonlinear third-order susceptibility of graphene, has ...so far proven challenging. Due to its gapless nature, several interfering electronic and phononic transitions concur to generate its optical response, preventing to retrieve spectral profiles analogous to those of spontaneous Raman. Here we report stimulated Raman spectroscopy of the G-phonon in single and multi-layer graphene, through coherent anti-Stokes Raman Scattering. The nonlinear signal is dominated by a vibrationally non-resonant background, obscuring the Raman lineshape. We demonstrate that the vibrationally resonant coherent anti-Stokes Raman Scattering peak can be measured by reducing the temporal overlap of the laser excitation pulses, suppressing the vibrationally non-resonant background. We model the spectra, taking into account the electronically resonant nature of both. We show how coherent anti-Stokes Raman Scattering can be used for graphene imaging with vibrational sensitivity.
Relativistic magnetized jets are key elements in active galactic nuclei and in other astrophysical environments. Their structure and evolution involve a complex non-linear physics that can be ...approached by numerical studies only. Still, owing to a number of challenging computational aspects, only a few numerical investigations have been undertaken so far. In this paper, we present high-resolution three-dimensional numerical simulations of relativistic magnetized jets carrying an initially toroidal magnetic field. The presence of a substantial toroidal component of the field is nowadays commonly invoked and held responsible for the process of jet acceleration and collimation. We find that the typical nose cone structures, commonly observed in axisymmetric two-dimensional simulations, are not produced in the three-dimensional case. Rather, the toroidal field gives rise to strong current-driven kink instabilities leading to jet wiggling. However, it appears to be able to maintain a highly relativistic spine along its full length. By comparing low- and high-resolution simulations, we emphasize the impact of resolution on the jet dynamical properties.
We present a new numerical code, PLUTO, for the solution of hypersonic flows in 1, 2, and 3 spatial dimensions and different systems of coordinates. The code provides a multiphysics, multialgorithm ...modular environment particularly oriented toward the treatment of astrophysical flows in presence of discontinuities. Different hydrodynamic modules and algorithms may be independently selected to properly describe Newtonian, relativistic, MHD, or relativistic MHD fluids. The modular structure exploits a general framework for integrating a system of conservation laws, built on modern Godunov-type shock-capturing schemes. Although a plethora of numerical methods has been successfully developed over the past two decades, the vast majority shares a common discretization recipe, involving three general steps: a piecewise polynomial reconstruction followed by the solution of Riemann problems at zone interfaces and a final evolution stage. We have checked and validated the code against several benchmarks available in literature. Test problems in 1, 2, and 3 dimensions are discussed.
The quest for materials capable of realizing the next generation of electronic and photonic devices continues to fuel research on the electronic, optical and vibrational properties of graphene. ...Few-layer graphene (FLG) flakes with less than ten layers each show a distinctive band structure. Thus, there is an increasing interest in the physics and applications of FLGs. Raman spectroscopy is one of the most useful and versatile tools to probe graphene samples. Here, we uncover the interlayer shear mode of FLGs, ranging from bilayer graphene (BLG) to bulk graphite, and suggest that the corresponding Raman peak measures the interlayer coupling. This peak scales from ~43 cm(-1) in bulk graphite to ~31 cm(-1) in BLG. Its low energy makes it sensitive to near-Dirac point quasiparticles. Similar shear modes are expected in all layered materials, providing a direct probe of interlayer interactions.
We investigate graphene and graphene layers on different substrates by monochromatic and white-light confocal Rayleigh scattering microscopy. The image contrast depends sensitively on the dielectric ...properties of the sample as well as the substrate geometry and can be described quantitatively using the complex refractive index of bulk graphite. For a few layers (<6), the monochromatic contrast increases linearly with thickness. The data can be adequately understood by considering the samples behaving as a superposition of single sheets that act as independent two-dimensional electron gases. Thus, Rayleigh imaging is a general, simple, and quick tool to identify graphene layers, which is readily combined with Raman scattering, that provides structural identification.
Context. We studied the imaging of exoplanetary systems using starshades, which are externally occulted coronagraphs in space. Aims. We provide a new method for precisely evaluating the stray light ...due to the star and a rapid calculation of the point spread functions in the presence of vignetting effects from the external occulter. Our study used shaped occulter configurations published in the literature, in particular, the SISTER NI2 and NW2 systems. Methods. The wavefront at the telescope aperture was computed using the classic Fresnel filtering method. The Fourier transform of the occulter was obtained with the highest possible precision using an approach initially developed for radio antennas, known as the polygonal shape factor. Results. We show that the Fresnel diffraction for a finite spatial field operates at very low frequencies only, and that it is sufficient to calculate the Fourier transforms there. Diffraction patterns computed numerically fully agree with theoretical predictions. The central parts of diffractions of petal and apodized occulters are identical over a large central area that increases in size with the number of petals. These diffraction patterns are used to compute the point spread functions. We computed the stray light for a non-point source star; this shows that starshades are not sensitive to star leakage, with a star diameter limit for a given configuration. We also computed signal-to-noise ratios for a perfect experiment limited by photon noise.
A general treatment of acoustic cavitation was presented, including both fluid dynamics instabilities that can occur at cavitation inception as well as non-equilibrium thermal and mechanical effects ...during bubble dynamics. Different approaches to cavitation modelling were considered and compared.
A novel barotropic cavitation model has been developed, based on the partial differential equations governing the mass-conservation and momentum balance. The fluid has been taken as a homogenous mixture of a pure liquid, its vapor and a quantity of gas, both dissolved and undissolved. The analytical expression for the vapor source term driving cavitation has been carried out by means of the energy conservation equation and a general formula for the sound speed in homogeneous bubbly flows has been derived.
A recently developed conservative, implicit, high-resolution, second-order accurate numerical scheme was applied to solve the equations governing the pipe flow. The resultant computational algorithm was assessed through comparison with experimental data referring to a system made up of a pipe connecting two constant-pressure reservoirs of water. The model predictions were examined and discussed in order to underline the most interesting fluid-dynamic phenomena, such as the dynamics of shock waves arising at cavitation collapse.
The influence of the frequency-dependent friction on the simulation of the pressure wave dynamics in the presence of cavitation was also analyzed and discussed.
PDF
