Direct Numerical Simulations of Hotspot-induced Ignition in Homogeneous Hydrogen-air Pre-mixtures and Ignition Spot Tracking

A systematic study relying on Direct Numerical Simulations (DNS) of premixed hydrogen-air mixtures has been performed to investigate the hotspot ignition characteristics and ignition probability under turbulent conditions. An ignition diagram is first obtained under laminar conditions by a parametric study. The impact of turbulence intensity on ignition delays and ignition probability is then quantified in a statistically-significant manner by repeating a large number of independent DNS realizations. By tracking in a Lagrangian frame the ignition spot, the balance between heat diffusion and heat of chemical reaction is observed as function of time. The evolution of each chemical species and radicals at the ignition spot is checked and the mechanism leading to ignition or misfire are analyzed. It is observed that successful ignition is mostly connected to a sufficient build-up of a HO₂ pool, ultimately initiating production of OH. Turbulence always delays ignition, and ignition probability goes to zero at sufficiently high turbulence intensity when keeping temperature and size of the initial hotspot constant.     

Gear shifting multi-objective optimization to improve vehicle performance,fuel consumption,and engine emissions

In the present work, the Adaptive-Weight Genetic Algorithm was employed in order to determine the gear shifting strategies that allow an automobile to work in the best compromise among fuel consumption, engine emissions, and vehicle performance. For the assessment of each of the three objective functions, a simulation model based on engine data and on the well-established equations of the longitudinal dynamics was developed. The driving cycle chosen for the calculations was the FTP-75, which takes into account both cold and hot starts, meaning that the transient operation during the warm-up of the catalyst is also considered.     

Fermi field and Dirac oscillator in a Som–Raychaudhuri space-time

We investigate the relativistic dynamics of a Dirac field in the Som–Raychaudhuri space-time, which is described by a Gödel-type metric and a stationary cylindrical symmetric solution of Einstein field equations for a charged dust distribution in rigid rotation. In order to analyze the effect of various physical parameters of this space-time, we solve the Dirac equation in the Som–Raychaudhuri space-time and obtain the energy levels and eigenfunctions of the Dirac operator by using the Nikiforov–Uvarov method. We also examine the behaviour of the Dirac oscillator in the Som–Raychaudhuri space-time, in particular, the effect of its frequency and the vorticity parameter.     

Refinements of the Weyl Pure Geometrical Thick Branes From Information‐Entropic Measure

This work aims to analyse the so‐called configurational entropy in the Weyl pure geometrical thick brane model. The Weyl structure plays a prominent role in the brane thickness of this model. We find a set of parameters associated to the brane width where the configurational entropy exhibits critical points. The information‐theoretical measure sets bounds into parameter of Weyl pure geometrical brane model. In addition, we also argue that a similar approach can be useful to analyze the corrections to Newtonian and Coulombian potentials in Weyl scenarios.     

Optical Absorption in Periodic Graphene Superlattices: Perpendicular Applied Magnetic Field and Temperature Effects

A detailed study of the magneto‐optical absorption is presented for graphene superlattices (SLs) subjected to a perpendicular magnetic field. For a given temperature, this quantity exhibits a resonant peak structure whose characteristics depend on the magnetic field regime, circular polarization of light and SL barrier height. For the intermediate field regime, we demonstrated that the resonant peak structure of is directly correlated to the partial joint density of states. Specifically, the latter exhibits van Hove‐like singularities and peaks at energies where takes its maximum values. We also investigated the magnetoabsorption in the weak field regime for SLs exhibiting one and extra Dirac points in the absence of the field. It was found that for SLs with only one Dirac point, the absorption spectra consist of resonant peaks satisfying the same circular polarization dependent selection rule as that for pristine graphene, except for one of them. For SLs with extra Dirac points, the resonant peaks arise from transitions between singlet subbands or between doublet subbands and satisfy a circular polarization and peak intensity dependent selection rule. It was also found that the resonant structure of can be observed experimentally at room temperature in clean SLs.     

Electromagnetic Propagation in a Relativistic Electron Gas at Finite Temperatures

The electromagnetic propagation in a relativistic electron gas at finite temperatures and carrier densities is described. Using quantum electrodynamics at finite temperatures, electric and magnetic responses and general constitutive relations are obtained. Rewriting the propagator for the electromagnetic field in terms of the electric and magnetic responses, the modes that propagate in the gas are identified. As expected, the usual collective excitations are obtained, i.e., a longitudinal electric and two transverse magnetic plasmonic modes. In addition, a purely photonic mode is found, which satisfies the wave equation in vacuum, for which the electron gas is transparent. Dispersion relations for the plasmon modes at zero and finite temperatures are presented and the intervals of frequency and wavelength where both electric and magnetic responses are simultaneously negative are identified, a behavior previously thought not to occur in natural systems. The investigation of the electromagnetic responses of a relativistic electron gas shows that, apart from the usual longitudinal electric plasmon mode and the two transverse magnetic plasmon modes, there is also a pure photonic mode that propagates with the speed of light, as if the medium were transparent. Furthermore, there is a region of frequencies and wavenumbers of the external fields where both the longitudinal electric permittivity and magnetic permeability are simultaneously negative, a property found in artificially constructed metamaterials.     

Detection of Single Quantum Dots in Model Systems with Sheet Illumination Microscopy

Single molecule detection and tracking provides at times the only possible method to observe the interactions of low numbers of biomolecules, inlcuding DNA, receptors and signal mediating proteins in living systems. However, most existing imaging methods do not enable both high sensitivity and non-invasive imaging of large specimens. In this study we report a new setup for selective plane illumination microscopy (SPIM), which enables fast imaging and single molecule tracking with the resolution of confocal microscopy and the optical penetration beyond 300 μm. We detect and report our instrumental figures of merit, control values of fluorescence properties of single nano crystals in comparison to both standard widefield configurations, and also values of nanocrystals in multicellular “fruiting bodies” of Dictyostelium, an excellent control as a model developmental system. In the Dictyostelium , we also report some of our first tracking of single nanocrystals with SPIM. The new SPIM setup represents a new technique, which enables fast single molecule imaging and tracking in living systems.     

The influence of nanocrystal size on optical second harmonic generation by <Emphasis Type="Italic">p</Emphasis>ara-nitroanaline embedded in electro-spun polymeric fibers

Poly(methyl methacrylate) electro-spun fibers with embedded nanocrystals of the paradigmatic donor–acceptor nonlinear chromophore para-nitroaniline have been recently demonstrated to be efficient generators of second harmonic light. To understand the influence of the size and local strain experienced by the embedded para-nitroaniline nanocrystals, a Williamson?Hall analysis was carried out on the X-ray diffraction intensity. Both the mean crystal size and strain can be tuned by simple changes in the deposition parameters of flow rate and applied voltage. The observed second harmonic signal is well correlated with the ratio of the fiber diameter to the mean para-nitroaniline crystal size suggesting that surface effects are the main source of the strong nonlinear optical response. Adjusting the electro-spinning deposition parameters when producing polymeric fibers doped with strong nonlinear organic chromophores with high dipole moments has the potential to provide a versatile and efficient method for developing second-order nonlinear optical materials.