A model for enhanced and selective transport through biological membranes with alternating pores

We investigate the outflux of ions through the channels in a cell membrane. The channels undergo an open/close cycle according to a periodic schedule. Our study is based both on theoretical considerations relying on homogenization theory, and on Monte Carlo numerical simulations. We examine the onset of a limiting boundary behavior characterized by a constant ratio between the outflux and the local density, in the thermodynamics limit. The focus here is on the issue of selectivity, that is on the different behavior of the ion currents through the channel in the cases of the selected and non-selected species.Comment: arXiv admin note: text overlap with arXiv:1307.418

Relaxation Height in Energy Landscapes: an Application to Multiple Metastable States

The study of systems with multiple (not necessarily degenerate) metastable states presents subtle difficulties from the mathematical point of view related to the variational problem that has to be solved in these cases. We introduce the notion of relaxation height in a general energy landscape and we prove sufficient conditions which are valid even in presence of multiple metastable states. We show how these results can be used to approach the problem of multiple metastable states via the use of the modern theories of metastability. We finally apply these general results to the Blume--Capel model for a particular choice of the parameters ensuring the existence of two multiple, and not degenerate in energy, metastable states

A phenomenological model of a downdraft biomass gasifier flexible to the feedstock composition and the reactor design

The development of a one-dimensional (1D) phenomenological model for biomass gasification in downdraft reactors is presented in this study; the model was developed with the aim of highlighting the main advantages and limits related to feedstocks that are different from woodchip, such as hydro-char derived from the hydrothermal carbonization of green waste, or a mix of olive pomace and sawdust. An experimental validation of the model is performed. The numerically evaluated temperature evolution along the reactor gasifier is found to be in agreement with locally measured values for all the considered biomasses. The model captures the pressure drop along the reactor axis, despite an underestimation with respect to the performed measurements. The producer gas composition resulting from the numerical model at the exit section is in quite good agreement with gas-chromatograph analyses (12% maximum error for CO and CO2 species), although the model predicts lower methane and hydrogen content in the syngas than the measurements show. Parametric analyses highlight that lower degrees of porosity enhance the pressure drop along the reactor axis, moving the zones characterized by the occurrence of the combustion and gasification phases towards the bottom. An increase in the biomass moisture content is associated with a delayed evolution of the temperature profile. The high energy expenditure in the evaporation phase occurs at the expense of the produced hydrogen and methane in the subsequent phases

Coupling of Josephson flux-flow oscillators to an external RC load

We investigate by numerical simulations the behavior of the power dissipated in a resistive load capacitively coupled to a Josephson flux flow oscillator and compare the results to those obtained for a d.c. coupled purely resistive load. Assuming realistic values for the parameters R and C, both in the high- and in the low-Tc case the power is large enough to allow the operation of such a device in applications.Comment: uuencoded, gzipped tar archive containing 11 pages of REVTeX text + 4 PostScript figures. To appear in Supercond. Sci. Techno

CFD modelling of a spark ignition internal combustion engine fuelled with syngas for a mCHP system

Micro Combined Heat and Power (mCHP) powered with biomass is nowadays a technology attracting increasing interest to develop a local supply chain to produce, process and valorise the available material in territorial areas as much as possible circumscribed, with a considerable reduction also of the CO2 related to transportation. Application for biomass powered mCHP produces environmental benefits by reducing primary energy consumption and associated greenhouse gas emissions and complies with the need for increased decentralization of energy supply. Of particular relevance is mCHP based on biomass gasification due to the negligible particulate matter release with respect to combustion. The present work describes a 3D CFD model of the spark ignition (SI) internal combustion engine (ICE) fuelled with syngas installed in the mCHP pilot system ECO20 manufactured by the Italian company Costruzioni Motori Diesel S.p.A. (CMD). The considered system is made of a gasifier combined with proper syngas cleaning devices, an ICE and a generator to deliver a maximum electrical and thermal power of 20 kW and 40 kW, respectively. For the proper initialisation of the 3D CFD model, the syngas composition is experimentally characterised using a gas-chromatograph on samples collected under real operation. The calculated pressure cycle is verified by comparison with the one calculated through a properly developed 1D ICE model. Main goals of the performed numerical analysis are to study into detail the combustion process and to assess the engine performance characteristics related to the use of syngas

Magnetic order in the Ising model with parallel dynamics

It is discussed how the equilibrium properties of the Ising model are described by an Hamiltonian with an antiferromagnetic low temperature behavior if only an heat bath dynamics, with the characteristics of a Probabilistic Cellular Automaton, is assumed to determine the temporal evolution of the system.Comment: 9 pages, 3 figure

Optimization of the Efficiency in a Syngas Powered Si Engine Through Numerical Studies Related to the Geometry of the Combustion Chamber

The combustion process occurring in an alternative Spark Ignition (SI) engine powered with bio-syngas from biomass gasification was previously studied by authors through the development of two different numerical models: a 0-1D model developed in the GT-Suite® environment, aimed at gaining a first look upon the main features of the heat release by the syngas and engine performances; a 3D Computational Fluid Dynamics (CFD) model developed within the AVL FireTM software reproducing the engine combustion cycle within a Reynolds Averaged Navier Stokes (RANS) schematization and employing a detailed chemical reaction mechanism to highlight the interaction between the fluid dynamics and the kinetics of the specific biofuel oxidation chain. The numerical results were validated with respect to experimental measurements in a baseline condition, where the presence of a relatively high amount of CO in the exhaust gases was noticed as related to an engine low combustion efficiency, mainly due to the peripheral spark plug position that determines the persistence of residual gases on the opposite side of the combustion chamber wall. The proposed work presents a numerical analysis made through the developed models on the effects of proper changes in the spark plug position. A multi-objective optimization problem is conducted also by varying the Start of Spark (SOS) and the mixture air-to-fuel ratio with the aim of reducing the engine environmental impact without affecting its performances. A centrally mounted spark, along with a correct calibration of the SOS and mixture ratio, allows a reduction of more than 90% of CO emission with respect to the baseline condition without penalizing the engine brake power and efficienc

Multiple-Choice Testing Using Immediate Feedback-Assessment Technique (IF AT®) Forms: Second-Chance Guessing vs. Second-Chance Learning?

Multiple choice testing is a common but often ineffective method for evaluating learning. A newer approach, however, using Immediate Feedback Assessment Technique (IF AT®, Epstein Educational Enterprise, Inc.) forms, offers several advantages. In particular, a student learns immediately if his or her answer is correct and, in the case of an incorrect answer, has an opportunity to provide a second response and receive partial credit for a correct second attempt. For a multiple choice question with five possible answers, the IF AT® form covers spaces labeled A through E with a thin opaque film; when the film is scratched away, a star indicates the correct answer. This study was conducted in order to assess learning after an initial incorrect answer. Based on random chance, students should have mathematically a 25% chance of guessing a correct second answer (i.e. 1 of 4 remaining answers on the IF AT® form). Analysis of second responses for 8775 questions on IF AT® forms in 22 classes over 3 years showed that the percent of correct second answers was 44.9%, significantly higher than one might expect from random guessing. This indicates that students learned from an incorrect answer and, possibly by re-reading the problem, were able to demonstrate some level of mastery of the material. This data leads us to conclude that IF AT® forms are useful assessment tools