Modeling of Unsteady Turbulent Flow in a Buried Co-Axial Exchanger—The Use of Green's Functions Theory

This work presents an analytical and numerical investigation of the heat transfer problem of turbulent flow under forced convection in a buried co-axial exchanger. A hybrid model consisting of a finite element method at the boundary (BEM) for the heat transfer problem on the boundary, and a finite volume method (FVM) to solve the turbulent flow inside solves this complicated problem. The development of the BEM method is based on Green's functions theory. The mathematical model employed makes a scientific contribution to a similar practical situation. The results can be of great interest for industrial processes requiring the estimation of the heating time necessary to obtain steady states. Essentially, the axial evolution of the heat transfer coefficient versus Reynolds number and transfer duration is treated here. The temperature field and the heat flux density at the wall are also investigated.     

Ab initio modelling of localized corrosion: Study of the role of surface steps in the interaction of chlorides with passivated nickel surfaces

Periodic DFT calculations have been performed on a (5 3 3)-oriented hydroxylated NiO surface to model the influence of step edges on the adsorption and sub-surface insertion of Cl at the surface of the passive film on nickel. With increasing Cl coverage, substructures of Ni(OH)₂, Ni(OH)Cl or Ni(Cl)₂ composition are formed and detached from the surface, suggesting a major role of the step edges in the Cl adsorption-induced thinning mechanism of the oxide film. The calculated energies show the promoting effect of the adsorption of Cl on the energy of detachment of the substructures. At surface saturation, sub-surface insertion becomes energetically more favourable than adsorption. The results suggest a possible bifurcation from the Cl adsorption-induced oxide thinning mechanism to the penetration-induced mechanism of passivity breakdown at saturation in adsorbed Cl of the step edges.     

The design synthesis of periodical microstrip structures using adaptive genetic algorithm and a multiresolution MoM analysis

A multiresolution MoM analysis for the design of periodical microstrip structures is presented. An adaptive genetic algorithm (AGA) is introduced for the synthesis of planar structure designs that can have nearly any desired frequency response, and to optimize the choice of trial functions used on the MoM method. Results for a bandstop structure are presented to show the effectiveness of the proposed method.     

Electromagnetic wave propagation in anisotropic metamaterial waveguides

In this paper, we present the formalism of the transverse operator method (TOM). A rigorous study of propagation in anisotropic and multi‐layer medium using the tensor character of the permeability and permittivity is presented. With the application of the Galerkin method, the propagating modes in metallic rectangular waveguides filled with anisotropic metamaterial are exploited. The results are compared with those previously published and show a good agreement. The complex modes have been obtained. The advantages of the techniques used in this paper lie in the proper analytical formulation of the problem studied, on the one hand, and the speed of convergence, on the other hand. TOM offers a fast convergence of the propagation constant. This shows the effectiveness of our numerical model. As such, the formulation of the transverse operator could be a useful tool for microwave engineers. This type of materials known as metamaterial is widely used and needed by industries and information technology, especially in microwave and radiofrequency devices such as patch antennas, antennas waveguides, resonators, circulators, isolators, phase shifters, and filters. Copyright © 2014 John Wiley & Sons, Ltd.     

Fatty acid methyl and ethyl esters obtained from rare seeds from Tunisia: Ammi visnaga,Citrullus colocynthis,Datura stramonium,Ecballium elaterium,and Silybum marianum

This work aims to study the properties of fatty acid methyl and ethyl esters from Tunisian oils in order to assess their potential use as biofuel sources. The oils chosen for this study have been scarcely researched by other authors and are very interesting for a possible exploitation as fuels. Most of them are nonedible sources and all of them can be grown in arid places with no need of supplementary water: the bishop’s weed Ammi visnaga, the bitter apple Citrullus colocynthis, the stramonium Datura stramonium, the devil pickle Ecballium elaterium, and the milk thistle Silybum marianum.     

IoT scheduling for higher throughput and lower transmission power

Wireless Networks - The massive increase in the Internet of Things (IoT) has brought a diverse long-range, low-power, and low bit-rate wireless network technologies. The LoRa a low-power wide area...     

Optimal Decay Rate for Unidimensional Thermoelastic Problem within the Green-Lindsay Model

In this article we consider the system of generalized thermoelasticity under Green–Lindsay model in one dimension with Dirichlet–Neumann boundary conditions. First, the roots of the characteristic polynomial are investigated by applying an approach based on the implicit function theorem. Then we prove the exponential decay of the associated energy and describe the optimal decay rate. The numerical calculation of the corresponding characteristic roots is done for different real materials. Then another approach based on Hurwitz criterion is applied to obtain the decay rate analytically. Finally, we present a discussion of the decay rate given by both approaches, as well as a comparison with already-existing results for Lord–Shulman and classical models of thermoelasticity.     

H/D Exchange on Silica-Grafted Tantalum(V) Imido Amido [(≡SiO)<Subscript>2</Subscript>Ta<Superscript>(V)</Superscript>(NH)(NH<Subscript>2</Subscript>)] Synthesized from Either Ammonia or Dinitrogen: IR and DFT Evidence for Heterolytic Splitting of D<Subscript>2</Subscript>

The silica-grafted Ta^(V) imido amido complex [(≡SiO)₂Ta(NH)(NH₂)], 2, obtained from the reaction of either ammonia or dinitrogen plus hydrogen with the silica-grafted hydrides [(≡SiO)₂Ta^(III)H], 1a, and [(≡SiO)₂Ta^(V)H₃], 1b, undergoes H/D exchange with D₂. In situ IR spectroscopy shows that the fully labelled compound [(≡SiO)₂Ta(ND)(ND₂)], 2-d, can be obtained by moderate heating (60 °C, 3 h) under D₂ atmosphere (550 torr, 300 eq. with respect to Ta), and that the exchange is reversible. The observed stretching and bending frequencies of 2-d are in agreement with the expected isotopic shift upon H/D replacement with respect to literature values reported for 2 and have been corroborated by the independent synthesis of 2-d by reaction of deuterated 1a and 1b with N₂ and D₂. Density functional theory (DFT) calculations, performed using a periodic or a cluster model, explored the structures and energetics of all minima involved in the reaction with H₂ and showed that among the explored pathways the energetically preferred mechanisms for H₂ reaction with [{(μ-O)[(HO)₂SiO]₂}Ta^(V)(NH)(NH₂)], 2q, is the heterolytic cleavage of either the imido Ta=N or the amido Ta-N bonds, to yield respectively [{(μ-O)[(HO)₂SiO]₂}TaH(NH₂)₂], 3q (ΔE = −9.5 kcal mol⁻¹ and ΔG_298K = +2.6 kcal mol⁻¹ with respect to 2q) and [{(μ-O)[(HO)₂SiO]₂}Ta(NH)(NH₃)], 4q (ΔE = −6.0 kcal mol⁻¹ and ΔG_298K = +7.9 kcal mol⁻¹ with respect to 2q). All activation barriers are moderate (between 17.7 and 30.2 kcal mol⁻¹) in agreement with the observed mild heating conditions necessary for the reaction to occur.     

Location of Cu2+ ions in some protoned Nasicon-type phosphates

Nasicon-type phosphates Cu_1–xH_xZr₂(PO₄)₃ (0 < x <1) and Cu_1–xH_2x–1Zr₂(PO₄)₃ (0.5 < x <1) have been investigated by magnetic susceptibility and electron paramagnetic resonance (EPR). Room-temperature EPR spectra at X-band (9.5 GHz) exhibit relatively different local information about paramagnetic environments in the two sets. Analysis by computer simulations of Cu_1–xH_xZr₂(PO₄)₃ spectra reveals that Cu²⁺ ion is located in an axially distorted octahedron, which can be assigned to the M(1) site. However, in the case of Cu_1–xH_2x–1Zr₂(PO₄)₃, EPR parameters suggest that Cu²⁺ ions are distributed in two types of sites with axial and lower than axial symmetries; these latter can be attributed to M(1) and M(2) sites respectively. g and A components are related to structural properties using molecular orbital method. Data are obtained on variations of the bond covalence with the composition.     

Ab initio study of the interaction of chlorides with defect-free hydroxylated NiO surfaces

DFT periodic calculations have been performed to model the adsorption and sub-surface insertion of Cl atoms at increasing coverage on undefective hydroxylated NiO(1 1 1) terraces characteristic of the surface of the passive film on nickel. The results evidence that structural relaxation, splitting the mixed topmost OH/Cl anionic plane, releases the repulsive interactions induced by Cl adsorption, but the effect decreases with increasing Cl coverage with a loss of ∼1.1 eV in stability at surface saturation. Cl sub-surface insertion in the first inner anionic plane also releases the repulsive interactions in the topmost plane but the effect becomes favourable only at high Cl coverage because of inter-layer mixing after surface reconstruction. At low Cl coverage, an unrelaxed diatomic Ni-OH layer weakly bonded to the oxide and likely to dissolve is formed after sub-surface insertion, suggesting a hybrid mechanism of local oxide thinning. Adsorption of Cl by place exchange is energetically favourable but does not seem to promote dissolution. Insertion into the lattice may promote dissolution but it is energetically favourable only at high Cl surface coverage, which is not a likely situation due to Cl repulsive interaction on the surface. For undefective terraces of the passive film, the reported calculations do not confirm the existing hypotheses of adsorption-induced oxide thinning or chloride sub-surface penetration.