X-ray tomography applied to the characterization of cellular materials. Related finite element modeling problems

The analyses of several materials exhibiting a cellular structure have been carried out using X-ray tomography. This new technique allows the three dimensional and non destructive visualisation of the studied materials at the scale of their cellular microstructure. Qualitative examples are given for metal foams, bread and cellular concrete. The similarity between these materials is striking. It has been measured by quantitative 3D image processing. The different Finite Element Methods available today to produce meshes from these images are presented and discussed in the final part of this paper.     

Dusty debris in tornadoes modelled by high Reynolds number two cells vortices

Exact solutions of Navier–Stokes equations enable to describe nice features of atmospherical flows as tornadoes. For this very particular and singular kind of fluid motions, the flows are very often modelled by conical solutions. Famous authors as Burgers (1948) [J.M. Burgers, A mathematical model illustrating the theory of turbulence, Advan. Appl. Mech. 1 (1948) 197–199], Rott (1958) [N. Rott, On the viscous core of a line vortex, Z. Angew. Math. Phys. 96 (1958) 543–553], Serrin (1972) [J. Serrin, The swirling vortex, Phil. Trans. Roy. Soc. Lond. A 271 (1972) 320–360; J. Serrin, The swirling vortex, Phil. Trans. Roy. Soc. Lond. A 271 (1972) 357–358] and Goldshtik and Shtern (1990) [M. Goldshtik, V. Shtern, J. Fluid Mech. 218 (1990) 483–508], improved greatly research. Most of the time, except Serrin’s model, these models applied to tornadoes, rather produced fields of velocity, with orders of magnitudes which are way too low to be appropriate for describing thunderstorms observed in meteorology. Moreover, these models do not include any explicit mechanisms which take into account the potential actions of dust particles often present in tornadoes. Here, we do suggest a new way of modelling the mature phase of a tornado by considering two swirling cells separated by a intermediate cone, which position results by the equilibrium of normal stresses exerted on the two sides of the cone. This choice of modelling is motivated by considering two kinds of flows, inside and outside the cone, to get significant and different characteristics of the associated resulting vortices. This equilibrium condition, completed by the discontinuity of the tangential stresses on the cone, leads to realistic magnitude of the velocity field, i.e., about 10² m s⁻¹. This discontinuity is motivated by several observations which show the ejection of dust particles along a specific cone direction. At our modelling scale, we do want to model the integral of the dragging forces, resulting from the motion of those particles inside the fluid, by a discontinuity of the tangential stresses on the cone which split the flows into two separate cells.     

Circularly polarized metallic EBG antenna

This letter describes the concept and the realization of a directive and circularly polarized antenna using an electromagnetic band gap material whose circular polarization is generated by the structure itself. Experimental and simulated results are presented for an antenna operating at 5GHz.     

Relationship between cutting conditions and chips morphology during milling of aluminium Al-2050

In the aeronautical field, some parts are forged and consequently some fibers orientation occurs. However, the thin walls are hard to get with forging method. Consequently, some machining operations are done to get a thin wall and reduce the weight of part. However, because of the size of part of the volume of chips can be significant. The ratio between the final weight of part and the final weight of chips is approximately 80 %. To improve the economical and the environmental sustainability, a method consists of recycling directly the chip after machining operations. However, studies show a great impact of different points on chip recyclability like chip thickness, chip roughness, or chip density. Consequently, control of the chip morphology is a first point in ensuring a good recyclability. This article concerns the relationship between the cutting conditions, the cutting tool geometries, the lubricant conditions, and the chips morphology. This analysis is decomposed on several points: chip length, chip morphology, chip thickness, and the chip formation. The observations show also a strong influence of cutting conditions on chip morphology where an increase of cutting conditions reduces drastically the chip size. Concerning the chip formation, because the chip is produced during milling where the uncut chip thickness varies, the chip microstructure is then affected.     

Effect of Nitrogen on Properties of Na2O–CaO–SrO–ZnO–SiO2 Glasses

Glasses in the Na₂O–CaO–SrO–ZnO–SiO₂ system have previously been investigated for suitability as a reagent in Al‐free glass polyalkenoate cements (GPCs). These materials have many properties that offer potential in orthopedics. However, their applicability has been limited, to date, because of their poor strength. This study was undertaken with the aim of increasing the mechanical properties of a series of these Zn‐based GPC glasses by doping with nitrogen to give overall compositions of: 10Na₂O–10CaO–20SrO–20ZnO–(40?3x)SiO₂–xSi₃N₄ (x is the no. of moles of Si₃N₄). The density, glass‐transition temperature, hardness, and elastic modulus of each glass were found to increase fairly linearly with nitrogen content. Indentation fracture resistance also increases with nitrogen content according to a power law relationship. These increases are consistent with the incorporation of N into the glass structure in threefold coordination with silicon resulting in extra cross‐linking of the glass network. This was confirmed using ²⁹Si MAS‐NMR which showed that an increasing number of Q² units and some Q³ units with extra bridging anions are formed as nitrogen content increases at the expense of Q¹ units. A small proportion of Zn ions are found to be in tetrahedral coordination in the base oxide glass and the proportion of these increases with the presence of nitrogen.     

Experimental analysis of propagation regimes during the autoignition of a fully premixed methane–air mixture in the presence of temperature inhomogeneities

The present work is devoted to the study of the combustion processes of a homogeneous methane–air mixture subject to thermal stratification within a rapid compression machine (RCM). Temperature fields obtained in nonreactive conditions have been documented in a previous study and the present work aims at correlating these data with the combustion process. The analysis of chemiluminescence images enables the delineation of two propagation regimes, namely spontaneous ignition fronts and deflagrations. The first is observed for short ignition delays, as the fluid features a fairly large and homogeneous hot core zone. The second dominates the combustion process for longer ignition delays. Indeed, despite global homogenization of the temperature fields, the hottest zones are fairly narrow and surrounded by non-negligible thermal gradients, which favors the formation of deflagration. The results thus clearly show a strong correlation between the preignition temperature field and the subsequent combustion process. They are commented on in the light of recent literature. In a second part, quantitative predictions of the occurrence of autoignition fronts and deflagrations are performed by employing a criterion derived from the analysis of direct numerical simulation data (Sankaran et al., 2005). The results are in good agreement with others previously obtained through chemiluminescence imaging for early and intermediate stages of combustion. It is more difficult to reach definitive conclusions for later instants. The present work highlights the relevance but also suggests some limitations of the corresponding criterion for the analysis of homogeneous charge compression ignition (HCCI) combustion processes at the cylinder scale. Furthermore, the quantitative data gathered within the RCM demonstrate the relevance of this device for further investigation of these fundamental issues.     

Renewable fuels: Policy effectiveness and project risk

This paper examines the impact of government policy on the risk profile of a small ethanol production facility. We derive four key results from a simulation model. First, we show that commodity price risk may discourage investment in a project, despite a positive expected rate of return. Second, we show that political uncertainty may have significant impacts on the risk profile of a project. Next, we show that using only production subsidies to attract investors is expensive, since the financial assistance is paid regardless of whether the plant is operating under positive or negative financial conditions. Finally, we show that a capital grant provides a valuable complement to a subsidy, because the grant reduces the amount of value investors must put at risk, and increases their leverage thereby enhancing returns, while the subsidy mitigates commodity price risk. Our results show that compared to a subsidy-only approach, a grant and subsidy combination provides an investment environment with similar downside protection and expected returns for less than 60% of the cost. Further, we show that the two policy tools combined yield a superior investment environment to that created by an equivalent or greater total investment deployed entirely in either of the policy tools without the other.     

GISMOO: A new hybrid genetic/immune strategy for multiple-objective optimization

In this paper, we propose a new Pareto generic algorithm, called GISMOO, which hybridizes genetic algorithm and artificial immune systems. GISMOO algorithm is generic in the sense that it can be used to solve both combinatorial and continuous optimization problems. The proposed approach offers an original iterative process in two phases: a Genetic Phase and an Immune Phase. The Immune Phase is used to identify and to emphasize the solutions located in less crowded regions found during the iterative process of the algorithm. Simulation results on difficult test problems, both in combinatorial and continuous optimization, show that the proposed approach, in most problems, is able to obtain better results than state of the art algorithms.