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Terahertz time-domain spectroscopy of boron nitride nanotube-reinforced PMMA composites
No full textInternational audiencePolymethyl methacrylate (PMMA) reinforced with boron nitride nanotubes (BNNT) at weight fill factors of 0.2 and 0.5 wt % are characterized by terahertz time-domain spectroscopy (THz-TDS). It is known that the introduction of BNNTs in polymer composites can provide thermal conduction paths and significantly reduce interface thermal resistance. Despite the low values of , the presence of the BNNTs has a significant effect on the optical constants, either due to the intrinsic contribution of the BNNTs or due to extrinsic effects such as induced strain in the PMMA matrix. Negligible THz birefringence observed in the polarization-resolved THz transmission measurements indicates a high degree of isotropy of the BNNT dispersed in the PMMA. Assuming an intrinsic effect and the validity of modified Maxwell–Garnett theory regardless of the inclusion shape, the frequency-dependent optical constants of BNNTs are extracted. The refractive index of BNNTs varies roughly linearly from n (ν = 500 GHz) ≈ 1.8 to n(ν = 1.5 THz) ≈ 1.75, while the absorption coefficient increases from α(ν = 500 GHz) ≈ 10 cm−1 to α(ν = 1.5THz) ≈ 45 cm−1. The extracted THz dielectric properties of the BNNTs trend toward reported values for bulk h-BNs
Recueil de communications école d'hiver 2025 GDR ARCHI-META - DN METACMED
No full textNational audienceThe last 20 years have seen a remarkable growth in scientific interest in(elastic and mechanical) metamaterials. One of the reasons for this is thegreat potential that this subject area has in itself as a “link between differentdisciplines”, particularly acoustics and solid mechanics. In addition, through abetter mastery of architecture, it makes it possible to create complex enginee-ring systems with unconventional dynamic and quasi-static behavior.However, the literature seems to show that since its introduction in the early2000s, two almost independent macro-communities have formed around thissubject: that of acousticians, more focused on phenomenology, and that ofmechanics, more focused on methodology.However, it is clear that 1) these two communities are dealing with thesame scientific problem, and that 2) certain differences in purpose and lan-guage have given, and still give, the impression that these are two looselycoupled fields of research. This has made intercommunity dialogue insufficientuntil now. However, it is easy to see that the subjects investigated and the me-thodologies used are generally complementary. The time has come for a newcommunity to emerge, particularly at the international level.In this context, the ARCHI-META GdR proposes to seize this spontaneousconvergence and act as a catalyst to go beyond the specificities related toacoustics and mechanics, and thus to bring together a new community aroundarchitectured metamaterials. The overall objective is to get the communities(mechanics and acousticians) to work together to identify and resolve thecommon scientific issues that have come to light in recent years and to sharethe theoretical tools specific to the two communities in order to establish acommon language. This synergy will make it possible to address more effec-tively the scientific issues limiting the development of these promising techno-logies
Analytical investigation of self-heating phenomenon of polymers considering frequency, liquid ageing and molecular-chain motion
No full textInternational audienceThe aim of this paper is to present a 1D thermodynamic model describing the rise in temperature induced by self-heating phenomenon occurring in cyclic solicitations of polymers and reinforced thermoplastics. The proposed model was validated for a neat Polyamide 6.6 (PA6.6) for various frequencies and water absorption configurations. This work showed that the presence of water molecules in the network cannot be the only explanation for the observed rise in temperature of the studied aged PA6.6. A physical explanation of this exacerbated rise in temperature is then proposed by modelling the unrecoverable molecular-chain network reorganisation and damage, generally known as the “plasticising” effect of water
Implementation of the sliding-mode controller on overhead cranes via the deadbeat method
No full textInternational audienceThis paper addresses the controller design for the overhead cranes. To this end, a nonlinear model of the overhead cranes is considered, and a continuous-time sliding-mode controller is designed for such a model, ensuring global robust stability. Subsequently, the deadbeat implementation method is employed to obtain a discrete-time equivalent of the designed controller, which is a crucial step to implement any continuous-time controller on digital processors. Comparative analyses based on numerical simulations show that the developed sliding-mode controller shows several advantages over the forward Euler discretization method, which is widely used in the literature
Investigation of asymmetric heating in Poiseuille-Rayleigh-Bénard water flow: A numerical study
No full textInternational audienceIn this paper, a numerical investigation of the impact of asymmetric heating on laminar mixed convection in Poiseuille-Rayleigh-Bénard water flow within parallel horizontal channels is presented. The study has been carried out in a rectangular channel with a transverse aspect ratio of 10, and considered both low (Ra = 1.28 × 10^4) and high (Ra = 1.4 × 10^5) Rayleigh numbers, with Reynolds numbers of 50 and 100. A uniform heat flux was applied to the top and bottom walls of the heated region to assess its effect on the system's thermoconvective behavior and heat transfer efficiency. Two flux ratio scenarios were considered: qt/qb = 1 and qt/qb = 2.The results indicate that increasing the flux ratio intensifies the destabilizing temperature gradient and significantly enhances buoyancy-induced flow, thereby influencing the patterns of thermoconvective structures. Specifically, flux ratios lead to an increased number of plumes originating from the bottom of the channel, while reducing their height and confining them between the bottom wall and the upper thermal boundary layer. It is also observed that flux ratios do not affect the mechanisms involved in the formation of longitudinal rolls. Furthermore, at low Rayleigh numbers, asymmetric heating has a pronounced impact on the establishment length. In contrast, this effect diminishes and becomes negligible at higher Rayleigh numbers. Numerical computations further reveal that near the bottom wall, the Nusselt number exhibits singular behavior, approaching infinity. Regardless of Reynolds and Rayleigh numbers, flux ratios significantly enhance heat transfer within the system. Additionally, near the top wall, the buoyancy effects from the bottom wall have negligible impact on heat transfer, except in the case where qt/qb = 2, Re = 50 and Ra = 1.4 × 10^5, where instability in the upper thermal layer was observed
Very high order finite volume solver for multi component two-phase flow with phase change using a posteriori Multi-dimensional Optimal Order Detection
No full textInternational audienceIn this work we propose a very high-order compressible finite volume scheme with a posteriori stabilization for the computation of multi-component two-phase flow with phase change. It is based on finite volume approach using moving least squares (MLS) reproducing kernels for high order reconstruction of the Riemann states. Increased robustness is achieved by using the multi-dimensional optimal order detection (MOOD) method to get a high-accurate and low-dissipation scheme while maintaining boundedness and preventing numerical oscillations at interfaces and strong gradient zones. The properties of the proposed framework are demonstrated on classical test problems starting with convergence order verification on simple scalar advection test cases. More complex shock and more stringent tube tests with various water, steam and air concentration are then simulated and compared with available references in the literature. Finally, the ability of the proposed approach to compute multi-component flows with phase change is illustrated with the simulation of a liquid oxygen jet in gaseous hydrogen
Numerical study of a novel jet-grid approach for Li-ion batteries cooling
No full textInternational audienceClimate change is driving new and more efficient ways of producing and storing energy. In particular, Lithium-ion batteries demonstrate to be a worthwhile storage system for their high specific power and energy density. Due to electrochemical processes inside batteries, high temperatures are achieved during fast charge and discharge. Herein, a novel jet-grid cooling technique, named ImpFilm, featuring fluid impingement and fluid film is proposed. The idea is to introduce an innovative system able to guarantee stable and uniform temperature for Lithium-ion batteries with the purpose to reduce weight and costs. Firstly, the system has been designed by means of a preliminary 0D thermodynamic analysis. Then, 3D CFD simulations have been run on a single module to test its feasibility and effectiveness by the standpoint of fluid and thermodynamics. Mass flow rate, velocity field, volume fraction and temperature distribution are analyzed in the module by focusing on the impact of the geometry grid on both flow dynamics and cell temperature evolution. Results show that a parametric study on the grid design is necessary to balance the flow rate subdivision and to uniform the temperature of all the batteries. Eventually, new grid features prove to be effective in keeping battery temperature uniform and below hazardous thresholds
Transient rheology and morphology in sheared nanolayer polymer films
No full textInternational audienceThe rheology of coextruded layered films of polystyrene/poly(methyl methacrylate) (PS/PMMA) has been studied with small and large amplitude oscillations at a temperature above their glass transition. While the complex viscosity remains constant over the experimental time window for the micron-sized layered films, a decrease has been observed for the nanolayered films. The rheological behavior has then been correlated to the morphological evolution of the multilayer films while the nanolayers dewet. Layer breakup followed by retraction and coalescence leading to a lamellar-like blend morphology followed by a nodular-like morphology has been evidenced in the nanolayer films, for all compositions and conditions tested. The analysis of the microscopic images of the Lfilm cross-sections also provided the droplet size distribution. The nodular morphology is achieved more rapidly when the initial layers are the thinnest at low strains, while at high strains the formation of these droplets is prevented
