Search

Vortex generators (VG) are widely used in enhancing the heat transfer coefficients in heat exchangers due to the development of longitudinal and transverse vortices. Therefore, understanding the development of these vortices has a high importance for the design and optimization of heat exchangers. When using numerical simulations, the choice of an appropriate turbulence model that can better predict the flow structure downstream a VG is fundamental. In the present study, three-dimensional numerical simulations, with two different commonly used eddy viscosity turbulence models, are performed for channel flow fitted with rectangular-winglet pairs (RWP) vortex generators. The numerical results are compared to experimental data obtained by stereoscopic particle image velocimetry (SPIV). The shear-stress transport (SST) κ-ω model and the re-normalization-group (RNG) κ-e model are used for modeling turbulence. Validation is conducted by comparing the flow structure topology and velocity field obtained from numerical simulations to those obtained using the SPIV method. It is found that the SST κ-ω model is better than the RNG κ-e turbulence model in predicting the flow characteristics downstream the RWP.

A nonparametric method assessing the error and variability margins in solutions depicted in a separated form using experimental results is illustrated in this work. The method assess the total variability of the solution including the modeling error and the truncation error when experimental results are available. The illustrated method is based on the use of the PGD separated form solutions, enriched by transforming a part of the PGD basis vectors into probabilistic one. The constructed probabilistic vectors are restricted to the physical solution’s Stiefel manifold. The result is a real-time parametric PGD solution enhanced with the solution variability and the confidence intervals.

International audience; This work aims at proposing a new procedure for parametric problems whose separated representation has been considered difficult, or whose SVD compression impacted the results in terms of performance and accuracy. The proposed technique achieves a fully separated representation for layered domains with interfaces exhibiting waviness or – more generally – deviating from planar surfaces, parallel to the coordinate plane. This will make possible a simple separated representation, equivalent to others, already analyzed in some of our former works. To prove the potentialities of the proposed approach, two benchmarks will be addressed, one of them involving an efficient space–time separated representation achieved by considering the same rationale.

Arsenic is amongst the most hazardous heavy metals released into the environment by natural or human-induced sources. A known carcinogen and irritant, its trivalent and pentavalent forms are the most common in aqueous media rendering their onsite monitoring and removal crucial. In this work, different adsorbents based on iron oxide nanoparticles (Fe­3O4 NPs) coated with (3-aminopropyl)triethoxysilane (APTES) were prepared. The nanoparticles were then functionalized with one of the three nucleobase derivatives: adenine hydrazide (AH), guanine hydrazide (GH) or uracil hydrazide (UH). The successful functionalization of the nanoparticles was confirmed using Fourier transform infrared spectroscopy. Boron-doped diamond electrodes were modified using the different functionalized nanoparticles, and the interaction of the nucleobases with trivalent arsenic ions was assessed using square wave voltammetry, the adsorption efficiency being extracted from the decrease of the nucleobase peak maximum intensity. The electrochemical evaluation of adsorption isotherms showed that the Langmuir model was a better fit compared to Freundlich, and that the adsorption capacity increased in the following order: AH < UH < GH. Furthermore, it was shown that the adsorption follows a pseudo-second order kinetic model implying the involvement of chemisorption in the process. The electrochemical detection of arsenic utilizing the magnetic nanoparticles functionalized with guanine hydrazide showed a better analytical performance compared to adenine and uracil hydrazide, with a sensitivity of 1.92 µA.µg-1.L and limit of detection of 1.6 µg/L.

A novel Model Order Reduction (MOR) technique is developed to compute high-dimensional parametric solutions for electromagnetic fields in synchronous machines. Specifically, the intrusive version of the Proper Generalized Decomposition (PGD) is employed to simulate a Permanent-Magnet Synchronous Motor (PMSM). The result is a virtual chart allowing real-time evaluation of the magnetic vector potential as a function of the operation point of the motor, or even as a function of constructive parameters, such as the remanent flux in permanent magnets. Currently, these solutions are highly demanded by the industry, especially with the recent developments in the Electric Vehicle (EV). In this framework, standard discretization techniques require highly time-consuming simulations when analyzing, for instance, the noise and vibration in electric motors. The proposed approach is able to construct a virtual chart within a few minutes of off-line simulation, thanks to the use of a fully separated representation in which the solution is written from a series of functions of the space and parameters coordinates, with full space separation made possible by the use of an adapted geometrical mapping. Finally, excellent performances are reported when comparing the reduced-order model with the more standard and computationally costly Finite Element solutions. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

International audience; Most heavy metal ions are known to be toxic and carcinogenic when present in high amounts. Thus, rapid and reliable on-site detection of these ions is crucial. Voltammetry is a highly sensitive electrochemical method that has been widely used for heavy metal detection offering the advantages of sensitivity and rapidity. On the other hand, nanoparticles offer the advantages of high surface area and high selectivity. Thus, this review aims to highlight the application of metallic and metallic oxide nanoparticles for the voltammetric detection of heavy metals. The nanoparticles used were either applied solely on the electrode or as modifiers with various materials. In all cases, the synthesized devices showed an enhanced analytical performance, such that the limits of detection were lowered and the sensitivities were increased as compared to voltammetric systems not using nanoparticles. Moreover, the applicability of some of these systems was investigated in real samples.

International audience; This paper analyzes the ability of different machine learning techniques, able to operate in the low-data limit, for constructing the model linking material and process parameters with the properties and performances of parts obtained by reactive polymer extrusion. The use of data-driven approaches is justified by the absence of reliable modeling and simulation approaches able to predict induced properties in those complex processes. The experimental part of this work is based on the in situ synthesis of a thermoset (TS) phase during the mixing step with a thermoplastic polypropylene (PP) phase in a twin-screw extruder. Three reactive epoxy/amine systems have been considered and anhydride maleic grafted polypropylene (PP-g-MA) has been used as compatibilizer. The final objective is to define the appropriate processing conditions in terms of improving the mechanical properties of these new PP materials by reactive extrusion.

International audience; Mixing in laminar flow regimes is crucial for many engineering applications in which highly viscous and fragile fluids are used. Moreover, the compactness of laminar mixers is a great challenge due to the large mixing time required to obtain the desired homogeneity. The "Split And Recombine" (SAR) static mixers are a promising solution for this challenge. This type of mixers consists of a network of separated and then recombined channels in which two fluids are introduced separately and mixed by a multi-lamination process. The SAR static mixers perform a series of baker's transforms on the concentration profile enhancing thus the mixing process at very low Reynolds numbers. In the present study, numerical simulations are carried out to analyze the mixing process in a new topology of SAR mixer with double separation and recombination in order to increase the lateral gradients and destroy the concentration profile faster. The new geometry proposed here is compared to two SAR configurations widely studied in the open literature namely the Gray and the Chen SAR configurations. The results show a good enhancement of the mixing process in the new double SAR configuration with decrease in the power dissipation. INTRODUCTION Efficient mixing remains a challenging process in many engineering applications such as food processing

International audience; Recommender systems have a better performance whenever they have more information about the users. When little to no information is available for a certain user, the system fails to provide them with accurate recommendations, and this is referred to as the cold-start problem. To solve this issue, non- personalized and personalized active learning strategies were proposed, with an aim to obtain some ratings from the new user on specific items. In this paper, we tackle the non-personalized category, we compare the strategies that were previously suggested in the literature, and we propose a new strategy that combines some selected previous strategies. We use the public MovieLens dataset in the experimentations and we use MAE and RMSE to compare the strategies. The results show that our strategy performs better than the individual strategies previously used.

This study characterizes a new mixing process based on chaotic advection for the production of water/oil (w/o) emulsified engine fuel. At low and intermediate Reynolds numbers, in curved pipes, Dean roll-cells induce radial convective mass transfer, a mechanism exploited in the technology of helical static mixers. The succession of bends of alternating curvature planes produces in addition spatially chaotic flow trajectories that enhance the mixing over and above that in the reference helically coiled geometry made of coplanar bends. This feature is assessed in the present work by comparison of the droplet sizes obtained in helical (regular) and alternating (chaotic) static mixers of the same cross section and the same tube length. The coils are assembled from 90 degrees bends, and the chaotic configuration is obtained by turning each bend by +/- 90 degrees with respect to the previous one. To cover a large range of Reynolds numbers [30-350] and capillary numbers [0.1-1], a lipophilic solvent (Butanol) is added to the continuous phase (oil) to decrease the viscosity and a surfactant (Tween 20) is used beyond the critical concentration (covering the whole w/o interface) to decrease the interfacial tension. Drop-size measurements at the exit of the mixer in the chaotic advection configuration show substantial drop-size reduction and tightening in drop-size distribution. Moreover, it is shown that mixing intensification by chaotic advection is almost independent of Reynolds number in the studied range, with a mixing efficiency enhancement around 30%. (C) 2013 Elsevier Ltd. All rights reserved.

Nevertheless the huge amount of numerical and experimental works on non-colloidal suspension flows, most studies in the literature were limited to the dynamics in different geometries like in channels and pipes. To author's knowledge, mixing enhancement quantification and suspension mixing behavior in static mixers has been rarely studied before. In the present contribution, the mixing enhancement in isothermal monodisperse neutrally buoyant non-Brownian suspensions is investigated through transient 3D CFD simulations. It is quantified for suspension Reynolds (ReS) numbers varying between 50 and 150, for spheres’ diameters (dp) between 25 and 400 μm and for initial volume fractions of particles (ϕ) between 0.25 and 0.45. The influences of dp, ReS and ϕ on the overall mixing enhancement and on pressure drop are quantified for a suspension flow in a chaotic and helical separate pipes (static mixers of equal lengths). It is found that for all cases, the chaotic flow configuration has better mixing performances than the helical pipe flow with almost no significant increase in pressure losses. Moreover, good mixing quality is observed in both static mixers (better at larger particles) where the mixing index ranges between 10 and 75% relative to the initial unmixed particles leaving the straight pipe.

International audience; Passive methods are widely used for flow control in engineering processes for heat and mass transfer enhancement. Using flexible vortex generators (FVGs) in such applications in order to destabilize the flow can be thought to achieve higher performances taking advantage of the fluid-structure interaction. In this paper, we discuss the assessment of getting self-sustained large oscillation amplitudes of multiple FVGs from an upstream confined laminar flow. The FVGs are located on the opposite channel walls in alternated positions, separated by a distance equal to their span and inclined in the upstream direction with an angle of 30° with respect to the wall. Five cases are studied which differ by the number of alternating FVGs in the system and investigations are also performed adding two co-planar FVGs upstream. The Reynolds number is held constant with a value of 2000 (based on the hydraulic diameter) for all the cases. The effect of increasing the degree of freedom of the system, on creating a large displacement flapping motion is numerically investigated. The results show that a minimum of three alternating FVGs is needed to produce a self-sustained and periodic flow instability, leading to large FVG displacement when the co-planar FVGs are not present. The introduction of upstream co-planar FVGs destabilizes the flow by producing vortices which act as periodic forces on the downstream FVGs. In this case, large displacement amplitudes are thus observed with two alternating FVGs added downstream. A phenomenon of inverted drafting is observed in all the cases: upstream FVGs display smaller drag force values than the downstream ones. Since the downstream FVGs oscillate in resonance with the incoming flow, motion amplitudes become higher. Moreover, it has been observed that for all the configurations studied here, the FVGs located at the same wall location oscillate in phase with each others and out-of-phase with the ones located on the opposite channel wall.

Resonance period is a key parameter in the seismic design of a structure, thus dynamic parameters of buildings in Beirut (Lebanon) were investigated based on ambient vibration method for risk and vulnerability assessment. Lebanon is facing high seismic hazard due to its major faults, combined to a high seismic risk caused by dense urbanization in addition to the lack of a seismic design code implementation. For this study, ambient vibration recordings have been performed on 330 RC buildings, period parameters extracted and statistically analyzed to identify correlations with physical building parameters (height, horizontal dimensions, age) and site characteristics (rock sites or soft sites). The study shows that (1) the building height or number of floors (N) is the primary statistically robust parameter for the estimation of the fundamental period T; (2) the correlation between T and N is linear and site dependent: T ae N/23 for rock sites and N/18 for soft sites; (3) the measured damping is inversely proportional to the period: the taller the building the lower is the damping; (4) a significant overestimation of the period exists in current building codes. However part of the large discrepancy with building code recommendations may be due to the very low level of loading.

Given a Riemannian spin $$^c$$ manifold whose boundary is endowed with a Riemannian flow, we show that any solution of the basic Dirac equation satisfies an integral inequality depending on geometric quantities, such as the mean curvature and the O’Neill tensor. We then characterize the equality case of the inequality when the ambient manifold is a domain of a Kahler–Einstein manifold or a Riemannian product of a Kahler–Einstein manifold with $$\mathbb R$$ (or with the circle $$\mathbb S^1$$ ).

In this paper, we consider a compact Riemannian manifold whose boundary is endowed with a Riemannian flow. Under a suitable curvature assumption depending on the O'Neill tensor of the flow, we prove that any solution of the basic Dirac equation is the restriction of a parallel spinor field defined on the whole manifold. As a consequence, we show that the flow is a local product. In particular, in the case where solutions of the basic Dirac equation are given by basic Killing spinors, we characterize the geometry of the manifold and the flow., Comment: 22 pages

The complex projective space $\mathbb C P^2$ of complex dimension $2$ has a Spin$^c$ structure carrying K\"ahlerian Killing spinors. The restriction of one of these K\"ahlerian Killing spinors to a surface $M^2$ characterizes the isometric immersion of $M^2$ into $\mathbb C P^2$ if the immersion is either Lagrangian or complex., Comment: 18 pages

Several vortex generator shapes are used to increase heat and mass transfer in open and internal flows. Here we report a three-dimensional numerical study investigating the effects of longitudinal and transverse vortices on the heat and mass transfer mechanisms generated by rows of trapezoidal vortex generators. The turbulent macrostructures of these streamwise vortices appear greatly to enhance radial convective transfer. Due to Kelvin-Helmholtz instability, the shear layer shed from the tab's edge becomes unstable and generates periodic transverse vortices that enhance fluid mixing and heat transfer. A global performance analysis of the high-efficiency vortex (HEV) heat exchanger designed to exploit these embedded vortices, shows that the HEV is very competitive with other multifunctional heat exchangers/reactors, especially due to its very low energy consumption. (C) 2010 Elsevier Ltd. All rights reserved.

International audience; The behavioral service composition problem arises when no available service can achieve a target behavior. The composition problem consists in building a special service, an orchestrator, which schedules the actions of the services to produce a behavior equivalent to the target one. In this paper, services are modeled as nondeterministic transition systems and the behavior of the composition realizes the behavior of the target. We propose an algorithm that avoids the full computation of the product of services, and instead constructs the orchestrator incrementally. Unlike most algorithms where the whole state space, which is exponential in the number of services, is visited, the proposed algorithm can find a solution by visiting only the pertinent portion of the state space. We also show that this on-the-fly behavior can be paired with a heuristic to speed up the synthesis.

International audience; The influence of the wake vortex arising behind a perforated tab on the mixing processin heat exchangers and chemical reactors is analyzed. The preliminary step of this study, i.e., investigation of the turbulent field generated by a single perforated tab, is presented here. For this aim, laser Doppler velocimetry measurements are conducted downstream from a perforated trapezoidal vortex generator placed in a wind tunnel. It is shown that two shear layers are generated by the tab. The first shear layer is located at the upper edge of the tab, and the other is ejected from the perforation edges. These shear layers are characterized by high turbulent kinetic energy levels, which are profitable for meso-mixing enhancement. Finally, a spectral study shows that the turbulent macro-scale is nearly the same for typical locations in the shear layers shed from the taband perforation edges.

In the process industry, vortex generators are being increasingly incorporated in modern multifunctional heat exchangers/reactors to enhance heat and mass transfer and thus increase energy efficiency. Longitudinal and transverse pressure-driven vortices and shear-instability-driven flow structures generated by flow separation behind the vortex generators play a crucial role in convective transport phenomena. The purpose of this work is to demonstrate experimentally the effects of hydrodynamics on the transfer processes accompanying such flows. The high-efficiency vortex (HEV) is an innovative static mixer and a low energy consumption heat exchanger designed to exploit these types of vortices. Heat transfer results obtained in turbulent flow with embedded vorticity in this multifunctional heat exchanger are compared with numerical results in the literature. Both numerical and experimental results confirm the high energy efficiency of the HEV static mixer flow. (C) 2012 Elsevier Ltd. All rights reserved.

We study the X-ray properties of LINER sources with definite detection of a broad H(alpha) emission line in their optical spectra, LINER 1s from Ho et al. sample. These objects preferentially harbor a low luminosity active nucleus at the center and show small or no intrinsic absorption (, 22 pages, 14 figures, accepted for publication in Astronomy and Astrophysics

(Abridged) Based on UV to X-ray and radio to UV flux ratios, some argue that low ionization emission line regions (LINERs) and low luminosity AGN (LLAGN) are a scaled-down version of their more luminous predecessors Seyfert galaxies. Others, based on the lack of X-ray short (hours) time-scale variability, the non-detection of an iron line at 6.4 keV, and the faint UV emission, suggest the truncation of the classical thin accretion disk in the inner regions of the AGN where a radiatively inefficient accretion flow (RIAF) structure forms. We investigate the LINER-Seyfert connection by studying the unabsorbed, AGN powered, LINER galaxy NGC 4278. We analyzed one XMM-Newton and seven Chandra X-ray observations of NGC 4278 spread over a three year period. We detected a flux increase by a factor of ~3 on a ~3 months time-scale and by a factor of 5 between the faintest and the brightest observation separated by ~3 years. During only the XMM-Newton observation, where the highest flux level is detected, we found a 10% flux increase on a ~1 hour time-scale. A combination of an absorbed power law (N(H)~10^20 cm^-2, Gamma~2.2) plus a thermal component (kT~0.6 keV) were able to fit the Chandra spectra. The XMM-Newton spectra, where the highest X-ray flux is detected, are well fitted with a single absorbed power-law. No Fe K(alpha) emission line is detected at 6.4 keV. We constructed SEDs based on simultaneous or quasi simultaneous observations and compared them to LINER, radio-loud, and radio-quiet quasar SEDs. We find that at a low X-ray flux the NGC 4278 SED resembles that of typical LINER sources where the radio to X-ray emission can be considered as originating from a jet and/or RIAF, whereas at a high X-ray flux, NGC 4278 SED is more like a low luminosity Seyfert SED. Consequently, NGC 4278 could exhibit both LINER and Seyfert nuclear activity depending on the strength of its X-ray emission., 14 pages, 15 figures, accepted for publication in Astronomy and Astrophysics

Longitudinal vortices and hairpin-like structures are generated in an open loop flow by a row of vortex generators inserted on the inner wall of a circular pipe; the vortex generator row is made up of four diametrically opposed trapezoidal tabs tilted from the wall. Steady counter-rotating vortex pairs and periodic hairpin-like structures develop downstream from each tab. The flow pattern of these vortical structures has been studied extensively [D. Dong and H. Meng, Flow past a trapezoidal tab, J. Fluid Mech. 510 (2004), pp. 219-242]; nevertheless, the specific contributions of these structures to the mixing process have not yet been elucidated, especially with regard to global improvement of the transfer coefficients compared to a straight pipe. This study aims at exploring the turbulent mixing mechanisms caused by artificially generated vorticity, especially at the different mixing scales (macro-, meso- and micro-mixing), using both numerical simulations and laboratory experiments. Instantaneous velocities and spectral analysis using Laser Doppler Velocimetry are carried out for axial velocity components. Numerical simulations using the Reynolds stress turbulence model are also performed to investigate the effect of the different flow structures on the averaged Reynolds stress tensor and the turbulent kinetic energy dissipation rate. The development and decay of the counter-rotating vortices are also investigated using a recent pseudo-viscous model [O. Logdberg, J.H.M. Fransson, and P.H. Alfredsson, Streamwise evolution of longitudinal vortices in a turbulent boundary layer, J. Fluid Mech. 623 (2009), pp. 27-58]. Here we modify this model to predict the center path of the streamwise vortices in a turbulent boundary layer. It is also shown that the hairpin-like structures govern both meso- and micro-mixing mechanisms, while the counter-rotating vortices act as internal agitators in the flow by creating convective transfer between the wall region and the flow core. This investigation is fundamental for optimizing static mixers based on vortex generators and for control of separation in aerodynamic applications.

Vortex-induced heat transfer enhancement exploits longitudinal and transverse pressure-driven vortices through the deliberate artificial generation of large-scale vortical flow structures. Thermal-hydraulic performance, Nusselt number and friction factor are experimentally investigated in a HEV (high-efficiency vortex) mixer, which is a tubular heat exchanger and static mixer equipped with trapezoidal vortex generators. Pressure gradients are generated on the trapezoidal tab initiating a streamwise swirling motion in the form of two longitudinal counter-rotating vortex pairs (CVP). Due to the Kelvin Helmholtz instability, the shear layer generated at the tab edges, which is a production site of turbulence kinetic energy (TKE), becomes unstable further downstream from the tabs and gives rise to periodic hairpin vortices. The aim of the study is to quantify the effects of hydrodynamics on the heat-and mass-transfer phenomena accompanying such flows for comparison with the results of numerical studies and validate the high efficiency of the intensification process implementing such vortex generators. The experimental results reflect the enhancement expected from the numerical studies and confirm the high status of the HEV heat exchanger and static mixer.