Incipient sediment transport for non-cohesive landforms by the discrete element method (DEM)

The determination of the shear stress at which a sediment grain of a given size and density starts to move has been treated with theoretical, experimental and numerical procedures by many authors. The seminal contribution of Shields [7] addresses a relationship for the non-dimensional critical shear stress in terms of the friction Reynolds number for a single particle in a flat bed. This work focusses on the incipient transport of particles for bedforms. The proposed numerical approach to the problem integrates the Discrete Element Method (DEM) [9] with a continuous finite element approximation. The DEM simulates the motion of the landform, defined as an aggregate of rigid discs that interact by contact and friction. The continuous finite element approach predicts the boundary shear stress field coming from the fluid flow over the bed (for basic formulation, see [4] and reference therein). Both methods are coupled through the flow-particle force transmission using drag coefficients. While for single particles (or very simple sets of particles) incipient motion (and consequently, the threshold stress) is clearly defined, for complex forms the use of the concept of incipient transport becomes necessary, and critical shear stress is established in terms of a threshold sediment flux over the bed surface. We present a series of numerical experiments for single particles, showing good agreement with Shields curve for the whole range of Reynolds number. In this communication we show some of these results, in compare with the basic Shields curves for flat bed and single grains

Elasto-thermoelectric non-linear, fully coupled, and dynamic finite element analysis of pulsed thermoelectrics

This paper presents a numerical study on the influence of pulsed electric signals applied to the overcooling of thermoelectric devices. To this end, an experimental setup taken from the literature and a commercial cell are simulated using a complete, specially developed research finite element code. The electro-thermal coupling is extended to include the elastic field, demonstrating that the increment of cooling can produce mechanical failure. Numerical results are developed and the variation of overcooling versus pulse gain and versus duration is validated towards a new analytical expression and the experimental data. The issue of optimal intensity at steady-state is also newly developed. Thermal and mechanical trends are presented using constant and variable (with temperature) material properties for a single thermoelement. While some of the first trends are similar to those of published works, others are different or directly new, all closer to those of the experiments. The mechanical results have not been thoroughly studied until recently. The three-dimensional finite element mesh includes non-thermoelectric materials that are fundamental for the current study. Distribution of stresses during steady and transient states are shown inside the thermoelement, for five components and for the combined Tresca stress. Concentrations at corners of the lower side appear close to the cold face. Due to these concentrations, 27-node isoparametric, quadratic brick elements are used. It is shown that the mechanical field is an important factor in the design of pulsed thermoelectrics, since for practical applications the stress levels are close or slightly above the admissible limits.This research was partially supported by grants CSD2008-00037 Canfranc Underground Physics and Polytechnic University of Valencia under programs PAID 02-11-1828 and 05-10-2674.Pérez-Aparicio, JL.; Palma, R.; Moreno-Navarro, P. (2016). Elasto-thermoelectric non-linear, fully coupled, and dynamic finite element analysis of pulsed thermoelectrics. Applied Thermal Engineering. 107:398-409. https://doi.org/10.1016/j.applthermaleng.2016.05.114S39840910

Study of hysteretic thermoelectric behavior in photovoltaic materials using the finite element method, extended thermodynamics and inverse problems

The main objective of the present work is to develop and prove a theoretical explanation based on the Extended Non-Equilibrium Thermodynamics (ENETs) for the hysteretical thermoelectric behavior observed in certain thin-film photovoltaic materials. The ENET introduces dissipative fluxes in the entropy balance that could explain this behavior. To verify this explanation from a numerical point of view, results are generated using a Finite Element (FE) formulation based on the ENET and already developed in previous publications by the authors. In addition, an identification Inverse Problem (IP) is formulated; a cost function is defined as the quadratic difference between experimental and numerical results and the IP is solved minimizing the cost function using genetic algorithms. The conclusion is that the loop-like distributions are due to energy dissipation introduced by dissipative fluxes that are closely related with relaxation times. Also, the FE-IP combination permits to find an approximated characterization of properties for several materials from single experimental curves. Finally, several numerical simulations are proposed for laboratory experiments to further validate the theoretical interpretation and to confirm the relation between relaxation times and hysteresis.This research was partially supported by the Grants CSD2008-00037 Canfranc Underground Physics, Excelencia Junta Andalucia P08-TEP-03641 and Polytechnic University of Valencia under programs PAID 02-11-1828 and 05-10-2674.Palma, R.; Pérez-Aparicio, JL.; Bravo, R. (2013). Study of hysteretic thermoelectric behavior in photovoltaic materials using the finite element method, extended thermodynamics and inverse problems. Energy Conversion and Management. 65(92):557-563. https://doi.org/10.1016/j.enconman.2012.07.009S557563659

Local chain deformation and overstrain in reinforced elastomers: An NMR study

A molecular-level understanding of the strain response of elastomers is a key to connect microscopic dynamics to macroscopic properties. In this study we investigate the local strain response of vulcanized, natural rubber systems and the effect of nanometer-sized filler particles, which are known to lead to highly improved mechanical properties. A multiple-quantum NMR approach enables the separation of relatively low fractions of network defects and allows to quantitatively and selectively study the local deformation distribution in the strained networks matrix on the microscopic (molecular) scale. We find that the presence of nondeformable filler particles induces an enhanced local deformation of the matrix (commonly referred to as overstrain), a slightly increased local stress/strain heterogeneity, and a reduced anisotropy. Furthermore, a careful analysis of the small nonelastic defect fraction provides new evidence that previous NMR and scattering results of strained defect-rich elastomers cannot be interpreted without explicitly taking the nonelastic defect fraction into account

Incipient sediment transport for non-cohesive landforms by the discrete element method (DEM)

[EN] We introduce a numerical method for incipient sediment transport past bedforms. The approach is based on the discrete element method (DEM) [1], simulating the micro-mechanics of the landform as an aggregate of rigid spheres interacting by contact and friction. A continuous finite element approximation [2] predicts the boundary shear stress field due to the fluid flow, resulting in drag and lift forces acting over the particles. Numerical experiments verify the method by reproducing results by Shields [3] and other authors for the initiation of motion of a single grain. A series of experiments for sediments with varying compacity and constituting piles yields enhanced relationships between threshold shear stress and friction Reynolds number, to define incipient sediment transport criterion for flows over small-scale bed morphologies.This work was partially supported by the MICIIN Grants #BIA-2008-00522 and #BIA-2012-32918.Bravo, R.; Ortiz Rossini, P.; Pérez Aparicio, JL. (2014). Incipient sediment transport for non-cohesive landforms by the discrete element method (DEM). Applied Mathematical Modelling. 38(4):1326-1337. https://doi.org/10.1016/j.apm.2013.08.010S1326133738

Refined Element Discontinuous Numerical Analysis of Dry-Contact Masonry Arches

The behavior of buried masonry arches is studied in this article using the Discontinuous Deformation Analysis (DDA), a numerical method that allows for the physical simulation of the intrinsic structure dis- continuities since it is based on contact and friction among pseudo-rigid blocks. Two types of arches (or vaults) are studied with a specially developed computer program, one of semicircular and another of ovoidal shape. The loads are self-weight, lateral filling, embankment thrusts and concentrated (through a short distribution) forces close to the peak. These loads are transformed into point forces applied to the center of gravity of each block with simple formulae from classical mechanics. Equilibrium is reached in the whole structure through contact forces calculated with a standard contact algorithm: penalty plus Coulomb friction. DDA-macroblocks composed of linked (through penalty contact springs) pseudo-rigid blocks are for- mulated. This linkage allows for the simulation of collapse by instability or by stress compressive failure more accurately than traditional DDA analyses, for instance funicular polygons. The numerical results are compared with those of the experiments taken from the literature with, for most cases, very good agreement given the uncertainties on geometry and material properties and given the intrinsic quality dispersion of masonry structures. Collapse loads as function of number of joints, safety factors and limit point forces from the numerical and experimental results are compared. The hinges that appear prior to collapse are also compared, obtaining again for most cases very good agreement.J.L. Perez-Aparicio, R. Bravo were partially supported by the MFOM I+D (2004/38), all authors by MICIIN #BIA-2008-00522 and the first also by Polytechnic University of Valencia under Grant PAID 05-10-2674.Pérez-Aparicio, JL.; Bravo, R.; Ortiz Rossini, P. (2013). Refined Element Discontinuous Numerical Analysis of Dry-Contact Masonry Arches. Engineering Structures. 48:578-587. https://doi.org/10.1016/j.engstruct.2012.09.027S5785874

Numerical sedimentation particle-size analysis using the Discrete Element Method

Sedimentation tests are widely used to determine the particle size distribution of a granular sample. In this work, the Discrete Element Method interacts with the simulation of flow using the well known one-way- coupling method, a computationally affordable approach for the time-consuming numerical simulation of the hydrometer, buoyancy and pipette sedimentation tests. These tests are used in the laboratory to determine the particle-size distribution of fine-grained aggregates. Five samples with different particle-size distributions are modeled by about six million rigid spheres pro- jected on two-dimensions, with diameters ranging from 2.5 × 10−6 m to 70 × 10−6 m, forming a water sus- pension in a sedimentation cylinder. DEM simulates the particle s movement considering laminar flow in- teractions of buoyant, drag and lubrication forces. The simulation provides the temporal/spatial distributions of densities and concentrations of the suspension. The numerical simulations cannot replace the laboratory tests since they need the final granulometry as initial data, but, as the results show, these simulations can identify the strong and weak points of each method and eventually recommend useful variations and draw conclusions on their validity, aspects very difficult to achieve in the laboratory.R. Bravo and J.L. Perez-Aparicio were partially supported by the project MICIIN #BIA-2012-32918. The second researcher used the grant GV BEST/2014/232 for the completion of this work. J. Jaime Gomez-Hernandez acknowledges the financial aid from project MINECO CGL2011-23295.Bravo, R.; Pérez Aparicio, JL.; Gómez Hernández, JJ. (2015). Numerical sedimentation particle-size analysis using the Discrete Element Method. Advances in Water Resources. 86:58-72. https://doi.org/10.1016/j.advwatres.2015.09.024S58728

Results of the material screening program of the NEXT experiment

[EN] The Neutrino Experiment with a Xenon TPC (NEXT), intended to investigate neutrinoless double beta decay, requires extremely low background levels. An extensive material screening and selection process to assess the radioactivity of components is underway combining several techniques, including germanium γ-ray spectrometry performed at the Canfranc Underground Laboratory; recent results of this material screening program are presented here.Dafni, T.; Álvarez-Puerta, V.; Bandac, I.; Bettini, A.; Borges, FIGM.; Camargo, M.; Carcel, S.... (2016). Results of the material screening program of the NEXT experiment. Nuclear and Particle Physics Proceedings. 273-275:2666-2668. https://doi.org/10.1016/j.nuclphysbps.2015.10.024S26662668273-27

An improved measurement of electron-ion recombination in high-pressure xenon gas

We report on results obtained with the NEXT-DEMO prototype of the NEXT-100 high-pressure xenon gas time projection chamber (TPC), exposed to an alpha decay calibration source. Compared to our previous measurements with alpha particles, an upgraded detector and improved analysis techniques have been used. We measure event-by-event correlated fluctuations between ionization and scintillation due to electron-ion recombination in the gas, with correlation coeffcients between -0.80 and -0.56 depending on the drift field conditions. By combining the two signals, we obtain a 2.8% FWHM energy resolution for 5.49 MeV alpha particles and a measurement of the optical gain of the electroluminescent TPC. The improved energy resolution also allows us to measure the specific activity of the radon in the gas due to natural impurities. Finally, we measure the average ratio of excited to ionized atoms produced in the xenon gas by alpha particles to be 0:561 0:045, translating into an average energy to produce a primary scintillation photon ofWex = (39:2 3:2) eV.This work was supported by the following agencies and institutions: the European Research Council under the Advanced Grant 339787-NEXT; the Ministerio de Economia y Competitividad of Spain under grants CONSOLIDER-Ingenio 2010 CSD2008-0037 (CUP), FPA2009-13697-C04 and FIS2012-37947-C04; the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under contract no. DE-AC02-05CH11231; and the Portuguese FCT and FEDER through the program COMPETE, project PTDC/FIS/103860/2008.Serra, L.; Sorel, M.; Alvarez, V.; Borges, FIG.; Camargo, M.; Carcel, S.; Cebrian, S.... (2015). An improved measurement of electron-ion recombination in high-pressure xenon gas. Journal of Instrumentation. 10:1-19. https://doi.org/10.1088/1748-0221/10/03/P03025S1191

Near-intrinsic energy resolution for 30-662 keV gamma rays in a high pressure xenon electroluminescent TPC

We present the design, data and results from the NEXT prototype for Double Beta and Dark Matter (NEXT-DBDM) detector, a high-pressure gaseous natural xenon electroluminescent time projection chamber (TPC) that was built at the Lawrence Berkeley National Laboratory. It is a prototype of the planned NEXT-100 136Xe neutrino-less double beta decay (0νββ) experiment with the main objectives of demonstrating near-intrinsic energy resolution at energies up to 662 keV and of optimizing the NEXT-100 detector design and operating parameters. Energy resolutions of ∼1% FWHM for 662 keV gamma rays were obtained at 10 and 15 atm and ∼5% FWHM for 30 keV fluorescence xenon X-rays. These results demonstrate that 0.5% FWHM resolutions for the 2,459 keV hypothetical neutrino-less double beta decay peak are realizable. This energy resolution is a factor 7 to 20 better than that of the current leading 0νββ experiments using liquid xenon and thus represents a significant advancement. We present also first results from a track imaging system consisting of 64 silicon photo-multipliers recently installed in NEXT-DBDM that, along with the excellent energy resolution, demonstrates the key functionalities required for the NEXT-100 0νββ search