14 research outputs found
Band Formation during Gaseous Diffusion in Aerogels
We study experimentally how gaseous HCl and NH_3 diffuse from opposite sides
of and react in silica aerogel rods with porosity of 92 % and average pore size
of about 50 nm. The reaction leads to solid NH_4Cl, which is deposited in thin
sheet-like structures. We present a numerical study of the phenomenon. Due to
the difference in boundary conditions between this system and those usually
studied, we find the sheet-like structures in the aerogel to differ
significantly from older studies. The influence of random nucleation centers
and inhomogeneities in the aerogel is studied numerically.Comment: 7 pages RevTex and 8 figures. Figs. 4-8 in Postscript, Figs. 1-3 on
request from author
Inexact spectral deferred corrections
Implicit integration methods based on collocation are attractive for a number of reasons, e.g. their ideal (for Gauss-Legendre nodes) or near ideal (Gauss-Radau or Gauss-Lobatto nodes) order and stability properties. However, straightforward application of a collocation formula with M nodes to an initial value problem with dimension d requires the solution of one large Md Ă— Md system of nonlinear equations
Adaptive Macro Finite Elements for the Numerical Solution of Monodomain Equations in Cardiac Electrophysiology
[EN] Many problems in Biology and Engineering are
governed by anisotropic reaction diffusion equations with a
very rapidly varying reaction term. This usually implies the
use of very fine meshes and small time steps in order to
accurately capture the propagating wave while avoiding the
appearance of spurious oscillations in the wave front. This
work develops a family of macro finite elements amenable for
solving anisotropic reaction diffusion equations with stiff
reactive terms. The developed elements are incorporated on a
semi-implicit algorithm based on operator splitting that
includes adaptive time stepping for handling the stiff reactive
term. A linear system is solved on each time step to update
the transmembrane potential, whereas the remaining ordinary
differential equations are solved uncoupled. The method
allows solving the linear system on a coarser mesh thanks to
the static condensation of the internal degrees of freedom
(DOF) of the macroelements while maintaining the accuracy
of the finer mesh. The method and algorithm have been
implemented in parallel. The accuracy of the method has
been tested on two- and three-dimensional examples demonstrating
excellent behavior when compared to standard
linear elements. The better performance and scalability of
different macro finite elements against standard finite elements
have been demonstrated in the simulation of a human
heart and a heterogeneous two-dimensional problem with
reentrant activity. Results have shown a reduction of up to
four times in computational cost for the macro finite
elements with respect to equivalent (same number of DOF)
standard linear finite elements as well as good scalability
properties.Heidenreich, E.; Ferrero De Loma-Osorio, JM.; DoblarĂ© Castellano, M.; RodrĂguez Matas, JF. (2010). Adaptive Macro Finite Elements for the Numerical Solution of Monodomain Equations in Cardiac Electrophysiology. Annals of Biomedical Engineering. 38(7):2331-2345. https://doi.org/10.1007/s10439-010-9997-2S23312345387Aliev, R., and A. Panfilov. A simple two-variable model of cardiac excitation. Chaos Solitons Fractals 7:293–301, 1996.Barad, M., and P. Colella. A fourth-order accurate local refinement method for Poisson’s equation. J. Comput. Phys. 209:1–18, 2005.Bendahmane, M., R. BĂĽrguer, and R. Ruiz-Baier. A multiresolution space-time adaptive scheme for the bidomain model in electrocardiology. Numer. Methods Partial Differ. Equ. 2010. doi: 10.1002/num.20495Bernabeu, M. O., R. Bordas, P. Pathmanathan, J. Pitt-Francis, J. Cooper, A. Garny, D. J. Gavaghan, B. Rodriguez, J. A. Southern, and J. P. Whiteley. 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Sinterização de cerâmicas em microondas. Parte III: Sinterização de zircônia, mulita e alumina Microwave sintering of ceramics. Part III: Sintering of zirconia, mullite and alumina
O aquecimento utilizando microondas possui muitas vantagens com relação aos mĂ©todos convencionais de aquecimento, como redução no tempo de processamento, economia de energia e melhora na uniformidade microestrutural dos corpos cerâmicos. Assim esse trabalho tem por objetivo a utilização da sinterização hĂbrida com microondas na queima rápida de materiais cerâmicos. Foram utilizados materiais que requerem altas temperaturas para densificação. Foram sinterizados materiais que apresentam bruscas e acentuadas mudanças nas suas propriedades dielĂ©tricas com a elevação da temperatura, zircĂ´nia, e materiais com baixas perdas dielĂ©tricas na temperatura ambiente, que apresentam dificuldades de aquecimento com microondas em baixas temperaturas, alumina e mulita. Foi utilizando material susceptor como agente auxiliar de aquecimento. Com base nos resultados obtidos pode-se concluir que o sistema de sinterização hĂbrida desenvolvido pode ser utilizado com sucesso na sinterização rápida e uniforme dos materiais estudados, sendo possĂvel a sinterização de zircĂ´nia em ciclos de 20 min, mulita em ciclos de 16 min e alumina em ciclos de 40 min.<br>Thermal processing by microwaves offers several advantages over conventional heating methods, such as shorter processing times, energy savings and improved microstructural homogeneity of ceramic bodies. Thus, this work focused on the fast hybrid microwave sintering of ceramic materials that require high sintering temperatures for densification. The materials studied here were zirconia, which displays abrupt and severe increases in dielectric loss with rising temperature, and alumina and mullite, which show low dielectric losses at ambient temperature and are difficult to microwave at low temperatures. A susceptor was used as an auxiliary heating agent. The results indicate that the sintering system developed here can be used efficiently for the rapid, homogeneous sintering of all the ceramics in question. Zirconia was sintered in 20 min, mullite in 16 min and alumina in 40 min heating cycles