398 research outputs found
The actual impedance of non-reflecting boundary conditions : implications for the computation of resonators
Non-reflecting boundary conditions are essential elements in the computation of many compressible flows: such simulations are very sensitive to the treatment of acoustic waves at boundaries. Non-reflecting conditions allow acoustic waves to propagate through boundaries with zero or small levels of reflection into the domain. However, perfectly non-reflecting conditions must be avoided because they can lead to ill-posed problems for the mean flow. Various methods have been proposed to construct boundary conditions which can be sufficiently non-reflecting for the acoustic field while still making the mean-flow problem well posed. This paper analyses a widely-used technique for non-reflecting outlets (Rudy and Strikwerda, Poinsot and Lele). It shows that the correction introduced by these authors can lead to large reflection levels and non-physical resonant behaviors. A simple scaling is proposed to evaluate the relaxation coefficient used in theses methods for a non-reflecting outlet. The proposed scaling is tested for simple cases (ducts) both theoretically and numerically
Procedimento prático para elaboração de uma tabela de produção.
bitstream/CNPF-2009-09/43533/1/Doc165.pd
Uso do dendrograma de manejo de densidade: uma aplicação prática com Hovenia dulcis Thunberg.
O objetivo principal deste trabalho é fazer uma aplicação prática dos dendrogramas de manejo de densidade em uma população de HovenÃa dulcÃs localizada na região central do estado do Rio Grande do Sul. Para tanto, foram utilizados dados de um povoamento de Hovenia dutcis de aproximadamente 1,3 hectares com 25 anos de idade, localizado em área da Fundação de Pesquisa Agropecuária (FEPAGRO). O dendrograma de densidade utilizado para a simulação das intervenções foi elaborado por Selle (2009), para o local e espécie. Estabeleceu-se que a melhor forma de conduzir o povoamento é com a aplicação de duas intervenções silviculturais, sendo uma aplicada imediatamente e outra, quando o diâmetro da árvore de área basal média atingir 27 em. No primeiro desbaste devem ser retiradas 36,6% e no segundo 33,3% das árvores, correspondendo a 33,6 e 45,6% da área basal, respectivamente
High Performance Algorithms for Counting Collisions and Pairwise Interactions
The problem of counting collisions or interactions is common in areas as
computer graphics and scientific simulations. Since it is a major bottleneck in
applications of these areas, a lot of research has been carried out on such
subject, mainly focused on techniques that allow calculations to be performed
within pruned sets of objects. This paper focuses on how interaction
calculation (such as collisions) within these sets can be done more efficiently
than existing approaches. Two algorithms are proposed: a sequential algorithm
that has linear complexity at the cost of high memory usage; and a parallel
algorithm, mathematically proved to be correct, that manages to use GPU
resources more efficiently than existing approaches. The proposed and existing
algorithms were implemented, and experiments show a speedup of 21.7 for the
sequential algorithm (on small problem size), and 1.12 for the parallel
proposal (large problem size). By improving interaction calculation, this work
contributes to research areas that promote interconnection in the modern world,
such as computer graphics and robotics.Comment: Accepted in ICCS 2019 and published in Springer's LNCS series.
Supplementary content at https://mjsaldanha.com/articles/1-hpc-ssp
Numerical framework for transcritical real-fluid reacting flow simulations using the flamelet progress variable approach
An extension to the classical FPV model is developed for transcritical
real-fluid combustion simulations in the context of finite volume, fully
compressible, explicit solvers. A double-flux model is developed for
transcritical flows to eliminate the spurious pressure oscillations. A hybrid
scheme with entropy-stable flux correction is formulated to robustly represent
large density ratios. The thermodynamics for ideal-gas values is modeled by a
linearized specific heat ratio model. Parameters needed for the cubic EoS are
pre-tabulated for the evaluation of departure functions and a quadratic
expression is used to recover the attraction parameter. The novelty of the
proposed approach lies in the ability to account for pressure and temperature
variations from the baseline table. Cryogenic LOX/GH2 mixing and reacting cases
are performed to demonstrate the capability of the proposed approach in
multidimensional simulations. The proposed combustion model and numerical
schemes are directly applicable for LES simulations of real applications under
transcritical conditions.Comment: 55th AIAA Aerospace Sciences Meeting, Dallas, T
Martini coarse-grained models of imidazolium-based ionic liquids:from nanostructural organization to liquid-liquid extraction
Ionic liquids (ILs) are remarkable green solvents, which find applications in many areas of nano- and biotechnology including extraction and purification of value-added compounds or fine chemicals. These liquid salts possess versatile solvation properties that can be tuned by modifications in the cation or anion structure. So far, in contrast to the great success of theoretical and computational methodologies applied to other fields, only a few IL models have been able to bring insights towards the rational design of such solvents. In this work, we develop coarse-grained (CG) models for imidazolium-based ILs using a new version of the Martini force field. The model is able to reproduce the main structural properties of pure ILs, including spatial heterogeneity and global densities over a wide range of temperatures. More importantly, given the high intermolecular compatibility of the Martini force field, this new IL CG model opens the possibility of large-scale simulations of liquid-liquid extraction experiments. As examples, we show two applications, namely the extraction of aromatic molecules from a petroleum oil model and the extraction of omega-3 polyunsaturated fatty acids from a fish oil model. In semi-quantitative agreement with the experiments, we show how the extraction capacity and selectivity of the IL could be affected by the cation chain length or addition of co-solvents
Using LES to Study Reacting Flows and Instabilities in Annular Combustion Chambers
Great prominence is put on the design of aeronautical gas turbines due to increasingly stringent regulations and the need to tackle rising fuel prices. This drive towards innovation has resulted sometimes in new concepts being prone to combustion instabilities. In the particular field of annular combustion chambers, these instabilities often take the form of azimuthal modes. To predict these modes, one must compute the full combustion chamber, which remained out of reach until very recently and the development of massively parallel computers. Since one of the most limiting factors in performing Large Eddy Simulation (LES) of real combustors is estimating the adequate grid, the effects of mesh resolution are investigated by computing full annular LES of a realistic helicopter combustion chamber on three grids, respectively made of 38, 93 and 336 million elements. Results are compared in terms of mean and fluctuating fields. LES captures self-established azimuthal modes. The presence and structure of the modes is discussed. This study therefore highlights the potential of LES for studying combustion instabilities in annular gas turbine combustors
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