12,172 research outputs found
An efficient flamelet progress-variable method for modeling non-premixed flames in weak electric fields
Combustion stabilization and enhancement of the flammability limits are
mandatory objectives to improve nowadays combustion chambers. At this purpose,
the use of an electric field in the flame region provides a solution which is,
at the same time, easy to implement and effective to modify the flame
structure. The present work describes an efficient flamelet progress-variable
approach developed to model the fluid dynamics of flames immersed in an
electric field. The main feature of this model is that it can use complex
ionization mechanisms without increasing the computational cost of the
simulation. The model is based on the assumption that the combustion process is
not directly influenced by the electric field and has been tested using two
chemi-ionization mechanisms of different complexity in order to examine its
behavior with and without the presence of heavy anions in the mixture. Using a
one- and two-dimensional numerical test cases, the present approach has been
able to reproduce all the major aspects encountered when a flame is subject to
an imposed electric field and the main effects of the different chemical
mechanisms. Moreover, the proposed model is shown to produce a large reduction
in the computational cost, being able to shorten the time needed to perform a
simulation up to 40 times.Comment: 26 pages, 13 figures, paper accepted for publication on Computers and
Fluid
Nonlinear cellular instabilities of planar premixed flames: numerical simulations of the Reactive Navier-Stokes equations
Two-dimensional compressible Reactive Navier-Stokes numerical simulations of intrinsic planar, premixed flame instabilities are performed. The initial growth of a sinusoidally perturbed planar flame is first compared with the predictions of a recent exact linear stability analysis, and it is shown the analysis provides a necessary but not sufficient test problem for validating numerical schemes intended for flame simulations. The long-time nonlinear evolution up to the final nonlinear stationary cellular flame is then examined for numerical domains of increasing width. It is shown that for routinely computationally affordable domain widths, the evolution and final state is, in general, entirely dependent on the width of the domain and choice of numerical boundary conditions. It is also shown that the linear analysis has no relevance to the final nonlinear cell size. When both hydrodynamic and thermal-diffusive effects are important, the evolution consists of a number of symmetry breaking cell splitting and re-merging processes which results in a stationary state of a single very asymmetric cell in the domain, a flame shape which is not predicted by weakly nonlinear evolution equations. Resolution studies are performed and it is found that lower numerical resolutions, typical of those used in previous works, do not give even the qualitatively correct solution in wide domains. We also show that the long-time evolution, including whether or not a stationary state is ever achieved, depends on the choice of the numerical boundary conditions at the inflow and outflow boundaries, and on the numerical domain length and flame Mach number for the types of boundary conditions used in some previous works
Ginzburg-Landau approximation for self-sustained oscillators weakly coupled on complex directed graphs
A normal form approximation for the evolution of a reaction-diffusion system
hosted on a directed graph is derived, in the vicinity of a supercritical Hopf
bifurcation. Weak diffusive couplings are assumed to hold between adjacent
nodes. Under this working assumption, a Complex Ginzburg-Landau equation (CGLE)
is obtained, whose coefficients depend on the parameters of the model and the
topological characteristics of the underlying network. The CGLE enables one to
probe the stability of the synchronous oscillating solution, as displayed by
the reaction-diffusion system above Hopf bifurcation. More specifically,
conditions can be worked out for the onset of the symmetry breaking instability
that eventually destroys the uniform oscillatory state. Numerical tests
performed for the Brusselator model confirm the validity of the proposed
theoretical scheme. Patterns recorded for the CGLE resemble closely those
recovered upon integration of the original Brussellator dynamics
A Multiscale Thermo-Fluid Computational Model for a Two-Phase Cooling System
In this paper, we describe a mathematical model and a numerical simulation
method for the condenser component of a novel two-phase thermosyphon cooling
system for power electronics applications. The condenser consists of a set of
roll-bonded vertically mounted fins among which air flows by either natural or
forced convection. In order to deepen the understanding of the mechanisms that
determine the performance of the condenser and to facilitate the further
optimization of its industrial design, a multiscale approach is developed to
reduce as much as possible the complexity of the simulation code while
maintaining reasonable predictive accuracy. To this end, heat diffusion in the
fins and its convective transport in air are modeled as 2D processes while the
flow of the two-phase coolant within the fins is modeled as a 1D network of
pipes. For the numerical solution of the resulting equations, a Dual
Mixed-Finite Volume scheme with Exponential Fitting stabilization is used for
2D heat diffusion and convection while a Primal Mixed Finite Element
discretization method with upwind stabilization is used for the 1D coolant
flow. The mathematical model and the numerical method are validated through
extensive simulations of realistic device structures which prove to be in
excellent agreement with available experimental data
Accurate Reaction-Diffusion Operator Splitting on Tetrahedral Meshes for Parallel Stochastic Molecular Simulations
Spatial stochastic molecular simulations in biology are limited by the
intense computation required to track molecules in space either in a discrete
time or discrete space framework, meaning that the serial limit has already
been reached in sub-cellular models. This calls for parallel simulations that
can take advantage of the power of modern supercomputers; however exact methods
are known to be inherently serial. We introduce an operator splitting
implementation for irregular grids with a novel method to improve accuracy, and
demonstrate potential for scalable parallel simulations in an initial MPI
version. We foresee that this groundwork will enable larger scale, whole-cell
stochastic simulations in the near future.Comment: 33 pages, 10 figure
Enhanced reaction kinetics and reactive mixing scale dynamics in mixing fronts under shear flow for arbitrary Damk\"ohler numbers
Mixing fronts, where fluids of different chemical compositions mix with each
other, are typically subjected to velocity gradients, ranging from the pore
scale to the catchment scale due to permeability variations and flow line
geometries. A common trait of these processes is that the mixing interface is
strained by shear. Depending on the P\'eclet number , which represents the
ratio of the characteristic diffusion time to the characteristic advection
time, and the Damk\"ohler number , which represents the ratio of the
characteristic diffusion time to the characteristic reaction time, the local
reaction rates can be strongly impacted by the dynamics of the mixing
interface. This impact has been characterized mostly either in kinetics-limited
or in mixing-limited conditions, that is, for either very low or very high
. Here the coupling of shear flow and chemical reactivity is investigated
for arbitrary Damk\"ohler numbers, for a bimolecular reaction and an initial
interface with separated reactants. Approximate analytical expressions for the
global production rate and reactive mixing scale are derived based on a
reactive lamella approach that allows for a general coupling between stretching
enhanced mixing and chemical reactions. While for , reaction kinetics
and stretching effects are decoupled, a scenario which we name "weak
stretching", for , we uncover a "strong stretching" scenario where new
scaling laws emerge from the interplay between reaction kinetics, diffusion,
and stretching. The analytical results are validated against numerical
simulations. These findings shed light on the effect of flow heterogeneity on
the enhancement of chemical reaction and the creation of spatially localized
hotspots of reactivity for a broad range of systems ranging from kinetic
limited to mixing limited situations
CFD modeling of a fixed-bed biofilm reactor coupling hydrodynamics and biokinetics
Peer ReviewedPostprint (author's final draft
Simulated single molecule microscopy with SMeagol
SMeagol is a software tool to simulate highly realistic microscopy data based
on spatial systems biology models, in order to facilitate development,
validation, and optimization of advanced analysis methods for live cell single
molecule microscopy data. Availability and Implementation: SMeagol runs on
Matlab R2014 and later, and uses compiled binaries in C for reaction-diffusion
simulations. Documentation, source code, and binaries for recent versions of
Mac OS, Windows, and Ubuntu Linux can be downloaded from
http://smeagol.sourceforge.net.Comment: v2: 14 pages including supplementary text. Pre-copyedited,
author-produced version of an application note published in Bioinformatics
following peer review. The version of record, and additional supplementary
material is available online at:
https://academic.oup.com/bioinformatics/article-lookup/doi/10.1093/bioinformatics/btw10
The effect of temperature-dependent solubility on the onset of thermosolutal convection in a horizontal porous layer
We consider the onset of thermosolutal (double-diffusive) convection of a binary fluid in a horizontal porous layer subject to fixed temperatures and chemical equilibrium on the bounding surfaces, in the case when the solubility of the dissolved component depends on temperature. We use a linear stability analysis to investigate how the dissolution or precipitation of this component affects the onset of convection and the selection of an unstable wavenumber; we extend this analysis using a Galerkin method to predict the structure of the initial bifurcation and compare our analytical results with numerical integration of the full nonlinear equations. We find that the reactive term may be stabilizing or destabilizing, with subtle effects particularly when the thermal gradient is destabilizing but the solutal gradient is stabilizing. The preferred spatial wavelength of convective cells at onset may also be substantially increased or reduced, and strongly reactive systems tend to prefer direct to subcritical bifurcation. These results have implications for geothermal-reservoir management and ore prospecting
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