arXiv subjects

Andrzej Pekalski

Publications and source records attributed to Andrzej Pekalski.

12 recordsLinked to original sources

arXiv2018

Mechanism of flame acceleration and detonation transition from the interaction of a supersonic turbulent flame with an obstruction

The present paper seeks to determine the mechanism of flame acceleration and transition to detonation when a turbulent flame preceded by a shock interacts with a single obstruction in its path, taken as a cylindrical obstacle or a wall in the present study. The problem is addressed experimentally in a mixture of propane-oxygen at sub-atmospheric conditions. The turbulent flame was generated by passing a detonation wave through a perforated plate, yielding flames with turbulent burning velocities 10 to 20 larger than the laminar values and incident shock Mach numbers ranging between 2 and 2.5. Time resolved schlieren videos recorded at approximately 100 kHz and numerical reconstruction of the flow field permitted to determine the mechanism of flame acceleration and transition to detonation. It was found to be the enhancement of the turbulent burning rate of the flame through its interaction with the shock reflection on the obstacle. The amplification of the burning rate was found to drive the flame burning velocity close to the speed of sound with respect to the fresh gases, resulting in the amplification of a shock in front of the flame. The acceleration through this regime resulted in the strengthening of this shock. Detonation was observed in regions of non-planarity of this internal shock, inherited by the irregular shape of the turbulent flame itself. Auto-ignition at early times of this process was found to be negligibly slow compared with the flow evolution time scale in the problem investigated, suggesting that the relevant time scale is primarily associated with the increase in turbulent burning rate by the interaction with reflected shocks.

physics.flu-dyn↗

arXiv2017

Investigation of Turbulent Mixing and Local Reaction Rates on Deflagration to Detonation Transition in Methane-Oxygen

In the current study, the influence of turbulent mixing and local reaction rates on deflagration to detonation transition (DDT) was investigated using a state-of-the-art large eddy simulation (LES) strategy. Specifically, detonation attenuation by a porous medium, and the subsequent re-initiation for methane-oxygen, a moderately unstable mixture, was considered. The purpose of the investigation was to validate the numerical strategy with previous experimental observations, and to determine what specific roles turbulent mixing and shock compression have on flame acceleration during the final stages of DDT. The modelling procedure adopted was a grid-within-a-grid approach: The compressible linear eddy model for large eddy simulation (CLEM-LES). It was found that average turbulent fluctuations greater than the laminar flame speed were required in order to maintain wave velocities above the Chapman-Jouguet (CJ)-deflagration velocity threshold, a precursor requirement for DDT to occur. It was also found that sufficient turbulent burning on the flame surface was required in order to drive pressure waves to sufficiently strengthen the leading shock wave, locally, in order to trigger auto-ignition hot spots in the wave front. These local explosion events, which were found to burn out through turbulent surface reactions, drive transverse pressure waves outward. Upon subsequent shock reflections or interactions of the transverse waves, new local explosion events occurred, which further strengthened the adjacent leading shock wave above the CJ-detonation speed. Eventually, through this process, the wave sustained the CJ-detonation speed, on average, through the cyclic mechanism of local explosion events followed by turbulent surface reactions. Finally, combustion of the flame acceleration process was found to lie predominantly within the thin-reaction zones regime.

physics.flu-dyn↗

arXiv2015

Chapman-Jouguet deflagrations and their transition to detonation

We study experimentally fast flames and their transition to detonation in mixtures of methane, ethane, ethylene, acetylene, and propane mixtures with oxygen. Following the interaction of a detonation wave with a column of cylinders of varying blockage ratio, the experiments demonstrate that the fast flames established are Chapman-Jouguet deflagrations, in excellent agreement with the self-similar model of Radulescu et al. (2015). The experiments indicate that these Chapman-Jouguet deflagrations dynamically restructure and amplify into fewer stronger modes until the eventual transition to detonation. The transition length to a self-sustained detonation was found to correlate very well with the mixtures' sensitivity to temperature fluctuations, reflected by the $χ$ parameter introduced by Radulescu, which is the product of the non-dimensional activation energy $E_a/RT$ and the ratio of chemical induction to reaction time $t_i/t_r$. Correlation of the measured DDT lengths determined that the relevant characteristic time scale from chemical kinetics controlling DDT is the energy release or excitation time $t_r$. Correlations with the cell size also capture the dependence of the DDT length on $χ$ for fixed blockage ratios.

physics.flu-dyn↗

arXiv2008

How bad is to be slow-reacting ? On the effect of the delay in response to a changing environment on a population's survival

We consider a simple-model population, whose individuals react with a certain delay to temporal variations of their habitat. We investigate the impact of such a delayed-answer on the survival chances of the population, both in a periodically changing environment, and in the case of an abrupt change of it. It is found that for population with low degree of mutation-induced variability, being "slow-reacting" decreases the extinction risk face to environmental changes. On the contrary, for populations with high mutation amplitude, the delayed reaction reduces the survival chances.

q-bio.PE↗

arXiv2008

Risk of Population Extinction from Periodic and Abrupt Changes of Environment

A simulation model of a population having internal (genetic) structure is presented. The population is subject to selection pressure coming from the environment which is the same in the whole system but changes in time. Reproduction has a sexual character with recombination and mutation. Two cases are considered - oscillatory changes of the environment and abrupt ones (catastrophes). We show how the survival chance of a population depends on maximum allowed size of the population, the length of the genotypes characterising individuals, selection pressure and the characteristics of the climate changes, either their period of oscillations or the scale of the abrupt shift.

nlin.AO↗

arXiv2007

Complex population dynamics as a competition between multiple time-scale phenomena

The role of the selection pressure and mutation amplitude on the behavior of a single-species population evolving on a two-dimensional lattice, in a periodically changing environment, is studied both analytically and numerically. The mean-field level of description allows to highlight the delicate interplay between the different time-scale processes in the resulting complex dynamics of the system. We clarify the influence of the amplitude and period of the environmental changes on the critical value of the selection pressure corresponding to a phase-transition "extinct-alive" of the population. However, the intrinsic stochasticity and the dynamically-built in correlations among the individuals, as well as the role of the mutation-induced variety in population's evolution are not appropriately accounted for. A more refined level of description, which is an individual-based one, has to be considered. The inherent fluctuations do not destroy the phase transition "extinct-alive", and the mutation amplitude is strongly influencing the value of the critical selection pressure. The phase diagram in the plane of the population's parameters -- selection and mutation is discussed as a function of the environmental variation characteristics. The differences between a smooth variation of the environment and an abrupt, catastrophic change are also addressesd.

q-bio.PE↗

arXiv2006

Model of Wealth and Goods Dynamics in a Closed Market

A simple computer simulation model of a closed market on a fixed network with free flow of goods and money is introduced. The model contains only two variables : the amount of goods and money beside the size of the system. An initially flat distribution of both variables is presupposed. We show that under completely random rules, i.e. through the choice of interacting agent pairs on the network and of the exchange rules that the market stabilizes in time and shows diversification of money and goods. We also indicate that the difference between poor and rich agents increases for small markets, as well as for systems in which money is steadily deduced from the market through taxation.

nlin.AO↗

arXiv2005

Monte Carlo simulations of the inside-intron recombination

Biological genomes are divided into coding and non-coding regions. Introns are non-coding parts within genes, while the remaining non-coding parts are intergenic sequences. To study the evolutionary significance of recombination inside introns we have used two models based on the Monte Carlo method. In our computer simulations we have implemented the internal structure of genes by declaring the probability of recombination between exons. One situation when inside-intron recombination is advantageous is recovering functional genes by combining proper exons dispersed in the genetic pool of the population after a long period without selection for the function of the gene. Populations have to pass through the bottleneck, then. These events are rather rare and we have expected that there should be other phenomena giving profits from the inside-intron recombination. In fact we have found that inside-intron recombination is advantageous only in the case when after recombination, besides the recombinant forms, parental haplotypes are available and selection is set already on gametes.

q-bio.PE↗

arXiv2004

A (reactive) lattice-gas approach to economic cycles

A microscopic approach to macroeconomic features is intended. A model for macroeconomic behavior under heterogeneous spatial economic conditions is reviewed. A birth-death lattice gas model taking into account the influence of an economic environment on the fitness and concentration evolution of economic entities is numerically and analytically examined. The reaction-diffusion model can be also mapped onto a high order logistic map. The role of the selection pressure along various dynamics with entity diffusion on a square symmetry lattice has been studied by Monte-Carlo simulation. The model leads to a sort of phase transition for the fitness gap as a function of the selection pressure and to cycles. The control parameter is a (scalar) ''business plan''. The business plan(s) allows for spin-offs or merging and enterprise survival evolution law(s), whence bifurcations, cycles and chaotic behavior.

cond-mat.stat-mech↗

arXiv2004

Model of macroeconomic evolution in stable regionally dependent economic fields

We develop a model for the evolution of economic entities within a geographical type of framework. On a square symmetry lattice made of three (economic) regions, firms, described by a scalar fitness, are allowed to move, adapt, merge or create spin-offs under predetermined rules, in a space and time dependent economic environment. We only consider here one timely variation of the ''external economic field condition''. For the firm fitness evolution we take into account a constraint such that the disappearance of a firm modifies the fitness of nearest neighboring ones, as in Bak-Sneppen population fitness evolution model. The concentration of firms, the averaged fitness, the regional distribution of firms, and fitness for different time moments, the number of collapsed, merged and new firms as a function of time have been recorded and are discussed. Also the asymptotic values of the number of firms present in the three regions together with their average fitness, as well as the number of respective births and collapses in the three regions are examined. It appears that a sort of $critical$ selection pressure exists. A power law dependence, signature of self-critical organization is seen in the birth and collapse asymptotic values for a high selection pressure only. A lack of self-organization is also seen at region borders.

cond-mat.stat-mech↗

arXiv2003

Evolution of economic entities under heterogeneous political/environmental conditions within a Bak-Sneppen-like dynamics

A model for economic behavior, under heterogeneous spatial economic conditions is developed. The role of selection pressure in a Bak-Sneppen-like dynamics with entity diffusion on a lattice is studied by Monte-Carlo simulation taking into account business rule(s), like enterprise - enterprise short range location "interaction"(s), business plan(s) through spin-offs or merging and enterprise survival evolution law(s). It is numerically found that the model leads to a sort of phase transition for the fitness gap as a function of the selection pressure.

physics.soc-ph↗

arXiv2002

Simple Model for the Dynamics of Correlations in the Evolution of Economic Entities Under Varying Economic Conditions

From some observations on economic behaviors, in particular changing economic conditions with time and space, we develop a very simple model for the evolution of economic entities within a geographical type of framework. We raise a few questions and attempt to investigate whether some of them can be tackled by our model. Several cases of interest are reported. It is found that the model even in its simple forms can lead to a large variety of situations, including: delocalization and cycles, but also pre-chaotic behavior.

nlin.AO↗