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Moshe Matalon - One of the best experts on this subject based on the ideXlab platform.
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Propagation Speed and stability of spherically expanding hydrogen air flames experimental study and asymptotics
Proceedings of the Combustion Institute, 2017Co-Authors: Joachim Beeckmann, Raik Hesse, Stephan Kruse, Andre Berens, N Peters, Heinz Pitsch, Moshe MatalonAbstract:Abstract Here, outwardly propagating spherical hydrogen/air flames are examined theoretically and experimentally with respect to flame Propagation Speed and the onset of instabilities which develop due to thermal expansion and non-equal diffusivities. Instabilities increase the surface area of the spherical flame, and hence the flame Propagation Speed. The theory applied here accounts for both hydrodynamic and diffusive-thermal effects, incorporating temperature dependent transport coefficients. Experiments are performed in a spherical combustion chamber over a wide range of equivalence ratios (0.6–2.0), initial temperatures (298–423 K), and initial pressures (1 atm to 15 bar). The evolution of the flame Propagation Speed as a function of flame radius is compared to predictions from theory showing excellent agreement. Also the wrinkling of hydrogen/air flames is examined under increased pressure and temperature for various equivalence ratios. Critical flame radii, defined as the point of transition to cellular flames, are extracted from high-Speed Schlieren flame imaging. Overall, the critical radius is found to decrease with increasing pressure. The predictions yield the growth rate of small disturbances and the critical flame radius. Experimental flame radii, as expected, are underpredicted by the theoretical findings. Experimental data are provided in the form of an approximation formula.
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Numerical study of unstable hydrogen/air flames: Shape and Propagation Speed
Proceedings of the Combustion Institute, 2015Co-Authors: Christos E. Frouzakis, Navin Fogla, Ananias G. Tomboulides, C. Altantzis, Moshe MatalonAbstract:Abstract Extensive numerical simulations with detailed chemistry and transport are performed to identify the range of dominance (in terms of equivalence ratio and domain size) of the hydrodynamic instability, the shape of the structures that evolve at long times, and their Propagation Speed. The calculations were performed in two-dimensional domains of lateral extent 3–100 flame thicknesses. Hydrogen/air mixtures ranging from rich ( ϕ = 2 ) to lean conditions ( ϕ = 0.5 ) were considered, expecting that thermo-diffusive effects will start becoming important only at the lean end. The initial growth of a perturbed planar flame front is found to agree qualitatively, and to a large extent even quantitatively, with the asymptotic theoretical predictions. Beyond linearity it is shown that the dynamics depend strongly on the equivalence ratio (or on the effective Lewis number of the mixture) and the domain lateral size. For stoichiometric and rich mixtures, the flame shape is generally characterized by a single-cusp structure that propagates at a constant Speed. The Propagation Speed increases with increasing lateral domain size and asymptotes to a value nearly 24% larger than the laminar flame Speed. For the lean mixtures, the flame does not assume a well-defined structure even after a long time. It is regularly contaminated by small cells that result from thermo-diffusive effects and cause a significant increase in the Propagation Speed (nearly 60% above the laminar flame Speed) that varies continuously in time. Except for the lean cases, the simulation results compare well with the asymptotic hydrodynamic theory both in the flame shape and Propagation Speed.
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numerical study of unstable hydrogen air flames shape and Propagation Speed
Proceedings of the Combustion Institute, 2015Co-Authors: Christos E. Frouzakis, Navin Fogla, Ananias G. Tomboulides, C. Altantzis, Moshe MatalonAbstract:Abstract Extensive numerical simulations with detailed chemistry and transport are performed to identify the range of dominance (in terms of equivalence ratio and domain size) of the hydrodynamic instability, the shape of the structures that evolve at long times, and their Propagation Speed. The calculations were performed in two-dimensional domains of lateral extent 3–100 flame thicknesses. Hydrogen/air mixtures ranging from rich ( ϕ = 2 ) to lean conditions ( ϕ = 0.5 ) were considered, expecting that thermo-diffusive effects will start becoming important only at the lean end. The initial growth of a perturbed planar flame front is found to agree qualitatively, and to a large extent even quantitatively, with the asymptotic theoretical predictions. Beyond linearity it is shown that the dynamics depend strongly on the equivalence ratio (or on the effective Lewis number of the mixture) and the domain lateral size. For stoichiometric and rich mixtures, the flame shape is generally characterized by a single-cusp structure that propagates at a constant Speed. The Propagation Speed increases with increasing lateral domain size and asymptotes to a value nearly 24% larger than the laminar flame Speed. For the lean mixtures, the flame does not assume a well-defined structure even after a long time. It is regularly contaminated by small cells that result from thermo-diffusive effects and cause a significant increase in the Propagation Speed (nearly 60% above the laminar flame Speed) that varies continuously in time. Except for the lean cases, the simulation results compare well with the asymptotic hydrodynamic theory both in the flame shape and Propagation Speed.
Georgios Rodolakis - One of the best experts on this subject based on the ideXlab platform.
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Highway Vehicular Delay Tolerant Networks: Information Propagation Speed Properties
IEEE Transactions on Information Theory, 2012Co-Authors: Emmanuel Baccelli, Philippe Jacquet, Bernard Mans, Georgios RodolakisAbstract:In this paper, we provide a full analysis of the information Propagation Speed in bidirectional vehicular delay tolerant networks such as roads or highways. The provided analysis shows that a phase transition occurs concerning the information Propagation Speed, with respect to the vehicle densities in each direction of the highway. We prove that under a certain threshold, information propagates on average at vehicle Speed, while above this threshold, information propagates dramatically faster at a Speed that increases quasi-exponentially when the vehicle density increases. We provide the exact expressions of the threshold and of the average information Propagation Speed near the threshold, in case of finite or infinite radio Propagation Speed. Furthermore, we investigate in detail the way information propagates under the threshold, and we prove that delay tolerant routing using cars moving on both directions provides a gain in Propagation distance, which is bounded by a sub-linear power law with respect to the elapsed time, in the referential of the moving cars. Combining these results, we thus obtain a complete picture of the way information propagates in vehicular networks on roads and highways, which may help designing and evaluating appropriate VANET routing protocols. We confirm our analytical results using simulations carried out in several environments (The One and Maple).
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highway vehicular delay tolerant networks information Propagation Speed properties
IEEE Transactions on Information Theory, 2012Co-Authors: Emmanuel Baccelli, Bernard Mans, P Jacquet, Georgios RodolakisAbstract:In this paper, we provide a full analysis of the information Propagation Speed in bidirectional vehicular delay tolerant networks such as roads or highways. The provided analysis shows that a phase transition occurs concerning the information Propagation Speed, with respect to the vehicle densities in each direction of the highway. We prove that under a certain threshold, information propagates on average at vehicle Speed, while above this threshold, information propagates dramatically faster at a Speed that increases quasi-exponentially when the vehicle density increases. We provide the exact expressions of the threshold and of the average information Propagation Speed near the threshold, in case of finite or infinite radio Propagation Speed. Furthermore, we investigate in detail the way information propagates under the threshold, and we prove that delay tolerant routing using cars moving on both directions provides a gain in Propagation distance, which is bounded by a sublinear power law with respect to the elapsed time, in the referential of the moving cars. Combining these results, we thus obtain a complete picture of the way information propagates in vehicular networks on roads and highways, which may help designing and evaluating appropriate vehicular ad hoc networks routing protocols. We confirm our analytical results using simulations carried out in several environments (The One and Maple).
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Information Propagation Speed in Bidirectional Vehicular Delay Tolerant Networks
2011Co-Authors: Emmanuel Baccelli, Philippe Jacquet, Bernard Mans, Georgios RodolakisAbstract:In this paper, we provide an analysis of the informa- tion Propagation Speed in bidirectional vehicular delay tolerant networks on highways. We show that a phase transition occurs concerning the information Propagation Speed, with respect to the vehicle densities in each direction of the highway. We prove that under a certain threshold, information propagates on average at vehicle Speed, while above this threshold, information propagates dramatically faster at a Speed that increase exponentially when vehicle density increases. We provide the exact expressions of the threshold and of the average Propagation Speed near the threshold. We show that under the threshold, the information propagates on a distance which is bounded by a sub-linear power law with respect to the elapsed time, in the referential of the moving cars. On the other hand, we show that information prop- agation Speed grows quasi-exponentially with respect to vehicle densities in each direction of the highway, when the densities become large, above the threshold. We confirm our analytical results using simulations carried out in several environments.
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INFOCOM - Information Propagation Speed in bidirectional vehicular delay tolerant networks
2011 Proceedings IEEE INFOCOM, 2011Co-Authors: Emmanuel Baccelli, Philippe Jacquet, Bernard Mans, Georgios RodolakisAbstract:In this paper, we provide an analysis of the information Propagation Speed in bidirectional vehicular delay tolerant networks on highways. We show that a phase transition occurs concerning the information Propagation Speed, with respect to the vehicle densities in each direction of the highway. We prove that under a certain threshold, information propagates on average at vehicle Speed, while above this threshold, information propagates dramatically faster at a Speed that increase exponentially when vehicle density increases. We provide the exact expressions of the threshold and of the average Propagation Speed near the threshold. We show that under the threshold, the information propagates on a distance which is bounded by a sub-linear power law with respect to the elapsed time, in the referential of the moving cars. On the other hand, we show that information Propagation Speed grows quasi-exponentially with respect to vehicle densities in each direction of the highway, when the densities become large, above the threshold. We confirm our analytical results using simulations carried out in several environments.
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information Propagation Speed in mobile and delay tolerant networks
International Conference on Computer Communications, 2009Co-Authors: Philippe Jacquet, Bernard Mans, Georgios RodolakisAbstract:The goal of this paper is to increase our under- standing of the fundamental performance limits of mobile and Delay Tolerant Networks (DTNs), where end-to-end multihop paths may not exist and communication routes may only be available through time and mobility. We use analytical tools to derive generic theoretical upper bounds for the information Propagation Speed in large scale mobile and intermittently con- nected networks. In other words, we upper-bound the optimal performance, in terms of delay, that can be achieved using any routing algorithm. We then show how our analysis can be applied to specific mobility models to obtain specific analytical estimates. In particular, in 2-D networks, when nodes move at a maximum Speed and their density is small (the network is sparse and asymptotically almost surely disconnected), we prove that the information Propagation Speed is upper bounded by in random waypoint-like models, while it is upper bounded by for other mobility models (random walk, Brownian motion). We also present simulations that confirm the validity of the bounds in these scenarios. Finally, we generalize our results to 1-D and 3-D networks.
Suk Ho Chung - One of the best experts on this subject based on the ideXlab platform.
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Effect of electric fields on the Propagation Speed of tribrachial flames in coflow jets
Combustion and Flame, 2008Co-Authors: Sang Hee Won, S. K. Ryu, Munki Kim, Min Suk Cha, Suk Ho ChungAbstract:The effect of electric fields on the Propagation Speed of tribrachial (or triple) flames has been investigated in a coflow jet by observing the transient flame Propagation behavior after ignition. The Propagation Speed of tribrachial edges when no electric fields were applied showed typical behavior by having an inverse proportionality to the mixture fraction gradient at the flame edge. The behavior of flame Propagation with electric fields was investigated by applying high voltage to the central fuel nozzle, thereby having a single-electrode configuration. The enhancement of Propagation Speed has been observed by varying the applied voltage and frequency for ac electric fields. The Propagation Speed of tribrachial flames was also investigated by applying positive and negative dc voltages to the nozzle, and similar improvements of the Propagation Speed were also observed. The Propagation Speeds of tribrachial flames in both the ac and dc electric fields correlated well with the electric field intensity, defined by the applied electric voltage divided by the distance between the nozzle electrode and the edge of the tribrachial flame.
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Effect of velocity gradient on Propagation Speed of tribrachial flames in laminar coflow jets
Proceedings of the Combustion Institute, 2007Co-Authors: Munki Kim, Sang Hee Won, Suk Ho ChungAbstract:Abstract The effect of velocity gradient on the Propagation Speed of tribrachial flame edge has been investigated experimentally in laminar coflow jets for propane fuel. It was observed that the Propagation Speed of tribrachial flame showed appreciable deviations at various jet velocities in high mixture fraction gradient regime. From the similarity solutions, it was demonstrated that the velocity gradient varied significantly during the flame Propagation. To examine the effect of velocity gradient, detail structures of tribrachial flames were investigated from OH LIF images and Abel transformed images of flame luminosity. It was revealed that the tribrachial point was located on the slanted surface of the premixed wing, and this slanted angle was correlated with the velocity gradient along the stoichiometric contour. The temperature field was visualized qualitatively by the Rayleigh scattering image. The Propagation Speed of tribrachial flame was corrected by considering the direction of flame Propagation with the slanted angle and effective heat conduction to upstream. The corrected Propagation Speed of tribrachial flame was correlated well. Thus, the mixture fraction gradient together with the velocity gradient affected the Propagation Speed.
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effect of electric fields on reattachment and Propagation Speed of tribrachial flames in laminar coflow jets
Proceedings of the Combustion Institute, 2007Co-Authors: Sang Hee Won, Min Suk Cha, C S Park, Suk Ho ChungAbstract:Abstract The effects of electric fields on the reattachment of lifted flames have been investigated experimentally in laminar coflow jets with propane fuel by applying high voltages to the fuel nozzle. In case of AC, the frequency has also been varied. Results showed that reattachment occurred at higher jet velocity when applying the AC voltages, thus the stabilization limit of attached flames was extended by the AC electric field. Higher voltage and lower frequency of the AC were found to be more effective. On the contrary, the effect of DC was found to be minimal. To understand the early onset of the reattachment with the AC, occurring at higher jet velocity, the influence of AC electric fields on the Propagation Speed of tribrachial flame edge was investigated during the transient reattachment processes. The Propagation Speed increased reasonably linearly with the applied AC voltage and decreased inversely to the distance between the flame edge and the nozzle electrode. Consequently, the enhancement in the Propagation Speed of tribrachial flame edge was correlated well with the electric field intensity, defined as the applied AC voltage divided by the distance.
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Propagation Speed of tribrachial (triple) flame of propane in laminar jets under normal and micro gravity conditions
Combustion and Flame, 2003Co-Authors: Jong-soo Lee, Sang Hee Won, Seong-ho Jin, Suk Ho Chung, Osamu Fujita, Kenichi ItoAbstract:The Propagation Speed of tribrachial (triple) flames in laminar propane jets has been investigated experimentally under normal and micro gravity conditions. We found in the present experiment that the displacement Speed varied nonlinearly with axial distance because the flow velocity along the stoichiometric contour was comparable to the Propagation Speed of tribrachial flame. Approximate solutions for the velocity and concentration accounting density difference and virtual origins have been used in determining the Propagation Speed of tribrachial flame and the concentration field was validated from the measurement of Raman scattering. Under the microgravity condition, the results showed that the Propagation Speed of tribrachial flame decreased with the mixture fraction gradient, in agreement with previous studies. The limiting maximum Propagation Speed under the microgravity condition is in good agreement with the theoretical prediction, ie, the ratio of maximum Propagation Speed to the stoichiometric laminar burning velocity is proportional to the square root of the density ratio of unburned to burnt mixture.
Emmanuel Baccelli - One of the best experts on this subject based on the ideXlab platform.
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Highway Vehicular Delay Tolerant Networks: Information Propagation Speed Properties
IEEE Transactions on Information Theory, 2012Co-Authors: Emmanuel Baccelli, Philippe Jacquet, Bernard Mans, Georgios RodolakisAbstract:In this paper, we provide a full analysis of the information Propagation Speed in bidirectional vehicular delay tolerant networks such as roads or highways. The provided analysis shows that a phase transition occurs concerning the information Propagation Speed, with respect to the vehicle densities in each direction of the highway. We prove that under a certain threshold, information propagates on average at vehicle Speed, while above this threshold, information propagates dramatically faster at a Speed that increases quasi-exponentially when the vehicle density increases. We provide the exact expressions of the threshold and of the average information Propagation Speed near the threshold, in case of finite or infinite radio Propagation Speed. Furthermore, we investigate in detail the way information propagates under the threshold, and we prove that delay tolerant routing using cars moving on both directions provides a gain in Propagation distance, which is bounded by a sub-linear power law with respect to the elapsed time, in the referential of the moving cars. Combining these results, we thus obtain a complete picture of the way information propagates in vehicular networks on roads and highways, which may help designing and evaluating appropriate VANET routing protocols. We confirm our analytical results using simulations carried out in several environments (The One and Maple).
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highway vehicular delay tolerant networks information Propagation Speed properties
IEEE Transactions on Information Theory, 2012Co-Authors: Emmanuel Baccelli, Bernard Mans, P Jacquet, Georgios RodolakisAbstract:In this paper, we provide a full analysis of the information Propagation Speed in bidirectional vehicular delay tolerant networks such as roads or highways. The provided analysis shows that a phase transition occurs concerning the information Propagation Speed, with respect to the vehicle densities in each direction of the highway. We prove that under a certain threshold, information propagates on average at vehicle Speed, while above this threshold, information propagates dramatically faster at a Speed that increases quasi-exponentially when the vehicle density increases. We provide the exact expressions of the threshold and of the average information Propagation Speed near the threshold, in case of finite or infinite radio Propagation Speed. Furthermore, we investigate in detail the way information propagates under the threshold, and we prove that delay tolerant routing using cars moving on both directions provides a gain in Propagation distance, which is bounded by a sublinear power law with respect to the elapsed time, in the referential of the moving cars. Combining these results, we thus obtain a complete picture of the way information propagates in vehicular networks on roads and highways, which may help designing and evaluating appropriate vehicular ad hoc networks routing protocols. We confirm our analytical results using simulations carried out in several environments (The One and Maple).
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Information Propagation Speed in Bidirectional Vehicular Delay Tolerant Networks
2011Co-Authors: Emmanuel Baccelli, Philippe Jacquet, Bernard Mans, Georgios RodolakisAbstract:In this paper, we provide an analysis of the informa- tion Propagation Speed in bidirectional vehicular delay tolerant networks on highways. We show that a phase transition occurs concerning the information Propagation Speed, with respect to the vehicle densities in each direction of the highway. We prove that under a certain threshold, information propagates on average at vehicle Speed, while above this threshold, information propagates dramatically faster at a Speed that increase exponentially when vehicle density increases. We provide the exact expressions of the threshold and of the average Propagation Speed near the threshold. We show that under the threshold, the information propagates on a distance which is bounded by a sub-linear power law with respect to the elapsed time, in the referential of the moving cars. On the other hand, we show that information prop- agation Speed grows quasi-exponentially with respect to vehicle densities in each direction of the highway, when the densities become large, above the threshold. We confirm our analytical results using simulations carried out in several environments.
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INFOCOM - Information Propagation Speed in bidirectional vehicular delay tolerant networks
2011 Proceedings IEEE INFOCOM, 2011Co-Authors: Emmanuel Baccelli, Philippe Jacquet, Bernard Mans, Georgios RodolakisAbstract:In this paper, we provide an analysis of the information Propagation Speed in bidirectional vehicular delay tolerant networks on highways. We show that a phase transition occurs concerning the information Propagation Speed, with respect to the vehicle densities in each direction of the highway. We prove that under a certain threshold, information propagates on average at vehicle Speed, while above this threshold, information propagates dramatically faster at a Speed that increase exponentially when vehicle density increases. We provide the exact expressions of the threshold and of the average Propagation Speed near the threshold. We show that under the threshold, the information propagates on a distance which is bounded by a sub-linear power law with respect to the elapsed time, in the referential of the moving cars. On the other hand, we show that information Propagation Speed grows quasi-exponentially with respect to vehicle densities in each direction of the highway, when the densities become large, above the threshold. We confirm our analytical results using simulations carried out in several environments.
Kazuyuki Aihara - One of the best experts on this subject based on the ideXlab platform.
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theory of localized synfire chain characteristic Propagation Speed of stable spike pattern
Neural Information Processing Systems, 2004Co-Authors: Kosuke Hamaguchi, Masato Okada, Kazuyuki AiharaAbstract:Repeated spike patterns have often been taken as evidence for the synfire chain, a phenomenon that a stable spike synchrony propagates through a feedforward network. Inter-spike intervals which represent a repeated spike pattern are influenced by the Propagation Speed of a spike packet. However, the relation between the Propagation Speed and network structure is not well understood. While it is apparent that the Propagation Speed depends on the excitatory synapse strength, it might also be related to spike patterns. We analyze a feedforward network with Mexican-Hat-type connectivity (FMH) using the Fokker-Planck equation. We show that both a uniform and a localized spike packet are stable in the FMH in a certain parameter region. We also demonstrate that the Propagation Speed depends on the distinct firing patterns in the same network.
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NIPS - Theory of localized synfire chain: characteristic Propagation Speed of stable spike pattern
2004Co-Authors: Kosuke Hamaguchi, Masato Okada, Kazuyuki AiharaAbstract:Repeated spike patterns have often been taken as evidence for the synfire chain, a phenomenon that a stable spike synchrony propagates through a feedforward network. Inter-spike intervals which represent a repeated spike pattern are influenced by the Propagation Speed of a spike packet. However, the relation between the Propagation Speed and network structure is not well understood. While it is apparent that the Propagation Speed depends on the excitatory synapse strength, it might also be related to spike patterns. We analyze a feedforward network with Mexican-Hat-type connectivity (FMH) using the Fokker-Planck equation. We show that both a uniform and a localized spike packet are stable in the FMH in a certain parameter region. We also demonstrate that the Propagation Speed depends on the distinct firing patterns in the same network.
