The Experts below are selected from a list of 162 Experts worldwide ranked by ideXlab platform
Peter R Lindstedt - One of the best experts on this subject based on the ideXlab platform.
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impact of molecular mixing and Scalar dissipation rate closures on turbulent bluff body flames with increasing local extinction
Combustion and Flame, 2019Co-Authors: Lu Tian, Peter R LindstedtAbstract:Abstract Bluff-body turbulent CH4: H2 (1:1) flames at 50% (HM1), 75% (HM2) and 91% (HM3) of the blow-off velocity (235 m s − 1 ) were studied experimentally by Masri and co-workers and found to exhibit gradually increasing periodic and shear layer instabilities. The latter are coupled with increasing levels of local extinction with subsequent re-ignition further downstream. This study provides a systematic evaluation of the sensitivity of predictions to molecular mixing and Scalar dissipation rate closures. The latter include extended forms of the Euclidean Minimum Spanning Tree (EMST) and modified Curl’s (MC) models, applicable to premixed turbulent flames via a closure that accounts for local Damkohler number effects (EEMST and EMC), and a conceptually related blended Scalar Time-scale approach (BEMST and BMC). Computations are performed using a hybrid finite volume (FV) – transported Joint Probability Density Function (JPDF) algorithm featuring stochastic Lagrangian particles, a comprehensive 48-Scalar systematically reduced C/H/N/O mechanism, and a second moment method based on the Generalised Langevin Model that provides a partial resolution of the unsteady fluid motion. The sensitivity to solution parameters affecting the temporal resolution is quantified using Fourier transforms of the Time histories of velocity and Scalar traces. Radial profiles, conditional means and scatter plots are compared to the experimental data along with burning indices based on the conditional mean temperature. Vortex related instabilities ∼ 1 kHz in the outer shear layer appear for all closures with EMC showing periodic local extinction and re-ignition in the neck region for HM3 and flame turbules (i.e., discrete pockets of hot gas) separating periodically at frequencies ∼ 85 Hz. Results are similar to well–resolved JPDF/LES simulations for HM1. It is shown that the EMC and (E)EMST models essentially enclose the experimental data for HM2 and HM3. For HM3, emissions of NO are controlled by local extinction events that become increasingly sensitive to the molecular mixing closure as blow-off is approached.
Lu Tian - One of the best experts on this subject based on the ideXlab platform.
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impact of molecular mixing and Scalar dissipation rate closures on turbulent bluff body flames with increasing local extinction
Combustion and Flame, 2019Co-Authors: Lu Tian, Peter R LindstedtAbstract:Abstract Bluff-body turbulent CH4: H2 (1:1) flames at 50% (HM1), 75% (HM2) and 91% (HM3) of the blow-off velocity (235 m s − 1 ) were studied experimentally by Masri and co-workers and found to exhibit gradually increasing periodic and shear layer instabilities. The latter are coupled with increasing levels of local extinction with subsequent re-ignition further downstream. This study provides a systematic evaluation of the sensitivity of predictions to molecular mixing and Scalar dissipation rate closures. The latter include extended forms of the Euclidean Minimum Spanning Tree (EMST) and modified Curl’s (MC) models, applicable to premixed turbulent flames via a closure that accounts for local Damkohler number effects (EEMST and EMC), and a conceptually related blended Scalar Time-scale approach (BEMST and BMC). Computations are performed using a hybrid finite volume (FV) – transported Joint Probability Density Function (JPDF) algorithm featuring stochastic Lagrangian particles, a comprehensive 48-Scalar systematically reduced C/H/N/O mechanism, and a second moment method based on the Generalised Langevin Model that provides a partial resolution of the unsteady fluid motion. The sensitivity to solution parameters affecting the temporal resolution is quantified using Fourier transforms of the Time histories of velocity and Scalar traces. Radial profiles, conditional means and scatter plots are compared to the experimental data along with burning indices based on the conditional mean temperature. Vortex related instabilities ∼ 1 kHz in the outer shear layer appear for all closures with EMC showing periodic local extinction and re-ignition in the neck region for HM3 and flame turbules (i.e., discrete pockets of hot gas) separating periodically at frequencies ∼ 85 Hz. Results are similar to well–resolved JPDF/LES simulations for HM1. It is shown that the EMC and (E)EMST models essentially enclose the experimental data for HM2 and HM3. For HM3, emissions of NO are controlled by local extinction events that become increasingly sensitive to the molecular mixing closure as blow-off is approached.
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The impact of pressure fluctuation and Scalar dissipation rate closures in turbulent flames
Mechanical Engineering Imperial College London, 2018Co-Authors: Lu TianAbstract:The present thesis features computational studies of turbulent flames with different degrees of premixing. The flow field is computed using Reynolds–Averaged Navier–Stokes (RANS) equations closed at the second–moment level and the Scalar field is modelled by either a presumed probability density function (pdf) or a joint Scalar transported pdf approach. The focus is on the turbulence–chemistry interactions for the different types of flames. For turbulent premixed flames, the impact of combustion–induced variable density on the pressure fluctuation correlations is investigated. Derivations are presented for pressure dilatation, flamelet scrambling and pressure transport terms in the flamelet regime where “dilatation effects” are found to be significant. A bimodal presumed pdf and a bridged algebraic reaction rate closure are adopted for illustration purposes. The complete closure is assessed by comparisons with direct numerical simulations (DNS) of statistically “steady" fully developed premixed turbulent planar flames at different expansion ratios and the prediction of lean premixed turbulent methane–air flames featuring fractal grid generated turbulence in an opposed jet geometry. Results show that “dilatation" effects contribute to counter–gradient transport and the overall agreement is promising in both cases. Overall, the derived models offer significant improvements and can readily be applied to the modelling of premixed turbulent flames at practical rates of heat release. For turbulent partially premixed flames and turbulent diffusion flames, the impact of mixing models and Scalar Time–scale forms is studied within a hybrid joint Scalar transported pdf (jpdf)/RANS approach with systematically reduced chemistry. The hybrid approach features a naturally closed chemical reaction source term and the study on the molecular mixing term is important for the understanding of combustion regime independent methods. The sensitivity to molecular mixing closures is investigated using variants of the Euclidean Minimum Spanning Tree (EMST) and modified Curl’s (MC) models. Extensions to Scalar Time–scale forms for premixed turbulent flames are included via an algebraic Scalar dissipation closure that accounts for local Damköhler number effects and a conceptually related blended Scalar Time-scale approach. The hybrid jpdf/RANS approach is first applied to turbulent partially premixed flames with inhomogeneous jets. Different composition profiles (homogeneous or inhomogeneous) at the burner exit lead to diverse combustion modes and both transit to diffusion–dominated combustion downstream. The MC model with standard Scalar Time–scale is found to achieve the best agreement and applied to investigate the joint statistics of mixture fraction and reaction progress variable. These two parameters, assumed to be independent in flamelet–based models, are investigated for both types of composition profiles. The correlation of mixture fraction and reaction progress variable is found to be related to local extinction and combustion modes. A second application of the hybrid jpdf/RANS approach features bluff-body turbulent flames approaching blow-off (HM1, HM2 and HM3). Such flames exhibit gradually increasing periodic and shear layer instabilities. The current second moment closure provides a partial resolution of the unsteady fluid motion. The impact of mixing models and Scalar Time–scale forms is investigated by comparing MC, EMST and two extended Scalar Time–scale forms. The latter include Extended Scalar Time–scale coupled with MC (EMC) and Blended Scalar Time–scale used with EMST (BEMST). The sensitivity to solution parameters affecting the temporal resolution is quantified using Fourier transforms of the Time histories of velocity and Scalar traces. Results are found be to similar to well–resolved jpdf /Large Eddy Simulation (LES) for two flames with relatively lower bulk velocities (HM1 and HM2). For flame HM3, the results of EMC and BEMST/EMST essentially enclose the experimental data. In addition, vortex related instabilities ~ 1 kHz in the outer shear layer are captured by all thermochemical closures. However, only the MC based models show periodic extinction/re-ignition in the neck region. The latter result in flame turbules (i.e. discrete pockets of hot gas) separating periodically at frequencies ~ 85 Hz.Open Acces
Mestre Xavier - One of the best experts on this subject based on the ideXlab platform.
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On the asymptotic behaviour of the eigenvalue distribution of block correlation matrices of high-dimensional Time series
2021Co-Authors: Loubaton Philippe, Mestre XavierAbstract:We consider linear spectral statistics built from the block-normalized correlation matrix of a set of $M$ mutually independent Scalar Time series. This matrix is composed of $M \Times M$ blocks that contain the sample cross correlation between pairs of Time series. In particular, each block has size $L \Times L$ and contains the sample cross-correlation measured at $L$ consecutive Time lags between each pair of Time series. Let $N$ denote the total number of consecutively observed windows that are used to estimate these correlation matrices. We analyze the asymptotic regime where $M,L,N \rightarrow +\infty$ while $ML/N \rightarrow c_\star$, $0
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On the asymptotic behaviour of the eigenvalue distribution of block correlation matrices of high-dimensional Time series
2020Co-Authors: Loubaton Philippe, Mestre XavierAbstract:We consider a set of M mutually independent Gaussian Scalar Time series that are observed between Time 1 to Time N. In this paper, we study the behaviour of linear statistics of the eigenvalues of a certain large random matrix built from the available data when both M and N are large. Our results are potentially useful in order to address the problem of testing whether a large number of Time series are uncorrelated or not
Yoshitaka Saiki - One of the best experts on this subject based on the ideXlab platform.
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machine learning construction of a model for a macroscopic fluid variable using the delay coordinate of a Scalar observable
arXiv: Fluid Dynamics, 2019Co-Authors: Kengo Nakai, Yoshitaka SaikiAbstract:We construct a data-driven dynamical system model for a macroscopic variable the Reynolds number of a high-dimensionally chaotic fluid flow by training its Scalar Time-series data. We use a machine-learning approach, the reservoir computing for the construction of the model, and do not use the knowledge of a physical process of fluid dynamics in its procedure. It is confirmed that an inferred Time-series obtained from the model approximates the actual one in each of various Time-intervals, and that some characteristics of the chaotic invariant set mimic the actual ones. We investigate the appropriate choice of the delay-coordinate, especially the delay-Time and the dimension, which enables us to construct a model having a relatively high-dimensional attractor easily.
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machine learning construction of a model for a macroscopic fluid variable using the delay coordinate of a Scalar observable
Discrete & Continuous Dynamical Systems - S, 2018Co-Authors: Kengo Nakai, Yoshitaka SaikiAbstract:We construct a data-driven dynamical system model for a macroscopic variable the Reynolds number of a high-dimensionally chaotic fluid flow by training its Scalar Time-series data. We use a machine-learning approach, the reservoir computing for the construction of the model, and do not use the knowledge of a physical process of fluid dynamics in its procedure. It is confirmed that an inferred Time-series obtained from the model approximates the actual one and that some characteristics of the chaotic invariant set mimic the actual ones. We investigate the appropriate choice of the delay-coordinate, especially the delay-Time and the dimension, which enables us to construct a model having a relatively high-dimensional attractor with low computational costs.
Johannes Kerimo - One of the best experts on this subject based on the ideXlab platform.
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preconditions and limitations of the postulate of Scalar dissipation conductivity independence in a variable conductivity medium
Physical Review E, 2011Co-Authors: Gaurav Kumar, Sharath S Girimaji, Johannes KerimoAbstract:Classical turbulent mixing paradigm--ingrained in scaling laws and closure models--is revisited in a variable-conductivity inhomogeneous medium. We perform direct numerical simulations to study the evolution of a passive Scalar (temperature) field in a fluid with large conductivity gradients and investigate the behavior of Scalar dissipation, conditional Scalar dissipation, and velocity-to-Scalar Time scale ratio. Subject to the conditions of the investigation, it is found that these mixing characteristics become reasonably insensitive to conductivity after about one-third eddy turnover Time. While the results support the classical paradigm, important preconditions and limitations are clearly identified.
