The Experts below are selected from a list of 195 Experts worldwide ranked by ideXlab platform
Suresh Menon - One of the best experts on this subject based on the ideXlab platform.
-
subgrid combustion modeling of 3 d premixed flames in the thin reaction zone regime
Proceedings of the Combustion Institute, 2005Co-Authors: Vaidyanathan Sankaran, Suresh MenonAbstract:Abstract Large eddy simulation (LES) is used to investigate three-dimensional (3D) lean premixed turbulent methane–air flames in the thin-reaction-zone regime. In this regime, the Kolmogorov Scale is smaller than the preheat zone thickness, but larger than the reaction zone thickness. Past numerical studies of similar flames were primarily direct numerical simulation either in two-dimensions or using the artificially thickened flame approach in 3D. For an LES the effect of small (unresolved) Scales on the scalar field must be, modeled accurately to capture the correct flame structure. A subgrid combustion model based on the linear-eddy-mixing (LEM) model is used within an LES framework (called LEM–LES hereafter) to capture the 3D flame-structure of the highly stretched premixed flames. A finite-rate, one-step methane–air chemistry with a non-unity Lewis number formulation is used in this study. The simulated flame structure resembles flames experimentally studied in the thin-reaction-zone regime. Even though the preheat zone is broadened by the penetration of small eddies, the chemical reaction zone remains thin and localized. This feature is captured properly in the current LEM–LES approach. The flame structure and other statistics such as the flame area evolution, curvature, and strain-rate statistics computed using the LEM–LES are also in good agreement with the past DNS studies.
Vaidyanathan Sankaran - One of the best experts on this subject based on the ideXlab platform.
-
subgrid combustion modeling of 3 d premixed flames in the thin reaction zone regime
Proceedings of the Combustion Institute, 2005Co-Authors: Vaidyanathan Sankaran, Suresh MenonAbstract:Abstract Large eddy simulation (LES) is used to investigate three-dimensional (3D) lean premixed turbulent methane–air flames in the thin-reaction-zone regime. In this regime, the Kolmogorov Scale is smaller than the preheat zone thickness, but larger than the reaction zone thickness. Past numerical studies of similar flames were primarily direct numerical simulation either in two-dimensions or using the artificially thickened flame approach in 3D. For an LES the effect of small (unresolved) Scales on the scalar field must be, modeled accurately to capture the correct flame structure. A subgrid combustion model based on the linear-eddy-mixing (LEM) model is used within an LES framework (called LEM–LES hereafter) to capture the 3D flame-structure of the highly stretched premixed flames. A finite-rate, one-step methane–air chemistry with a non-unity Lewis number formulation is used in this study. The simulated flame structure resembles flames experimentally studied in the thin-reaction-zone regime. Even though the preheat zone is broadened by the penetration of small eddies, the chemical reaction zone remains thin and localized. This feature is captured properly in the current LEM–LES approach. The flame structure and other statistics such as the flame area evolution, curvature, and strain-rate statistics computed using the LEM–LES are also in good agreement with the past DNS studies.
James R. Debonis - One of the best experts on this subject based on the ideXlab platform.
-
Navier-Stokes analysis methods for turbulent jet flows with application to aircraft exhaust nozzles
Progress in Aerospace Sciences, 2007Co-Authors: Nicholas J Georgiadis, James R. DebonisAbstract:This article presents the current status of computational fluid dynamics (CFD) methods as applied to the simulation of turbulent jet flowfields issuing from aircraft engine exhaust nozzles. For many years, Reynolds-averaged Navier-Stokes (RANS) methods have been used routinely to calculate such flows, including very complex nozzle configurations. RANS methods replace all turbulent fluid dynamic effects with a turbulence model. Such turbulence models have limitations for jets with significant three-dimensionality, compressibility, and high temperature streams. In contrast to the RANS approach, direct numerical simulation (DNS) methods calculate the entire turbulent energy spectrum by resolving all turbulent motion down to the Kolmogorov Scale. Although this avoids the limitations associated with turbulence modeling, DNS methods will remain computationally impractical in the foreseeable future for all but the simplest configurations. Large-Eddy simulation (LES) methods, which directly calculate the large-Scale turbulent structures and reserve modeling only for the smallest Scales, have been pursued in recent years and may offer the best prospects for improving the fidelity of turbulent jet flow simulations. A related approach is the group of hybrid RANS/LES methods, where RANS is used to model the small-Scale turbulence in wall boundary layers and LES is utilized in regions dominated by the large-Scale jet mixing. The advantages, limitations, and applicability of each approach are discussed and recommendations for further research are presented. © 2007 Elsevier Ltd. All rights reserved.
Joerg Schumacher - One of the best experts on this subject based on the ideXlab platform.
-
stretching of polymers around the Kolmogorov Scale in a turbulent shear flow
Physics of Fluids, 2006Co-Authors: J Davoudi, Joerg SchumacherAbstract:We present numerical studies of stretching of Hookean dumbbells in a turbulent Navier-Stokes flow with a linear mean profile, ⟨ux⟩=Sy. In addition to the turbulence features beyond the viscous Kolmogorov Scale η, the dynamics at the equilibrium extension of the dumbbells significantly below η is well resolved. The variation of the constant shear rate S causes a change of the turbulent velocity fluctuations on all Scales and thus of the intensity of local stretching rate of the advecting flow. The latter is measured by the maximum Lyapunov exponent λ1 which is found to increase as λ1∼S3∕2, in agreement with a dimensional argument. The ensemble of up to 2×106 passively advected dumbbells is advanced by Brownian dynamics simulations in combination with a pseudospectral integration for the turbulent shear flow. Anisotropy of stretching is quantified by the statistics of the azimuthal angle ϕ which measures the alignment with the mean flow axis in the x-y shear plane, and the polar angle θ which determines the...
Toshisuke Hirano - One of the best experts on this subject based on the ideXlab platform.
-
Turbulence characteristics within the local reaction one of a high-intensity turbulent premixed flame
Symposium (International) on Combustion, 2007Co-Authors: Junichi Furukawa, Kyoko Okamoto, Toshisuke HiranoAbstract:An attempt has been made to measure the Kolmogorov Scale of turbulence within the local reaction zone of a turbulent premixed flame established in a regime in which the Kolmogorov Scale of turbulence in the nonreacting flow is smaller than the laminar premixed flame thickness. In orde to measure the Kolmogorov Scale of turbulence within the local reaction zone, the specially arranged diagnostics composed of an laser-doppler velocimeter (LDV) system and a microelectrostatic probe have been adopted. By using this technique, the velocity fluctuation within the local reaction zone can be successfully distinguished from that in unburned mixture or burned gas stream. Thus, the Kolmogorov Scale of tubulence in the local reaction zone could be evaluated on the basis of the power spectrum density function derived from the velocity fluctuations. The power spectrum density function, the turbulence intensity, and the Kolmogorov Scale of turbulence derived from velocity fluctuations in the approach flow are shown to be the same as those in the nonreacting flow. Also, small-Scale eddies in the nonreacting flow, which are smaller than the laminar premixed flame thickness, are confirmed to exist in the approach flow. It is found that the approach flow turbulence increase in the local reactionzone because of the expansion of gases due to heat release. The Kolmogorov Scale of turbulence derived from velocity fluctuations in the local reaction zone is shown to be much larger than the laminar premixed flame thickness. This result implies that the small-Scale eddies in the approach flow may not survive through the preheat zone.
-
Flame front configuration of turbulent premixed flames
Combustion and Flame, 1998Co-Authors: Junichi Furukawa, Kaoru Maruta, Toshisuke HiranoAbstract:Abstract The present study is performed to explore dependence of the wrinkle Scale of propane-air turbulent premixed flames on the characteristics of turbulence in the nonreacting flow burner size, and mixture ratio. The wrinkle Scales are examined and expressed in the frequency distribution of the radii of flame front curvatures. The average wrinkle Scale depends not only on the characteristics of turbulence in the nonreacting flow but also on burner diameter and mixture ratio. The average wrinkle Scale of a lean propane-air flame is larger than those of the near stoichiometric and rich flames. The smallest wrinkle Scale of turbulent premixed flame is in the range of 0.75–1.0 mm, which is much larger than the Kolmogorov Scale of turbulence in the nonreacting flow.
