The Experts below are selected from a list of 321 Experts worldwide ranked by ideXlab platform
Habib N. Najm - One of the best experts on this subject based on the ideXlab platform.
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a second order coupled immersed boundary samr construction for chemically Reacting Flow over a heat conducting cartesian grid conforming solid
Journal of Computational Physics, 2014Co-Authors: Kushal S Kedia, Habib N. Najm, Cosmin Safta, Ahmed F GhoniemAbstract:Abstract In this paper, we present a second-order numerical method for simulations of Reacting Flow around heat-conducting immersed solid objects. The method is coupled with a block-structured adaptive mesh refinement (SAMR) framework and a low-Mach number operator-split projection algorithm. A “buffer zone” methodology is introduced to impose the solid–fluid boundary conditions such that the solver uses symmetric derivatives and interpolation stencils throughout the interior of the numerical domain; irrespective of whether it describes fluid or solid cells. Solid cells are tracked using a binary marker function. The no-slip velocity boundary condition at the immersed wall is imposed using the staggered mesh. Near the immersed solid boundary, single-sided buffer zones (inside the solid) are created to resolve the species discontinuities, and dual buffer zones (inside and outside the solid) are created to capture the temperature gradient discontinuities. The development discussed in this paper is limited to a two-dimensional Cartesian grid-conforming solid. We validate the code using benchmark simulations documented in the literature. We also demonstrate the overall second-order convergence of our numerical method. To demonstrate its capability, a Reacting Flow simulation of a methane/air premixed flame stabilized on a channel-confined bluff-body using a detailed chemical kinetics model is discussed.
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Uncertainty Quantification in Reacting Flow.
2011Co-Authors: Habib N. NajmAbstract:Chemically Reacting Flow models generally involve inputs and parameters that are determined from empirical measurements, and therefore exhibit a certain degree of uncertainty. Estimating the propagation of this uncertainty into computational model output predictions is crucial for purposes of Reacting Flow model validation, model exploration, as well as design optimization. Recent years have seen great developments in probabilistic methods and tools for efficient uncertainty quantification (UQ) in computational models. These tools are grounded in the use of Polynomial Chaos (PC) expansions for representation of random variables. The utility and effectiveness of PC methods have been demonstrated in a range of physical models, including structural mechanics, transport in porous media, fluid dynamics, aeronautics, heat transfer, and chemically Reacting Flow. While high-dimensionality remains nominally an ongoing challenge, great strides have been made in dealing with moderate dimensionality along with non-linearity and oscillatory dynamics. In this talk, I will give an overview of UQ in chemical systems. I will cover both: (1) the estimation of uncertain input parameters from empirical data, and (2) the forward propagation of parametric uncertainty to model outputs. I will cover the basics of forward PC UQ methods with examples of their use. I will also highlight the needmore » for accurate estimation of the joint probability density over the uncertain parameters, in order to arrive at meaningful estimates of model output uncertainties. Finally, I will discuss recent developments on the inference of this density given partial information from legacy experiments, in the absence of raw data.« less
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Adaptive chemistry computations of Reacting Flow
Journal of Physics: Conference Series, 2007Co-Authors: Jose Ortega, Habib N. Najm, Mauro Valorani, Dimitris A. Goussis, Michael FrenklachAbstract:We present a new tabulation strategy for the numerical integration of chemical Reacting Flow processes on the basis of a non-stiff system of equations. Both the tabulation and the identification of the non-stiff system are adaptive and are based on the Computational Singular Perturbation (CSP) method. The tabulation strategy is implemented in order to store and reuse the CSP quantities required for the construction of the non-stiff model. In this paper we describe a particular feature of this algorithm, the 'homogeneous correction', that allows for an accurate and efficient identification of the manifold on which the solution moves according to the slow time scales. The improved efficiency in constructing the slow model and simulating the system dynamics along the manifold during run-time calculations is demonstrated.
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modeling unsteady Reacting Flow with operator splitting and isat
Combustion and Flame, 2006Co-Authors: Michael A Singer, S B Pope, Habib N. NajmAbstract:We examine the utility of in situ adaptive tabulation (ISAT) for the simulation of two-dimensional unsteady laminar Reacting Flow. The numerical scheme used to solve the low-Mach-number Reacting Flow equations is an operator-split projection scheme which incorporates ISAT by a Strang subsplitting procedure. The scheme is parallelized using a combination of OpenMP and MPI. ISAT is used for the pure reaction substeps, while convection and diffusion are treated explicitly by a stabilized Runge–Kutta method. We apply the scheme to a two-dimensional problem involving a laminar premixed methane–air flame interacting with a counterrotating vortex pair using detailed GRIMech3.0 chemical kinetics. Computational performance is examined; we observe an overall speed-up factor due to ISAT of approximately 2.5–3.
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A computational facility for Reacting Flow science
Journal of Physics: Conference Series, 2006Co-Authors: Habib N. Najm, Mauro Valorani, Francesco Creta, Dimitris A. GoussisAbstract:We discuss recent developments in the application of high-order adaptive mesh refinement constructions in Reacting Flow computations. We present results pertaining to the time integration of coupled diffusive-convective terms in this context using a stabilized explicit Runge-Kutta-Chebyshev scheme. We also discuss chemical model reduction strategies, with a focus on the utilization of computational singular perturbation theory for generation of simplified chemical models. Starting from a detailed chemical mechanism for methane-air combustion, we examine a posteriori errors in flame species computed with a range of simplified mechanisms corresponding to a varying degree of model reduction.
Helmut Pitsch - One of the best experts on this subject based on the ideXlab platform.
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development and application of a comprehensive soot model for 3d cfd Reacting Flow studies in a diesel engine
Combustion and Flame, 2005Co-Authors: Sangjin Hong, Margaret S Wooldridge, Hong G. Im, D. N. Assanis, Helmut PitschAbstract:A three-dimensional Reacting Flow modeling approach is presented for diesel engine studies that can be used for predictions of trends in soot emissions for a wide range of operating conditions. The modeling framework employs skeletal chemistry for n-heptane for ignition and combustion, and links acetylene chemistry to the soot nucleation process. The soot model is based on integration and modification of existing submodels for soot nucleation, agglomeration, oxidation, and surface growth. With the optimized modeling parameters, the simulations agree well with results of high-pressure shock tube studies of rich n-heptane mixtures, reproducing the trends for soot mass over a range of temperature and pressure conditions (T = 1550–2050 K, P = 20, 40, and 80 MPa). Engine simulation results for soot mass are in excellent agreement with diesel engine smoke number measurements over a range of injection timings (−11 ◦ ATDC–2.4 ◦ ATDC) and two exhaust gas recirculation levels (16 and 26–27%). The model results demonstrate that correct description of the soot formation, as well as the soot transport processes, is critical for achieving reliable predictive capabilities in engine simulations.
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Development and application of a comprehensive soot model for 3D CFD Reacting Flow studies in a diesel engine
Combustion and Flame, 2005Co-Authors: Sangjin Hong, Margaret S Wooldridge, Hong G. Im, D. N. Assanis, Helmut PitschAbstract:A three-dimensional Reacting Flow modeling approach is presented for diesel engine studies that can be used for predictions of trends in soot emissions for a wide range of operating conditions. The modeling framework employs skeletal chemistry for n-heptane for ignition and combustion, and links acetylene chemistry to the soot nucleation process. The soot model is based on integration and modification of existing submodels for soot nucleation, agglomeration, oxidation, and surface growth. With the optimized modeling parameters, the simulations agree well with results of high-pressure shock tube studies of rich n-heptane mixtures, reproducing the trends for soot mass over a range of temperature and pressure conditions (T=1550-2050 K, P=20, 40, and 80 MPa). Engine simulation results for soot mass are in excellent agreement with diesel engine smoke number measurements over a range of injection timings (-11°ATDC-2.4°ATDC) and two exhaust gas recirculation levels (16 and 26-27%). The model results demonstrate that correct description of the soot formation, as well as the soot transport processes, is critical for achieving reliable predictive capabilities in engine simulations. © 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
Koyo Norinaga - One of the best experts on this subject based on the ideXlab platform.
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a cfd study on the Reacting Flow of partially combusting hot coke oven gas in a bench scale reformer
Fuel, 2015Co-Authors: Chengyi Li, Srinivas Appari, Ryota Tanaka, Kyoko Hanao, Shinji Kudo, Junichiro Hayashi, Vinod M Janardhanan, Hiroaki Watanabe, Koyo NorinagaAbstract:Abstract A computational fluid dynamics (CFD) approach to simulate Reacting Flow in a hot coke oven gas (HCOG) reformer is presented. The HCOG was reformed by non-catalytic partial oxidation in a tubular reactor (0.6 m i.d. and ∼4.1 m long) with four oxygen nozzles (0.0427 m i.d.), which was installed on a platform of an operating coke oven. The reforming of HCOG, a multi-component mixture, in a turbulent Flow was simulated numerically by considering both chemical reactions and fluid dynamics. The detailed chemical kinetic model, originally consisting of more than 2000 elementary reactions with 257 species, was reduced to 410 reactions with 47 species for realising a kinetic model of finite rate reactions with a k–e turbulence model. The calculation was carried out using the eddy dissipation concept (EDC) coupled with the kinetic model, and accelerated using the in situ adaptive tabulation (ISAT) algorithm. Numerical simulations could reproduce the reformed gas compositions fairly well, such as H2, CO, CO2, and CH4, as well as the temperature profile in a HCOG reformer as measured by thermocouples.
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predicting the temperature and reactant concentration profiles of Reacting Flow in the partial oxidation of hot coke oven gas using detailed chemistry and a one dimensional Flow model
Chemical Engineering Journal, 2015Co-Authors: Srinivas Appari, Chengyi Li, Ryota Tanaka, Shinji Kudo, Junichiro Hayashi, Vinod M Janardhanan, Hiroaki Watanabe, Koyo NorinagaAbstract:A numerical approach is presented for predicting the species concentrations and temperature profiles of chemically Reacting Flow in the non-catalytic partial oxidation of hot coke oven gas (HCOG) in a pilot-scale reformer installed on an operating coke oven. A detailed chemical kinetic model consisting of 2216 reactions with 257 species ranging in size from the hydrogen radical to coronene was used to predict the chemistries of HCOG reforming and was coupled with a plug model and one-dimensional (1D) Flow with axial diffusion model. The HCOG was a multi-component gas mixture derived from coal dry distillation, and was approximated with more than 40 compounds: H2, CO, CO2, CH4, C2 hydrocarbons, H2O, aromatic hydrocarbons such as benzene and toluene, and polycyclic aromatic hydrocarbons up to coronene. The measured gas temperature profiles were reproduced successfully by solving the energy balance equation accounting for the heat change induced by chemical reactions and heat losses to the surroundings. The approach was evaluated critically by comparing the computed results with experimental data for exit products such as H2, CO, CO2, and CH4, in addition to the total exit gas Flow rate. The axial diffusion model slightly improves the predictions of H2, CO, and CO2, but significantly improves those of CH4 and total exit Flow rate. The improvements in the model predictions were due primarily to the improved temperature predictions by accounting for axial diffusion in the Flow model.
Omar M Knio - One of the best experts on this subject based on the ideXlab platform.
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modeling low mach number Reacting Flow with detailed chemistry and transport
Journal of Scientific Computing, 2005Co-Authors: Habib N. Najm, Omar M KnioAbstract:An efficient projection scheme is developed for the simulation of Reacting Flow with detailed kinetics and transport. The scheme is based on a zero-Mach-number formulation of the compressible conservation equations for an ideal gas mixture. It relies on Strang splitting of the discrete evolution equations, where diffusion is integrated in two half steps that are symmetrically distributed around a single stiff step for the reaction source terms. The diffusive half-step is integrated using an explicit single-step, multistage, Runge---Kutta---Chebyshev (RKC) method. The resulting construction is second-order convergent, and has superior efficiency due to the extended real-stability region of the RKC scheme. Two additional efficiency-enhancements are also explored, based on an extrapolation procedure for the transport coefficients and on the use of approximate Jacobian data evaluated on a coarse mesh. We demonstrate the construction in 1D and 2D flames, and examine consequences of splitting errors. By including the above enhancements, performance tests using 2D computations with a detailed C1C2 methane-air mechanism and a mixture-averaged transport model indicate that speedup factors of about 15 are achieved over the starting split-stiff scheme
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uncertainty quantification in Reacting Flow simulations through non intrusive spectral projection
Combustion and Flame, 2003Co-Authors: Matthew T Reagan, Habib N. Najm, Roger Ghanem, Omar M KnioAbstract:Abstract A spectral formalism has been developed for the “non-intrusive” analysis of parametric uncertainty in Reacting-Flow systems. In comparison to conventional Monte Carlo analysis, this method quantifies the extent, dependence, and propagation of uncertainty through the model system and allows the correlation of uncertainties in specific parameters to the resulting uncertainty in detailed flame structure. For the homogeneous ignition chemistry of a hydrogen oxidation mechanism in supercritical water, spectral projection enhances existing Monte Carlo methods, adding detailed sensitivity information to uncertainty analysis and relating uncertainty propagation to reaction chemistry. For 1-D premixed flame calculations, the method quantifies the effect of each uncertain parameter on total uncertainty and flame structure, and localizes the effects of specific parameters within the flame itself. In both 0-D and 1-D examples, it is clear that known empirical uncertainties in model parameters may result in large uncertainties in the final output. This has important consequences for the development and evaluation of combustion models. This spectral formalism may be extended to multidimensional systems and can be used to develop more efficient “intrusive” reformulations of the governing equations to build uncertainty analysis directly into Reacting Flow simulations.
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regular article a semi implicit numerical scheme for Reacting Flow
Journal of Computational Physics, 1999Co-Authors: Omar M Knio, Habib N. Najm, Peter S WyckoffAbstract:A stiff,^1 operator-split projection scheme is constructed for the simulation of unsteady two-dimensional Reacting Flow with detailed kinetics. The scheme is based on the compressible conservation equations for an ideal gas mixture in the zero-Mach-number limit. The equations of motion are spatially discretized using second-order centered differences and are advanced in time using a new stiff predictor-corrector approach. The new scheme is a modified version of the additive, stiff scheme introduced in a previous effort by H. N. Najm, P. S. Wyckoff, and O. M. Knio (1998, J. Comput. Phys.143, 381). The predictor updates the scalar fields using a Strang-type operator-split integration step which combines several explicit diffusion sub-steps with a single stiff step for the reaction terms, such that the global time step may significantly exceed the critical diffusion stability limit. Convection terms are explicitly handled using a second-order multi-step scheme. The velocity field is advanced using a projection scheme which consists of a partial convection-diffusion update followed by a pressure correction step. A split approach is also adopted for the convection-diffusion step in the momentum update. This splitting combines an explicit treatment of the convective terms at the global time step with several explicit fractional steps for diffusion. Finally, a corrector step is implemented in order to couple the evolution of the density and velocity fields and to stabilize the computations. The corrector acts only on the convective terms and the pressure field, while the predicted updates due to diffusion and reaction are left unchanged. The correction of the scalar fields is implemented using a single-step non-split, non-stiff, second-order time integration. A similar procedure is used for the velocity field, which is followed by a pressure projection step. The performance and behavior of the operator-split scheme are first analyzed based on tests for a nonlinear reaction-diffusion equation in one space dimension, followed by computations with a detailed C"1C"2 methane-air mechanism in one and two dimensions. The tests are used to verify that the scheme is effectively second order in time, and to suggest guidelines for selecting integration parameters, including the number of fractional diffusion steps and tolerance levels in the stiff integration. For two-dimensional simulations with the present reaction mechanism, flame conditions, and resolution parameters, speedup factors of about 5 are achieved over the previous additive scheme, and about 25 over the original explicit scheme.
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a semi implicit numerical scheme for Reacting Flow
Journal of Computational Physics, 1998Co-Authors: Habib N. Najm, Peter S Wyckoff, Omar M KnioAbstract:An additive semi-implicit projection scheme for the simulation of unsteady combustion in two dimensions is constructed. The scheme relies on a zero-Mach number formulation of the compressible conservation equations with detailed chemistry. The governing equations are discretized in space using second-order differences and integrated in time using a semi-implicit approach. Time integration of the evolution equations for species mass fraction, thermodynamic pressure, and density is performed using a semi-implicit, nonsplit scheme that combines a second-order predictor?corrector treatment of convection and diffusion terms, and a stiff integrator for the reaction source terms. Meanwhile, the momentum equations are integrated using a second-order projection scheme. The projection scheme is based on a predictor?corrector approach that couples the evolution of the velocity and density fields in order to stabilize computations of Reacting Flows with large density variations. A pressure Poisson equation is inverted following both the predictor and corrector steps using a fast solver. The advantages of the stiff integration of reaction source terms are analyzed by comparing the performance of the scheme to that of a predictor?corrector scheme in which reaction and diffusion are integrated in a similar nonstiff fashion. The comparison in based on both one-dimensional (1D) unsteady tests of a premixed methane?air flame, and unsteady two-dimensional tests of the same flame interacting with a counterrotating vortex pair. In both cases, the GRImech1.2 reaction mechanism with 32 species and 177 elementary reactions is used. Computed results show that the stiff reaction scheme enables selection of larger time steps and thus leads to substantial improvement in the performance of the computations. For the present reaction mechanism and flame conditions, speedup factors of about 10 are achieved in the 1D tests and about five in two dimensions. Possible extensions of the present scheme to further improve efficiency are also discussed.
Sangjin Hong - One of the best experts on this subject based on the ideXlab platform.
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development and application of a comprehensive soot model for 3d cfd Reacting Flow studies in a diesel engine
Combustion and Flame, 2005Co-Authors: Sangjin Hong, Margaret S Wooldridge, Hong G. Im, D. N. Assanis, Helmut PitschAbstract:A three-dimensional Reacting Flow modeling approach is presented for diesel engine studies that can be used for predictions of trends in soot emissions for a wide range of operating conditions. The modeling framework employs skeletal chemistry for n-heptane for ignition and combustion, and links acetylene chemistry to the soot nucleation process. The soot model is based on integration and modification of existing submodels for soot nucleation, agglomeration, oxidation, and surface growth. With the optimized modeling parameters, the simulations agree well with results of high-pressure shock tube studies of rich n-heptane mixtures, reproducing the trends for soot mass over a range of temperature and pressure conditions (T = 1550–2050 K, P = 20, 40, and 80 MPa). Engine simulation results for soot mass are in excellent agreement with diesel engine smoke number measurements over a range of injection timings (−11 ◦ ATDC–2.4 ◦ ATDC) and two exhaust gas recirculation levels (16 and 26–27%). The model results demonstrate that correct description of the soot formation, as well as the soot transport processes, is critical for achieving reliable predictive capabilities in engine simulations.
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Development and application of a comprehensive soot model for 3D CFD Reacting Flow studies in a diesel engine
Combustion and Flame, 2005Co-Authors: Sangjin Hong, Margaret S Wooldridge, Hong G. Im, D. N. Assanis, Helmut PitschAbstract:A three-dimensional Reacting Flow modeling approach is presented for diesel engine studies that can be used for predictions of trends in soot emissions for a wide range of operating conditions. The modeling framework employs skeletal chemistry for n-heptane for ignition and combustion, and links acetylene chemistry to the soot nucleation process. The soot model is based on integration and modification of existing submodels for soot nucleation, agglomeration, oxidation, and surface growth. With the optimized modeling parameters, the simulations agree well with results of high-pressure shock tube studies of rich n-heptane mixtures, reproducing the trends for soot mass over a range of temperature and pressure conditions (T=1550-2050 K, P=20, 40, and 80 MPa). Engine simulation results for soot mass are in excellent agreement with diesel engine smoke number measurements over a range of injection timings (-11°ATDC-2.4°ATDC) and two exhaust gas recirculation levels (16 and 26-27%). The model results demonstrate that correct description of the soot formation, as well as the soot transport processes, is critical for achieving reliable predictive capabilities in engine simulations. © 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
