Showing papers by "Parviz Moin published in 2004"
TL;DR: In this article, a new approach to chemistry modelling for large-eddy simulation of turbulent reacting flows is developed, whereby all of the detailed chemical processes are mapped to a reduced system of tracking scalars.
Abstract: A new approach to chemistry modelling for large-eddy simulation of turbulent reacting flows is developed. Instead of solving transport equations for all of the numerous species in a typical chemical mechanism and modelling the unclosed chemical source terms, the present study adopts an indirect mapping approach, whereby all of the detailed chemical processes are mapped to a reduced system of tracking scalars. Here, only two such scalars are considered: a mixture fraction variable, which tracks the mixing of fuel and oxidizer, and a progress variable, which tracks the global extent of reaction of the local mixture. The mapping functions, which describe all of the detailed chemical processes with respect to the tracking variables, are determined by solving quasi-steady diffusion-reaction equations with complex chemical kinetics and multicomponent mass diffusion. The performance of the new model is compared to fast-chemistry and steady-flamelet models for predicting velocity, species concentration, and temperature fields in a methane-fuelled coaxial jet combustor for which experimental data are available. The progress-variable approach is able to capture the unsteady, lifted flame dynamics observed in the experiment, and to obtain good agreement with the experimental data, while the fast-chemistry and steady-flamelet models both predict an attached flame.
1,102 citations
TL;DR: In this article, the authors discuss the development of a numerical algorithm and solver capable of performing large-eddy simulation in very complex geometries often encountered in industrial applications, such as unstructured hybrid grids.
502 citations
TL;DR: In this paper, the authors used numerical simulations of turbulent polymer solutions using the FENE-P model to characterize the action of polymers on turbulence in drag-reduced flows and found that polymers are found to store and to release energy to the flow in a well-organized manner.
Abstract: Numerical simulations of turbulent polymer solutions using the FENE-P model are used to characterize the action of polymers on turbulence in drag-reduced flows. The energetics of turbulence is investigated by correlating the work done by polymers on the flow with turbulent structures. Polymers are found to store and to release energy to the flow in a well-organized manner. The storage of energy occurs around near-wall vortices as has been anticipated for a long time. Quite unexpectedly, coherent release of energy is observed in the very near-wall region. Large fluctuations of polymer work are shown to re-energize decaying streamwise velocity fluctuations in high-speed streaks just above the viscous sublayer. These distinct behaviours are used to propose an autonomous regeneration cycle of polymer wall turbulence, in the spirit of Jimenez & Pinelli (1999).
266 citations
TL;DR: A numerical formulation for the treatment of nonlinear instabilities in shock-free compressible turbulence simulations is presented and is capable of capturing the statistical equilibrium of low Mach number compressible turbulent fluctuations at infinite Reynolds number.
192 citations
01 Nov 2004
TL;DR: In this article, a high-order skew-symmetric splitting of the nonlinear terms is proposed for the treatment of nonlinear instabilities in shock-free compressible turbulence simulations, which achieves the statistical equilibrium of low Mach number compressible turbulent fluctuations at infinite Reynolds number.
Abstract: We present a numerical formulation for the treatment of nonlinear instabilities in shock-free compressible turbulence simulations. The formulation is high order and contains no artificial dissipation. Numerical stability is enhanced through semi-discrete satisfaction of global conservation properties stemming from the second law of thermodynamics and the entropy equation. The numerical implementation is achieved using a conservative skew-symmetric splitting of the nonlinear terms. The robustness of the method is demonstrated by performing unresolved numerical simulations and large eddy simulations of compressible isotropic turbulence at a very high Reynolds number. Results show the scheme is capable of capturing the statistical equilibrium of low Mach number compressible turbulent fluctuations at infinite Reynolds number. Comparisons with the entropy splitting technique [J. Comput. Phys. 162 (2000) 33; J. Comput. Phys. 178 (2002) 307], staggered method [J. Comput. Phys. 191(2) (2003) 392], and skew-symmetric like schemes [J. Comput. Phys. 161 (2000) 114] confirm the superiority of the current approach. We also discuss a flaw in the skew-symmetric splitting implemented in the literature. Very good results are obtained based on the proper splitting.
181 citations
TL;DR: A method to provide LES inflow conditions through auxiliary, a priori LES computations, where an L ES inflow database is generated and modified to account for the unsteadiness of the interface flow statistics is defined.
Abstract: Hybrid approaches using a combination of Reynolds-averaged Navier-Stokes (RANS) approaches and large eddy simulations (LES) have become increasingly popular. One way to construct a hybrid approach is to apply separate flow solvers to components of a complex system and to exchange information at the interfaces of the domains. For the LES flow solver, boundary conditions then have to be defined on the basis of the Reynolds-averaged flow statistics delivered by a RANS flow solver. This is a challenge, which also arises, for instance, when defining LES inflow conditions from experimental data. The problem for the coupled RANS-LES computations is further complicated by the fact that the mean flow statistics at the interface may vary in time and are not known a priori but only from the RANS solution. The present study defines a method to provide LES inflow conditions through auxiliary, a priori LES computations, where an LES inflow database is generated. The database is modified to account for the unsteadiness of the interface flow statistics
172 citations
TL;DR: In this paper, a non-gradient based pattern search method is used for shape optimization to minimize aerodynamic noise in a laminar flow past an acoustically compact airfoil.
Abstract: Shape optimization is applied to time-dependent trailing-edge flow in order to minimize aerodynamic noise. Optimization is performed using the surrogate management framework (SMF), a non-gradient based pattern search method chosen for its efficiency and rigorous convergence properties. Using SMF, design space exploration is performed not with the expensive actual function but with an inexpensive surrogate function. The use of a polling step in the SMF guarantees that the algorithm generates a convergent subsequence of mesh points in the parameter space. Each term of this subsequence is a weak local minimizer of the cost function on the mesh in a sense to be made precise later. We will discuss necessary optimality conditions for the design problem that are satisfied by the limit of this subsequence. Results are presented for an unsteady laminar flow past an acoustically compact airfoil. Constraints on lift and drag are handled within SMF by applying the filter pattern search method of Audet and Dennis, within which a penalty function is used to form and optimize a surrogate function. Optimal shapes that minimize noise have been identified for the trailing-edge problem in constrained and unconstrained cases. Results show a significant reduction (as much as 80%) in acoustic power with reasonable computational cost using several shape parameters. Physical mechanisms for noise reduction are discussed.
136 citations
TL;DR: In this paper, a study of the drag reduction induced by rigid fibres in a turbulent channel flow using direct numerical simulation is presented, where the extra stresses due to the fibres are calculated with the well-known constitutive equation involving the moments of the orientation vector.
Abstract: We present a study of the drag reduction induced by rigid fibres in a turbulent channel flow using direct numerical simulation. The extra stresses due to the fibres are calculated with the well-known constitutive equation involving the moments of the orientation vector. Drag reductions of up to 26% are calculated, with the largest drag reductions observed using non-Brownian fibres and semi-dilute concentrations. These findings suggest that elasticity is not necessary to achieve turbulent drag reduction. Flow statistics show trends similar to those observed in simulation of polymeric drag reduction: Reynolds stresses are reduced, velocity fluctuations in the wall-normal and spanwise directions are reduced while streamwise fluctuations are increased, and streamwise vorticity is reduced. We observe strong correlations between the fibre stresses and inter-vortex extensional flow regions. Based on these correlations and instantaneous visualizations of the flow field, we propose a mechanism for turbulent drag reduction by rigid fibre additives.
101 citations
TL;DR: A large-eddy-simulation-based flow solver that combines an immersed boundary technique with a curvilinear structured grid has been developed to study the temporal and spatial dynamics of an incompressible rotor-tip-clearance flow as discussed by the authors.
Abstract: A large-eddy-simulation-based flow solver that combines an immersed-boundary technique with a curvilinear structured grid has been developed to study the temporal and spatial dynamics of an incompressible rotor-tipclearance flow. The overall objective of these simulations is to determine the underlying mechanisms for lowpressure fluctuations downstream of the rotor near the end wall. Salient features of the numerical methodology, including the mesh topology, the immersed boundary method, the treatment of numerical instability for nondissipative schemes on highly skewed meshes, and the parallelization of the code for shared memory computer platforms, are discussed. The computational approach is shown to be capable of capturing the evolution of the highly complicated flowfield characterized by the interaction of distinct blade-associated vortical structures with the turbulent end-wall boundary layer. Simulation results are compared with experiments, and qualitative as well as quantitative agreement is observed.
91 citations
TL;DR: In this article, the authors examined the dynamics of a large number of single polymer chains along their trajectories in a Newtonian turbulent channel flow and found that at moderate Weissenberg number Wi the polymers, that are stretched to a large fraction of their maximum extensibility, have experienced a strong biaxial extensional flow.
Abstract: We examine the phenomenon of polymer drag reduction in a turbulent flow through Brownian dynamics simulations. The dynamics of a large number of single polymer chains along their trajectories is investigated in a Newtonian turbulent channel flow. In particular, the FENE, FENE-P and multimode FENE models with realistic parameters are used to investigate the mechanisms of polymer stretching. A topological methodology is applied to characterize the ability of the flow to stretch the polymers. It is found using conditional statistics that at moderate Weissenberg number Wi the polymers, that are stretched to a large fraction of their maximum extensibility, have experienced a strong biaxial extensional flow. When Wi is increased other flow types can stretch the polymers but the few highly extended molecules again have, on average, experienced a biaxial extensional flow. Moreover, highly extended polymers are found in the near-wall regions around the quasi-streamwise vortices, essentially in regions of strong biaxial extensional flow.
84 citations
TL;DR: Using the method of large-eddy simulation, the authors studied decaying homogeneous turbulence of a conductive flow under the influence of an applied external magnetic field at low magnetic Reynolds number.
Abstract: Using the method of large-eddy simulation, we study decaying homogeneous turbulence of a conductive flow under the influence of an applied external magnetic field at low magnetic Reynolds number. In order to assess the performance of large-eddy simulation, comparison with high resolution (512 3 ) direct numerical simulation is performed. Results show that the modeling of subgrid scales using the dynamic Smagorinsky model is very effective in the present context.
TL;DR: In this paper, the authors describe the application of a derivative-free optimization technique, the surrogate management framework (SMF), for designing the shape of an airfoil trailing edge which minimizes the noise of vortex shedding.
Abstract: In this Letter we describe the application of a derivative-free optimization technique, the surrogate management framework (SMF), for designing the shape of an airfoil trailing edge which minimizes the noise of vortex shedding. Constraints on lift and drag are enforced within SMF using a filter. Several optimal shapes have been identified for the case of laminar vortex shedding with reasonable computational cost using several shape parameters, and results show a significant reduction in acoustic power. Physical mechanisms for noise reduction are discussed.
TL;DR: A powerful computational technique, large-eddy simulation, has been used to study the detailed flow dynamics in the tip-gap region of hydraulic turbomachines as discussed by the authors, which can lead to reduced performance, increased noise, and structural vibration and erosion.
Abstract: A powerful computational technique, large-eddy simulation, helps researchers study the detailed flow dynamics in the tip-gap region of hydraulic turbomachines. LES also helps researchers investigate ways to mitigate undesirable effects, such as cavitation, which can lead to reduced performance, increased noise, and structural vibration and erosion.
TL;DR: In this paper, a non-dissipative, massively parallel, unstructured grid solver (CDP) is used to perform LES in complex combustor geometries.
Abstract: An overview is provided of the recent advances in performing LES in complex combustor geometries using a non-dissipative, massively parallel, unstructured grid solver (CDP). A new numerical algorithm that is discretely energy conserving on hybrid unstructured grids is developed. This allows simulations at high Reynolds numbers without the use of numerical dissipation. This LES methodology has been extended to include advanced turbulent combustion models and a Lagrangian particle tracking methodology for simulating turbulent multiphase flows. Results for single and multi-phase flow simulations in realistic Pratt & Whitney (P&W) combustor geometries are discussed.
05 Jan 2004
TL;DR: A novel data management architecture is introduced to take advantage of the quasi-structured nature of the grids and to obtain fully implicit, fast and robust solutions when several levels of refinement are introduced.
Abstract: A RANS solver based on the Immersed Boundary technique is extended to handle locally refined grids in order to increase the resolution close to the boundaries for high Reynolds number simulations. A novel data management architecture is introduced to take advantage of the quasi-structured nature of the grids and to obtain fully implicit, fast and robust solutions when several levels of refinement are introduced. The mesh refinement is fully anisotropic, and can handle n-to-one cell connectivity. It is generated in a fully automatic way by coarsening a fine structured grid. A conjugate gradient-based algorithm is used to solve the Navier-Stokes equations and validation cases include a twoand a three-dimensional problem.
01 Jan 2004
TL;DR: In this article, the authors developed a practical computational model of a carbonate fuel cell stack and validated its accuracy and predictive capabilities with experimental data, which has proven to be a valuable asset for design evaluation and optimization of fuel cell stacks.
Abstract: The objective of this work is the development of a practical computational model of a carbonate fuel cell stack. Previously published carbonate fuel cell models have focused more on the fundamental mechanisms of fuel cell operation than on evaluation of practical fuel cell product designs. Efficient development of a fuel cell product requires a predictive tool that couples all the important mechanisms with the capability to evaluate the performance of many design iterations quickly. The important mechanisms typically include three dimensional fluid flow, heat and mass transfer, gas-phase and surface chemistry, electrochemistry and structural mechanics. For large-scale fuel cells with applications in the power generation industry, thermal management is of significant interest for stack performance, reliability and life. Minimizing peak cell temperatures improves cell life and minimizing stack temperature gradients reduces stack thermal stress and improves cell performance. In this paper, the fuel cell model is presented along with experimental data validating its accuracy and predictive capabilities. The tool has proven to be a valuable asset for design evaluation and optimization of fuel cell stack designs at FuelCell Energy.
07 Jun 2004
TL;DR: In this article, a large-eddy simulation (LES) solver was used to analyze the dynamics of rotor tip-clearance flow and determine the underlying mechanism for the tip-leakage cavitation.
Abstract: In order to analyze the dynamics of rotor tip-clearance flow and determine the underlying mechanism for the tip-leakage cavitation, a newly developed large-eddy simulation (LES) solver which combines an immersed-boundary method with a generalized curvilinear structured grid has been employed. An analysis of the LES results has been performed to understand the mean flow field, turbulence characteristics, vortex dynamics, and pressure fluctuations in the turbomachinery cascade with tip gap. Based on thorough analysis of the flow field, a guideline for reducing viscous losses in the cascade is provided. Analyses of the energy spectra and space-time correlations of the velocity fluctuations suggest that the tip-leakage vortex is subject to pitchwise wandering motion. The largest pressure drop and most intense pressure fluctuations due to the formation of the tip-leakage vortex are found in the region where the tip-leakage vortex is strongest. The effects of tip-gap size and an end-wall groove on the tip-clearance vortical structures and on the velocity and pressure fields have been investigated to explore ways for minimizing the detrimental effects of cavitation. Attempts to investigate the vortex-rotor interaction and to enhance the LES capability for realistic rotors are also discussed.
01 Dec 2004
TL;DR: The Proceedings volume as mentioned in this paper contains 32 papers that span a wide range of topics that reflect the ubiquity of turbulence, including solar simulation, magnetohydrodynamics, large eddy simulation, numerical simulation, stability and acoustics, and combustion and multi-phase flow.
Abstract: This Proceedings volume contains 32 papers that span a wide range of topics that reflect the ubiquity of turbulence. The papers have been divided into six groups: 1) Solar Simulations; 2) Magnetohydrodynamics (MHD); 3) Large Eddy Simulation (LES) and Numerical Simulations; 4) Reynolds Averaged Navier Stokes (RANS) Modeling and Simulations; 5) Stability and Acoustics; 6) Combustion and Multi-Phase Flow.
05 Aug 2004
TL;DR: In this article, a particle tracking scheme is used to perform high fidelity multiphase flow simulations with models to capture liquid-sheet breakup, droplet evaporation, and droplet deformation and drag.
Abstract: Large-eddy simulation (LES) is a promising technique to accurately predict reacting multi-phase flows in practical combustors involving complex physical phenomena of turbulent mixing and combustion dynamics. Our goal in the present work is to develop a computational tool based on particle-tracking schemes capable of performing hi-fidelity multiphase flow simulations with models to capture liquid-sheet breakup, droplet evaporation, droplet deformation and drag. An Eulerian low-Mach number formulation on arbitrary shaped unstructured grids is used to compute the gaseous phase. The dispersed phase is solved in a Lagrangian framework by tracking a large number of particles on the unstructured grid. The interphase mass, momentum, and energy transport are modeled using two-way coupling of point-particles. A series of validation simulations are performed in coaxial and realistic gas-turbine combustor geometries to evaluate the predictions made for multiphase, turbulent flow.
05 Jan 2004
TL;DR: Mahesh et al. as discussed by the authors proposed a time-accurate finite-volume fractional-step algorithm for predicting turbulent flows on unstructured hybrid meshes, which is based on large-eddy simulation.
Abstract: Large-eddy simulation is a promising technique for accurate prediction of reacting multiphase flows in practical gas-turbine combustion chambers involving complex physical phenomena of turbulent mixing and combustion dynamics. Development of advanced models for liquid fuel atomization, droplet evaporation, droplet deformation and drag, and turbulent combustion is discussed specifically for gas-turbine applications. The nondissipative, yet robust numerical scheme for arbitrary shaped unstructured grids developed by Mahesh et al. (Mahesh, K., Constantinescu, G., and Moin, P., "A New Time-Accurate Finite-Volume Fractional-Step Algorithm for Prediction of Turbulent Flows on Unstructured Hybrid Meshes," Journal of Computational Physics, Vol. 197, No. 1, 2004, pp. 215-240) is modified to account for density variations due to chemical reactions. A systematic validation and verification study of the individual spray models and the numerical scheme is performed in canonical and complex combustor geometries. Finally, a multiscale, multi physics, turbulent reacting flow simulation in a real gas-turbine combustor is performed to assess the predictive capability of the solver.
01 Jan 2004
TL;DR: The Center for Turbulence Research (CTR) as mentioned in this paper has been a major research center for turbulence analysis and control in the field of computer science and applied mathematics, chemical kinetics and fluid mechanics.
Abstract: This report contains the 2004 annual progress reports of the Research Fellows and students of the Center for Turbulence Research in its eighteenth year of operation. Since its inception in 1987, the objective of the CTR has been to advance the physical understanding of turbulent flows and development of physics based predictive tools for engineering analysis and turbulence control. Turbulence is ubiquitous in nature and in engineering devices. The studies at CTR have been motivated by applications where turbulence effects are significant; these include a broad range of technical areas such as planetary boundary layers, formation of planets, solar convection, magnetohydrodynamics, environmental and eco systems, aerodynamic noise, propulsion systems and high speed transportation. Numerical simulation has been the predominant research tool at CTR which has required a critical mass of researchers in numerical analysis and computer science in addition to core disciplines such as applied mathematics, chemical kinetics and fluid mechanics. Maintaining and promoting this interdisciplinary culture has been a hallmark of CTR and has been responsible for the realization of the results of its basic research in applications. The first group of reports in this volume are directed towards development, analysis and application of novel numerical methods for ow simulations. Development of methods for large eddy simulation of complex flows has been a central theme in this group. The second group is concerned with turbulent combustion, scalar transport and multi-phase ows. The nal group is devoted to geophysical turbulence where the problem of solar convection has been a new focus of considerable attention recently at CTR.