Showing papers by "Parviz Moin published in 2003"

A numerical method for large-eddy simulation in complex geometries

Abstract: Publisher Summary This chapter discusses the development of a numerical algorithm and solver capable of performing large-eddy simulation (LES) in geometries as complex as the combustor of a gas-turbine engine. The algorithm developed for unstructured grids is nondissipative yet robust at high Reynolds numbers on highly skewed grids. Results from validation in simple geometries are shown in the chapter along with the simulation results in the exceedingly complex geometry of a Pratt & Whitney gas-turbine combustor.

Optimal aeroacoustic shape design using the surrogate management framework

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.

Thermal Conduction in Magnetized Turbulent Gas

Abstract: Using numerical methods, we systematically study in the framework of ideal MHD the effect of magnetic fields on heat transfer within a turbulent gas. We measure the rates of passive scalar diffusion within magnetized fluids and make the comparisons (1) between MHD and hydrodynamic simulations, (2) between different MHD runs with different values of the external magnetic field (up to the energy equipartition value), and (3) between thermal conductivities parallel and perpendicular to the magnetic field. We do not find apparent suppression of diffusion rates by the presence of magnetic fields, which implies that magnetic fields do not suppress heat diffusion by turbulent motions.

Large-eddy simulation of conductive flows at low magnetic Reynolds number

Abstract: In this paper we study the LES method with dynamic procedure in the context of conductive flows subject to an applied external magnetic field at low magnetic Reynolds number R(sub m). These kind of flows are encountered in many industrial applications. For example, in the steel industry, applied magnetic fields can be used to damp turbulence in the casting process. In nuclear fusion devices (Tokamaks), liquid-lithium flows are used as coolant blankets and interact with the surrounding magnetic field that drives and confines the fusion plasma. Also, in experimental facilities investigating the dynamo effect, the flow consists of liquid-sodium for which the Prandtl number and, as a consequence, the magnetic Reynolds number is low. Our attention is focused here on the case of homogeneous (initially isotropic) decaying turbulence. The numerical simulations performed mimic the thought experiment described in Moffatt in which an initially homogeneous isotropic conductive flow is suddenly subjected to an applied magnetic field and freely decays without any forcing. Note that this flow was first studied numerically by Schumann. It is well known that in that case, extra damping of turbulence occurs due to the Joule effect and that the flow tends to become progressively independent of the coordinate along the direction of the magnetic field. Our comparison of filtered direct numerical simulation (DNS) predictions and LES predictions show that the dynamic Smagorinsky model enables one to capture successfully the flow with LES, and that it automatically incorporates the effect of the magnetic field on the turbulence. Our paper is organized as follows. In the next section we summarize the LES approach in the case of MHD turbulence at low R(sub m) and recall the definition of the dynamic Smagorinsky model. In Sec. 3 we describe the parameters of the numerical experiments performed and the code used. Section 4 is devoted to the comparison of filtered DNS results and LES results. Conclusions are presented in Sec. 5.

Unstructured LES of Reacting Multiphase Flows in Realistic Gas Turbine Combustors

Abstract: As part of the Accelerated Strategic Computing Initiative (ASCI) program, an accurate and robust simulation tool is being developed to perform high-fidelity LES studies of multiphase, multiscale turbulent reacting flows in aircraft gas turbine combustor configurations using hybrid unstructured grids. In the combustor, pressurized gas from the upstream compressor is reacted with atomized liquid fuel to produce the combustion products that drive the downstream turbine. The Large Eddy Simulation (LES) approach is used to simulate the combustor because of its demonstrated superiority over RANS in predicting turbulent mixing, which is central to combustion. This paper summarizes the accomplishments of the combustor group over the past year, concentrating mainly on the two major milestones achieved this year: 1) Large scale simulation: A major rewrite and redesign of the flagship unstructured LES code has allowed the group to perform large eddy simulations of the complete combustor geometry (all 18 injectors) with over 100 million control volumes; 2) Multi-physics simulation in complex geometry: The first multi-physics simulations including fuel spray breakup, coalescence, evaporation, and combustion are now being performed in a single periodic sector (1/18th) of an actual Pratt & Whitney combustor geometry.

Combustion instability due to the nonlinear interaction between sound and flame

Abstract: It has been observed in experiments that significant levels of sound may be produced when a curved flame propagates downwards along a tube in a gravity field In this paper, we present a mathematical description of this acoustic amplification process, which represents a simple form of combustion instability First, based on the large-activation-energy and small-Mach-number assumptions, a general asymptotic formulation is derived, in which the nature of flame-sound coupling is brought out explicitly This framework is then employed to study the weakly nonlinear coupling between a Darrieus-Landau (D-L) instability mode of the flame and an acoustic mode of the tube, which is the main mechanism for sound generation in the experiments In order to provide a somewhat unified description, the linear coupling via the direct pressure effect has also been included in our analysis A set of coupled equations which govern the evolution of the acoustic and D-L modes was derived The solutions show that the nonlinear coupling leads to very rapid amplification of sound After reaching an appreciable level, the sound inhibits the flame, causing the latter to flatten The sound then saturates at an almost constant level, or continues to grow at a smaller rate owing to the pressure effect The above theoretical predictions are in good qualitative agreement with experiments The present study also considered the influence of weak vortical disturbances in the oncoming flow It is shown that certain components in these perturbations may form resonant triads with the acoustic and D-L modes, thereby providing an additional coupling mechanism

Towards Multi-Component Analysis of Gas Turbines by CFD: Integration of RANS and LES Flow Solvers

Abstract: The numerical prediction of the entire aero-thermal o w through an entire gas turbine is currently limited by its high computational costs. The approach presented here intends to use several specialized o w solvers based on the Reynolds-averaged Navier-Stokes equations (RANS) as well as Large Eddy Simulations (LES) running simultaneously and exchanging information at the interfaces. This study documents the development of the interface and proves its accuracy and efcienc y on simple testcases. Compressor computation currently underway

Study of Rotor Tip-Clearance Flow Using Large-Eddy Simulation

Abstract: A large-eddy simulation has been performed to study the temporal and spatial dynamics of a rotor tip-clearance flow, with the objective of determining the underlying mechanisms for low pressure fluctuations downstream of the tip-gap. Simulation results are compared with experimental measurements, and favorable agreements are observed in both qualitative and quantitative sense. Typical vortical structures such as the tip-leakage vortex and tip-separation vortices are revealed, and their evolution is shown to be strongly influenced by the moving endwall and the blade wake. These vortical structures are the main sources of turbulence energy and Reynolds stresses as well as low-pressure fluctuations. Cavitation-inception analysis shows a high correlation between cavitation and the tip-leakage vortex.

A Computational Methodology for Large-Eddy Simulation of Tip-Clearance Flows

Abstract: A large-eddy simulation (LES) solver which 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. The overall objective of these simulations is to determine the underlying mechanisms for low-pressure fluctuations downstream of the rotor near the endwall. Salient features of the numerical methodology, including the mesh topology, the immersed boundary method, the treatment of numerical instability for non-dissipative schemes on highly skewed meshes, and the parallelization of the code for shared memory 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 endwall boundary layer. Simulation results are compared with experiments and qualitative as well as quantitative agreement is observed.Copyright © 2003 by ASME

Integration of rans and les flow solvers for simultaneous flow computations

Abstract: Numerical investigations of highly complex flow systems, such as the aero-thermal flow through an entire aircraft gas turbine engine, require the application of multiple specialized flow solvers in order to compute the flow efficiently and accurately. The flow solvers have to run simultaneously in order to capture unsteady multi-component effects. The present study reports the coupling of two flow solvers, one based on a RANS approach, the other on a LES approach. Procedures to set up an efficient data exchange are described. The processing of the flow data to meaningful boundary conditions is discussed. As a validation case for integrated RANS/LES computations, a swirl flow at an expansion with a subsequent contraction is computed and compared to LES results of the entire domain.

A New Paradigm for Simulation of Turbulent Combustion in Realistic Gas Turbine Combustors Using LES

Abstract: This paper presents a new paradigm for numerical simulation of turbulent combustion in realistic gas turbine combustors Advanced CFD methods using Large Eddy Simulation (LES) turbulence models are central to this paradigm in fluid dynamics where engineers can apply the full predictive abilities of numerical simulations to the design of realistic gas turbine combustors The use of LES models is motivated by their demonstrated superiority over RANS to predict turbulent mixing The subgrid scale models incorporated in LES are based on the dynamic approach where the model coefficients are computed rather than prescribed by the user This has provided unparalleled robustness to modern turbulent flow computations using LES A new numerical algorithm was derived that is discretely energy conserving on hybrid unstructured grids, thus allowing numerical simulations at high Reynolds numbers corresponding to operating conditions without using artificial numerical dissipation This paper deals specifically with the simulation of the gas phase flow through realistic gas turbine combustors and the implementation of combustion and spray models that are needed to predict and control the combustion phenomena in these geometries Results from several simulations and comparison with experimental data are used to validate this approach In particular, a complete simulation of the unsteady flow field in a realistic combustor geometry is carried out Some preliminary results for reacting flow simulations in gas turbine combustors are also discussed We discuss several challenges related to large-scale simulations of the flow in realistic combustors, including methods to further accelerate the algorithm’s convergence (eg, use of multigrid techniques), improvement of the parallel performance of the flow solver for two-phase flow simulations (eg, use of dynamic load balancing that accounts for the additional CPU time spent in the spray module when particles are present in the cells)Copyright © 2003 by ASME

Large-Eddy Simulation of Atomizing Spray With Stochastic Modeling of Secondary Breakup

Abstract: Large-eddy simulation (LES) of reacting multi-phase flows in practical combustor geometries is essential to accurately predict complex physical phenomena of turbulent mixing and combustion dynamics This necessitates use of Lagrangian particle-tracking methodology for liquid phase in order to correctly capture the droplet evaporation rates in the sparse-liquid regime away from the fuel injector Our goal in the present work is to develop a spray-atomization methodology which can be used in conjuction with the standard particle-tracking schemes and predict the droplet-size distribution accurately The intricate process of primary atomization and lack of detailed experimental observations close to the injector requires us to model its global effects and focus on secondary breakup to capture the evolution of droplet sizes Accordingly, a stochastic model for LES of atomizing spray is developed Following Kolmogorov’s idea of viewing solid particle-breakup as a discrete random process, atomization of liquid blobs at high relative liquid-to-gas velocity is considered in the framework of uncorrelated breakup events, independent of the initial droplet size Kolmogorov’s discrete model of breakup is represented by Fokker-Planck equation for the temporal and spatial evolution of droplet radius distribution The parameters of the model are obtained dynamically by relating them to the local Weber number A novel hybrid-approach involving tracking of individual droplets and a group of like-droplets known as parcels is developed to reduce the computational cost and maintain the essential features and dynamics of spray evolution The present approach is shown to capture the complex fragmentary process of liquid atomization in idealized and realistic Diesel and gas-turbine combustorsCopyright © 2003 by ASME

Large-eddy simulation and analysis of tip-clearance flows in turbomachinery applications

Abstract: The tip-leakage flow in axial turbomachines is studied using large-eddy simulation with an emphasis on understanding the underlying mechanisms for low-pressure fluctuations and cavitations downstream of the tip-gap. Simulation results are validated against experimental measurements, and reasonable agreements are obtained. Dominant vortical structures such as the tip-leakage vortex and tip-separation vortices are examined, and their effects on the turbulent flow characteristics as well as on the low-pressure statistics are investigated. Analysis of the velocity and pressure fields suggests a high correlation between cavitations inception and the tip-leakage vortex. To suppress the tip-leakage vortex and the associated low-pressure events, the use of a grooved casing wall is explored, and preliminary simulation results show good promise. Additional simulations to investigate the effects of inflow vortices and tip-gap size are also discussed.

Brownian Dynamics Simulation in a Turbulent Channel Flow

Abstract: The dynamics of different bead-spring models is investigated in a turbulent channel flow. In particular, the FENE, the FENE-P and the FENE multichain models are compared. In the case of the FENE-P model, both the Brownian Dynamics and the constitutive equations are used. It is shown that the different models produce qualitatively similar results for the mean extension and the mean stresses. This qualitative behaviour is also reproduced for different extensibility parameters. It is also found that the action of polymers is confined in the near wall region where the polymers are mainly oriented in the streamwise direction.Copyright © 2003 by ASME

Numerical Simulation of High Drag Reduction Regime in Polymer Solutions (Keynote)

Abstract: The simulation of drag reduced channel flows has to rely on consitituve models such as FENE-P. Their implementation is not straightforward to achieve convergence and stability of the solution. This paper discusses the problem of advection embedded in the FENE-P equation and the issue of the domain size. Finally we present results of simulations for High Drag Reduction regime, and show the subsequent modification of the vortical structures.Copyright © 2003 by ASME

Simulating Turbulent Flows in Complex Geometries

Abstract: We discuss development of a numerical algorithm, and solver capable of performing large-eddy simulation (LES) in geometries as complex as the combustor of a gas-turbine engine. The algorithm is developed for unstructured grids, is non-dissipative, yet robust at high Reynolds numbers on highly skewed grids. Results from validation in simple geometries is shown along with simulation results in the exceedingly complex geometry of a Pratt & Whitney gas turbine combustor.© 2003 ASME