Application Of Large Eddy Simulation Research Articles

Mind the gap: a guideline for large eddy simulation

This paper briefly reviews some of the fundamental ideas of turbulence as they relate to large eddy simulation (LES). Of special interest is how our thinking about the so-called 'spectral gap' has evolved over the past decade, and what this evolution implies for LES applications.

High-fidelity simulations for clean and efficient combustion of alternative fuels

There is an urgent and growing demand for high-fidelity simulations that capture complex turbulence-chemistry interactions in propulsion and power systems, and in particular, that capture and discriminate the effects of fuel variability. This project addresses this demand using the Large Eddy Simulation (LES) technique (led by Oefelein) and the Direct Numerical Simulation (DNS) technique (led by Chen). In particular, we are conducting research under the INCITE program that is tightly coupled with funded projects established under the DOE Basic Energy Sciences and Energy Efficiency and Renewable Energy programs that will provide the foundational science required to develop a predictive modeling capability for design of advanced engines for transportation. Application of LES provides the formal ability to treat the full range of multidimensional time and length scales that exist in turbulent reacting flows in a computationally feasible manner and thus provides a way to simulate reacting flow phenomena in complex internal-combustion engine geometries at device relevant conditions. Application of DNS provides a way to study fundamental issues related to small-scale combustion processes in canonical configurations to understand dynamics that occur over a range of reactive-diffusive scales. Here we describe the challenges and present representative examples of the types of simulations each respective tool has been used for as part of the INCITE program. We focus on recent experiences on the Oak Ridge National Laboratory (ORNL) National Center for Computational Sciences (NCCS) Cray-XT Platform (i.e., Jaguar).

One‐Way Nested Large‐Eddy Simulation over the Askervein Hill

Large‐eddy simulation (LES) models have been used extensively to study atmospheric boundary layer turbulence over flat surfaces; however, LES applications over topography are less common. We evaluate the ability of an existing model – COAMPS®‐LES – to simulate flow over terrain using data from the Askervein Hill Project. A new approach is suggested for the treatment of the lateral boundaries using one‐way grid nesting. LES wind profile and speed‐up are compared with observations at various locations around the hill. The COAMPS‐LES model performs generally well. This case could serve as a useful benchmark for evaluating LES models for applications over topography.

Subgrid-scale contributions to Lagrangian time correlations in isotropic turbulence

The application of large-eddy simulation (LES) to particle-laden turbulence raises such a fundamental question as whether the LES with a subgrid scale (SGS) model can correctly predict Lagrangian time correlations (LTCs). Most of the currently existing SGS models are constructed based on the energy budget equations. Therefore, they are able to correctly predict energy spectra, but they may not ensure the correct prediction on the LTCs. Previous researches investigated the effect of the SGS modeling on the Eulerian time correlations. This paper is devoted to study the LTCs in LES. A direct numerical simulation (DNS) and the LES with a spectral eddy viscosity model are performed for isotropic turbulence and the LTCs are calculated using the passive vector method. Both a priori and a posteriori tests are carried out. It is observed that the subgrid-scale contributions to the LTCs cannot be simply ignored and the LES overpredicts the LTCs than the DNS. It is concluded from the straining hypothesis that an accurate prediction of enstrophy spectra is most critical to the prediction of the LTCs.

Evaluation of numerical strategies for large eddy simulation of particulate two-phase recirculating flows

Predicting particle dispersion in recirculating two-phase flows is a key issue for reacting flows and a potential application of large eddy simulation (LES) methods. In this study, Euler/Euler and Euler/Lagrange LES approaches are compared in the bluff body configuration from Borée et al. [J. Borée, T. Ishima, I. Flour, The effect of mass loading and inter-particle collisions on the development of the polydispersed two-phase flow downstream of a confined bluff body, J. Fluid Mech. 443 (2001) 129–165] where glass beads are injected into a complex recirculating flow. These tests are performed for non-reacting, non-evaporating sprays but are mandatory validations before computing realistic combustion chambers. Two different codes (one explicit and compressible and the other implicit and incompressible) are also tested on the same configuration. Results show that the gas flow is well predicted by both codes. The dispersed phase is also well predicted by both codes but the Lagrangian approach predicts root-mean-square values more accurately than the Eulerian approach. The effects of mesh, solvers and numerical schemes are discussed for each method.

Optimal artificial neural networks and tabulation methods for chemistry representation in LES of a bluff-body swirl-stabilized flame

Large-eddy simulations (LES) of the Sydney bluff-body swirl-stabilized methane–hydrogen flame are performed, employing two chemistry representation methods, namely a conventional structured tabulation technique and artificial neural networks (ANNs). A generalized method for the generation of optimal artificial networks (OANNs) has been proposed by Ihme et al. [M. Ihme, A.L. Marsden, H. Pitsch, Neural Comput. 20 (2) (2008) 573–601]. This method is, for the first time, applied in LES of turbulent reactive flows, guaranteeing an optimal chemistry representation with error control, which was previously not possible. The network performance with respect to accuracy, data retrieval time, and storage requirements is compared with the structured tabulation of increasing resolution, and effects of long-time error accumulation on the statistical results during a numerical simulation are discussed. Using the optimization algorithm, it is demonstrated that ANN accuracies can be achieved which are comparable with structured tables of moderate to fine resolution. Furthermore, it is shown that for a comparable number of synaptic weights, the network fitness increases with increasing number of hidden layers. Compared to the tabulation technique, data retrieval from the network is computationally more expensive; however, the additional overhead associated with the ANN evaluation remains acceptable in LES applications. Results for flow field statistics and scalar quantities which are obtained from LES are in good agreement with experimental data, and possible reasons for the differences between computed and measured temperature profiles near the bluff-body are discussed. The difference in the velocity statistics between simulations employing structured table and network representation are small, and deviations in the CO2 profiles on the fuel-rich side of the flame are mainly attributed to the sensitivity of CO2 with respect to changes in progress variable.

Large eddy simulation of city micro-atmospheric environment

Air quality is one of the important conditions for a better residence life in the populated urban area and it is closed related to the micro-atmospheric environment. Atmospheric environment is controlled by air motion with multi-scales in the city, while air flows in the residence area are of micro-scale atmospheric motion. This paper introduces a modern numerical simulation method, i.e. large eddy simulation (LES), for studying micro-atmospheric flows in the city residence area. For the complex flow features in the residence area, the proper application of LES is studied and various numerical methods are compared in order to investigate their effects on the prediction accuracy of micro-atmospheric flows, for instance, roughness elements and immersed boundary method for complex terrain, different subgrid models and so on. The wind field (including turbulence properties) and contaminant dispersion are computed by the proposed method for a model and a realistic residence area, and the numerical results are in good agreement with the experimental measurements.

High-fidelity simulations for clean and efficient combustion of alternative fuels

The monolithic nature of transportation technologies offers opportunities for significant improvements in efficiency of 25-50% through strategic technical investments in both advanced fuels and new low-temperature engine concepts. The application of direct numerical simulation (DNS) provides a way to study fundamental issues related to small-scale combustion processes in well-defined canonical configurations, whereas the application of large eddy simulation (LES) provides a formal treatment of the full range of time and length scales that exist in turbulent reacting flows, and thus provides a direct link to experimental studies of relevant combustion devices. In the present study, through DOE INCITE and Oak Ridge National Laboratory 250 Tflop Transition-to-Operations grants in 2008, DNS is performed to understand how a lifted autoignitive flame is stabilized, and LES is performed to understand the high-pressure injection and mixing processes in internal combustion engines. Understanding of these and other fundamental issues is needed to develop robust and reliable ignition and combustion models for the combustion regimes observed under low-temperature combustion engine environments using alternative fuels.

Prediction of extinction and reignition in nonpremixed turbulent flames using a flamelet/progress variable model: 2. Application in LES of Sandia flames D and E

An extension of the flamelet/progress variable (FPV) model for the prediction of extinction and reignition is applied in large-eddy simulation (LES) of flames D and E of the Sandia piloted turbulent jet flame series. This model employs a presumed probability density function (PDF), in which the marginal PDF of a reactive scalar is modeled by a statistically most likely distribution. This provides two advantages. First of all, the shape of the distribution depends on chemical and mixing time-scale information, and second, an arbitrary number of moments can be enforced. This model was analyzed in an a priori study in the first part of this work. In the present LES application, the first two moments of mixture fraction and reaction progress variable are used to constrain the shape of the presumed PDF. Transport equations for these quantities are solved, and models for the residual scalar dissipation rates, which appear as unclosed terms in the equations for the scalar variances, are provided. Statistical flow field quantities for axial velocity, mixture fraction, and temperature, obtained from the extended FPV model, are in good agreement with experimental data. Mixture-fraction-conditioned data, conditional PDFs, and burning indices are computed and compared with the delta-function flamelet closure model, which employs a Dirac distribution as a model for the marginal PDF of the reaction progress parameter. The latter model considerably underpredicts the amount of local extinction, which shows that the consideration of second-moment information in the presumed PDF of the reaction progress parameter is important for the accurate prediction of extinction and reignition. Mixture-fraction-conditioned results obtained from the extended FPV model are in good agreement with experimental data; however, the overprediction of the consumption of fuel and oxidizer on the fuel-rich side results in an overprediction of minor species. The predictions for the conditional PDFs and burning indices are in good agreement with measurements.

Preconditioning method and engineering application of large eddy simulation

On applying large eddy simulation (LES) to engineering interest, one of the keys is to obtain two-order low-dissipation shock-capturing schemes adapted to LES. Therefore, the preconditioning method for all speed flows is adopted. However, the accuracy of preconditioned schemes is not satisfactory because of the poor performance of stability of preconditioning techniques especially in viscous flows. The reason of unstability is attributed to the unstability structure in the preconditioned eigenvalue matrix. Based on Roe scheme and two assumptions for low-Mach-Number flows, the new scheme named Low-Speed-Roe scheme is deduced for removing the unstability structure. Numerical experiments show that this scheme has the reasonable computational stability. For general-precision problems, Low-Speed-Roe scheme has similar behavior as the classical preconditioned Roe scheme. For simulations of high-accuracy requirement such as LES, Low-Speed-Roe scheme can obtain better results of complex flows, such as the laminar separation bubble on the suction surface of the high-loaded turbine blade T106.

Computation of Indirect Combustion Noise by a CAA Method

Indirect combustion noise, which is generated by the acceleration of hot gases from a turbulent swirling flame, has become an important issue to understand turbo-machinery core noise. This indirect combustion noise it is dominated by a strong interaction of hydrodynamic and acoustic perturbations due to the transonic base flow in the turbine stages and combustion chamber exit nozzle. Current numerical simulation methods for the phenomenon based on compressible Large Eddy Simulation (LES) are very expansive. Thus LES applications are still limited to one design or few variations with immense computational costs. The most simple model fully covering indirect combustion noise, are the linearized non-isentropic Euler equations over an arbitrary mean flow. Therefore the gap between theory and LES is closed by a method adopted from Computational Aeroacoustics (CAA). It combines a high cost efficiency in terms of required points per wavelength with a high adaptability to realistic mean flow conditions. The CAA method is first validated with a simplified experiment and then applied to predict the acoustic properties of two model combustion chambers, one of them with transonic termination nozzle. By means of the acoustic intensity the sources of the noise are located.

Large Eddy Simulations of Swirling Non-premixed Flames With Flamelet Models: A Comparison of Numerical Methods

This work investigates the application of large eddy simulation (LES) to selected cases of the turbulent non-premixed Sydney swirl flames. Two research groups (Loughborough University, LU and Imperial College, IC) have simulated these cases for different parameter sets, using two different and independent LES methods. The simulations of the non-reactive turbulent flow predicted the experimental results with good agreement and both simulations captured the recirculation structures and the vortex breakdown without major difficulties. For the reactive cases, the LES predictions were less satisfactory, and using two independent simulations has helped to understand the shortcomings of each. Furthermore one of the flames (SMH2) was found to be exceptionally hard to predict, which was supported by the lower amount of turbulent kinetic energy that was resolved in this case. However, the LES has identified modes of flame instability that were similar to those observed in some of the experiments.

Special issue on large-eddy simulation of complex flows

Large-eddy simulation (LES) often appears to be the perpetual aspirant to replace Reynolds-averaged simulations (RANS) in many industrial applications. While LES certainly still requires a strong research activity to improve existing subgrid-scale modeling approaches and develop new approaches, currently available models have achieved a level of maturity that invites exploration of their limitations for the reliable prediction of complex flows. Flows can be complex due to the geometric configuration of the flow boundaries, extra physical phenomena present besides hydrodynamic turbulence or the complexity of the fluid properties. In this Special Issue of Theoretical and Computational Fluid Dynamics, a cross section of the current status of LES for complex flows of the first and second kind is provided. This cross section comprises 9 papers out of 42 presented at the Euromech Colloquium 469 “LES of Complex Flows”, which was held in October of 2005, in Dresden, Germany [jointly supported by the German (DFG) and the French (CNRS) research councils]. These papers include the application of LES in complex geometric configurations, the coupling of LES with RANS, LES in compressible flows; LES wall modeling, the application of LES to combustion problems and the development of improved numerical methods. Each has undergone the normal review process to ensure conformance to the high scientific standards of the Journal.

Towards practical use of LES in wind engineering

This paper reviews large-scale computing results currently obtained by the large eddy simulation (LES) technique for various wind engineering problems. The LES or direct numerical simulation (DNS) techniques should be used to numerically simulate unsteady flow phenomena. LES is appropriate for wind engineering applications because its computational power and memory requirements are reasonable. The present study discusses the applicability of LES to several issues such as wind-resistant design, prediction of wind velocity affected by terrain or ground surface conditions, and estimation of turbulence structures and atmospheric diffusion in urban areas. Thus far, numerical validation has been performed mainly by comparison with experimental data. However, when considering the complexity of actual conditions and the natural wind environment, direct comparison with full-scale measurement data is very important in all cases to verify the effectiveness of LES in wind engineering.

Numerical Calculation of Particle-Laden Cyclone Separator Flow Using Les

Numerical flow calculations were carried out at various axial positions of a gas cyclone separator for industrial applications. Due to the nature of cyclone flows, which exhibit highly curved streamlines and anisotropic turbulence, we used the advanced turbulence model of Large Eddy Simulation (LES). The application of LES reveals better agreement with the experimental data, however, it requires higher computer capacity and longer running times when compared to standard turbulence models. These calculations of the continuous phase flow were the basis for modeling the behavior of the solid particles in the cyclone. Particle trajectories, pressure drop and the cyclone separation efficiency have been studied in some details. The paper is organized into five sections. The first section consists of an introduction and a summary of previous work. Section 2 deals with the LES turbulence calculations of the continuous phase flow. The third section treats modeling of the dispersed phase behavior. In section 4, computational issues are presented and discussed as applied grids, boundary conditions and the solution algorithm. In section 5, prediction profiles of the gas flow at axial positions are presented and discussed in some details. Moreover, pressure drop, particle trajectories and cyclone efficiency are discussed. Section 6 summarizes and concludes the paper.

ANALYSIS OF TURBULENT FLOW OVER ROUGHNESS BY LES AND PIV WITH WATER SURFACE FLUCTUATION

In the applications of the Large eddy simulation (LES) to open-channel flows with roughness, water surface is usually treated as a fixed boundary by neglecting the effects of surface fluctuation generated by boil vortices or relatively large surface variations caused by the drag force at each roughness element. In the present study, in order to expand the applicability of LES to such general conditions, the bottom topography is treated by introducing the immersed boundary method and the free surface variation is simulated by introducing a density function. It was made clear that the developed model is capable to represent relatively large surface variations in the strip-roughness flows with various roughness spacings. The numerical results agreed fairly well with those obtained by PIV.

Effects of subgrid-scale modeling on Lagrangian statistics in large-eddy simulation

The application of large-eddy simulation (LES) to turbulent transport processes requires accurate prediction of the Lagrangian statistics of flow fields. However, in most existing SGS models, no explicit consideration is given to Lagrangian statistics. In this paper, we focus on the effects of SGS modeling on Lagrangian statistics in LES ranging from statistics determining single-particle dispersion to those of pair dispersion and multiparticle dispersion. Lagrangian statistics in homogeneous isotropic turbulence are extracted from direct numerical simulation (DNS) and the LES with a spectral eddy-viscosity model. For the case of longtime single-particle dispersion, it is shown that, compared to DNS, LES overpredicts the time scale of the Lagrangian velocity correlation but underpredicts the Lagrangian velocity fluctuation. These two effects tend to cancel one another leading to an accurate prediction of the longtime turbulent dispersion coefficient. Unlike the single-particle dispersion, LES tends to underestimate significantly the rate of relative dispersion of particle pairs and multiple-particles, when initial separation distances are less than the minimum resolved scale due to the lack of subgrid fluctuations. The overprediction of LES on the time scale of the Lagrangian velocity correlation is further confirmed by a theoretical analysis using a turbulence closure theory.

Large eddy simulation of wind field and plume dispersion in building array

This paper presents numerical simulation of wind field and contaminant dispersion in the flow over a group of buildings by large eddy simulation (LES) with higher accuracy finite volume method (FVM) for numerical discretization of governing equation and with immersed boundary method (IBM) at building surfaces. The paper is aimed at proposing a numerical method which can deal with the complex turbulent flow and contaminant dispersion inside a group of buildings. The geometry of the building layout and flow parameters are taken from the wind tunnel experiments by Davidson et al. [1996. Wind tunnel simulations of plume dispersion through groups of obstacles. Atmospheric Environment 30 (22), 3715–3731] so that the feasibility and reliability of the numerical method and code can be examined by comparison between numerical results and experimental measurements with confidence. The flow patterns and contaminant dispersion inside the group of buildings are shown in agreement with the description of Davidson et al. [1996. Wind tunnel simulations of plume dispersion through groups of obstacles. Atmospheric Environment 30 (22), 3715–3731]. The numerical prediction of the statistical properties of contaminant dispersion, e.g. the mean concentration distributions, lateral and vertical spreads of the contaminant an so forth, is in good agreement with wind tunnel experiment measurements. The numerical study of this testing case reveals a number of issues which should be considered in application of LES to such complex turbulent flows in environment engineering, e.g. spatial resolution, reasonable subgrid stress model and the turbulent intensity in atmospheric boundary layer.

Usability of explicit filtering in large eddy simulation with a low‐order numerical scheme and different subgrid‐scale models

AbstractBased on a priori tests, in large eddy simulation (LES) of turbulent fluid flow, the numerical error related to low‐order finite‐difference‐type methods can be large in comparison with the effect of subgrid‐scale (SGS) model. Explicit filtering has been suggested to reduce the error, and it has shown promising results in a priori studies and in some simulations with fourth‐order method. In this paper, the effect of explicit filtering on the total simulation error is studied together with a second‐order scheme, where the numerical error should be even larger. The fully developed turbulent channel flow between two parallel walls is used as a test case. Rather simple SGS models are applied, because these models are most likely used in practical applications of LES. Explicit filtering is here applied to the non‐linear convection term of the Navier–Stokes equations, four three‐dimensional filter functions are applied, and the effect of filtering is separated from the effect of SGS modelling. It is shown that the effect of filtering is rather large and smooth filters introduce an additional error component that increases the total simulation error. Finally, filtering via subfilter‐scale modelling is applied, and it is shown that this approach performs better. However, the large‐frequency components of the resolved flow field are not as effectively damped as when the non‐linear convection term is filtered. Copyright © 2007 John Wiley & Sons, Ltd.

Numerical simulation of phase separation and a priori two-phase LES filtering

This work reports on the potential application of Large Eddy Simulation (LES) in the calculation of turbulent isothermal two-phase flows, in the case where the large scales of each phase are resolved and small interface structures can be smaller than the mesh size. In comparison with single phase flows, application of LES to two-phase flow problems should account for the complex interaction between the interface and the turbulent motion. The complete filtered two-phase flow equations are formulated to deal with turbulence at the interface. Explicit filtering of 3D direct numerical simulations of a phase separation problem has been employed to evaluate the order of magnitude of the specific subgrid contributions. Analyses of the numerical results have been conducted to derive conclusions on the relative importance of the different subgrid scale contributions. Modeling issues and turbulent energy transfer across the interface are discussed.