Development Of Large-eddy Simulation Research Articles

A Graphics Processing Unit (GPU) Approach to Large Eddy Simulation (LES) for Transport and Contaminant Dispersion

Recent advances in the development of large eddy simulation (LES) atmospheric models with corresponding atmospheric transport and dispersion (AT&D) modeling capabilities have made it possible to simulate short, time-averaged, single realizations of pollutant dispersion at the spatial and temporal resolution necessary for common atmospheric dispersion needs, such as designing air sampling networks, assessing pollutant sensor system performance, and characterizing the impact of airborne materials on human health. The high computational burden required to form an ensemble of single-realization dispersion solutions using an LES and coupled AT&D model has, until recently, limited its use to a few proof-of-concept studies. An example of an LES model that can meet the temporal and spatial resolution and computational requirements of these applications is the joint outdoor-indoor urban large eddy simulation (JOULES). A key enabling element within JOULES is the computationally efficient graphics processing unit (GPU)-based LES, which is on the order of 150 times faster than if the LES contaminant dispersion simulations were executed on a central processing unit (CPU) computing platform. JOULES is capable of resolving the turbulence components at a suitable scale for both open terrain and urban landscapes, e.g., owing to varying environmental conditions and a diverse building topology. In this paper, we describe the JOULES modeling system, prior efforts to validate the accuracy of its meteorological simulations, and current results from an evaluation that uses ensembles of dispersion solutions for unstable, neutral, and stable static stability conditions in an open terrain environment.

Assessment of Turbulence Models on Bridge-Pier Scour Using Flow-3D

The main purpose of this paper is to analyze the influence of different turbulence flow models on scouring pit of bridge-pier. Flow-3D software is applied in line with the purpose. The key motivation for this study is to contribute to the Flow-3D software by means of some modification and adjustment in the sediment scour model and shallow water model. An assessment of turbulence model adopted with the parameters of the Melville experiment to estimate the maximum scour-depth was performed. In the simulation results, the alternate eddy formation and shedding were repeated while the Karman vortex street formed behind the pier for the large eddy simulation LES turbulence model is more realistic in the flow phenomenon. The results of the scour development of large eddy simulation (LES) turbulence model were found to be more satisfied than the Renormalized group (RNG) turbulence model and close to the prior experiment results. The simulated scour results were significantly different with the observed data collected from previous literature in the reason of some unsuitability of meshing method in Flow-3D software.

Editorial: IFP Energies nouvelles International Conference LES4ICE 2014 – Large-Eddy Simulation for Internal Combustion Engine Flows

Further improving the environmental performances of Internal Combustion Engines (ICE) increasingly requires moving beyond traditional multidimensional simulation tools based on a cycle averaged approach (Reynolds-Averaged Navier–Stokes, RANS), and to reliably predict and control individual engine cycles under realistic operating conditions. Large-Eddy Simulation (LES) offers this unique potential by predicting spatially filtered flow realizations, thus opening up new perspectives for extending the scope of application of Computational Fluid Dynamics (CFD) for ICE. Since its 1 edition in 2008, the LES4ICE conference provides a forum for exchange concerning research and development of LES and related experimental techniques for their application to ICE flows. It brings together researchers and engineers working in the field of piston engine combustion to debate the state of the art in LES applied to ICE and examine advanced experimental techniques capable of supporting and validating its development. The present special issue proposes an extract from the contributions to the 2014 edition of LES4ICE, following a selection by the conference’s Scientific Committee. The selected four articles provide a view on some recent advances in the domain of LES and related experimental techniques, and on their application to ICE flows. The study of Cyclic Combustion Variability (CCV) in Spark-Ignition Engines (SIE) has become a prominent topic for LES research in the past decade. It has been demonstrated how combining advanced optical diagnostics and state-of-the-art LES can allow identifying the causes of CCV, and that the acquired understanding could efficiently be capitalized in the form of reduced models for 1D CFD simulations of complete engine systems. This on-going research opened exciting perspectives for the application of LES to other yet poorly understood and mastered non-cyclic phenomena in SIE, as in particular the link between CCV and engine knock. A longstanding issue when studying the causes of CCV in SI engines is the relative importance of the variability of large-scale coherent structures, and of small scale turbulent fluctuations. A method aiming at addressing this question was proposed by S. Buhl, F. Hartmann and C. Hasse, in their contribution “Identification of Large-Scale Structure Fluctuations in IC Engines using POD-Based Conditional Averaging” [1]. They propose a method combining POD and conditional averaging, and discuss its application to the analysis of the origins of CCV for two different LES datasets. Oil & Gas Science and Technology – Rev. IFP Energies nouvelles (2016) 71, E1 C. Angelberger and C. Mounaim-Rousselle, published by IFP Energies nouvelles, 2016 DOI: 10.2516/ogst/2015048 IFP Energies nouvelles International Conference Rencontres Scientifiques d'IFP Energies nouvelles

Investigation of the 3D flow field in an IC engine using tomographic PIV

In-cylinder measurements of the instantaneous 3D flow field are required for further understanding of turbulent mixing and combustion processes in internal combustion (IC) engines as well provide valuable data for the development of large eddy simulation (LES) models for combustion simulation. Tomographic particle image velocimetry (TPIV) is applied within a motored direct-injection spark-ignition (DISI) optical engine to measure the instantaneous volumetric flow field at selected crank-angle degrees (CAD) during the intake and compression stroke. Measurements were conducted using a double-pulse Nd:YAG laser with an average pulse energy of 375mJ for illumination and four CCD cameras arranged circularly around the optically accessible cylinder. A volume of 47×35×4mm3 was imaged located centrally between the intake valves. The uncertainty of the 3D velocity data was addressed using the principle of mass conservation for the inlet flow. A precision within 9% was found for the calculation of the velocity gradient tensor and is comparable to experiments in generic configurations, indicating that TPIV measurements within a motored IC engine are feasible. Based on the volumetric velocity data, the average structure of the flow field was analyzed showing a clear orientation of the average velocity, which changes during the different phases of the cycle. The 3D turbulent kinetic energy (TKE) is computed and compared to the 2D-TKE determined from in-plane velocity components and reveals distinct regions of large out-of-plane velocity fluctuations. Results from volumetric velocimetry furthermore provides insight into the occurrence and orientation of vortical structures that here are identified by using the Q-criterion. Differences between flow structures during intake and compression strokes are discussed on basis of ensemble-averaged and individual cycle velocity maps as well as the full vorticity vector.

Large eddy simulation modelling of combustion for propulsion applications

Predictive modelling of turbulent combustion is important for the development of air-breathing engines, internal combustion engines, furnaces and for power generation. Significant advances in modelling non-reactive turbulent flows are now possible with the development of large eddy simulation (LES), in which the large energetic scales of the flow are resolved on the grid while modelling the effects of the small scales. Here, we discuss the use of combustion LES in predictive modelling of propulsion applications such as gas turbine, ramjet and scramjet engines. The LES models used are described in some detail and are validated against laboratory data-of which results from two cases are presented. These validated LES models are then applied to an annular multi-burner gas turbine combustor and a simplified scramjet combustor, for which some additional experimental data are available. For these cases, good agreement with the available reference data is obtained, and the LES predictions are used to elucidate the flow physics in such devices to further enhance our knowledge of these propulsion systems. Particular attention is focused on the influence of the combustion chemistry, turbulence-chemistry interaction, self-ignition, flame holding burner-to-burner interactions and combustion oscillations.

Implicit LES applied to zero-pressure-gradient and adverse-pressure-gradient boundary-layer turbulence

Further development of large-eddy simulation (LES) faces as major obstacles the strong coupling between subgrid-scale (SGS) modeling and the truncation error of the numerical discretization. One can exploit this link by developing discretization methods where the truncation error itself functions as an implicit SGS model. The adaptive local deconvolution method (ALDM) is an approach to LES of turbulent flows that represents a full coupling of SGS model and discretization scheme. To provide evidence for the validity of this new SGS model, well resolved large-eddy simulations of a fully turbulent flat-plate boundary-layer flow subjected to a constant adverse pressure gradient are conducted. Flow parameters are adapted to an available experiment. The Reynolds number based on the local free-stream velocity and momentum thickness is 670 at the inflow and 5100 at the separation point. Clauser’s pressure-gradient parameter increases monotonically from 0 up to approximately 100 since a constant pressure gradient is prescribed. The adverse pressure gradient leads to a highly unsteady and massive separation of the boundary layer. The numerical predictions agree well with theory and experimental data.

Implicit subgrid-scale modeling for large-eddy simulation of passive-scalar mixing

Further development of large-eddy simulation (LES) faces as major obstacles the strong coupling between subgrid-scale (SGS) modeling and the truncation error of the numerical discretization. One can exploit this link by developing discretization methods where the truncation error itself functions as an implicit SGS model. The name “implicit LES” is used for approaches that merge the SGS model and numerical discretization. In this paper, the implicit SGS modeling environment provided by the adaptive local deconvolution method is extended to LES of passive-scalar mixing. The resulting adaptive advection algorithm is discussed with respect to its numerical and turbulence-theoretical background. We demonstrate that the new method allows for reliable predictions of the turbulent transport of passive scalars in isotropic turbulence and in turbulent channel flow for a wide range of Schmidt numbers.

Development of large eddy simulation for modeling of decaying compressible magnetohydrodynamic turbulence

The large eddy simulation technique is developed for the study of decaying compressible magnetohydrodynamic turbulence. In the present paper the obtained results of numerical computations for large eddy simulation are compared with the results of direct numerical simulation of three-dimensional compressible magnetohydrodynamic turbulence under various similarity parameters, namely, magnetic Reynolds numbers, hydrodynamic Reynolds numbers, and Mach numbers. The comparison of five subgrid-scale closures of large eddy simulation for the magnetohydrodynamic case is made. The comparison between large eddy simulation and direct numerical simulation is carried out regarding the time evolution of kinetic and magnetic energy, cross helicity, subgrid-scale and molecular dissipations for kinetic and magnetic energy, turbulent intensities and quantities that describe anisotropy of flow, that is, skewness and kurtosis of velocity and magnetic field. It is shown that some subgrid-scale models proposed in the paper provide sufficient dissipation of kinetic and magnetic energy, reduce computational efforts and produce adequate results of magnetohydrodynamic turbulent modeling for various values of similarity parameters of flows.

Numerical study of the subgrid fluid turbulence effects on the statistics of heavy colliding particles

The main purpose of this article is to investigate the effects of the subgrid fluid turbulence on the motion of nonsettling colliding particles suspended in steady homogeneous isotropic turbulent flow. An additional goal is to characterize the statistical properties of the subgrid fluid turbulence “viewed” by inertial particles to support the development of large eddy simulation (LES) approach for particle-laden turbulent flows. Two types of numerical experiments have been carried out: first, the discrete particle trajectories were computed using the fluid velocity field given by direct numerical simulation (DNS) in order to characterize the small-scale fluid velocity fluctuations “seen” by the particles. In a second stage, the particle trajectory simulations were performed using several filtered velocity fields computed from the DNS data to evaluate the effect of the subgrid fluid turbulence on the particle statistics (turbulent dispersion, kinetic energy, accumulation efficiency, particle-particle relative velocity and collision frequency). The first part of this study shows that particle inertia has a limited effect on the subgrid fluid turbulent statistics: subgrid kinetic energy and Lagrangian integral time scale measured along the particle trajectories. In addition, the subgrid fluid Lagrangian integral time scale is found to be nearly equal to the subgrid Eulerian integral time scale and proportional to the ratio of the filter width to the whole fluid turbulent velocity. The second part shows that the particle turbulent dispersion and kinetic energy are affected by the filtering only when a significant percentage of the turbulent kinetic energy was removed from the velocity field seen by the particles. In contrast, accumulation and collision phenomena are found to be significantly influenced by the subgrid fluid velocity fluctuations when the particle response time is of the same order or smaller than the subgrid Lagrangian integral time scale measured along particle paths. Finally, these results are used to characterize, in terms of particle response time to subgrid time scale ratio, the validity range of LES for the computation of gas-solid turbulent flow.

Optically Accessible Pressurized Research Combustor for Computational Fluid Dynamics Model Validation

The development of, and results from, the operation of an optically accessible industrial-scale research combustor, designed to provide comprehensive data sets to assist the development of large eddy simulation codes for turbulent combustion applications is discussed. The combustor can be operated at elevated pressure and inlet air temperature, with precisely determined thermal, acoustic, and flow boundary conditions. The effect of fuel composition is investigated by augmenting the natural gas baseline fuel with hydrogen using a lean-premixed combustor configuration. A detailed description of the research combustor and test facility is provided, and recent data on the effects of hydrogen content and combustor heat losses on lean extinction limits, dynamic stability, and pollutant emissions are included. Analysis of the results confirm that heat losses in the combustor play an important role in pollutant formation and that characterization of the thermal boundary conditions is crucial in providing explicit data sets for model validation.

Ten questions concerning the large-eddy simulation of turbulent flows

In the past 30 years, there has been considerable progress in the development of large-eddy simulation (LES) for turbulent flows, which has been greatly facilitated by the substantial increase in computer power. In this paper, we raise some fundamental questions concerning the conceptual foundations of LES and about the methodologies and protocols used in its application. The 10 questions addressed are stated at the end of the introduction. Several of these questions highlight the importance of recognizing the dependence of LES calculations on the artificial parameter Δ (i.e. the filter width or, more generally, the turbulence resolution length scale). The principle that LES predictions of turbulence statistics should depend minimally on Δ provides an alternative justification for the dynamic procedure.

A low-diffusion MUSCL scheme for LES on unstructured grids

We are interested in the development of large-eddy simulation (LES) methods for compressible flows in complex geometries. The starting point is a numerical scheme applicable to unstructured tetrahedrizations, conservative, upwind of MUSCL type and vertex centered. A low-diffusion version stabilized with sixth-order spatial derivatives is proposed. The obtained scheme is combined with two LES models, derived from the Smagorinsky model and its dynamic version. The basic test-case chosen is the flow around a square cylinder. Calculations around a forward-swept wing and a business jet are also presented.