Validation Of Large Eddy Simulation Research Articles

Large eddy simulation of flame extinction in a turbulent line fire exposed to air-nitrogen co-flow

The general objective of this project is to support the development and validation of large eddy simulation (LES) models used to simulate the response of fires to the activation of suppression systems. The focus here is on suppression by gaseous agents. The present experimental configuration is a two-dimensional, plane, buoyancy-driven, methane-fueled, turbulent diffusion flame with a controlled co-flow. The co-flow is an air-nitrogen mixture with variable oxygen dilution conditions, including conditions that lead to full flame extinction. Experimental measurements include the global combustion efficiency and global radiative loss fraction. The numerical simulations are performed with a LES-based fire model developed by FM Global and called FireFOAM. In this study, FireFOAM is modified to include a flame extinction model based on the concept of a critical flame Damköhler number and a flame reignition model based on the concept of a critical gas temperature. The numerical simulations are found to successfully reproduce the rapid change that is observed experimentally when exposing the flame to a co-flow with decreasing oxygen strength: the change corresponds to an abrupt transition from a strong flame with a global combustion efficiency close to one to a residual flame with a global combustion efficiency close to zero.

Profiles of second- to fourth-order moments of turbulent temperature fluctuations in the convective boundary layer: first measurements with rotational Raman lidar

Abstract. The rotational Raman lidar (RRL) of the University of Hohenheim (UHOH) measures atmospheric temperature profiles with high resolution (10 s, 109 m). The data contain low-noise errors even in daytime due to the use of strong UV laser light (355 nm, 10 W, 50 Hz) and a very efficient interference-filter-based polychromator. In this paper, the first profiling of the second- to fourth-order moments of turbulent temperature fluctuations is presented. Furthermore, skewness profiles and kurtosis profiles in the convective planetary boundary layer (CBL) including the interfacial layer (IL) are discussed. The results demonstrate that the UHOH RRL resolves the vertical structure of these moments. The data set which is used for this case study was collected in western Germany (50°53'50.56'' N, 6°27'50.39'' E; 110 m a.s.l.) on 24 April 2013 during the Intensive Observations Period (IOP) 6 of the HD(CP)2 (High-Definition Clouds and Precipitation for advancing Climate Prediction) Observational Prototype Experiment (HOPE). We used the data between 11:00 and 12:00 UTC corresponding to 1 h around local noon (the highest position of the Sun was at 11:33 UTC). First, we investigated profiles of the total noise error of the temperature measurements and compared them with estimates of the temperature measurement uncertainty due to shot noise derived with Poisson statistics. The comparison confirms that the major contribution to the total statistical uncertainty of the temperature measurements originates from shot noise. The total statistical uncertainty of a 20 min temperature measurement is lower than 0.1 K up to 1050 m a.g.l. (above ground level) at noontime; even for single 10 s temperature profiles, it is smaller than 1 K up to 1020 m a.g.l. Autocovariance and spectral analyses of the atmospheric temperature fluctuations confirm that a temporal resolution of 10 s was sufficient to resolve the turbulence down to the inertial subrange. This is also indicated by the integral scale of the temperature fluctuations which had a mean value of about 80 s in the CBL with a tendency to decrease to smaller values towards the CBL top. Analyses of profiles of the second-, third-, and fourth-order moments show that all moments had peak values in the IL around the mean top of the CBL which was located at 1230 m a.g.l. The maximum of the variance profile in the IL was 0.39 K2 with 0.07 and 0.11 K2 for the sampling error and noise error, respectively. The third-order moment (TOM) was not significantly different from zero in the CBL but showed a negative peak in the IL with a minimum of −0.93 K3 and values of 0.05 and 0.16 K3 for the sampling and noise errors, respectively. The fourth-order moment (FOM) and kurtosis values throughout the CBL were not significantly different to those of a Gaussian distribution. Both showed also maxima in the IL but these were not statistically significant taking the measurement uncertainties into account. We conclude that these measurements permit the validation of large eddy simulation results and the direct investigation of turbulence parameterizations with respect to temperature.

A general framework for verification and validation of large eddy simulations

A general framework (methodology and procedures) for verification and validation (V&V) of large eddy simulations in computational fluid dynamics (CFD) is derived based on two hypotheses. The framework allows for quantitative estimations of numerical error, modeling error, their coupling, and the associated uncertainties. To meet different needs of users based on their affordable computational cost, various large eddy simulation (LES) V&V methods are proposed. These methods range from the most sophisticated seven equation estimator to the simplest one-grid estimator, which will be calibrated using factors of safety to achieve the objective reliability and confidence level. Evaluation, calibration and validation of various LES V&V methods in this study will be performed using rigorous statistical analysis based on an extensive database. Identification of the error sources and magnitudes has the potential to improve existing or derive new LES models. Based on extensive parametric studies in the database, it is expected that guidelines for performing large eddy simulations that meet pre-specified quality and credibility criteria can be obtained. Extension of this framework to bubbly flow is also discussed.

有限体積法に基づくLarge Eddy Simulationのための対流項離散化スキームの検討

Validation of Large Eddy Simulation based on a Finite Volume Method was undertaken using data from our previous experiment. Four schemes for convection-term discretization were tested on a momentum transport equation, a passive scalar transport equation and an active scalar transport equation. The results support the use of a low-dissipation scheme for the momentum transport equation to avoid overestimating circulation on a rooftop. Such overestimation of circulation can lead to an underestimation of concentrations behind the building. Conversely, a bounded scheme, even in cases that resulted in excess numerical dissipation, is preferred for each scalar transport equation to maintain a profile and peak concentration value.

Mechanism of Particle Collection in a Cylindrical Cyclone Separator (1st Report, Validation of Large Eddy Simulation and Investigation on Detailed Flow Structures)

A cyclone separator separates particles by using centrifugal force that acts on the particles in a swirl flow and the separated particles finally fall to the bottom part of the separator by the gravity force. For a gas cyclone separator, the Reynolds number of the bulk flow is typically in the order of 104 to 105, and complicated vortical structures are generated. The particle behaviors in such vortical flows have not yet been fully understood. The final goal of the present study is to clarify the mechanism of particle separation in a cyclone separator and to improve the separation efficiency. To achieve this goal, the unsteady flow in cyclone separators is firstly computed by large-eddy simulation. The computed flow velocities will then be fed to the subsequent analysis of particle tracking. This first report describes results of validations for the large-eddy simulation, where we compared the computed velocity profiles with the experimental data. We also compared the precession frequency of the vortex rope with that reported in the literature. Regarding the vortical structures in the cyclone separator, we identified longitudinal vortices which are generated in the periphery of the large vortex rope. These vortices have one end attached to the inner wall and their strength is stronger than the longitudinal vortices in the boundary layer on the wall. The identified vortices are most likely to play an important role in the particle separation, which will be proven in the second report that describes particle motion in the cyclone separator.

On the experimental validation of combustion simulations in turbulent non-premixed jets

A Reynolds averaged Navier–Stokes (RANS) based combustion model, which incorporated the conditional source-term estimation (CSE) method for the closure of the chemical source term and the trajectory generated low-dimensional manifold (TGLDM) method for the reduction of detailed chemistry, was applied to predict the OH radical distribution in a combusting non-premixed methane jet. The results of the numerical prediction were compared with the results of a complementary experimental study in which the OH radical fields of combusting non-premixed methane jets were visualized using planar laser induced fluorescence (PLIF). It is well known within the modelling community that RANS based models are unable to capture the stochastic nature of turbulent combustion and autoignition, and are therefore unable to predict individual realizations of the flame. In this study, the agreement between the predicted OH field and a well-converged ensemble average of the experimental results was also shown to be poor. The lack of agreement between the numerical results and the ensemble averaged experimental results expose the potential significance of the known weakness in the RANS method. A statistical analysis of the experimental results was also performed. The results of the analysis showed that a minimum of 100 individual realizations was required to provide a well-converged average OH field for the combusting non-premixed jet under investigation. The significance of this result with respect to the validation of large-eddy simulations (LES) of combusting jets is discussed.

LES validation of turbulent rotating buoyancy- and surface tension-driven flow against DNS

The paper is concerned with the validation and error analysis of predictions for the flow and heat transfer in a silicon melt ( Pr = 0.013 ) found in a Czochralski (Cz) apparatus for crystal growth. This system resembles turbulent Rayleigh–Bénard–Marangoni convection. Since for practical applications predictions based on direct numerical simulations (DNS) require too many resources to conduct parametric studies or optimizations, nowadays in practice the method of choice is the large-eddy simulation (LES). The case considered consists of an idealized cylindrical crucible of 170 mm radius with a rotating crystal of 50 mm radius. Boundary conditions from experimental data were applied, which lead to the dimensionless numbers of Re = 4.7 × 10 4 , Gr = 2.2 × 10 9 , Ma = 2.8 × 10 4 , and Ra = 2.8 × 10 7 . The filtered Navier–Stokes equations were solved based on a finite-volume scheme for curvilinear block-structured grids and an explicit time discretization. For a comprehensive error analysis, different grid sizes, subgrid-scale models, and discretization schemes were employed. The results were compared to reference DNS data of the same case recently generated by the authors (Int J Heat Mass Transfer, 51 (2008) 6219–6234) for validation. For the finest LES grid ( 10 6 control volumes) using a standard Smagorinsky model with van Driest damping or a dynamic model, both with central discretization, the results agree well with the DNS reference while the computational effort could be reduced by a factor of 20. When using an upwind scheme even of formally second-order accuracy, significant deviations occur. Further stepwise reductions of the grid size decrease the CPU time drastically, but also lead to larger aberrations. When the grid is coarsened by a factor of 32 (resulting in ca. 130,000 CVs), even qualitative differences between the LES and the DNS solution appear. It could be shown in the present work that the LES method is an efficient tool to model the turbulent flow and heat transfer in Rayleigh–Bénard–Marangoni configurations. However, care should be taken in the choice of the grid resolution and discretization scheme for the nonlinear convective terms, as too coarse meshes in combination with upwind schemes lead to significant numerical errors. Finally, a quantified relation between the achievable accuracy and the necessary computational effort is presented.

Computational visualization of unsteady flow around vehicles using high performance computing

One of the largest-scale unstructured Large Eddy Simulation (LES) of flow around a full-scale road vehicle is conducted on the Earth Simulator in Japan. The main objective of our study is to look into the validity of LES for the assessment of vehicle aerodynamics, especially in the context of its possibility for unsteady or transient aerodynamic forces. Firstly, the aerodynamic LES proposed is quantitatively validated on the ASMO simplified model by comparing the mean pressure distributions on the vehicle surface with those obtained by a conventional Reynolds-Averaged Navier–Stokes simulation (RANS) or a wind tunnel measurement. Then, the method is applied to the full-scale vehicle with complicated geometry to qualitatively investigate the capability of capturing organized flow structures around the vehicle. Finally, unsteady aerodynamic forces acting on the vehicle in transient yawing angle change are estimated and relationship between the flow structures and the transient aerodynamic forces is mentioned. As a result, it is demonstrated that LES will be a powerful tool for the vehicle aerodynamic assessment in the foreseeable future, because it can provide precious aerodynamic data which conventional wind tunnel tests or RANS simulations are difficult to provide.

On the outer large-scale motions of wall turbulence and their interaction with near-wall structures using large eddy simulation

Large-scale structures typically observed at or above the logarithmic layer of fully developed turbulent channel flows were numerically studied. The potential validity of large eddy simulation (LES) to the large-scale analysis was focused on, and its applicability was investigated for the first time by carrying out an intensive grid resolution study to determine the minimum grid spacing necessary to properly capture such flows. It was found that rather fine grid spacing sufficient to resolve the near-wall streaky motions represented by λ x + × λ z + ∼ 1000 × 100 is required to reproduce the typical spectral features of large structures in the outer layer. Subsequently, the Reynolds-number scaling for such structures and their interaction with buffer-layer turbulence were examined. It was observed that the large structures in the outer layer remarkably appear only in the streamwise velocity fluctuation, basically obeying the outer scaling, and their spanwise size is approximately twice as large as the boundary layer thickness, independent of the Reynolds-number range tested here. It was also found that they penetrate deep into the buffer layer, where small streaky structures obey the inner scaling. These results clearly demonstrate that mixed inner–outer scaling instead of simple inner scaling for the streamwise velocity root mean square in the near-wall region is reasonable.

LES Validation from Experiments

This paper addresses issues relating to the validation of LES from experiments. It highlights the differences between large eddy simulations (LES) and Reynolds averaged Navier–Stokes simulations (RANS) and how LES puts higher demands on the validation process than RANS. Some popular validation experiments are outlined, which are part of the Workshop on Turbulent Non-Premixed Flames. Important features of validation experiments are discussed to help modellers with the selection of suitable experiments, and experimentalists with the design. An LES validation using the typical approach is then presented, together with some unusual, more innovative approaches that deviate from common RANS practice. The final section considers experimental errors, with examples of their effects, and how LES can contribute to their analysis and compensation. This paper does not aim to provide a complete review of the field, but rather to communicate some ideas that were presented at an invited Workshop presentation.

Idealized gas turbine combustor for performance research and validation of large eddy simulations

This paper details the design of a premixed, swirl-stabilized combustor that was designed and built for the express purpose of obtaining validation-quality data for the development of large eddy simulations (LES) of gas turbine combustors. The combustor features nonambiguous boundary conditions, a geometrically simple design that retains the essential fluid dynamics and thermochemical processes that occur in actual gas turbine combustors, and unrestrictive access for laser and optical diagnostic measurements. After discussing the design detail, a preliminary investigation of the performance and operating envelope of the combustor is presented. With the combustor operating on premixed methane/air, both the equivalence ratio and the inlet velocity were systematically varied and the flame structure was recorded via digital photography. Interesting lean flame blowout and resonance characteristics were observed. In addition, the combustor exhibited a large region of stable, acoustically clean combustion that is suitable for preliminary validation of LES models.

Numerical and Experimental Investigation of the Neutral Atmospheric Surface Layer

AbstractThis study combines the experimental measurements with large-eddy simulation (LES) data of a neutral planetary boundary layer (PBL) documented by a 60-m tower instrumented with eight sonic anemometers, and a high-resolution Doppler lidar during the 1999 Cooperative Atmospheric and Surface Exchange Study (CASES-99) experiment. The target of the paper is (i) to investigate the multiscale nature of the turbulent eddies in the surface layer (SL), (ii) to explain the existence of a −1 power law in the velocity fluctuation spectra, and (iii) to investigate the different nature of turbulence in the two sublayers within the SL, which are the eddy surface layer (ESL; lower sublayer of the SL lying between the surface and about 20-m height) and the shear surface layer (SSL; lying between the ESL top and the SL top). The sonic anemometers and Doppler lidar prove to be proper instruments for LES validation, and in particular, the Doppler lidar because of its ability to map near-surface eddies.This study shows the different nature of turbulence in the ESL and the SSL in terms of organized eddies, velocity fluctuation spectra, and second-order moment statistics. If quantitative agreement is found in the SSL between the LES and the measurements, only qualitative similarity is found in the ESL due to the subgrid-scale model, indicating that the LES captures part of the physics of the ESL. The LES helps explain the origin of the −1 power-law spectral subrange evidence in the velocity fluctuation spectra computed in the SL using the CASES-99 dataset: strong interaction between the mean flow and the fluctuating vorticities is found in the SL, and turbulent production is found to be larger than turbulent energy transfer for k1z > 1 (k1 being the longitudinal wavenumber and z the height), which is the condition of the −1 power-law existence.

Large-eddy simulation for urban micro-meteorology

Abstract Validation of large-eddy simulation (LES) computations of flows over an array of cubes with imposed periodic inflow-outflow condition is described. Then, a method of generation of appropriate inflow data for such flows is proposed and the results are compared with those using periodic boundary conditions in the streamwise direction and wind tunnel measurements [3]. Finally, some results obtained using the inflow method for an oscillatory through- flow combined with a steady current are discussed.

Experimental validation of large eddy simulations of flow and heat transfer in a stationary ribbed duct

Accurate prediction of ribbed duct flow and heat transfer is of importance to the gas turbine industry. The present study comprehensively validates the use of large eddy simulations (LES) for predicting flow and heat transfer with measured flowfield data in a stationary duct with 90° ribs and elucidates on the detailed physics encountered in the developing flow region, the fully developed region, and the 180° bend region. Among the major flow features predicted with accuracy are flow transition at the entrance of the duct; the distribution of mean and turbulent quantities in the developing, fully developed, and 180° bend; the development of secondary flows in the duct cross-section and the 180° bend; and friction and heat transfer augmentation. At the duct inlet, both the computations and experiments show that the peak turbulence intensities reach values as high as 40% in the streamwise and spanwise directions and 32% in the vertical direction, and a comparison of values along the centerline of the developing flow region shows that the mean flow and turbulent quantities do not become fully developed until they reach beyond the seventh rib of the duct. Turbulence intensities in the 180° bend are found to reach values as high as 50%, and local heat transfer comparisons show that the heat transfer augmentation shifts towards the outside wall downstream of the bend with little or no shift upstream. In addition to primary flow effects, secondary flow impingement on the smooth walls is found to develop by the third rib, while it continues to evolve downstream of the sixth rib. In all different aspects, it is found that LES produces the correct physics both qualitatively and quantitatively to within 10–15% of experiments.

Shallow cumulus convection: A validation of large‐eddy simulation against aircraft and Landsat observations

AbstractLarge‐eddy simulation (LES) results of shallow cumulus convection are directly evaluated against in‐cloud aircraft measurements, as made during the Small Cumulus Microphysics Study (SCMS). To this purpose an LES case is first constructed, based on available observations (during SCMS). Then the simulations are directly compared with the in‐cloud measurements by using conditionally sampled fields. An advantage of the SCMS dataset is the combination of a range of different surface measurements, in‐cloud measurements by an aircraft at many levels in the cloud layer, and the availability of high‐resolution Landsat images.The results show that given the correct initial and boundary conditions the LES concept is capable of realistically predicting the bulk thermodynamic properties of temperature, moisture and liquid‐water content of the cumulus‐cloud ensemble as observed in SCMS. Furthermore, the vertical component of the in‐cloud turbulent kinetic energy and the cloud size distribution in LES were in agreement with the observations. These results support the credibility of cloud statistics as produced by LES in general, and encourage its use as a tool for testing hypotheses and developing parametrizations of shallow cumulus cloud processes.Several hypotheses which make use of conditionally sampled fields were tested on the SCMS data. The magnitudes and the decrease with height of the bulk entrainment rate following from the SCMS data confirm the typical values of the order of magnitude of 10−3 m−1 as reported in several other recent LES studies. An alternative formulation of the lateral entrainment rate as a function of the liquid‐water content and the mean lapse rate agrees well with the original form based on the conserved variables. Applying an often‐used simplified equation for the cloud vertical velocity to the aircraft measurements results in a reasonably closed budget. Copyright © 2003 Royal Meteorological Society

水平環状空間内非定常３次元自然対流 （ＩＶ） 乱流自然対流へのＬＥＳの適用

The large eddy simulation (LES) method is applied to turbulent natural convection in a horizontal concentric annulus. Numerical results are obtained for Rayleigh numbers based on the gap width from 2.51×106 to 1.18×109. These results are compared with other researchers' experimental data and the validity of LES is investigated. It is found that LES is very useful even if at high Ra, at which the DNS is not applicable. However, in the cases where temperature differences between cylinders are large, effect of dependency of physical properties on temperature is not negligible, which causes the poor coincidence betwenn the experiments and the numerical results.