Large Eddy Simulation Code Research Articles

Large-eddy simulation of a utility-scale wind farm in complex terrain

Site-specific wind farm design must take into account the effects of site-specific terrain topography. Large-eddy simulation (LES) is a promising approach for simulating the site-specific characteristics of the wind fields and turbine wakes in complex terrain. However, to the best of our knowledge, the capability of LES in simulating utility-scale wind farms in complex terrain has not been systematically evaluated. In this work, we apply the state-of-art LES code Virtual Flow Simulator (VFS-Wind) to simulate the Invenergy Vantage wind farm (located in the Washington state, USA) in complex terrain. The computed power outputs are compared with field measurements and good agreement with the measured data is obtained both in terms of mean power and statistics of power generated by the wind farm. A simple analytical wind farm model without considering the complex terrain effects is also applied to predict the performance of the Vantage wind farm layout. The results show that such a model overestimates the performance of the actual Vantage wind farm and underscore the need for developing analytical models that account for terrain effects to enable wind farm design and optimization in complex terrain. LES can provide the data sets required to calibrate and validate such terrain-specific analytical models.

Towards an adjoint based 4D-Var state estimation for turbulent flow

The turbulent nature of the atmospheric boundary layer leads to wind-turbine power fluctuations and fatigue loading. The possibility to perform faster than real-time turbulent flow field simulations opens up applications in the forecasting and potentially actively controlling of turbulent structures. An important aspect of the forecasting, is the initialization of the simulation based on measurements, which will be the focus of this work. To this end, we use a 4D-Var approach, which is already being applied in numerical weather prediction applications. In this article, a fully developed pressure-driven atmospheric boundary layer is used as a case study. As a underlying model for the state estimation, we use a large eddy simulation (LES) code run on a relatively coarse grid, to decrease the model evaluation time. We take virtual measurements from a reference LES simulation, which is also used to benchmark the state estimation. For the 4D-Var optimization problem, we use a L-BFGS approach combined with an adjoint LES simulation for the gradient calculations.

A POD reduced-order model for wake steering control

Wake steering by active yawing of upstream wind turbines is a promising wind plant control technique. To enable the development of model-based wind plant control methods, there is a need for models that can marry the contrasting requirements of good fidelity and low computational cost. This paper presents a reduced-order model (ROM) obtained by directly compressing high-fidelity computational fluid dynamics (CFD) simulation data using the proper orthogonal decomposition (POD) method. At first, simulations of wake-interacting wind turbines are obtained for time-varying yaw settings using the lifting-line large-eddy simulation (LES) code SOWFA. Next, a ROM is synthesized from the CFD transient simulations, obtaining a discrete-time state-space model that captures the dominant dynamics of the underlying high-fidelity model with only a reduced number of states. The ROM is optionally augmented with a Kalman filter, which feeds back turbine power measurements from the plant to the model, enhancing its accuracy. Results obtained in realistic turbulent conditions show a good agreement between high-fidelity CFD solutions and the proposed POD-based ROM in terms of wake behavior and power output of waked turbines. Additionally, the ROM presents acceptable results when compared to wind tunnel experiments, including the capability of the model to partially correct for an intentionally built-in model mismatch.

Verification and Validation of a Low-Mach-Number Large-Eddy Simulation Code against Manufactured Solutions and Experimental Results

To investigate turbulent reacting flows, a low-Mach number large-eddy simulation (LES) code called ‘LESsCoal’ has been developed in our group. This code employs the Germano dynamic sub-grid scale (SGS) model and the steady flamelet/progress variable approach (SFPVA) on a stagger-structured grid, in both time and space. The method of manufactured solutions (MMS) is used to investigate the convergence and the order of accuracy of the code when no model is used. Finally, a Sandia non-reacting propane jet and Sandia Flame D are simulated to inspect the performance of the code under experimental setups. The results show that MMS is a promising tool for code verification and that the low-Mach-number LES code can accurately predict the non-reacting and reacting turbulent flows. The validated LES code can be used in numerical investigations on the turbulent combustion characteristics of new fuel gases in the future.

Detailed SGS atomization model and its implementation to two-phase flow LES

A novel turbulent atomization model, which is physically closed itself and free of case-by-case parameter tuning using experimental data, has been formulated and demonstrated in the framework of turbulent spray combustion large-eddy simulation (LES). Based on our accumulated research findings that elementary droplet/ligament generation is a deterministic phenomenon, not something random as considered in the conventional understanding, the model describes two dominant modes of turbulent atomization, i.e. the turbulent resonant mode and the Rayleigh–Taylor (RT) mode, in a physically straightforward manner. Extending the baseline theory proposed in Umemura (2016), to a hybrid turbulent spray LES formulation which includes both an Eulerian liquid jet core and Lagrangian droplets, the subgrid-scale (SGS) atomization characteristics are completely detailed in this study. Using the LES-resolved turbulent Weber and Bond numbers on the liquid core surface, the atomization mode and the SGS atomization characteristics such as droplet size, number, ejection velocity and core regression velocity are all identified locally, and the information is transferred back to the LES code as input information. Test cases of Diesel fuel jets demonstrate that the present formulation well reproduces the turbulent spray behavior. Thanks to the obtained detailed data, the spray formation process can be tracked both temporally and spatially, from the initial head formation with edge atomization to the later core atomization and spray spreading. It is essentially featured that the present turbulent atomization model works well without ambiguous user input, contrary to the conventional way of spray simulation. This is a significant breakthrough to urge paradigm shift in spray simulation, from unclosed/unpredictable to closed/predictable, which enables drastic improvement in the accuracy of spray simulation and may exert a large impact on both research studies and industrial applications.

Flow adjustment inside homogeneous canopies after a leading edge – An analytical approach backed by LES

A two-dimensional analytical model for describing the mean flow behavior inside a vegetation canopy after a leading edge in neutral conditions was developed and tested by means of large eddy simulations (LES) employing the LES code PALM. The analytical model is developed for the region directly after the canopy edge, the adjustment region, where one-dimensional canopy models fail due to the sharp change in roughness. The derivation of this adjustment region model is based on an analytic solution of the two-dimensional Reynolds averaged Navier–Stokes equation in neutral conditions for a canopy with constant plant area density (PAD). The main assumptions for solving the governing equations are separability of the velocity components concerning the spatial variables and the neglection of the Reynolds stress gradients. These two assumptions are verified by means of LES. To determine the emerging model parameters, a simultaneous fitting scheme was applied to the velocity and pressure data of a reference LES simulation. Furthermore a sensitivity analysis of the adjustment region model, equipped with the previously calculated parameters, was performed varying the three relevant length, the canopy height (h), the canopy length and the adjustment length (Lc), in additional LES. Even if the model parameters are, in general, functions of h/Lc, it was found out that the model is capable of predicting the flow quantities in various cases, when using constant parameters. Subsequently the adjustment region model is combined with the one-dimensional model of Massman [Bound. Layer Meteorol., 83(3):407–421, 1997], which is applicable for the interior of the canopy, to attain an analytical model capable of describing the mean flow for the full canopy domain. Finally the model is tested against an analytical model based on a linearization approach.

Large-eddy simulation of thermally stratified forest canopy flow for wind energy studies purposes

A Large Eddy Simulation (LES) code using Lagrangian Dynamic Smagorinsky subgrid models was developed for the simulation of atmospheric stratified flows over topography for wind energy assessment purposes. In this article, the code, which uses separated dynamic procedures for the calculation of eddy diffusivities of momentum and heat, was used to simulate horizontally homogeneous flow in forest canopies, for both unstable and stable regimes. The results were in good agreement with both numerical and experimental results for stable stratified wind regimes but, as far as unstable regimes are concerned, there are still unresolved issues to be addressed.

Numerical Investigation on Flame Stabilization in DLR Hydrogen Supersonic Combustor with Strut Injection

ABSTRACTFlame stabilization in the DLR hydrogen supersonic combustor with strut injection was numerically investigated by using an in-house large eddy simulation code developed on the OpenFoam platform. To facilitate the comparison and analysis of various hydrogen oxidation mechanisms with different levels of mechanism reduction, the proposed 2D calculation model was validated against both the 3D simulation and the experimental data. The results show that the 2D model can capture the DLR flow and combustion characteristics with satisfactorily quantitative accuracy and significantly less computational load. By virtue of the flow visualization and the analyses of species evolution and heat release, the supersonic combustion in the DLR combustor can be divided into three stages along the streamwise direction: the induction stage where ignition occurs and active radicals are produced, the transition stage through which radicals are advected to the downstream, and the intense combustion stage where most heat release occurs. Furthermore, the sensitivity analysis of key reaction steps identifies the important role of chain carrying and heat release reactions in numerically reproducing the three-stage combustion stabilization mode in the DLR combustor.

Numerics and subgrid-scale modeling in large eddy simulations of stratocumulus clouds.

Stratocumulus clouds are the most common type of boundary layer cloud; their radiative effects strongly modulate climate. Large eddy simulations (LES) of stratocumulus clouds often struggle to maintain fidelity to observations because of the sharp gradients occurring at the entrainment interfacial layer at the cloud top. The challenge posed to LES by stratocumulus clouds is evident in the wide range of solutions found in the LES intercomparison based on the DYCOMS‐II field campaign, where simulated liquid water paths for identical initial and boundary conditions varied by a factor of nearly 12. Here we revisit the DYCOMS‐II RF01 case and show that the wide range of previous LES results can be realized in a single LES code by varying only the numerical treatment of the equations of motion and the nature of subgrid‐scale (SGS) closures. The simulations that maintain the greatest fidelity to DYCOMS‐II observations are identified. The results show that using weighted essentially non‐oscillatory (WENO) numerics for all resolved advective terms and no explicit SGS closure consistently produces the highest‐fidelity simulations. This suggests that the numerical dissipation inherent in WENO schemes functions as a high‐quality, implicit SGS closure for this stratocumulus case. Conversely, using oscillatory centered difference numerical schemes for momentum advection, WENO numerics for scalars, and explicitly modeled SGS fluxes consistently produces the lowest‐fidelity simulations. We attribute this to the production of anomalously large SGS fluxes near the cloud tops through the interaction of numerical error in the momentum field with the scalar SGS model.

Validation of the actuator disc and actuator line techniques for yawed rotor flows using the New MEXICO experimental data

Experimental data acquired in the New MEXICO experiment on a yawed 4.5m diameter rotor model turbine are used here to validate the actuator line (AL) and actuator disc (AD) models implemented in the Large Eddy Simulation code EllipSys3D in terms of loading and velocity field. Even without modelling the geometry of the hub and nacelle, the AL and AD models produce similar results that are generally in good agreement with the experimental data under the various configurations considered. As expected, the AL model does better at capturing the induction effects from the individual blade tip vortices, while the AD model can reproduce the averaged features of the flow. The importance of using high quality airfoil data (including 3D corrections) as well as a fine grid resolution is highlighted by the results obtained. Overall, it is found that both models can satisfactorily predict the 3D velocity field and blade loading of the New MEXICO rotor under yawed inflow.

Benchmarking the Multidimensional Stellar Implicit Code MUSIC

We present the results of a numerical benchmark study for the MUlti-dimensional Stellar Implicit Code (MUSIC) based on widely applicable two- and three-dimensional compressible hydrodynamics problems relevant to stellar interiors. MUSIC is an implicit large eddy simulation code that uses implicit time integration, implemented as a Jacobian-free Newton Krylov method. A physics based preconditioning technique which can be adjusted to target varying physics is used to improve the performance of the solver. The problems used for this benchmark study include the Rayleigh-Taylor and Kelvin-Helmholtz instabilities, and the decay of the Taylor-Green vortex. Additionally we show a test of hydrostatic equilibrium, in a stellar environment which is dominated by radiative effects. In this setting the flexibility of the preconditioning technique is demonstrated. This work aims to bridge the gap between the hydrodynamic test problems typically used during development of numerical methods and the complex flows of stellar interiors. A series of multi-dimensional tests are performed and analysed. Each of these test cases is analysed with a simple, scalar diagnostic, with the aim of enabling direct code comparisons. As the tests performed do not have analytic solutions we verify MUSIC by comparing to established codes including ATHENA and the PENCIL code. MUSIC is able to both reproduce behaviour from established and widely-used codes as well as results expected from theoretical predictions. This benchmarking study concludes a series of papers describing the development of the MUSIC code and provides confidence in the future applications.

The effect of mountain wind on the falling snow deposition

The air-flow over morphologic prominence is of vital importance for the temporal and spatial variations of snow distribution, which may further affect the hydrological cycle, climate system, ecological evolution as well as other natural processes. Most previous studies have neglected the effect of non-uniform mountain wind on the trajectory of falling snow particle and thus the uneven snow distribution over complex terrain is little known. Here in this paper, we carry out a numerical study on the deposition process of mixed grain size snow particles over mountain terrains. A three-dimensional large eddy simulation (LES) code is used to predict the wind field and the fluid-solid coupling effect is considered, the Lagrangian particle tracking method is employed to track the movement of each falling snow particle. Our results suggest the boundary layer flow over steep slopes or complex terrains exhibits obvious uneven characteristics. The mountain wind has obvious effect on the motion of snow particles and finally results in a non-uniform distribution of snow. This research is of significance to understand the evolution of high resolution snow distribution in cold highland areas.

UCLALES–SALSA v1.0: a large-eddy model with interactive sectional microphysics for aerosol, clouds and precipitation

Abstract. Challenges in understanding the aerosol–cloud interactions and their impacts on global climate highlight the need for improved knowledge of the underlying physical processes and feedbacks as well as their interactions with cloud and boundary layer dynamics. To pursue this goal, increasingly sophisticated cloud-scale models are needed to complement the limited supply of observations of the interactions between aerosols and clouds. For this purpose, a new large-eddy simulation (LES) model, coupled with an interactive sectional description for aerosols and clouds, is introduced. The new model builds and extends upon the well-characterized UCLA Large-Eddy Simulation Code (UCLALES) and the Sectional Aerosol module for Large-Scale Applications (SALSA), hereafter denoted as UCLALES-SALSA. Novel strategies for the aerosol, cloud and precipitation bin discretisation are presented. These enable tracking the effects of cloud processing and wet scavenging on the aerosol size distribution as accurately as possible, while keeping the computational cost of the model as low as possible. The model is tested with two different simulation set-ups: a marine stratocumulus case in the DYCOMS-II campaign and another case focusing on the formation and evolution of a nocturnal radiation fog. It is shown that, in both cases, the size-resolved interactions between aerosols and clouds have a critical influence on the dynamics of the boundary layer. The results demonstrate the importance of accurately representing the wet scavenging of aerosol in the model. Specifically, in a case with marine stratocumulus, precipitation and the subsequent removal of cloud activating particles lead to thinning of the cloud deck and the formation of a decoupled boundary layer structure. In radiation fog, the growth and sedimentation of droplets strongly affect their radiative properties, which in turn drive new droplet formation. The size-resolved diagnostics provided by the model enable investigations of these issues with high detail. It is also shown that the results remain consistent with UCLALES (without SALSA) in cases where the dominating physical processes remain well represented by both models.

A priori and a posteriori analyses of algebraic flame surface density modeling in the context of Large Eddy Simulation of turbulent premixed combustion

ABSTRACTThe flame surface density (FSD) based reaction rate closure is one of the most important methodologies of turbulent premixed flame modeling in the context of Large Eddy Simulations (LES). The transport equation for the Favre-filtered reaction progress variable needs closure of the filtered reaction rate and the subgrid scalar flux (SGSF). The SGSF in premixed turbulent flames has both gradient and countergradient components, where the former is typically modeled using eddy diffusivity and the latter can be modeled either on its own or in combination with the filtered reaction rate term using an appropriate wrinkling factor. The scope of the present work is to identify an explicit SGSF closure for the optimum performance in combination with an already established LES FSD model. The performance of different SGSF models for premixed turbulent combustion has been assessed recently by the authors using a Direct Numerical Simulation (DNS) database of freely propagating turbulent premixed flames with a range of different values of turbulent Reynolds number. The two most promising models have been implemented in the LES code. The modeling methodology identified based on a priori DNS analysis is assessed further a posteriori by comparing the LES simulation results of turbulent methane Bunsen flames with the well-documented experimental data. A significant change of the overall flame speed is not observed for different SGSF models. However, the flame shape and thickness respond to the modeling of SGSF. Considering the fact that the SGSF models have very different characteristics, the overall effect on the LES results in this work is smaller than expected. An extension of a previous a priori DNS analysis provides detailed explanations for the observed behavior.

High-Resolution LES of Terrain-Induced Turbulence around Wind Turbine Generators by Using Turbulent Inflow Boundary Conditions

We have developed an LES (Large-Eddy Simulation) code called RIAM-COMPACT (Research Institute for Applied Mechanics, Kyushu University, Computational Prediction of Airflow over Complex Terrain). The analysis do-main of this numerical model extends from several meters to several kilometers. The model is able to predict airflow over complex terrain with high accuracy and is also now able to estimate the annual power output of wind turbine generators with the use of field observation data. In the present study, a numerical simulation of turbulent airflow over an existing wind farm was performed using RIAM-COMPACT and high-resolution elevation data. Based on the simulation results, suitable and unsuitable locations for the operation of WTGs (Wind Turbine Generators) were identified. The latter location was subject to the influence of turbulence induced by small topographical variations just upwind of the WTG location.

Simulation and wake analysis of a single vertical axis wind turbine

AbstractBecause of several design advantages and operational characteristics, particularly in offshore farms, vertical axis wind turbines (VAWTs) are being reconsidered as a complementary technology to horizontal axial turbines. However, considerable gaps remain in our understanding of VAWT performance since cross‐flow rotor configurations have been significantly less studied than axial turbines. This study examines the wakes of VAWTs and how their evolution is influenced by turbine design parameters. An actuator line model is implemented in an atmospheric boundary layer large eddy simulation code, with offline coupling to a high‐resolution blade‐scale unsteady Reynolds‐averaged Navier–Stokes model. The large eddy simulation captures the turbine‐to‐farm scale dynamics, while the unsteady Reynolds‐averaged Navier–Stokes captures the blade‐to‐turbine scale flow. The simulation results are found to be in good agreement with three existing experimental datasets. Subsequently, a parametric study of the flow over an isolated VAWT, carried out by varying solidities, height‐to‐diameter aspect ratios and tip speed ratios, is conducted. The analyses of the wake area and velocity and power deficits yield an improved understanding of the downstream evolution of VAWT wakes, which in turn enables a more informed selection of turbine designs for wind farms. Copyright © 2016 John Wiley & Sons, Ltd.

Mixing characteristics of a transverse jet injection into supersonic crossflows through an expansion wall

Mixing characteristics of a transverse jet injection into supersonic crossflows through an expansion plate are investigated using large eddy simulation (LES), where the expansion effects on the mixing are analyzed emphatically by comparing to the flat-plate counterpart. An adaptive central-upwind weighted essentially non-oscillatory (WENO) scheme along with multi-threaded and multi-process MPI/OpenMP parallel is adopted to improve the accuracy and efficiency of the calculations. Progressive mesh refinement study is performed to assess the grid resolution and solution convergence. Statistic results obtained are compared to the experimental data and recently performed classical numerical simulation, which validates the reliability of the present LES codes. Firstly, the jet mixing mechanisms in the flowfield with expansion plate are revealed. It indicates that the large-scale vortices in the windward side of jet plume induced by Kelvin–Helmholtz (K–H) instability contribute to the mixing in the near-field, while the entrainment by the counter-rotating vortices and molecular diffusion dominate the mixing process in the far-field. Furthermore, the effects of wall expansion on the flow and mixing characteristics are discussed. The boundary layer across the expansion corner is relaminarized and the profiles of streamwise velocity are distinctly changed. Then the separation region ahead of jet plume is more close to the wall, and the breaking process of large-scale vortices in the windward side of jet plume starts earlier. However, the favorable pressure gradient generated by wall expansion reduces the mixing efficiency and brings a greater total pressure loss.

Effect of stable stratification on dispersion within urban street canyons: A large-eddy simulation

This study employs a validated large-eddy simulation (LES) code with high tempo-spatial resolution to investigate the effect of a stably stratified roughness sublayer (RSL) on scalar transport within an urban street canyon. The major effect of stable stratification on the flow and turbulence inside the street canyon is that the flow slows down in both streamwise and vertical directions, a stagnant area near the street level emerges, and the vertical transport of momentum is weakened. Consequently, the transfer of heat between the street canyon and overlying atmosphere also gets weaker. The pollutant emitted from the street level ‘pools’ within the lower street canyon, and more pollutant accumulates within the street canyon with increasing stability. Under stable stratification, the dominant mechanism for pollutant transport within the street canyon has changed from ejections (flow carries high-concentration pollutant upward) to unorganized motions (flow carries high-concentration pollutant downward), which is responsible for the much lower dispersion efficiency under stable stratifications.

Inception and evolution of coherent structures in under-expanded supersonic jets

The purpose of this paper is to examine the generation and nature of the large coherent structures observed experimentally in an under-expanded supersonic impinging jet. More specifically, the questions to answer are: What mechanisms govern the receptivity process at the nozzle lip?, how does the underlying flow field affect the evolution of the large-scale coherent structure generated from the initial instability? and what are the interactions between the large-scale (forced) coherent structures and the developing turbulence in the jet shear layer? In order to answer some of these questions both alternatives, that these structures come from global modal flow instabilities or from convective instabilities, the latter, are considered in this work.The stability analysis considered in the former case is performed in this work near the nozzle around the temporal average of the flow obtained by using an in-house LES (Large Eddy Simulation) code. The flow in this region is considered laminar, steady and without non-linear effects. The well known feedback loop in the impinging jet, according to which acoustic waves propagate upstream and excite the jet shear-layer (see Figure 2), advises against some of the hypothesis considered previously in the global stability analysis (ie. non-linear approximation). However the acoustic waves are orders of magnitude smaller than the hydrodynamic waves and should be smoothed out in the temporal average used in the calculation of the mean flow.The results show that both, axisymmetrical (m = 0) and azimuthal modes (m ≥ 1) are stable to global modal analysis and only convective instability could justify the instabilities observed in experiments in the shear layer. A study on the receptivity problem confirms that external disturbances may enter and excite the shear layer, being responsible of the instabilities observed in both experiments and direct numerical simulations.

Accuracy requirements for unmanned aerial vehicle-based acoustic atmospheric tomography

If the acoustic signature is measured onboard a propeller-driven unmanned aerial vehicle and by an array of microphones on the ground, comparison between the projected and observed signals allows the propagation delays—and hence effective sound speed values of the intervening medium—to be determined. As the ray paths along which energy travels intersect within the atmosphere at multiple locations and angles, temperature and wind velocity fields may be estimated using tomography. This paper describes signal processing strategies for determining propagation delays at sub-millisecond levels of precision; the inverse problem and a model that allows meteorological observations to be incorporated into the tomographic solution; and the algebraic iterative reconstruction technique used to overcome the ill-posed nature of the inversion. A weakly sheared daytime convective atmospheric boundary layer—synthesised through use of large eddy simulation code that utilises pseudo-spectral differencing and solves an ellipt...