Coarse Large Eddy Simulation Research Articles

A new coupling of a GPU‐resident large‐eddy simulation code with a multiphysics wind turbine simulation tool

AbstractThe development of new wind farm control strategies can benefit from combined analysis of flow dynamics in the farm and the behavior of individual turbines within one simulation environment. In this work, we present such an environment by developing a new coupling between the large‐eddy simulation (LES) code GRASP and the multiphysics wind turbine simulation tool OpenFAST via an actuator line model (ALM). In addition, the implementation of the recently proposed filtered actuator line model (FALM) within the coupling is described. The new ALM implementation is cross‐verified with results from four other commonly used research LES codes. The results for the blade loads and the near wake obtained with the new coupling are consistent with the other codes. Deviations are observed in the far wake. The results further indicate that the FALM is able to reduce the lift and power overprediction from which the traditional ALM suffers on coarse LES grids. This new simulation environment paves the way for future wind farm simulations under realistic weather conditions by leveraging GRASP's ability to impose data from large‐scale meteorological models as boundary conditions.

Dynamic nonlocal passive scalar subgrid-scale turbulence modeling

Extensive experimental evidence highlights that scalar turbulence exhibits anomalous diffusion and stronger intermittency levels at small scales compared to that in fluid turbulence. This renders the corresponding subgrid-scale dynamics modeling for scalar turbulence a greater challenge to date. We develop a new large eddy simulation (LES) paradigm for efficiently and dynamically nonlocal LES modeling of the scalar turbulence. To this end, we formulate the underlying nonlocal model starting from the filtered Boltzmann kinetic transport equation, where the divergence of subgrid-scale scalar fluxes emerges as a fractional-order Laplacian term in the filtered advection–diffusion model, coding the corresponding superdiffusive nature of scalar turbulence. Subsequently, we develop a robust data-driven algorithm for estimation of the fractional (noninteger) Laplacian exponent, where we, on the fly, calculate the corresponding model coefficient employing a new dynamic procedure. Our a priori tests show that our new dynamically nonlocal LES paradigm provides better agreement with the ground-truth filtered direct numerical simulation data in comparison to the conventional static and dynamic Prandtl–Smagorinsky models. Moreover, in order to analyze the numerical stability and assessing the model's performance, we carry out comprehensive a posteriori tests. They unanimously illustrate that our new model considerably outperforms other existing functional models, correctly predicting the backscattering phenomena and, at the same time, providing higher correlations at small-to-large filter sizes. We conclude that our proposed nonlocal subgrid-scale model for scalar turbulence is amenable for coarse LES and very large eddy simulation frameworks even with strong anisotropies, applicable to environmental applications.

Simulation of flow and heat transfer in a differentially heated cubical cavity using coarse Large Eddy Simulation

Two widely used sub-grid scale models namely: the standard and the dynamic Smagorinsky models were tested in a simulation of the flow in a thermally driven 3D cavity at turbulent Rayleigh number Ra = 109. The cubical cavity of 0.7 m side-length is set to have a temperature difference of 39 K between the two facing cold and hot vertical walls. Experimentally measured temperature profiles for both the top and bottom walls were imposed as Dirichlet boundary conditions to implicitly account for wall-to-wall radiation effects. The focus of this research is the response of the subgrid-scale models and their ability to predict the flow in the thermally driven 3D cavity when the mesh resolution is coarse and below optimal LES standards. The research is motivated by investigating a feasible modeling strategy for particulate flows in the differentially heated cavity. As URANS and hybrid RANS-LES models fail to reproduce the flow accurately due to difficulty to model subtle physical mechanisms such as laminarization and three-dimensional effects, the alternative is LES applied on a coarse mesh. In a quantitative manner, the first and second-moment statistics are compared at different locations across the cavity against both previous fine mesh LES and experimental databases. For the first moment statistics, both models globally predict the flow fields well. However, for higher moments, the dynamic model outperforms the standard Smagorinsky model, and its predictions are globally in very good agreement with both reference LES and experimental measurements at a fraction of the CPU power needed for optimal LES. The presented results will motivate a further investigation of particle dispersion in the cubical cavity at moderate computational power.

CFD Simulation of Particle-Laden Flow in a 3D Differentially Heated Cavity Using Coarse Large Eddy Simulation

Particulate flow in closed space is involved in many engineering applications. In this paper, the prediction of particle removal is investigated in a thermally driven 3D cavity at turbulent Rayleigh number Ra = 109 using Coarse Large Eddy Simulation (CLES). The depletion dynamics of SiO2 aerosol with aerodynamic diameters between 1.4 and 14 µm is reported in an Euler/Lagrange framework. The main focus of this work is therefore to assess the effect of the subgrid-scale motions on the prediction of the particulate flow in a buoyancy driven 3D cavity flow when the mesh resolution is coarse and below optimal LES standards. The research is motivated by the feasibility of modeling more complex particulate flows with reduced CPU cost.The cubical cavity of 0.7 m side-length is set to have a temperature difference of 39 K between the two facing cold and hot vertical walls. As a first step, the carrier fluid flow was validated by comparing the first and second-moment statistics against both previous well-resolved LES and experimental databases [Kalilainen (J. Aero Sci. 100:73–87, 2016); Dehbi (J. Aero. Sci. 103:67–82, 2017)]. First moment Eulerian statistics show a very good match with the reference data both qualitatively and quantitatively, whereas higher moments show underprediction due to the lesser spatial resolution. In a second step, six particle swarms spanning a wide range of particle Stokes numbers were computed to predict particle depletion. In particular, predictions of 1.4 and 3.5 µm particles were compared to LES and available experimental data. Particles of low inertia i.e. dp < 3.5 µm are more affected by the SGS effects, while bigger ones i.e. dp = 3.5–14 µm exhibit much less grid-dependency. Lagrangian statistics reported in both qualitative and quantitative fashions show globally a very good agreement with reference LES and experimental databases at a fraction of the CPU power needed for optimal LES.

A dual-mesh hybrid Reynolds-averaged Navier-Stokes/Large eddy simulation study of the buoyant flow between coaxial cylinders

This study is concerned with the investigation of the suitability of the dual-mesh method for the buoyancy driven flow inside a cylindrical annulus with one internal cylinder. This flow situation can occur, for instance, in gas cooled nuclear reactors and is characterized by complex features, including a buoyant plume and a negatively buoyant wall-jet. The buoyancy forces in this flow are generated by the temperature difference between the inner and outer cylinders. The dual-mesh approach is a hybrid RANS-LES method in which an unsteady Reynolds-averaged Navier-Stokes (RANS) simulation and a coarse Large eddy simulation (i.e. an LES that is under-resolved near walls) are run simultaneously on two different grids. In this approach, a criterion is used to determine the locations at which each simulation is expected to perform better than the other. Consequently, at every location the less accurate simulation is forced and corrected towards the more accurate one. This correction is done using source terms that are included in the equations of the flow and thermal fields of the two simultaneous simulations. It is observed that the buoyancy-extended lengthscale resolution criterion behaves satisfactorily in defining the regions where the LES is corrected towards the RANS and vice versa. Moreover, both quantitative and qualitative analyses of the results are conducted using quasi direct numerical simulation results from the literature. It is shown that the dual-mesh method has the potential to yield results that are better than the results of the pure RANS and the pure coarse LES simulations.

Effects of Horizontal Resolution, Domain Size, Boundary Conditions, and Surface Heterogeneity on Coarse LES of a Convective Boundary Layer

Abstract Atmospheric properties in a convective boundary layer vary over a wide range of spatial scales and are commonly studied using large-eddy simulations (LES) in various configurations. We examine how the boundary layer depth and distribution of variability across scales are affected by LES grid spacing, domain size, inhomogeneity of surface properties, and external forcing. Two different setups of the Weather Research and Forecasting (WRF) model are analyzed. A semi-idealized configuration uses a periodic domain, flat surface, prescribed homogeneous surface heat fluxes, and horizontally uniform profiles of large-scale advective tendencies. A nested LES setup employs a larger domain and realistic initial and boundary conditions, including an interactive land surface model with representative topography and vegetation and soil types. Subdomains of identical size are analyzed for all simulations. Characteristic structure sizes are quantified using the variability scales L50 and L95, defined such that features smaller than that contain 50% and 95% of the total variance, respectively. Progressive increase in L50 from vertical velocity to temperature and moisture structures is systematically reproduced in all simulation configurations. This dependence of L50 on the considered variable complicates the development of scale-aware parameterizations for models with grid spacing in the “terra incognita”. In simulations using a larger domain with heterogeneous surface properties, the development of internal mesoscale patterns significantly affects variance distributions inside analyzed subdomains. Sizes of boundary layer structures also strongly depend on the LES grid spacing and, in case of heterogeneous surface and topography, on location of the subdomain inside a larger computational domain.

Erosion of a helium-rich layer by a steam jet in the presence of an inclined grid: Comparison of the predictions by URANS, STRUC-URANS and LES

Severe accidents in Light Water Reactors (LWRs) involve the production of hydrogen, which may accumulate in the upper regions of the containment and form flammable mixtures with the initial air inventory. To study phenomena related to light gas layer removal, the H2P1_10_2 HYMERES test (Paranjape et al., 2019, 2020) was conducted in the PANDA vessel with the aim of investigating the erosion of a helium-rich layer by a steam jet deflected by an inclined grid. In this paper, we make use of the test data to validate computational fluid dynamics (CFD) simulations of such a transient. The simulation models involve the standard k- ε URANS, the more advanced second generation STRUC- ε URANS, and a coarse Large Eddy Simulation (CLES). All simulations are conducted with the same numerical grid consisting of 3.5 million hexahedral cells. Results show that all models capture well the main physical quantities, namely: the helium erosion rate, the temperatures and the velocities downstream of the obstructing grid. Fine details such as the turbulent kinetic energy (TKE) are better reproduced by the CLES, although the latter miss somewhat the amplitudes due to lack of spatial resolution, especially in the near-field downstream of the grid region. The STRUC- ε model replicates qualitatively the flow field fluctuations in sections of the domain where the spatiotemporal resolution is adequate. Mesh refinement in the jet region shows a substantial quantitative improvement, demonstrating that the STRUC- ε model is superior to traditional URANS when spatial resolution is increased. Finally, the simulations show unmistakably that radiation modeling is necessary to achieve reasonable predictions of the temperature field, as the radiative flux can account for a substantial fraction of the heat transfer to the vessel walls, in particular when convective currents are weak.

Resolved and subgrid-scale crossing trajectory effects in Eulerian large eddy simulations of size-dependent droplet transport

We study the dispersion characteristics of slightly buoyant droplets in a turbulent jet using large eddy simulations (LES). The droplet number density fields are represented using an Eulerian approach, with the dispersed phase modelled using the Fast-Eulerian method (Ferry &amp; Balachandar, Intl J. Multiphase Flow, vol. 27, issue 7, pp. 1199–1226, 2001) that includes the droplet rise velocity. Radial concentration profiles and turbulent concentration fluxes for droplets of different sizes are analysed to quantify the ‘trajectory crossing effect’, when relative motions between particles and turbulent eddies tend to reduce turbulent diffusion. For finer LES grid resolutions, the model captures the differential, size-based dispersion characteristics of the droplets with the transverse dispersion of the larger droplet sizes suppressed, since trajectory crossing effects are explicitly resolved in LES. We examine a similarity solution model for the size-dependent radial concentration profiles based on a modified Schmidt number derived from the theory of turbulent diffusion of particles in the atmosphere proposed by Csanady (J. Atmos. Sci., vol. 20, pp. 201–208, 1963). The results are validated with the high resolution LES data and show good agreement. Then the size-dependent Schmidt number model is reformulated as a model for unresolved subgrid-scale trajectory crossing effects and used to calculate the subgrid concentration flux in a coarse LES of a turbulent jet, with slightly buoyant droplets injected at the centreline in the self-similar region of the jet. The results are compared to a simulation with higher grid resolution and a coarse simulation with a constant Schmidt number subgrid-scale model. We find that the subgrid model enhances the prediction accuracy of the concentration profiles and turbulent concentration flux for the coarse LES.

A dual-mesh hybrid RANS-LES simulation of the buoyant flow in a differentially heated square cavity with an improved resolution criterion

In this work, the dual-mesh hybrid RANS-LES approach was applied for the first time to a natural convection flow, namely a high Rayleigh number differentially heated square cavity flow. This approach involves running an unsteady Reynolds Averaged Navier-Stokes (RANS) simulation and a coarse Large Eddy Simulation (LES) simultaneously using two different grids that overlap each other (typically, a highly wall-refined grid is used for the RANS, whereas a more homogenous and isotropic mesh is used for the LES). The two simulations correct each other using a switching criterion that determines the driving and the driven simulations at every point in space. It is demonstrated that the flow unsteadiness and the coexistence of laminar and turbulent regions in the square cavity complicate the task of choosing a suitable switching criterion. Accordingly, a new criterion based on comparing the turbulence lengthscales to the grid size was developed to account for the presence of the aforementioned complex flow features. The behaviour of this criterion and comparisons of the dual-mesh predictions against pure RANS, pure coarse LES and Direct Numerical Simulation (DNS) results are also presented in the current paper.

Simulating Real Atmospheric Boundary Layers at Gray-Zone Resolutions: How Do Currently Available Turbulence Parameterizations Perform?

Recent computational and modeling advances have led a diverse modeling community to experiment with atmospheric boundary layer (ABL) simulations at subkilometer horizontal scales. Accurately parameterizing turbulence at these scales is a complex problem. The modeling solutions proposed to date are still in the development phase and remain largely unvalidated. This work assesses the performance of methods currently available in the Weather Research and Forecasting (WRF) model to represent ABL turbulence at a gray-zone grid spacing of 333 m. We consider three one-dimensional boundary layer parameterizations (MYNN, YSU and Shin-Hong) and coarse large-eddy simulations (LES). The reference dataset consists of five real-case simulations performed with WRF-LES nested down to 25 m. Results reveal that users should refrain from coarse LES and favor the scale-aware, Shin-Hong parameterization over traditional one-dimensional schemes. Overall, the spread in model performance is large for the cellular convection regime corresponding to the majority of our cases, with coarse LES overestimating turbulent energy across scales and YSU underestimating it and failing to reproduce its horizontal structure. Despite yielding the best results, the Shin-Hong scheme overestimates the effect of grid dependence on turbulent transport, highlighting the outstanding need for improved solutions to seamlessly parameterize turbulence across scales.

A Priori Assessment of Subgrid-Scale Models and Numerical Error in Forced Convective Flow at High Prandtl Numbers

An extensive a priori analysis has been carried out on data from Direct numerical simulation of fully developed heated turbulent pipe flow at high molecular Prandtl numbers Pr=10/20, testing three popular modelling candidates for subgrid-scale closure in Large-Eddy simulation (LES). Aside from assessing the models’ capabilities to describe quantitatively the unresolved turbulent fluxes, a special focus is also put on the role of the numerical error, which arises from the discretization of the filtered advective fluxes on a coarse LES grid. The present analysis extends here previous studies on subgrid-scale momentum transport in a isothermal mixing layer and channel flow carried out by Brandt (J Numer Methods Fluids 51: 635–657, 2006) and Vreman et al. (J Eng Math 29: 299–327, 1995), respectively, to the subgrid-scale transport of heat at high Prandtl numbers. The statistical dependence between the individual contributions (resolved, subgrid-scale, numerical discretization error) constituting the filtered advective flux divergence in the LES formulation is investigated as well, in terms of corresponding cross-correlations. The sensitivity of the tested sgs-models to a grid refinement is further examined performing also a posteriori LES, where the basically more sophisticated candidates turn out to be more demanding in terms of required grid resolution.

LES grid resolution requirements for the modelling of gravity currents

The influence of the grid resolution on the large eddy simulation (LES) of a lock-exchange turbidity current is investigated. The simulations are performed using a finite volume Boussinesq code with a Smagorinsky turbulence model for a range of buoyancy Reynolds numbers, ranging from transitional currents (Reb=1,000) to fully-developed turbulence (Reb=60,000). The general features of the flow and the relative independence of the current front for Reb>10,000 are correctly predicted. In agreement with previous research, the spanwise two-point correlations are found to be the most useful quantities to assess the mesh resolution. In addition, velocity power spectrum densities are used to provide information on the maximum cell size required to ensure the LES filter cutoff wavelength is inside the inertial range of the turbulence spectra. We show that at low Reynolds numbers, the turbulence model is too restrictive and direct resolution (DNS) is preferable. For 10,000<Reb<60,000, the combination of the different criteria lead to a minimum resolution of 1140 × 37 × 74 cells for coarse LES, and 1925 × 62 × 125 cells for well-resolved LES, regardless of the Reynolds number. Finally, recommendations are made on how to achieve a well-resolved LES based on examination of the vertical profiles of the ratio of SGS viscosity to molecular viscosity, and of the SGS shear-stress to the resolved Reynolds stress.

Enrichment methods for inflow turbulence generation in the atmospheric boundary layer

We investigate the feasibility of introducing synthetic turbulence into finite-domain large-eddy simulations (LES) of the wind plant operating environment. This effort is motivated by the need for a robust mesoscale-to-microscale coupling strategy in which a microscale (wind plant) simulation is driven by mesoscale data without any resolved microscale turbulence. A neutrally stratified atmospheric boundary layer was simulated in an LES with 10-m grid spacing. We show how such a fully developed turbulence field may be reproduced with spectral enrichment starting from an under-resolved coarse LES field (with 20-m and 40-m grid spacing). The velocity spectra of the under-resolved fields are enriched by superimposing a fluctuating velocity field calculated by two turbulence simulators: TurbSim and Gabor Kinematic Simulation. Both forms of enrichment accurately simulated the autospectra of all three velocity components at high wavenumbers, with agreement between the enriched fields and the full-resolution LES observed at 400 m from the inflow boundary. In contrast, the spectra of the unenriched fields reached the same fully developed state at four times the downstream distance.

Coarse large-eddy simulations in a transitional wake flow with flow models under location uncertainty

The focus of this paper is to perform coarse-grid large-eddy simulation (LES) using recently developed sub-grid scale (SGS) models of cylinder wake flow at Reynolds number (Re) of 3900. As we approach coarser resolutions, a drop in accuracy is noted for all LES models but more importantly, the numerical stability of classical models is called into question. The objective is to identify a statistically accurate, stable sub-grid scale (SGS) model for this transitional flow at a coarse resolution. The proposed new models under location uncertainty (MULU) are applied in a deterministic coarse LES context and the statistical results are compared with variants of the Smagorinsky model and various reference data-sets (both experimental and Direct Numerical Simulation (DNS)). MULU are shown to better estimate statistics for coarse resolution (at 0.46% the cost of a DNS) while being numerically stable. The performance of the MULU is studied through statistical comparisons, energy spectra, and sub-grid scale (SGS) contributions. The physics behind the MULU are characterised and explored using divergence and curl functions. The additional terms present (velocity bias) in the MULU are shown to improve model performance. The spanwise periodicity observed at low Reynolds is achieved at this moderate Reynolds number through the curl function, in coherence with the birth of streamwise vortices.

Salient drag reduction of a heavy vehicle using modified cab-roof fairings

Aerodynamic drag reduction is directly related to fuel consumption and CO2 emission and is thus a main research interest in heavy vehicles. Approximately half of the total aerodynamic drag is attributed to the flow at the forebody of a vehicle and the gap between the tractor and trailer. Therefore, flow control devices that can reduce aerodynamic drag on the forebodies of heavy vehicles offer a considerably practical significance. Cab-roof fairing (CRF) is one of the most widely used drag reduction devices installed at the roofs of trucks or tractor-trailers. However, the drag-reducing effect and the three-dimensional flow characteristics around forebodies as a function of the external shape of CRFs have yet to be fully investigated. In this study, the drag reduction effects of typical and modified CRF models are quantitatively examined through wind tunnel tests and numerical simulation (coarse large eddy simulation (LES)). The wind tunnel experiment at a Reynolds number of >5.5× 105 is conducted for a scaled-down model of a 15-tonne truck. The modified CRF significantly changes the flow structure, leading to approximately 19% drag reduction. PIV flow field measurement was conducted to figure out the differences in flow characteristics around the forebody of the vehicle with and without CRFs.Flow characteristics, including vortical structures, turbulent kinetic energy, mean pressure field, and mean velocity field around the forebody of the vehicle model (1/8 scale) with and without CRFs are numerically investigated using coarse LES for further understanding of the mechanism associated with drag reduction. The present results are expected to provide useful information for the design of new CRF models and the improvement of the aerodynamic performance of heavy vehicles, including trucks and tractor-trailers.

Simulation of heat and mass transfer in turbulent channel flow using the spectral-element method: effect of spatial resolution

We use the open-source code nek5000 to assess the accuracy of high-order spectral element large-eddy simulations (LES) of a turbulent channel flow depending on the spatial resolution compared to the direct numerical simulation (DNS). The Reynolds number Re = 6800 is considered based on the bulk velocity and half-width of the channel. The filtered governing equations are closed with the dynamic Smagorinsky model for subgrid stresses and heat flux. The results show very good agreement between LES and DNS for time-averaged velocity and temperature profiles and their fluctuations. Even the coarse LES grid which contains around 30 times less points than the DNS one provided predictions of the friction velocity within 2.0% accuracy interval.

Constant-energetics physical-space forcing methods for improved convergence to homogeneous-isotropic turbulence with application to particle-laden flows

This study investigates control-based forcing methods for incompressible homogeneous-isotropic turbulence forced linearly in physical space which result in constant turbulent kinetic energy, constant turbulent dissipation (also constant enstrophy), or a combination of the two based on a least-squares error minimization. The methods consist of proportional controllers embedded in the forcing coefficients. During the transient, the controllers adjust the forcing coefficients such that the controlled quantity achieves very early a minimal relative error with respect to its target stationary value. Comparisons of these forcing methods are made with the non-controlled approaches of Rosales and Meneveau [“Linear forcing in numerical simulations of isotropic turbulence: Physical space implementations and convergence properties,” Phys. Fluids 17, 095106 (2005)] and Carroll and Blanquart [“A proposed modification to Lundgren’s physical space velocity forcing method for isotropic turbulence,” Phys. Fluids 25, 105114 (2013)], using direct numerical simulations (DNS) and large-eddy simulations (LES). The results indicate that the proposed constant-energetics forcing methods shorten the transient period from a user-defined artificial flow field to Navier-Stokes turbulence while maintaining steadier statistics. Additionally, the proposed method of constant kinetic-energy forcing behaves more robustly in coarse LES when initial conditions are employed that favor the occurrence of subgrid-scale backscatter, whereas the other approaches fail to provide physical turbulent flow fields. For illustration, the proposed forcing methods are applied to dilute particle-laden homogeneous-isotropic turbulent flows; the results serve to highlight the influences of the forcing strategies on the disperse-phase statistics.

Performance of subgrid-scale models in coarse large eddy simulations of a laminar separation bubble

The flow over many blades and airfoils at moderate angles of attack and Reynolds numbers ranging from 104 to 105 undergoes separation due to the adverse pressure gradient generated by surface curvature. In many cases, the separated shear layer then transitions to turbulence and reattaches, closing off a recirculation region—the laminar separation bubble. An equivalent problem is formulated by imposing suitable boundary conditions for flow over a flat plate to avoid numerical and mesh generation issues. Recent work demonstrated that accurate large eddy simulation (LES) of such a flow is possible using only O(1%) of the direct numerical simulation (DNS) resolution but the performance of different subgrid-scale models could not be properly assessed because of the effects of unquantified numerical dissipation. LES of a laminar separation bubble flow over a flat plate is performed using a pseudo-spectral Navier-Stokes solver at resolutions corresponding to 3% and 1% of the chosen DNS benchmark by Spalart and Strelets (2000). The negligible numerical dissipation of the pseudo-spectral code allows an unambiguous assessment of the performance of subgrid-scale models. Three explicit subgrid-scale models—dynamic Smagorinsky, σ, and truncated Navier-Stokes (TNS)—are compared to a no-model simulation (under-resolved DNS) and evaluated against benchmark DNS data focusing on two quantities of critical importance to airfoil and blade designers: time-averaged pressure (Cp) and skin friction (Cf) predictions used in lift and drag calculations. Results obtained with explicit subgrid-scale models confirm that accurate LES of laminar separation bubble flows is attainable with as low as 1% of DNS resolution, and the poor performance of the no-model simulation underscores the necessity of subgrid-scale modeling in coarse LES with low numerical dissipation.

LES of a turbulent jet impinging on a heated wall using high-order numerical schemes

Large-eddy simulations (LES) of a turbulent impinging jet flow with a nozzle-to-plate distance of two jet diameters and a Reynolds number of Re=10,000 are presented in comparison with experimental data and results from a Direct Numerical Simulation (DNS). The impingement wall is heated and both dynamical and thermal features of the flow are discussed. It is shown that highly accurate numerical methods can lead to correct predictions of velocity statistics and heat transfer but only if a procedure is used to regularize the large-scale dynamics computed explicitly. A better regularization is obtained using a numerical dissipation that mimics a spectral vanishing viscosity in comparison to conventional subgrid-scale models based on an eddy viscosity assumption, even though the model is adapted to the wall through an explicit correction or using the dynamic procedure. These observations suggest that, in the present context of high-order schemes, a simple high-order artificial dissipation coherent with the numerical methods is more suitable than a physical subgrid-scale model that does not take the numerical errors into account. The radial evolution of the Nusselt number predicted by DNS is non-monotonous but this specific behavior is not captured by LES. Due to the complexity of the turbulent processes associated with the corresponding secondary peak in the Nusselt number distribution, an explicit calculation of all the significant scales seems to be required. This conclusion goes against the use of a coarse LES grid in this region of the flow.

Large Eddy Simulation of premixed turbulent combustion using approximate deconvolution and explicit flame filtering

Approximate deconvolution and explicit filtering of scalar fields are used to construct a new approach to sub-grid scale modeling in Large Eddy Simulation (LES) of premixed turbulent flames. This modeling is designed to handle any form of chemical description, Arrhenius or tabulated, as soon as the grid resolution is not too coarse. The closure of the unresolved terms relies on the explicit filtering of their exact expressions computed after an approximate deconvolution of the resolved scalars. A specific numerical treatment is needed to capture the deconvoluted peak burning rate over the coarse LES mesh. The method is first tested in the canonical laminar filtered flame context with two strategies for the unresolved fluxes, which are based either on a full approximate deconvolution, or, on the introduction of a corrective factor applied to the resolved diffusive flux. This factor is dynamically determined from the deconvoluted/filtered burning rates to preserve basic filtered flame front properties (propagation speed and thickness). Then, a three-dimensional turbulent Bunsen flame studied experimentally by Chen et al. (1996) [13] is simulated.