Large-eddy Simulation Cases Research Articles

Analysis of Wind Farms under Different Yaw Angles and Wind Speeds

Wind farm optimization is pivotal in maximizing energy output, reducing costs, and minimizing environmental impact. This study comprehensively explores wind farm behavior under varying wind conditions and yaw angles to achieve these objectives. The primary motivation is to optimize wind farm performance and efficiency through proper yaw adjustment in response to wind speed changes. A computational investigation using a three-by-three wind turbine array was conducted, employing large eddy simulation (LES) to evaluate wind farm performance. Nine LES cases were considered, incorporating three wind speeds (7.3 ms−1, 10.4 ms−1, and 4.3 ms−1) and three yaw angles (30°, 20°, and 0°), with nearly constant turbulence intensity (TI) at 12.0%. The impact of wind speed and yaw angles on wake characteristics and power outputs were analyzed. The findings reveal that wind speed has limited influence on wake characteristics and power outputs, except for lower wind speeds at a yaw angle of 20 degrees. These results contribute to understanding wind farm performance optimization, aiding in developing strategies to enhance energy extraction while minimizing costs and environmental implications.

The Effects of Differential Diffusion on Turbulent Non-Premixed Flames LO2/CH4 under Transcritical Conditions Using Large-Eddy Simulation

In this paper, a large-eddy simulation (LES) of turbulent non-premixed LO2/CH4 combustion under transcritical conditions is performed based on the Mascotte test rig from the Office National d’Etudes et de Recherches Ae´rospatiales (ONERA), and the aim is to understand the effects of differential diffusion on the flame behaviors. In the LES, oxygen was injected into the environment above the critical pressure while the temperature was below the critical temperature. The flamelet/progress variable (FPV) approach was used as the combustion model. Two LES cases with different species diffusion coefficient schemes—i.e., non-unity and unity Lewis numbers—for generating the flamelet tables were carried out to explore the effects of differential diffusion on the flame and flow structures. The results of the LES case with non-unity Lewis numbers were in good agreement with the experimental data. It was shown that differential diffusion had evident impacts on the flame structure and flow dynamics. In particular, when unity Lewis numbers were used to evaluate the species diffusion coefficient, the flame length was underestimated and the flame expansion was more significant. Compared to laminar counterflow flames, turbulence in jet flames allows chemical reactions to take place in a wider range of mixture fractions. The density distributions of the two LES cases in the mixture fraction space were very similar, indicating that differential diffusion had no significant effects on the phase transition under transcritical conditions.

Investigation of the derivation and consistency of the quasi-two-dimensional flamelet models for non-premixed flames

Three non-premixed quasi-two-dimensional flamelet (Q2DF) models can be derived via integrating one-dimensional flamelet libraries, which are generated by premixing the third stream with the fuel stream or (and) the oxidizer stream as the premix fraction (η) varied from zero to unity. These models are attractive and considered effective substitutes for two-dimensional flamelet (2DF) formulations; however, the connections between these Q2DF models and 2DF equations have not been clearly illustrated formerly. In this work, the 2DF formulations are converted from the space defined by two mixture fractions (Z1×Z2) to that used for each Q2DF model (ξ×η) in which ξ designates a newly defined mixture fraction. For moderate Reynolds number conditions, it is considered that the Q2DF models and the 2DF formulations are equivalents as the one-dimensional (1D) flamelet elemental libraries are integrated upon sufficient grids of η. To confirm this, the two-dimensional (2D) direct numerical simulations (DNS) and three-dimensional (3D) large-eddy simulation (LES) are performed to provide reference data since it is readily to associate the 2DF formulations with variable transports in physical space. The 2D DNS cases present various three-feed systems characterized by different third streams and ensure a well-mixed mixture in moderate η, while the 3D LES case is performed with a large flux of pure CO2 as the third stream. Three Q2DF models are compared by using the same tracking parameters obtained from, respectively, the DNSs and LES. The comparisons based on the DNS cases demonstrate that the Q2DF models can reproduce the DNS data with quite satisfying accuracy, and it is confirmed that the accuracy has nothing to do with the type of the third stream. Through the LES results and the related comparisons, it can be concluded that the Q2DF models are reliable to predict the experimental measurements, and on the condition of a sufficient η resolution, the three Q2DF models can give the same prediction in the whole range of η.

Neighborhood scale traffic pollutant dispersion subject to different wind-buoyancy ratios: A LES case study in Singapore

This study explores the buoyancy effect on traffic pollutant dispersion at neighborhood scale, in which a wide range of wind-buoyancy ratios is examined. Pollutant dispersion is solved using a Large Eddy Simulation (LES) turbulence model, where realistic urban morphology and historical traffic pollutant emission data are adopted. For no incoming wind condition, thermal plumes develop and entrain horizontal flow near ground level. This neighborhood scale buoyancy-driven flow leads to air pollutant dispersion, which is as strong as wind-driven dispersion in the target area. As a result, urban air pollutant accumulation is limited due to buoyancy, regardless of ambient wind conditions. A light incoming wind can flush upward thermal plumes downstream when the Richardson number (Ri) reaches 25.5. The interplay of the approaching wind and upward plumes from urban surfaces leads to an oscillatory flow, which intensifies local turbulence and enhances the pollutant removal rate. At the pedestrian level, the neighborhood scale average NO2 concentration due to the traffic emission ranges from 4.1μg/m3 to 4.8μg/m3. The minimum value is found for a weak incoming wind condition, (coupling with buoyancy, Ri = 25.5) and the maximum value is observed for the no incoming wind condition (Ri = ∞). The modelling result analysis clarifies how buoyancy-related dispersion, emission location, surrounding urban density, and incoming wind affect local pollutant concentrations. The research outputs highlight the importance of these coupling effects in the air pollutant dispersion evaluation in real urban areas.

Predictive and stochastic reduced-order modeling of wind turbine wake dynamics

Abstract. This article presents a reduced-order model of the highly turbulent wind turbine wake dynamics. The model is derived using a large eddy simulation (LES) database, which cover a range of different wind speeds. The model consists of several sub-models: (1) dimensionality reduction using proper orthogonal decomposition (POD) on the global database, (2) projection in modal coordinates to get time series of the dynamics, (3) interpolation over the parameter space that enables the prediction of unseen cases, and (4) stochastic time series generation to generalize the modal dynamics based on spectral analysis. The model is validated against an unseen LES case in terms of the modal time series properties as well as turbine performance and aero-elastic responses. The reduced-order model provides LES accuracy and comparable distributions of all channels. Furthermore, the model provides substantial insights about the underlying flow physics, how these change with respect to the thrust coefficient CT, and whether the model is constructed for single wake or deep array conditions. The predictive and stochastic capabilities of the reduced-order model can effectively be viewed as a generalization of a LES for statistically stationary flows, and the model framework can be applied to other flow cases than wake dynamics behind wind turbines.

CFD analysis of combustion and emission formation using URANS and LES under large two-stroke marine engine-like conditions

In this paper, the main aim is to examine the performance of turbulence models to shed light on the effect of turbulence modeling in capturing different in-cylinder phenomena under large two-stroke marine engine-like conditions. The Unsteady Reynolds Averaged Navier–Stokes (URANS) and Large Eddy Simulation (LES) turbulence models are utilized. The LES and URANS results are compared with experimental data, in which LES and URANS models show similar accuracy in capturing the pressure and heat release with a moderately better accuracy in the LES case. The predicted gas temperature at the liner wall is approximately 45% higher for URANS than LES during the expansion stroke, which may lead to different sulfuric acid formation and heat transfer prediction. The LES model predicts a 34% higher average swirl than that in the URANS case which leads to an earlier and a stronger interaction between the flame and the spray, decreasing the oxidation of the emissions. Due to the higher predicted in-cylinder temperature in the LES case, the NO emission amount at exhaust valve opening time (EVO) is 7% higher in the LES case. At EVO, the CO emission in the LES case is predicted to be 3-fold higher than that in the URANS case due to less oxidation of CO in the post oxidation stage in the LES case. The second cycle LES simulation shows that the solutions after the scavenging process are in-sensitive to the initial conditions.

The explicit filtering method for large eddy simulations of a turbulent premixed flame

This paper reports large eddy simulations (LES) of a premixed turbulent round methane-air jet flame at an equivalence ratio of 0.8 and unburnt gas temperature of 800 K using the explicit filtering LES (EFLES) method. The nominal 1D premixed flame speed, sL=2.05ms−1 and thermal thickness, δL=300μm. EFLES is formally derived from the approximate deconvolution method for LES computations. The nominal turbulent Reynolds number Ret=34 and the turbulent Karlovitz number Ka=25 places the present flame in the thin reaction zones regime of turbulent premixed combustion. LES and DNS results are obtained by solving species transport equations using a reduced 13-species chemical mechanism. Chemical source terms in the LES are computed using resolved species and temperature fields in all simulations. Conditional means of net heat release rate predicted by the LES and DNS show excellent agreement. The LES results show a marginally thicker flame brush and a shorter mean flame height when compared to the DNS results. Good agreement between LES and DNS for mean quantities is seen upstream of the tip. Larger quantitative differences are seen at the tip due to the difference in flame height between LES and DNS cases, as may be expected. Even at these positions, qualitative features of the DNS results and peak mean values are recovered correctly. Agreement between conditional mean statistics from both LES and DNS is good. RMS fluctuation amplitudes show larger quantitative differences between LES and DNS when compared to mean statistics while still capturing the correct qualitative features of the DNS profiles. The radial profiles of unconditioned RMS fluctuations are broader by O(δL) in the LES results, with increasing downstream distance. The smaller range of resolved scales in the LES causes the resolved flow motions to be more energetic resulting in increased turbulent transport rates for mean and RMS quantities. This results in the observed broadening of flow statistics in the LES when compared to the DNS. With mesh refinement, differences between LES and DNS results for all statistics are found to diminish smoothly.

Baroclinicity in stable atmospheric boundary layers: Characterizing turbulence structures and collapsing wind profiles via reduced models and large‐eddy simulations

AbstractBaroclinicity is a common, while often overlooked, phenomenon in the atmospheric boundary layer (ABL) that significantly impacts mean wind and turbulence structures. It adds another layer of complexity to the much‐studied stable barotropic ABLs by modulating the pressure gradient in height. Despite the prevalence of baroclinic and stable conditions in nature, our knowledge of their interacting effects on stratified ABL features is limited. The article aims to bridge this knowledge gap by systematically varying baroclinicity and stability using the large‐eddy simulation (LES) technique. The results of 15 conducted LESs indicate that baroclinicity alters the Ekman spiral shape and the low‐level jet's height in stable ABLs. Friction velocity, Obukhov length, shear production and the ABL height are highly influenced by baroclinicity in weakly stable ABLs, whereas they converge to a constant asymptote as the stability increases, independent of the baroclinicity regime. This is attributed to the strong turbulence destruction in very stable ABLs that decouples the surface from higher elevations where the baroclinicity effect is more significant. A reduced model for baroclinic wind profiles is developed and an analytical estimation of that for weakly baroclinic regimes is tested against LES results. Two rescaling methods in the inner and outer layers of stable baroclinic ABLs are used in a novel way to non‐dimensionalize the wind profiles. The suggested approaches reasonably collapse baroclinic/barotropic stable wind profiles in all considered LES cases. This study's findings elucidate the underlying physics of baroclinic stable ABLs and are useful for characterizing the wind profiles in weather/climate models, field measurements, and various industrial applications.

On the Impact of Bed-Bathymetry Resolution and Bank Vegetation on the Flood Flow Field of the American River, California: Insights Gained Using Data-Driven Large-Eddy Simulation

High-fidelity models can simulate many detailed hydrodynamic features of flow in natural rivers. However, it is challenging to explicitly capture the effects of highly complex bathymetry and bank line vegetation on the simulation results of models. Often detailed hydrographic data representing the channel bed is unavailable, requiring the use of more simplified representations of the channel bathymetry. Furthermore, explicit modeling of vegetation is generally not accounted for in most three-dimensional (3D) numerical models, which could lead to significant error in obtaining accurate results. In this study, we have investigated these issues by comparing the effect of (1) different sonar survey resolutions of the channel bed, and (2) trees on the large-eddy simulation (LES) results of a test case in a reach of the American River, California. This comparison has shown that the higher resolution survey reduced the near bed velocities and bed shear stresses and increased the number of vortical structures in areas with highly detailed surface structures. The hydrodynamic results from the tree-resolving LES were compared against a baseline LES case that excluded trees. The results indicate that the inclusion of trees in the simulations can significantly alter the flow dynamics of the river by decreasing the flow momentum and bed shear stress along the banks, while increasing the momentum and stresses in the middle of the river. While trees appear to increase turbulent fluctuations near the bank, these fluctuations probably do not contribute to erosion processes. These findings have the potential to improve the accuracy of high-fidelity modeling of natural streams and rivers and add to the general understanding of the flow dynamics in natural waterways.

Computational efficiency of CFD modeling for building engineering: An empty domain study

In computational wind engineering, Large Eddy Simulation (LES) operates with superior accuracy. However, this comes with a higher computational cost compared to Reynolds Averaged Navier-Stokes (RANS) models. We propose multiple strategies to reduce the computational cost without compromising accuracy. The turbulence synthesis approach is adopted for efficient inflow generation. To alleviate the issues with tedious and resource-consuming computations, the concept of ‘interpolation’ is endorsed. This approach allows the usage of identical inflow information for different arrangements of meshing. The effect of employing larger time-step and coarser mesh, on the performance of LES with wall functions, is also investigated. Several LES cases are studied, initially with special near-wall treatment, and later, with three different Subgrid Scale (SGS) models. The profiles of mean velocities, turbulence intensities, and integral length scales are examined, in addition to the velocity spectra. The horizontal homogeneity is assessed for all cases. The results show that the wall-adapting eddy viscosity (WALE) SGS model with LES can reduce the computational time at a lower error range. Besides, among three investigated hybrid models, the Detached Eddy Simulation (DES) and the Delayed Detached Eddy Simulation (DDES) provide a significant reduction in the computational cost, compared to LES with the dynamic one-equation eddy-viscosity SGS model, when the objective is to achieve a similar level of accuracy.

An investigation on early evolution of soot in n-dodecane spray combustion using large eddy simulation

Numerical simulations using large eddy simulation (LES) and Unsteady Reynolds Averaged Navier–Stokes (URANS) are carried out to identify the underlying mechanisms that govern the early soot evolution process in an n-dodecane spray flame at 21% O2 by molar concentration. A two-equation phenomenological soot model is used here to simulate soot formation and oxidation. Both ignition delay time (IDT) and lift-off length (LOL) are found to agree with experimental measurements. The transient evolution of soot mass, in particularly the soot spike phenomenon, is captured in the present LES cases, but not in the URANS cases. Hence, a comparison of numerical results from LES and URANS simulations is conducted to provide a better insight of this phenomenon. LES is able to predict the rapid increasing soot mass during the early stage of soot formation due to having a large favorable region of equivalence ratio (ϕ>1.5) and temperature (T>1800K) for soot formation. This favorable region increases and then decreases to reach a quasi-steady state in the LES case, while it continues to increase in the URANS simulation during the early time. In addition, the soot spike is a consequence of the competition between soot formation and oxidation rates. The time instance when the total soot mass reaches peak value coincides with the time instance when the total mass of soot precursor reaches a plateau. The soot spike is formed due to the continuous increase of oxidizing species in the LES case which leads to a more dominant oxidation process than the formation process.

Influence of fuel plenum on thermoacoustic oscillations inside a swirl combustor

Large eddy simulation (LES) of a lean methane/air flame exhibiting self-excited thermoacoustic oscillations inside a swirl combustor is performed. The influence of fuel plenum on the combustor dynamics is studied. The velocity, temperature and mixture fraction statistics computed from the two LES cases are compared with the measurements. The results show good overall agreement for both cases with slight improvements by including the fuel plenum. In particular, the near-field mixing and flame lift-off height are captured more accurately. The dominant unstable Helmholtz frequency (≈270 Hz) computed by including the plenum is closer to the measured value (≈290 Hz) compared to that (≈250 Hz) obtained without the plenum. Mode shape analysis shows that including the fuel plenum is necessary for obtaining the phase difference between the pressure oscillations in the combustion chamber and air plenum. It is also found that self-excited oscillations are amplified with an increase by 6% in the mean Rayleigh index and 50% in the gain of the flame transfer function due to the equivalence ratio fluctuations when the fuel plenum is included.

A comparative numerical investigation of reactivity controlled compression ignition combustion using Large Eddy Simulation and Reynolds-Averaged Navier-Stokes approaches

In this work, a comparative numerical study of Reactivity Controlled Compression Ignition (RCCI) combustion was conducted using both the Large Eddy Simulation (LES) and Reynolds-Averaged Navier-Stokes (RANS) approaches. In order to clarify the associated chemical kinetic processes, a data processing method that is able to calculate average reaction pathways, and identify representative reactions for the species creation, consumption, and heat release was adopted. The LES case yielded a wrinkled spray-flame structure, in contrast to the smoother and concentrated structure predicted by the RANS case. However, despite the different distribution features, both cases demonstrated similar reaction pathways and heat release characteristics. Modeling results showed that the low-temperature heat releases in RCCI combustion were controlled by the n-heptane spray-combustion process and dominated by the reactions R8 (C7H15O2 − 2 = C7H15 − 2 + O2), R95 (CH3COCH2 + O2 = CH3COCH2O2), and R285 (HCO + O2 = CO + HO2). In addition, low-temperature consumption pathways of n-heptane led to the substantial formation of a hydroxyl radical, which enhanced the consumption of iso-octane. As the combustion process progressed, high-temperature ignition pockets were found near the piston rim edge and squish regions, where heat release was dominated by the reaction R285. Intense high-temperature heat release was observed instantly from the ignition pockets, resulting in a rapid propagation of the reactive surfaces.

What Determines the Distribution of Shallow Convective Mass Flux through a Cloud Base?

Abstract The distribution of cloud-base mass flux is studied using large-eddy simulations (LESs) of two reference cases: one representing conditions over the tropical ocean and another one representing midlatitude conditions over land. To examine what sets the difference between the two distributions, nine additional LES cases are set up as variations of the two reference cases. It is found that the total surface heat flux and its changes over the diurnal cycle do not influence the distribution shape. The latter is also not determined by the level of organization in the cloud field. It is instead determined by the ratio of the surface sensible heat flux to the latent heat flux, that is, the Bowen ratio B. This ratio sets the thermodynamic efficiency of the moist convective heat cycle, which determines the portion of the total surface heat flux that can be transformed into mechanical work of convection against mechanical dissipation. The thermodynamic moist heat cycle sets the average mass flux per cloud 〈m〉, and through 〈m〉 it also controls the shape of the distribution. An expression for 〈m〉 is derived based on the moist convective heat cycle and is evaluated against LES. This expression can be used in shallow cumulus parameterizations as a physical constraint on the mass flux distribution. The similarity between the mass flux and the cloud area distributions indicates that B also has a role in shaping the cloud area distribution, which could explain its different shapes and slopes observed in previous studies.

Atmospheric Boundary Layer Turbulence Closure Scheme for Wind-Following Swell Conditions

Abstract Over the ocean, atmospheric boundary layer turbulence can be altered by underlying waves. Under swell conditions, the impact of waves on the atmosphere is more complicated compared to that under wind-wave conditions. Based on large-eddy simulation (LES), the wind-following swell impact on the atmospheric boundary layer is investigated through three terms: swell-induced surface momentum flux, the vertical profile of swell-induced momentum flux, and the swell impact on atmospheric mixing. The swell-induced surface momentum flux displays a decreasing trend with increasing atmospheric convection. The swell-induced momentum flux decays approximately exponentially with height. Compared with atmospheric convection, the decay coefficient is more sensitive to wave age. Atmospheric mixing is enhanced under swell conditions relative to a flat stationary surface. The swell impact on the atmospheric boundary layer is incorporated into a turbulence closure parameterization through the three terms. The modified turbulence closure parameterization is introduced into a single-column atmospheric model to simulate LES cases. Adding only the swell impact on the atmospheric mixing has a limited influence on wind profiles. Adding both the impact of swell on the atmospheric mixing and the profile of swell-induced momentum flux significantly improves the agreement between the 1D atmospheric simulation results and the LES results, to some extent simulating the wave-induced low-level wind jet. It is concluded that the swell impact should be included in atmospheric numerical models.

Large-eddy simulation of decaying isotropic turbulence using the flowfield dependent variation method

PurposeThe purpose of this study is to investigate the accuracy and applicability of the Flowfield Dependent Variation (FDV) method for large-eddy simulations (LES) of decaying isotropic turbulence.Design/methodology/approachIn an earlier paper, the FDV method was successfully demonstrated for simulations of laminar flows with speeds varying from low subsonic to high supersonic Mach numbers. In the current study, the FDV method, implemented in a finite element framework, is used to perform LESs of decaying isotropic turbulence. The FDV method is fundamentally derived from the Lax–Wendroff Scheme (LWS) by replacing the explicit time derivatives in LWS with a weighted combination of explicit and implicit time derivatives. The increased implicitness and the inherent numerical dissipation of FDV contribute to the scheme’s numerical stability and monotonicity. Understanding the role of numerical dissipation that is inherent to the FDV method is essential for the maturation of FDV into a robust scheme for LES of turbulent flows. Accordingly, three types of LES of decaying isotropic turbulence were performed. The first two types of LES utilized explicit subgrid scale (SGS) models, namely, the constant-coefficient Smagorinsky and dynamic Smagorinsky models. In the third, no explicit SGS model was employed; instead, the numerical dissipation inherent to FDV was used to emulate the role played by explicit SGS models. Such an approach is commonly known as Implicit LES (ILES). A new formulation was also developed for quantifying the FDV numerical viscosity that principally arises from the convective terms of the filtered Navier–Stokes equations.FindingsThe temporal variation of the turbulent kinetic energy and enstrophy and the energy spectra are presented and analyzed. At all grid resolutions, the temporal profiles of kinetic energy showed good agreement with t(−1.43) theoretical scaling in the fully developed turbulent flow regime, where t represents time. The energy spectra also showed reasonable agreement with the Kolmogorov’s k(−5/3) power law in the inertial subrange, with the spectra moving closer to the Kolmogorov scaling at higher-grid resolutions. The intrinsic numerical viscosity and the dissipation rate of the FDV scheme are quantified, both in physical and spectral spaces, and compared with those of the two SGS LES runs. Furthermore, at a finite number of flow realizations, the numerical viscosities of FDV and of the Streamline Upwind/Petrov–Galerkin (SUPG) finite element method are compared. In the initial stages of turbulence development, all three LES cases have similar viscosities. But, once the turbulence is fully developed, implicit LES is less dissipative compared to the two SGS LES runs. It was also observed that the SUPG method is significantly more dissipative than the three LES approaches.Research limitations/implicationsJust as any computational method, the limitations are based on the available computational resources.Practical implicationsSolving problems involving turbulent flows is by far the biggest challenge facing engineers and scientists in the twenty-first century, this is the road that the authors have embarked upon in this paper and the road ahead of is very long.Social implicationsUnderstanding turbulence is a very lofty goal and a challenging one as well; however, if the authors succeed, the rewards are limitless.Originality/valueThe derivation of an explicit expression for the numerical viscosity tensor of FDV is an important contribution of this study, and is a crucial step forward in elucidating the fundamental properties of the FDV method. The comparison of viscosities for the three LES cases and the SUPG method has important implications for the application of ILES approach for turbulent flow simulations.

Large-eddy simulation and parameterization of buoyant plume dynamics in stratified flow

Characteristics of laboratory-scale bubble-driven buoyant plumes in a stably stratified quiescent fluid are studied using large-eddy simulation (LES). As a bubble plume entrains stratified ambient water, its net buoyancy decreases due to the increasing density difference between the entrained and ambient fluids. A large fraction of the entrained fluid eventually detrains and falls along an annular outer plume from a height of maximum rise (peel height) to a neutral buoyancy level (trap height), during which less buoyant scalars (e.g. small droplets) are trapped and dispersed horizontally, forming quasi-horizontal intrusion layers. The inner/outer double-plume structure and the peel/intrusion process are found to be more distinct for cases with small bubble rise velocity, while weak and unstable when the slip velocity is large. LES results are averaged to generate distributions of mean velocity and turbulent fluxes. These distributions provide data for assessing the performance of previously developed closures used in one-dimensional integral plume models. In particular, the various LES cases considered in this study yield consistent behaviour for the entrainment coefficients for various plume cases. Furthermore, a new continuous peeling model is derived based on the insights obtained from LES results. Comparing to previous peeling models, the new model behaves in a more self-consistent manner, and it is expected to provide more reliable performance when applied in integral plume models.

CFD simulation of the wind environment around an isolated high-rise building: An evaluation of SRANS, LES and DES models

The choice of proper turbulence models in computational fluid dynamics is still a challenging issue for accurately predicting outdoor microclimate and thermal comfort conditions in urban planning. This study compared the performances of the Steady Reynolds Averaged Navier–Stokes (SRANS) RNG k-ε, Large Eddy Simulation (LES) and Detached Eddy Simulation (DES) modeling approaches in simulating the wind flow around an isolated building (with a 1:1:2 shape). The effects of the computational parameters were analyzed, including the grid resolution for all cases, and the discretization time step (Δt) and non-dimensional sampling time (t*) for the LES and DES cases. The results of the LES and DES simulations were affected by the gradual decrease in Δt and increase in t* until the two parameters reached 0.005 s and 288, respectively. The mean velocity fields on the windward side of the building predicted by the three models were in good agreement with the wind tunnel results. However, the results of the LES and DES cases were in better agreement with the experimental results for the leeward and lateral regions in both vertical and horizontal planes. The DDES (Delayed Detached Eddy Simulation) and LES models predicted similar results in the wake region, but the DDES has a lower overall mesh requirement. It is encouraging that the DDES model provides not only the mean flow field, but also the instantaneous wind characteristics, which can be useful for more accurate analysis of outdoor wind and thermal comfort.

LES and RANS of air and oxy-coal combustion in a pilot-scale facility: Predictions of radiative heat transfer

This study evaluates the use of large eddy simulation (LES) and Reynolds-averaged Navier Stokes (RANS) models for the prediction of turbulent coal combustion under air and oxyfuel environments in a pilot-scale 250kWth furnace. The furnace is part of the UKCCSRC Pilot-scale Advanced Capture Technology (PACT) facilities and was designed for detailed analysis of the combustion process. The prediction of thermal radiation is validated against experimental measurements under both air- and oxy-firing regimes. Two radiation models were evaluated during the RANS calculations, the widely used weighted sum of grey gases (WSGG) and the full-spectrum correlated k (FSCK) model, while the LES case was calculated using the FSCK radiation model. The results show that the choice in gas radiation model demonstrates only a small change in the temperature and heat flux predictions in the RANS calculations, while the LES solutions are able to achieve better agreement with measured values than the RANS predictions for both air-fired and oxyfuel coal combustion.

An efficient flamelet-based combustion model for compressible flows

A combustion model based on a flamelet/progress variable approach for high-speed flows is introduced. In the proposed formulation, the temperature is computed from the transported total energy and tabulated species mass fractions. Only three additional scalar equations need to be solved for the combustion model. Additionally, a flamelet library is used that is computed in a pre-processing step. This approach is very efficient and allows for the use of complex chemical mechanisms. An approximation is also introduced to eliminate costly iterative steps during the temperature calculation. To better account for compressibility effects, the chemical source term of the progress variable is rescaled with the density and temperature. The compressibility corrections are analyzed in an a priori study. The model is also tested in both Reynolds-averaged Navier–Stokes (RANS) and large-eddy simulation (LES) computations of a hydrogen jet in a supersonic transverse flow. Comparison with experimental measurements shows good agreement, particularly for the LES case. It is found that the disagreement between RANS results and experimental data is mostly due to the mixing model deficiencies and the presumed probability density functions used in the RANS formulation. A sensitivity study of the proposed model shows the importance of the compressibility corrections especially for the source term of the progress variable.