Large Eddy Simulation Simulations Research Articles

LES and RANS Spray Combustion Analysis of OME3-5 and n-Dodecane

Clean-burning oxygenated and synthetic fuels derived from renewable power, so-called e-fuels, are a promising pathway to decarbonize compression–ignition engines. Polyoxymethylene dimethyl ethers (PODEs or OMEs) are one candidate of such fuels with good prospects. Their lack of carbon-to-carbon bonds and high concentration of chemically bound oxygen effectively negate the emergence of polycyclic aromatic hydrocarbons (PAHs) and even their precursors like acetylene (C2H2), enabling soot-free combustion without the soot-NOx trade-off common for diesel engines. The differences in the spray combustion process for OMEs and diesel-like reference fuels like n-dodecane and their potential implications on engine applications include discrepancies in the observed ignition delay, the stabilized flame lift-off location, and significant deviations in high-temperature flame morphology. For CFD simulations, the accurate modeling and prediction of these differences between OMEs and n-dodecane proved challenging. This study investigates the spray combustion process of an OME3 − 5 mixture and n-dodecane with advanced optical diagnostics, Reynolds-Averaged Navier–Stokes (RANS), and Large-Eddy Simulations (LESs) within a constant-volume vessel. Cool-flame and high-temperature combustion were measured simultaneously via high-speed (50 kHz) imaging with formaldehyde (CH2O) planar laser-induced fluorescence (PLIF) representing the former and line-of-sight OH* chemiluminescence the latter. Both RANS and LES simulations accurately describe the cool-flame development process with the formation of CH2O. However, CH2O consumption and the onset of high-temperature reactions, signaled by the rise of OH* levels, show significant deviations between RANS, LES, and experiments as well as between n-dodecane and OME. A focus is set on the quality of the simulated results compared to the experimentally observed spatial distribution of OH*, especially in OME fuel-rich regions. The influence of the turbulence modeling is investigated for the two distinct ambient temperatures of 900 K and 1200 K within the Engine Combustion Network Spray A setup. The capabilities and limitations of the RANS simulations are demonstrated with the initial cool-flame propagation and periodic oscillations of CH2O formation/consumption during the quasi-steady combustion period captured by the LES.

Large eddy simulation of a turbulent channel flow using dynamic Smagorinsky subgrid scale model and differential equation wall model

A large eddy simulation (LES) is conducted in a plane channel flow which is turbulent. For understanding the mechanism of the turbulent motion more efficiently, the present study is an attempt to conduct the LES using a Dynamic Smagorinsky Model (DSM) and a Differential Equation Wall Model (DEWM). DSM is a subgrid-scale model and DEWM is for approximating the near wall layer. The simulation is performed in a domain of 2 π δ × 2 δ × π δwhich is discretized by 32 × 20 × 32grid points where δ is the channel half width. The discretization techniques have been used in the simulation, are the 3rd order low storage R-K method in time and 2nd order finite difference formulation in space. To assess the performance of the LES with DSM and DEWM, the simulation results are compared with direct numerical simulation (DNS) data and LES data using Standard Smagorinsky Model (SSM). Comparing the results in the essential turbulence statistical field it has been found that DSM performs better than SSM in most of the cases and shows better agreement with the DNS results. Flow structures in the turbulent flow field are compared in isosurfaces and contour plots, and found that tube-like vortical structures are randomly distributed over the entire flow field, and the number of the vortical structures are more for the DSM case than that of SSM.

Turbulent Particle Flow in Spent Nuclear Fuel Storage Systems

Deposition models are being developed with the commercial computational fluid dynamics software STAR-CCM+ to evaluate particulate deposition on spent nuclear fuel (SNF) canisters. The primary particulate of concern is chloride salts, which are dispersed in the atmosphere and then deposited onto the canisters. During dry storage, the primary degradation process is likely to be chloride-induced stress corrosion cracking (CISCC) at the heat-affected zones of the canister welds. It is known that stainless steel canisters are susceptible to CISCC; however, the rate of chloride deposition onto the canisters is poorly known, based on sparse field data from a small number of sites. This paper describes work looking at various approaches to modeling turbulence, such as Reynolds-averaged Navier Stokes (RANS) and large eddy simulation (LES), and its impact on particle flow and deposition within a ventilated SNF storage system. The deposition rate is determined for a vertical canister system and a horizontal canister system. LES has the potential to simulate turbulent flows more accurately versus RANS, but is much more computationally expensive. A k-omega version of the RANS turbulence model was used for this study. The computational efficient RANS steady-state model predicted a similar canister deposition result as the LES simulation for a vertical canister storage system. For a horizontal storage system, the RANS steady-state model predicted more particles depositing on the canister than the LES simulation, providing a conservative estimation for particle deposition. A wall correction factor was added to the RANS model to dampen the turbulence fluctuation normal to a surface that left undamped, leads to the RANS model overpredicting deposition along a surface for smaller particles. This work was done to further development of deposition models that could be used to plan and inform SNF canister aging management programs with predictive models for the timing and occurrence of canister CISCC. This work is part of a larger effort tasked with understanding the likelihood of canister degradation due to CISCC. These models are still under development, and testing is needed for validation. However, these models are being presented now to demonstrate to canister vendors, utilities, regulators, and stakeholders the value of this type of modeling.

Flow-induced Vibrations of Dual-cylinders in Axial Flow via LES Simulations

The axial-flow-induced vibration (AFIV) of fuel rods in the nuclear power plant is closely related to nuclear safety. In this article, a numerical study is performed on vibration of two elastic cylinders arranged side-by-side in axial flow. Large eddy simulation (LES) is employed to predict the turbulent flow. The numerical method has been verified using the experimental root-mean-square vibration amplitude of a single cylinder. A wide range of inflow velocities u*, incident turbulence intensity Tu and space ratio P/D have been examined, where D and P are the diameter and centre-to-centre distance of the cylinders, respectively. The results show that the vibration amplitudes increase with an increasing u*, comparable to the case of a single cylinder in axial flow. However, the two cylinders could bend outwards during a relatively high u* and low Tu. Although Tu significantly affects the amplitudes of the cylinders, it does not change the vibration frequency and the critical velocity at which buckling instability occurs. As the gap between the two cylinders is sufficiently small, the vibration amplitude enhances significantly due to the pronounced hydrodynamic interaction between the two elastic cylinders and surrounding fluid. The direction of buckling is no longer random but fixed.

Turbulent mixed convection in a horizontal cylindrical cavity with the off-lattice Boltzmann method

Large-eddy simulations (LES) of turbulent mixed convection in a horizontal concentric cylindrical cavity when cylinders are subjected to rotations are investigated in the present study. A characteristic-based finite-difference off-lattice Boltzmann method solver is employed to investigate whether rotation enhances heat transfer in a horizontal concentric cylindrical cavity configuration where buoyancy acts perpendicular to the cylinder axis. The study focuses on assessing heat transfer characteristics for different rotation intensities in both counter-rotating-cylinders (CRC) and outer-rotating-cylinder (ORC) configurations. The LES simulations are performed for Rayleigh number, Ra=106, and Reynolds numbers, Re=0,500,1000, and 5000 for air (Pr=0.71), employing a classical Smagorinsky sub-grid-scale (SGS) model. The numerical solver is verified using two benchmark problems, namely (i) Buoyancy-driven flow in a differentially heated air-filled cubic enclosure and (ii) turbulent Taylor-Couette flow. The CRC and ORC configurations are chosen for the three-dimensional simulations in this study, prompted by the notable improvement in heat transfer observed in their respective two-dimensional simulations. A thorough comparison between two- and three-dimensional mixed convection results reveals substantial influence from secondary flow in the axial direction. While three-dimensional CRC simulations exhibit a modest increase in heat transfer at Re=500, the ORC configuration experiences a decrease in heat transfer rate as the Reynolds number increases, contrasting with two-dimensional simulation trends. The CRC configuration enhances the axial motion of the flow with Reynolds number, and its magnitude is larger than that of the ORC configuration and natural convection. A significant difference between two-dimensional and three-dimensional mixed convection simulation results is observed because of considerable secondary flow. Furthermore, the flow and thermal characteristics of turbulent mixed convection are investigated using streamlines, isotherms, Q-criterion, Nusselt number variation along the cylinders, anisotropy analysis, turbulent kinetic energy (TKE), dissipation, contours of radial and axial components of velocity.

Role of turbulence in ozone chemistry: Segregation effect implicated from multiscale modeling over Hong Kong

By influencing the mixing of reactive species in the atmosphere, turbulence affects the rates at which chemical species react, specifically in the planetary boundary layer. To analyze the turbulent segregation effect on the atmospheric ozone photochemistry, coupled meso-to micro-scale simulations are performed over Hong Kong, a polluted megacity. The comparison of the ozone production rates between the mesoscale model and the large-eddy simulations (LES) indicates that the resolved turbulent processes inhibit the mixing of the reactants, and thereby slow down the reaction rate between NO and O3, resulting in a smaller net O3 production when represented by a LES simulation. The calculated segregation intensities show clear correlations with the emission distribution, revealing that the segregation effect is significant in urban areas with inhomogeneous pollution sources, including traffic emissions. The evolution of the eddy structure plays a significant role in the diurnal variation of the segregation intensity. Applying the obtained segregation intensities to the mesoscale output, the NO–O3 reaction rate decreases with results closer to those provided by the LES model. This confirms the importance of the segregation effect on fast chemical reactions and provides insights into the parameterization of the segregation effect in mesoscale models, which do not resolve turbulent eddies.

Impact of Momentum Perturbation on Convective Boundary Layer Turbulence

AbstractMesoscale‐to‐microscale coupling is an important tool for conducting turbulence‐resolving multiscale simulations of realistic atmospheric flows, which are crucial for applications ranging from wind energy to wildfire spread studies. Different techniques are used to facilitate the development of realistic turbulence in the large‐eddy simulation (LES) domain while minimizing computational cost. Here, we explore the impact of a simple and computationally efficient Stochastic Cell Perturbation method using momentum perturbation (SCPM‐M) to accelerate turbulence generation in boundary‐coupled LES simulations using the Weather Research and Forecasting model. We simulate a convective boundary layer (CBL) to characterize the production and dissipation of turbulent kinetic energy (TKE) and the variation of TKE budget terms. Furthermore, we evaluate the impact of applying momentum perturbations of three magnitudes below, up to, and above the CBL on the TKE budget terms. Momentum perturbations greatly reduce the fetch associated with turbulence generation. When applied to half the vertical extent of the boundary layer, momentum perturbations produce an adequate amount of turbulence. However, when applied above the CBL, additional structures are generated at the top of the CBL, near the inversion layer. The magnitudes of the TKE budgets produced by SCPM‐M when applied at varying heights and with different perturbation amplitudes are always higher near the surface and inversion layer than those produced by No‐SCPM, as are their contributions to the TKE. This study provides a better understanding of how SCPM‐M reduces computational costs and how different budget terms contribute to TKE in a boundary‐coupled LES simulation.

Large-Eddy Simulation Study of Injector Geometry and Parcel Injection Location on Spray Simulation of the Engine Combustion Network Spray G Injector

Abstract Recent improvements in computing power and numerical techniques have enabled us resolve minute details of spray plume behavior and its wall boundary interactions, and made detailed spray simulations using large-eddy simulation (LES) turbulence models available to many more engineers. However, guidelines for parcel-based spray simulation boundary and initial conditions are still based on results from lower-resolution and Reynolds averaged Navier–Stokes (RANS) simulations. Hence, it is necessary to critically examine those assumptions and compare their impact with results using a RANS turbulence model. Three different parameters, including whether a simulation includes a detailed injector tip geometry or a flat surface, whether parcels are initialized at the counterbore exit, which is more common, or at the nozzle exit, and whether to use an experimentally derived rate of injection or one-way coupling with a separate internal nozzle volume of fluid simulation, were examined with an LES turbulence model. Both local data close to the injector and global penetration results were used to compare simulations. Local data, such as the local liquid volume fraction, showed greater variation between the conditions, which may have an impact on mixing and combustion predictions in engine applications. Spray penetration and other global measures demonstrated limited sensitivity to the boundary conditions/initialization procedure. Results were also compared with prior results that used a RANS turbulence model. RANS simulations had overall smoother responses to the changes, as would be expected, but LES simulations showed similar trends in the effects for the measured variables.

NUMERICAL INVESTIGATION OF TURBULENCE CHARACTERISTICS AND SELF-SIMILARITY IN A HIGHLY AERATED STABLE HYDRAULIC JUMP USING LARGE EDDY SIMULATION

This work presents a numerical study of a stable hydraulic jump at Froude number 4.25 and Reynolds number 1.15×105 inside a horizontal and rectangular channel with a length of 3.2 m, a width of 0.5 m and a height of 0.4 m using large eddy simulation (LES). Classical hydraulic jump characteristics are obtained, such as conjugate depths, jump length, void fraction and velocity profiles. The hydraulic jump maximum streamwise velocity decay and shear layer spreading rate are simulated and compared with experimental data. For these parameters, numerical results demonstrate that is possible to stablish an analogy with other shear flows, such as the horizontal plane wall jet. Profiles of streamwise and vertical components of mean velocity are simulated, and self-similarity is observed for cross-sections located at the recirculation region of the jump. Self-similarity is also observed in terms of turbulent fluctuations, insofar as LES simulations indicate a high level of turbulence in the recirculation region. The simulated root mean square of streamwise velocity fluctuations, , ranges from 0.5 to 0.7 of the maximum cross-sections velocity, whereas the root mean square of vertical component of velocity fluctuations, , stays around 0.5 of the maximum cross-sections velocitiy. All validation comparisons show good agreement with the selected experimental data of Kramer and Valero (2020) and Wang (2014), presenting average deviations always lesser than 5%.

The effects of swirler vane configurations on the vortex-mixing interaction and combustion stability in a low-emission combustor

Particle Image Velocimetry (PIV) measurements and Large Eddy Simulation (LES) simulations are conducted for non-reacting and reacting flows to investigate the effects of swirler vane configurations on the vortex-mixing interaction and combustion stability in a low-emission tower-type coaxial-staged combustor (LETCC). Results show that the LES simulation results agree well with the experimental data. The combustor without vane lobes exhibits large mixture fraction bubbles in the inner shear layer (ISL) in non-reacting flow, and shows unstable combustion in reacting flow. Adding lobes to the first main stage swirler vanes eliminates instability factors in both non-reacting flow and reacting flow. Adding lobes to the second main stage swirler vanes suppresses mixture fraction bubbles in the ISL, but forms large areas with low mixture fraction in the outer shear layer (OSL) and outer recirculation zone (ORZ) in non-reacting flow, and induces localized flame extinction and re-ignition within the ORZ in reacting flow, leading to combustion instability.

Large Eddy Simulation of the Flow around a Generic Submarine under Straight-Ahead and 10° Yaw Conditions

Aiming towards a better understanding of the flow field around a fully appended Joubert BB2 submarine model, and in order to complement the experimental investigations of the wake of the hydroplanes and sail, large eddy simulation (LES) with the dynamic Smagorinsky model was conducted. Three sets of grids with a maximum grid number of up to 228 million were designed to perform the LES simulation for the Joubert BB2 under 10° yaw conditions, with a freestream Reynolds number based on the local freestream velocity and a hull length of ReL = 2.2 × 107. Comparisons of the wake of the cruciform appendage were made with experiments to verify the computational accuracy and to examine the influence of the spatial resolution. A satisfactory result was more representative of the experiments with the improvement in grid spatial resolution. The evolution characteristics of three co-rotating vortices originating from the cruciform appendage under the most refined grid arrangement are further described in detail under straight-ahead and 10° yaw conditions. The comparison results show that, in the core-flow region, the resultant velocity, vorticity magnitude, and TKE were stronger and the wake was more complicated under 10° yaw conditions. Tip vortex tracking under 10° yaw conditions exhibited significant three-dimensional characteristics as the wake developed downstream.

Fluid–structure interaction on vibrating square prisms considering interference effects

Existing research on interference effects predominantly focuses on rigid structures. However, studies based on rigid models tend to overlook the feedback of structural motions on the flow field, thus failing to capture the intrinsic dynamics of the interference effect induced by wind-induced structural vibrations. This paper provides a comprehensive analysis of the fluid–structure interaction mechanisms considering interference effects involving two parallel square prisms, employing large-eddy simulation (LES). Various factors, including wind speed, arrangement, and vibration amplitude, are meticulously considered in the analysis. The study utilized three-dimensional LES simulations, incorporating the narrowband synthesis random flow generator method for inlet turbulence generation and adjusted through the “feedback” approach to ensure accuracy and efficiency. The research highlighted different structural arrangements exhibited distinct interference effects, and the end effect of the structure could substantially modify the flow pattern at various heights. In the tandem arrangements, the study observed several flow phenomena, including early reattachment, attenuation of the end effect, premature formation of roll structures, increased turbulence in the flow field due to vibration, resulting in wider second leading-edge separation, and a fragmented wake flow on the downstream structure. For side-by-side arrangements, the “acceleration effect” was identified and found to be further intensified by structural vibrations. The vibration of the interfering structure was noted to cause changes in vortex shedding frequencies and alterations in the wake flow pattern. In addition, vibration would enhance the interference effect but increasing amplitude and wind speed might diminish the interference effect. Overall, this study offers valuable insight into the intricate interplay of factors influencing the aerodynamics of parallel structures across diverse arrangements and under varying conditions.

Application of computationally inexpensive CFD model in steady-state and transient simulations of pulverized sewage sludge combustion

The combustion process of pulverized fuels with high ash contents, such as sewage sludge, is significantly impacted by the amount of slag deposition in a furnace. A CFD model based on the Steady Diffusion Flamelet (SFM) approach was applied to numerically simulate the combustion of pulverized sewage sludge in a drop tube furnace. Steady-state solutions of the Reynolds-Averaged Navier–Stokes (RANS) equations already displayed good agreement with species and temperature measurements. Still, they failed to accurately predict the significant amount of ash trapped on the internal furnace surfaces (minimum deposition of 28% of the generated ash versus 5% predicted by the RANS simulations). The SFM-based CFD model’s computational efficiency enabled the conduction of large eddy simulations (LES), significantly improving the model’s predictive capabilities (27% of the generated ash deposited). Additionally, the transient simulations further improved the agreement with temperature measurement data. A novel initialization procedure was developed, which allowed the transient LES simulations to be conducted in a computationally efficient manner. The SFM-based model can effectively support research and development efforts, even for large-scale systems which require high cell counts. It provides valuable insights during the early design phases of industrial furnaces for pulverized sewage sludge combustion.

A Mapping Model of Propeller RANS and LES Flow Fields Based on Deep Learning Methods

In this work, we propose a deep-learning-based mapping model for simulating and predicting the flow field of Reynolds-averaged Navier–Stokes (RANS) and large eddy simulation (LES) of propellers. The model employs image processing and computer vision methods to process the two-dimensional propeller RANS and LES simulation data. First, images are obtained by simulating the flow fields with the location data used to acquire a set of features specific to the corresponding positions. Second, the regression models for the flow fields and the mapping between the two different flow fields are established to predict the LES flow field at that position. Specifically, we utilize a deep convolutional neural network (CNN) for feature extraction from the flow field, which is then integrated with a nonlinear module for the purposes of regression and mapping. The effectiveness and accuracy of the proposed model in flow field prediction are demonstrated by its application to propeller RANS and LES simulations. It is shown that the overall error rate between the LES flow field predictions generated using this method and actual flow field data is 7.92%. Additionally, we also evaluate the model’s generalization ability, stability, and robustness by testing it on the data of propeller flow fields at different Reynolds numbers. The results verify the applicability of the proposed model in various problems of flow field simulation and prediction.

Performance analysis of vertical-axis wind turbine clusters: Effect of inter-turbine spacing and turbine rotation

Wind energy has emerged as a viable alternative to fossil fuels, with vertical-axis wind turbines (VAWTs) gaining popularity due to their efficiency and adaptability. Combining the actuator line method (ALM) with large eddy simulation (LES) enables accurate performance evaluations, facilitating the design and optimization of wind turbines. The present study invokes ALM-based methodology to perform calculations for the VAWTs. The results of the LES simulations of the VAWTs have been extensively validated against the available experimental and numerical data. The study further explores a VAWT cluster of three turbines by investigating the influence of turbine spacing (in both inline and staggered configuration) on cluster performance. This study shows that the configuration with a streamwise separation (Xsep) of 0.34 D and a transverse separation (Ysep) of 2.5 D exhibits superior performance to other combinations owing to increased kinetic energy in the wake for the downstream turbines. Furthermore, we have presented the effect of varying the rotation direction (in combinations of Clockwise and Counterclockwise rotation) for the individual turbines in the 3-turbine cluster for the two configurations: inline (Xsep = 0 D, Ysep = 2.5 D) and staggered (Xsep = 0.34 D, Ysep = 2.5 D). Staggered counter-rotating turbine cases show reduced performance compared to co-rotating cases, specifically, the clockwise co-rotating (C-C-C) configuration. In the inline configuration, counter-rotating setups outperform co-rotating ones. Counter-rotation analysis reveals that reducing streamwise separation allows turbines to align in line without sacrificing performance, thereby increasing the power density of the turbine cluster.

LES MODELING OF HIGH-LIFT HIGH-WORK LPT BLADES: PART II—VALIDATION AND APPLICATION

Abstract Over the years, computational fluids dynamics (CFD) has matured to such a state so as to be indispensable in turbine design. In the past two decades, significant advances in turbine design have been made through the use of CFD—in particular, through the use of Reynolds-Averaged Navier-Stokes (RANS) simulations. Currently, turbine design is RANS-driven; however, significant gains in performance and efficiency are becoming more difficult to achieve using RANS. For this reason, the turbomachinery CFD community is moving toward Large-Eddy Simulations (LES). In the design of low-pressure turbine (LPT) blades, LES is particularly beneficial owing to its ability to capture accurately both transition and separation. In this paper, LES is used to characterize a new family of high-lift high-work LPT blades—designated the LXFHW-LS family—designed at the U.S. Air Force Research Laboratory (AFRL). LES simulations are conducted in accordance with the methodology outlined in Part I of this paper. The purpose of this paper is twofold: 1) to use LES to predict the performance of the LXFHW-LS family and compare to measurements in a low-speed linear cascade and, in doing so, 2) to illustrate how LES may be used in LPT design as it evolves from RANS-driven to LES-driven. For each blade in the family, the loading distribution and loss coefficient are computed for sixteen separate Reynolds numbers. Computational results are validated using detailed experimental measurements from a low-speed linear cascade wind tunnel.

Wind Tunnel and Grid Resolution Effects in Large-Eddy Simulations of the High-Lift Common Research Model

This paper describes the application of a compressible large-eddy simulation (LES) solver to the flow over the High-Lift Common Research Model (CRM-HL) in landing configuration, a complex external aerodynamic flow configuration with deployed slats, flaps, a flow-through nacelle, and the associated brackets/fairings on the high-lift devices. The bulk Mach number is (0.2) and the mean-aerodynamic-chord-based Reynolds number is typical of a wind tunnel experiment (5.49E6). A key development in these simulations relative to previous work is that this work serves to establish robustness of LES methods to Reynolds number and aircraft configuration. Previous studies focused on the Japan Aerospace Exploration Agency Standard Model, which featured a nearly 3× lower Reynolds number than the present CRM-HL investigation and less aggressive wing/slat leading-edge curvature than the CRM-HL geometry, which led to lower leading-edge pressure suction peak magnitudes, both of which led to less stringent grid resolution requirements. In these LES simulations, an algebraic equilibrium wall modeling approach is employed along with a dynamic implementation of the Smagorinsky subgrid-scale model. The calculations are carried out in both a free air setting and one that includes the wind tunnel facility at seven angles of attack at five grid resolution levels, ranging from ≈10 to 1500 million control volumes. In free air, the solutions show decreasing sensitivity to the grid with each successive refinement level and systematically approach the experimental lift coefficient data as the grid is refined, with the 1.5 billion control volume case showing excellent agreement with the corrected experimental data. The simulations in both free air and in the wind tunnel predict a stall mechanism featuring a large inboard juncture stall and a nose-down break in the pitching moment curve, both of which agree with the experimental observations. The accuracy of the simulations is assessed via comparisons of integrated forces/moments, surface pressures, and surface skin friction visualizations. Graphics processing unit (GPU) acceleration of the charLES solver results in tractable turnaround times that make LES a useful tool in the aerospace industry design cycle. Recent GPU acceleration of the flow solver has made LES solutions that are highly accurate in lift/drag/moment for relevant high-lift aircraft flows achievable within about 5 h of wall time on 600 GPU cores.

Characteristics of instantaneous intersecting airflow from main nozzle and various relay nozzles in an air jet loom

Due to the low insertion speed and poor weft motion stability with single round-hole relay nozzle in the air jet loom, this study is aims to investigate the characteristics of transient intersecting airflow from a main nozzle and tri-elliptical hole relay nozzles. The turbulent intersecting airflow was simulated based on a large eddy simulations (LES). The correctness of the LES model was evaluated by comparing the time-averaged axial velocity with experimental and the Reynolds-averaged Navier–Stokes (RANS) results. The LES simulation has shown that only one potential-core arose its height about 4 mm for single round hole relay nozzle, whereas three potential-core arouse its height about 4 mm for tri-elliptical hole relay nozzles. It is possible to enhance the mean axial velocity of the intersecting airflow by up to 10% and decrease fluctuation magnitude by 25% using a new type of tri-elliptical center array relay nozzle.

CFD simulation of an axial discharge rotor-stator mixer: LES versus RANS predictions

The large eddy simulation (LES) technique is used to model the turbulent flow of water in a two-stage, axial discharge, in-line rotor-stator mixer. The WALE sub-grid scale model is employed, and the resulting filtered velocity field is ensemble averaged to predict the mean velocity, turbulent kinetic energy, energy dissipation rate, and energy production rate. The results of these 3-D sliding mesh simulations are compared to analogous RANS predictions on similar meshes using the realizable k-ε turbulence model. Stator slot flow rate and rotor torque/power draw are also monitored. RANS and LES predict similar mean velocity profiles, flow rate and rotor torque. LES predicts higher turbulent kinetic energy, but lower dissipation rate compared to RANS. Results from a coarse and refined computational mesh are compared to discuss the significance of resolving smaller scales for LES. The effect of operating conditions on throughput, power draw, and other quantities of practical utility are also discussed. It was found that prediction of macroscopic quantities such as flow rate and power draw can be done efficiently using either RANS or LES on a relatively course mesh while capturing turbulence quantities requires a well refined LES simulation.

Impact of external turbulence on film cooling designs using large eddy simulations

Large Eddy Simulations (LES) of film cooling flows were conducted to investigate the impact of high turbulence intensity in the main flow, representative of jet-engine combustor conditions. A comparison was made with LES simulations at zero turbulence at the main flow inlet boundary. The study investigated three different film cooling configurations, which involved cylindrical holes and two variations with holes embedded in trenches known as transverse and segmented trench. Different momentum ratios (I=3.5 and 8.3) relevant to combustor flows in jet-engines were applied. The LES simulations utilized OpenFOAM and solved the energy and passive scalar transport equations to determine film cooling efficiency (η), heat transfer coefficient (hf), and net heat flux reduction (NHFR). The numerical results were validated through comparison with experimental data. The mean and instantaneous results revealed that high turbulence intensity in the main flow significantly influenced film cooling performance. The film cooling efficiency was found to decrease with increased turbulence intensity, while for the ordinary film cooling design exhibiting jet lift-off, the increased mixing led to higher η. The heat transfer coefficient increased with turbulence intensity and momentum ratio, with turbulence intensity having a greater impact on ordinary designs and momentum ratio on trenched designs. Moreover, increasing turbulence intensity or momentum ratio resulted in lower NHFR for all designs. Hot spots were observed in the instantaneous results, with spatial variations depending on the film cooling design.