Dynamic Large Eddy Simulation Research Articles

Hidden mechanism of dynamic large-eddy simulation models

Hidden mechanism of dynamic large-eddy simulation models

Numerical analysis of a parallel triple-jet of liquid-sodium in a turbulent forced convection regime

In the present study, we have applied a combined wall-resolving dynamic Large-Eddy Simulation (LES) (for the velocity field) and Direct Numerical Simulation (DNS) (for the temperature field) approach for mixing of parallel triple-jets with different temperatures of liquid sodium in a turbulent forced convection regime. Because of the high thermal conductivity of sodium (a low-Prandtl fluid), we adopted the dynamic Smagorinsky subgrid closure for the unresolved velocity scales, while the thermal scales are fully resolved. Furthermore, the Time-dependent Reynolds-Averaged Navier-Stokes (T-RANS) approach with the high-Reynolds number variant (i.e. with the wall functions as boundary conditions along solid boundaries) of the four-equation eddy viscosity model (k−ε−kθ−εθ) was applied. The fine-mesh LES/DNS provided a close agreement with the experimental data for both velocity and temperature fields (for both first- and second-moments). In contrast, the coarse-mesh LES/DNS overestimated the turbulent kinetic energy profiles at different distances from the inlet plane. The T-RANS results confirmed a good agreement with the mean streamwise velocity and turbulent kinetic energy, as well as the mean temperature profiles. Finally, the analysis of power spectral density distributions of the temperature signal revealed that all simulation techniques captured a dominant flow frequency originating from the induced Kelvin-Helmholtz instabilities between the side and central jets. The presented combined dynamic LES/DNS approach is recommended for future simulations of the turbulent forced convection flows of low Prandtl fluids, especially if thermal fatigue effects need to be predicted correctly.

Coarse grained simulation and dynamic bridging for turbulent mixing predictions

We revisit coarse-grained simulation strategies for turbulent material mixing driven by shocked/accelerated material interfaces, based on LANL’s Radiation Adaptive Grid Eulerian (xRAGE)–Large-Eddy Simulation (LES), and dynamic bridging xRAGE LES and Besnard–Harlow–Rauenzahn Reynolds-Averaged-Navier–Stokes using Low-Mach-Corrected hydrodynamics. Tests of the new simulation paradigms demonstrate improved scale-resolving, enabling higher simulated mixing and turbulence levels on coarser grids. Impact assessments are carried out based on simulations of shock-tube experiments.

SIMULATING SPRAY DYNAMICS WITH A FINITE ELEMENT METHOD FOR INTERNAL COMBUSTION ENGINES USING LARGE EDDY SIMULATIONS

Spray dynamics in an internal combustion engine is comprised of complex phenomena while interacting with unsteady turbulence. Physics requires detailed modeling of the dynamics for spray and carrier gases to accurately predict a spray's fate. Large eddy simulation (LES) turbulence modeling approaches are capable predictors of the turbulent processes and of dynamically modeling sub-grid scales, therefore enabling calculation of model coefficients for the smallest resolved scale. Dynamic LES methods are also well suited for unsteady flows associated with spray injection and engine fluid dynamics. In this study, for the first time, a dynamic Verman LES scheme employed in a stabilized a finite element (FE) framework is used to model the spray process with emphasis on injected fuels for simulating internal combustion engines. Spray modeling often comprises a coupled Eulerian-Lagrangian approach to capture the droplet/particle dynamics, where the droplets are modeled in the Lagrangian frame, interacting with the gas. The momentum and heat exchange between the fluid gases and the evaporating and atomizing spray droplets are modeled in a two-way coupling system as described in this paper. Direct injected liquid is modeled in this paper as a spherical ligament of fuel and ligament breakup to atomization use our version of the Kelvin Helmholtz break-up scheme. Discussed are models and methods of the whole system in some detail. The method for simulation of the fluid's momentum, heat transfer, and turbulence are discussed as is the system to evaluate droplet or ligament properties. Validation or results of the modeling are presented on test cases as determined by the Engine Combustion Network, base cases G along with some benchmarking cases. We demonstrate in the paper by using the FE method combined with the Verman LES model, and our version of the Kelvin Helmholtz-Rayleigh Taylor spray break-up model combined with two-way coupling is quite capable of modeling sprays, having distinct and significant advantages over the more traditional finite volume discretization framework using similar spray modeling methods.

Numerical study on aerodynamic characteristics and flow field of a circular cylinder equipped with a C-ring behind at Re = 1000

Slender cylindrical structures such as cables and signal poles often experience vortex-induced vibrations. Recently, scholars have developed a novel mitigating measure by equipping a C-ring on the structures, where the C-ring is always behind the cylinder by tail rudder, but the mitigation mechanism is still unclear. To this end, the OpenFOAM©, an open source software, has been adopted in this study to carry out the large-eddy simulations (LES) to investigate the influence of a C-ring on the aerodynamic forces, surface pressure, and flow around a circular cylinder at Re = 1000, where the dynamic k-equation LES model is employed. Although the Re is far smaller than that in the reality, the aerodynamic characteristics and flow is also representative and worth studying in this subcritical range. Furthermore, to determine the appropriate radii ( R) and arc lengths ( θ) of the C-ring, various values of R (0.7 D, 0.8 D, 0.9 D, and 1.0 D, where D is diameter of the cylinder) and θ (60°, 90°, 120°, 150°, and 180°) are considered and studied herein. The results indicate that a C-ring can reduce the SD of the lift coefficient by 40%–90% compared to that of a bare cylinder, whereas it has an unremarkable effect on the mean drag coefficient. The C-ring also has a significant influence on the fluctuating pressure coefficients, resulting in a small SD of the fluctuating lift. The spanwise flow on the leeward side of the cylinder under the influence of C-rings with different radii and arc lengths can be categorized into three modes. Meanwhile, the C-ring mitigates wind-induced vibrations by enhancing the spanwise flow on the leeward side of the cylinder. Finally, the appropriate parameters for the C-ring are recommended as follows: R = 0.7 D and θ = 180°; R = 0.8 D and θ = 150°; R = 0.9 D and θ = 150°; R = 1.0 D and θ = 120°, 150°.

Coaxial jets with disparate viscosity: mixing and laminarization characteristics

Mixing of fluids in a coaxial jet is studied under four distinct viscosity ratios, $m=1$ , $10$ , $20$ and $40$ , using highly resolved large-eddy simulations (LES), particle image velocimetry and planar laser-induced fluorescence. The accuracy of predictions is tested against data obtained by the simultaneous experimental measurements of velocity and concentration fields. For the highest and lowest viscosity ratios, standard RANS models with unclosed terms pertaining to viscosity variations are employed. We show that the standard Reynolds-averaged Navier–Stokes (RANS) approach with no explicit modelling for variable-viscosity terms is not applicable whereas dynamic LES models provide high-quality agreement with the measurements. To identify the underlying mixing physics and sources of discrepancy in RANS predictions, two distinct mixing modes are defined based on the viscosity ratio. Then, for each mode, the evolution of mixing structures, momentum budget analysis with emphasis on variable-viscosity terms, analysis of the turbulent activity and decay of turbulence are investigated using highly resolved LES data. The mixing dynamics is found to be quite distinct in each mixing mode. Variable viscosity manifests multiple effects that are working against each other. Viscosity gradients induce additional instabilities while increasing overall viscosity decreases the effective Reynolds number leading to laminarization of the turbulent jet, explaining the lack of dispersion and turbulent diffusion. Momentum budget analysis reveals that variable-viscosity terms are significant to be neglected. The scaling of the energy spectrum cascade suggests that in the TLL mode the unsteady laminar shedding is responsible for the eddies observed.

Effects of electrically conductive walls on turbulent magnetohydrodynamic flow in a continuous casting mold

In the present study, we have performed a series of numerical simulations of the turbulent liquid metal flow in a laboratory-scale setup of the continuous casting. The liquid metal flow was subjected to an external non-uniform magnetic field reproducing a realistic electromagnetic brake (EMBr) effect. The focus of this research was on the effects of the finite electrical conductivity of Hartmann walls on the flow and turbulence in the mold. To be able to simulate distributions of the electric potential and current in both the fluid and solid wall domains, we applied our recently developed and validated in-house conjugate MHD solver based on the open-source code OpenFOAM. The dynamic Large Eddy Simulation (LES) method was used to simulate the turbulent flow. The results obtained for the neutral (non-MHD) and MHD cases over a range of the imposed EMBr strengths – all for the perfectly electrically insulated walls – were compared with the available Ultrasound Doppler Velocimetry (UDV) measurements. A good agreement between simulations and experiments was obtained for all simulated cases. Next, we completed a series of simulations including a wide range of the finite electric conductivities (ranging from a weakly to perfectly conducting wall conditions) of the Hartmann walls for a fixed value of the imposed EMBr. The obtained results demonstrated a significant influence of the electric wall conductivities on the flow and turbulence reorganization. It is expected that here provided insights can be applicable for the new generation of the laboratory- and real-scale continuous casting setups.

Combined Large-Eddy and Direct Numerical Simulations of a Planar Jet with Heated Co-Flow with Medium and Low Prandtl Fluids

In the present work, we have applied a combined dynamic large-eddy simulation (LES) and direct numerical simulation (DNS) approach for a three-dimensional planar jet in a turbulent forced convection regime (Re = 18000) with a heated co-flow. Results from LES are compared with Reynolds Averaged Navier-Stokes (RANS) simulations and experimental data. We have analyzed flow and heat transfer features for four values of the characteristic Prandtl numbers (Pr = 0.71, 0.2, 0.025, and 0.006), which are representatives of air, He-Xe gas mixture, Lead-Bismuth Eutectic (LBE), and sodium, respectively. The latter two low-Prandtl fluids have been considered because of their role as primary coolants in advanced fast pool-type reactor prototypes (such as the Multi-purpose Hybrid Research Reactor for High-tech Applications (MYRRHA) at SCK•CEN, Belgium). We have provided detailed insights into instantaneous and long-term time-averaged behavior of the velocity and temperature fields (the first- and second-order moments). Furthermore, we have analyzed profiles of characteristic velocity and temperature time scales and dissipation rates, as well as the power spectra of the streamwise velocity component and temperature at several characteristic locations. The mean temperature profiles demonstrated rather low sensitivity for various values of the Prandtl number. In contrast, profiles of the temperature standard deviation exhibited larger variations, decreasing in magnitude with lower Prandtl values. Here presented results of the high fidelity numerical simulations (dynamic LES/DNS) for the low-Prandtl working fluids can be used for further development, testing, and validation of the advanced RANS-type turbulence models.

Evaluation of the mixing quality of high-viscosity yield stress fluids in a tubular reactor

Abstract In this study, the mixing quality of high-viscosity yield stress fluid (Carbopol aqueous solution) under laminar and turbulent flow regimes was evaluated through a numerical experimental study. A three-dimensional computational fluid dynamics large-eddy simulation (CFD-LES) model was employed to capture large-scale vortex structures. The proposed CFD model was validated by the experimental data in terms of mean velocity profiles and velocity-time history. Thereafter, the CFD model was applied to simulate the residence time distribution using the tracking technique: tracer pulse method and step method. In addition, the non-ideal flow phenomena caused by molecular diffusion and eddy diffusion were evaluated. The effects of the rheological properties on the mixing performance were also investigated. The presented results can provide useful guidance to enhance mass transfer in reactors with high-viscosity fluids.

An extensive numerical benchmark of the various magnetohydrodynamic flows

There is a continuous need for an updated series of numerical benchmarks dealing with various aspects of the magnetohydrodynamics (MHD) phenomena (i.e. interactions of the flow of an electrically conducting fluid and an externally imposed magnetic field). The focus of the present study is numerical magnetohydrodynamics (MHD) where we have performed an extensive series of simulations for generic configurations, including: (i) a laminar conjugate MHD flow in a duct with varied electrical conductivity of the walls, (ii) a back-step flow, (iii) a multiphase cavity flow, (iv) a rising bubble in liquid metal and (v) a turbulent conjugate MHD flow in a duct with varied electrical conductivity of surrounding walls. All considered benchmark situations are for the one-way coupled MHD approach, where the induced magnetic field is negligible. The governing equations describing the one-way coupled MHD phenomena are numerically implemented in the open-source code OpenFOAM. The novel elements of the numerical algorithm include fully-conservative forms of the discretized Lorentz force in the momentum equation and divergence-free current density, the conjugate MHD (coupling of the wall/fluid domains), the multi-phase MHD, and, finally, the MHD turbulence. The multi-phase phenomena are simulated with the Volume of Fluid (VOF) approach, whereas the MHD turbulence is simulated with the dynamic Large-Eddy Simulation (LES) method. For all considered benchmark cases, a very good agreement is obtained with available analytical solutions and other numerical results in the literature. The presented extensive numerical benchmarks are expected to be potentially useful for developers of the numerical codes used to simulate various types of the complex MHD phenomena.

Scale-resolving simulations and investigations of the flow in a hydraulic retarder considering cavitation

Cavitation has a significant influence on the accurate control of the liquid filling rate and braking performance of a hydraulic retarder; however, previous studies of the flow field in hydraulic retarders have provided insufficient information in terms of considering cavitation. Here, the volume of fluid (VOF) method and a scale-resolving simulation (SRS) were employed to numerically and more comprehensively calculate and analyze the flow field in a retarder considering the cavitation phenomenon. The numerical models included the improved delayed detached eddy simulation (IDDES) model, stress-blended eddy simulation (SBES) model, dynamic large eddy simulation (DLES) model, and shear stress transport (SST) model in the Reynolds-averaged Navier-Stokes (RANS) model. All the calculations were typically validated by the brake torque in the impeller rather than the internal flow. The unsteady flow field indicated that the SBES and DLES models could better capture unsteady flow phenomena, such as the chord vortex. The SBES and DLES models could also better capture bubbles than the SST and IDDES models. Since the braking torque error of the SBES model was the smallest, the transient variation of the bubble volume fraction over time on a typical flow surface was analyzed in detail with the SBES model. It was found that bubbles mainly appeared in the center area of the blade suction surface, which coincided with the experiments. The accumulation of bubbles resulted in a larger bubble volume fraction in the center of the blade over time. In addition, the temperature variations of the pressure blade caused by heat transfer were further analyzed. More bubbles precipitated in the center of the blade, leading to a lower temperature in this area.

Modeling of In-Line Tube Banks Inside Advanced Gas-Cooled Reactor Boilers

The objective of this research is the prediction of forced convection across confined in-line tube banks found in Advanced Gas-Cooled Reactor (AGR) boiler systems. The focus of this work is periodic flow about a 2 × 2 array. A number of Reynolds-Averaged Navier–Stokes (RANS) turbulence modeling closures have been applied to the challenging case of 1.6 pitch-to-diameter spacing at which flow undergoes transition from straight to diagonal. In contrast to Large Eddy Simulation (LES) predictions, those of RANS models fail to predict transition to diagonal flow. As LES computations are not anticipated to become routine enough for the purpose of flow diagnostics in industrial contexts for many years, further refinement to the RANS methodology in the form of an analytical wall function is one of the main objectives. Its eventual inclusion is not only expected to improve the representation of turbulent boundary layer behavior, but also has scope to account for augmented surface roughness effects experienced by AGR technologies nearing their end-of-life. This work is ultimately in aid of EDF Energy’s policy of AGR lifetime extensions, to allow the UK Government sufficient time to formulate a credible Nuclear New Build scheme to secure our energy requirements in the coming decades. It was found that all but dynamic LES adequately reproduces flow behavior across all criteria considered, and that standard wall functions left much to be desired in the case of flow and thermal predictions, while introducing the analytical wall function generates improvements for flow and heat transfer predictions that are not universally shared between high-Re models utilized.

Scale-resolving simulation and particle image velocimetry validation of the flow around a marine propeller

There are many unresolved issues in Reynolds-averaged Navier-Stokes (RANS) calculations of marine propeller performance, especially in the treatment of complex flow phenomena such as boundary-layer development, scale effects, and tip and hub vortices. The particular focus of this study was to apply three scale-resolving simulation (SRS) methods, i.e. dynamic large eddy simulation (DLES), delayed detached-eddy simulation (DDES), and stress-blended eddy simulation (SBES), to improve the prediction of flow characteristics. Firstly, the effectiveness of the SRS methods was verified by comparing numerical results with experimental data. The external performance of rotating machinery is determined by internal flow structures. Particle image velocimetry (PIV) measurement is established as a visualization tool to analyze the wake evolution of a scaled propeller by velocity and vorticity contours in a specified cross-section plane. We found that SRS methods, especially the SBES model, performed well in predicting characteristic parameters and capturing flow field information via quantitative and qualitative analyses. The ability to accurately predict flow characteristics can make computational tools more effective in meeting the needs of modern propeller design and analysis.

PIV and dynamic LES of the turbulent stream and mixing induced by a V-grooved blade axial agitator

The hydrodynamic behavior of a turbulent flow and the mixing characteristics generated by a V-grooved axial impeller inside an agitated tank reactor were investigated both experimentally and numerically. Angle resolved Particle Image Velocimetry (PIV) techniques with an angular displacement Δθ = 5° have been applied and two aerodynamic planes along the blades were considered. PIV-based results were compared to those obtained by Large Eddy Simulation (LES), used with the dynamic Smagorinsky-Lilly sub-grid scale (SGS) model. Results showed the existence of distinctive recirculation zones in the aerodynamic planes, and new additional frequencies in the impeller stream, induced by the grooves. A decrease of mixing time of about 11% was obtained experimentally, consequence of the better suction induced by the grooved blades in the early stages of mixing. Mean velocities, vorticity, TKE obtained from LES showed a good agreement with the PIV-based results. The distributions of turbulence dissipation rate ε were similar to those obtained from PIV, however showing high under-predicted magnitudes.

Buoyant wind-driven pollutant dispersion and recirculation behaviour in wedge-shaped roof urban street canyons.

The present study investigated the buoyant wind-driven pollutant plume dispersion and recirculation behaviour inside urban street canyons formed by buildings with wedge-shaped roofs. Numerical modelling was performed using a computational fluid dynamics (CFD) large eddy simulation (LES). Street canyon models with a strongly buoyant fire source located on the street and environmental winds perpendicular to the canyon were developed using the fire dynamics simulator (FDS). The complex interaction of buoyancy and wind, as well as their combined effects on the pollutant plume dispersion, was simulated inside the urban street canyon. The results showed that the flow pattern of pollutant plume dispersion inside the street canyon with increasing wind speed for different roof inclination angles could be divided into three regimes, including a recirculation regime, a quasi-recirculation regime and a non-recirculation regime. The pollutant levels in the street canyon, as indexed by carbon monoxide (CO) concentration, increased under the recirculation regime. For the quasi-recirculation regime, however, the leeward buildings primarily suffered from the higher pollutant levels. The critical wind speed needed to trigger recirculation was analysed for various roof inclination angles. A correlation was proposed to predict the critical wind speed of various wedge-shaped roof angles for recirculation regime and quasi-recirculation regimes.

Exploring the large-scale structure of Taylor–Couette turbulence through Large-Eddy Simulations

Large eddy simulations (LES) of Taylor-Couette (TC) flow, the flow between two co-axial and independently rotating cylinders are performed in an attempt to explore the large-scale axially-pinned structures seen in experiments and simulations. Both static and dynamic LES models are used. The Reynolds number is kept fixed at Re = 3.4 · 104, and the radius ratio η = ri/ro is set to η = 0.909, limiting the effects of curvature and resulting in frictional Reynolds numbers of around Reτ ≈ 500. Four rotation ratios from Rot = −0.0909 to Rot = 0.3 are simulated. First, the LES of TC is benchmarked for different rotation ratios. Both the Smagorinsky model with a constant of cs = 0.1 and the dynamic model are found to produce reasonable results for no mean rotation and cyclonic rotation, but deviations increase for increasing rotation. This is attributed to the increasing anisotropic character of the fluctuations. Second, “over-damped” LES, i.e. LES with a large Smagorinsky constant is performed and is shown to reproduce some features of the large-scale structures, even when the near-wall region is not adequately modeled. This shows the potential for using over-damped LES for fast explorations of the parameter space where large-scale structures are found.

Dynamic large eddy simulation: Stability via realizability

The concept of dynamic large eddy simulation (LES) is highly attractive: such methods can dynamically adjust to changing flow conditions, which is known to be highly beneficial. For example, this avoids the use of empirical, case dependent approximations (like damping functions). Ideally, dynamic LES should be local in physical space (without involving artificial clipping parameters), and it should be stable for a wide range of simulation time steps, Reynolds numbers, and numerical schemes. These properties are not trivial, but dynamic LES suffers from such problems over decades. We address these questions by performing dynamic LES of periodic hill flow including separation at a high Reynolds number Re = 37 000. For the case considered, the main result of our studies is that it is possible to design LES that has the desired properties. It requires physical consistency: a PDF-realizable and stress-realizable LES model, which requires the inclusion of the turbulent kinetic energy in the LES calculation. LES models that do not honor such physical consistency can become unstable. We do not find support for the previous assumption that long-term correlations of negative dynamic model parameters are responsible for instability. Instead, we concluded that instability is caused by the stable spatial organization of significant unphysical states, which are represented by wall-type gradient streaks of the standard deviation of the dynamic model parameter. The applicability of our realizability stabilization to other dynamic models (including the dynamic Smagorinsky model) is discussed.

ICCM2015: A Comparison of RANS and LES Computational Methods in Analyzing Ventilation Flow Through a Room Fitted with a Two-Sided Windcatcher

A windcatcher is a structure for providing natural ventilation using wind power; it is usually fitted on the roof of a building to exhaust the inside stale air to the outside and supplies the outside fresh air into the building interior space working by pressure difference between outside and inside of the building. In this paper, the behavior of free wind flow through a three-dimensional room fitted with a centered position two-canal bottom shape windcatcher model is investigated numerically, using a commercial computational fluid dynamics (CFD) software package and LES (Large Eddy Simulation) CFD method. The results have been compared with the obtained results for the same model but using RANS (Reynolds Averaged Navier–Stokes) CFD method. The model with its surrounded space has been considered in both method. It is found that the achieved results for the model from LES method are in good agreement with RANS method’s results for the same model.

Wind farm wake modeling using NWTC design codes

Wind turbines located in a wind farm are subject to a wind field that is substantially modified when compared to the ambient wind field due to wake effects. It is clear that in addition to the power, a tool that could model the loads of the waked turbine is needed. In this work, a wind farm wake model is created using a single wake model based on the dynamic wake meandering model to systematically model the wake effects and thus the power and loads of an entire wind farm with an arbitrary wind turbine layout and wind condition. This wind farm wake model is incorporated into the National Wind Technology Center design codes. Preliminary results demonstrate that this wind farm tool yields satisfactory results when compared with Simulator for Wind Farm Applications, a high fidelity computational fluid dynamics Large eddy simulation model, and with experimentally-obtained field data. The integration of the wind farm model with the National Wind Technology Center design codes is described in detail and validation results of the tool are provided.

Large Eddy Simulation of Normally Impinging Round Air-Jet Heat Transfer at Moderate Reynolds Numbers

This paper presents results of dynamic Smagorinsky-type model-based large eddy simulation of normally impinging round air-jet heat transfer at moderate Reynolds numbers (4,400, 104, and 2.3 × 104) with orifice-to-plate distance fixed at 5. Using software Open Source Field Operation and Manipulation (OpenFOAM), predicted distributions of mean velocity components, velocity fluctuations, and turbulent stresses in the vertical and radial directions are compared with existing empirical and numerical results. For the predicted heat flow from the target wall, there is satisfactory consistency of the mean Nusselt number in comparison with measured empirical results.