Very Large Eddy Simulation Research Articles

Posterior comparison of model dynamics in several hybrid turbulence model forms

Hybrid turbulence models that can accurately reproduce unsteady three-dimensional flow physics across the entire range of grid scales and turbulence dynamics from Reynolds-averaged Navier–Stokes (RANS), through large-eddy simulation (LES), down to direct numerical simulations (DNS) are of increasing interest to the turbulence modeling community. However, despite decades of research and development, the basic tasks of eliminating poor-performing hybrid RANS-LES models and accelerating adoption of superior models through well-designed validation and verification have yet to occur. As a step in this direction, in this work we evaluate thirteen different hybrid RANS-LES models via systematic grid refinement of decaying homogeneous isotropic turbulence. We further derive a novel mathematical framework for assessing the energy partitioning dynamics of each Hybrid RANS-LES model, wherein model-to-model variations in energy partitioning can be interpreted as different feedback mechanisms operating on a low-dimensional nonlinear dynamical system. We found that model forms similar to the flow simulation methodology—also often termed very-large eddy simulation—are dynamically inconsistent with DNS at all resolutions. Additionally, we found a strong dynamical similarity in the feedback mechanisms of all models related to detached eddy simulation and partially averaged Navier–Stokes that is inherent to their general model forms.

Internal flow stability analysis of vertical mixed-flow pump device based on EGT and FFT

With the purpose of researching the internal flow stability of the vertical mixed-flow pump device (VMFPD), the entire flow field was solved through the very large-eddy simulation (VLES) k- ω model and the internal flow characteristics of the pump device were quantitatively analyzed with the use of the energy gradient theory (EGT) along with the physical parameters of the flow field. The research elucidates the variety rule of the pressure along the VMFPD and the energy gradient function K as well as the time-frequency features of the pressure pulsation (PP) in the impeller and guide vane. The results demonstrate that the solid wall restriction of the guide vane and the impeller rotation cause a significant shift in the velocity and direction of the water flow. This causes a significant pressure difference between the water bodies, which makes it easier to create an unstable flow. The suction surface of the impeller blade possesses the great concentration of the area with energy gradient function K > 107, which is the critical area influencing the internal flow stability of the pump device.

Numerical investigation on effects of low Reynolds number conditions on fan shaped film cooling performances

This study investigates the effects of low Reynolds number (Re) conditions on the cooling performance of fan-shaped film cooling holes in the gas turbine engines of high-altitude long-endurance Unmanned Aerial Vehicles (UAVs). The significance of film cooling technology becomes paramount due to the low Re operational environment of UAVs. A novel Very Large Eddy Simulation (VLES) method was employed to systematically analyze a range of mainstream Reynolds numbers (ReD) from 480 to 32 000 and blowing ratios (M) from 1.0 to 2.5 while keeping the density ratio (DR) constant at 1.75. Compared to other turbulence models, the VLES model showed the highest similarity in predicting thermal field distributions and the most accurate prediction of film cooling performance at low Re. The results reveal that at low ReD, significant flow concentration within the hole leads to reduced velocity uniformity at the outlet, resulting in poor film coverage downstream. Additionally, the coolant jet tends to detach from the wall, adversely affecting cooling performance. In contrast, higher ReD exhibited improved coolant jet adhesion to the wall and cooling efficiency, attributed to the reduced intensity of counter-rotating vortex pair and decreased hot gas entrainment, thereby enhancing cooling effectiveness. Notably, for the same ReD values, cases with M = 1.5 consistently showed better cooling effectiveness compared to other M conditions. This study provides new insight into the cooling performance of fan-shaped film cooling holes under low Reynolds number conditions, which is crucial for enhancing the cooling efficiency of gas turbines in high-altitude UAVs.

Improved hybrid model for transitional separated flows over a rough compressor blade

It is known that boundary layer transition and turbulent separation flow after transition can be influenced significantly by surface roughness. Since the traditional hybrid RANS/LES method cannot predict the boundary layer transition, and the RANS-based transition model cannot accurately simulate the massively separated flow, the present study sought to build an effective modeling strategy for the laminar, roughness-induced-transition and attached turbulence/massively separated flows that couples the Very-Large-Eddy-Simulation (VLES) model and a transition model considering roughness effects. Because the hybrid function between the transition model and VLES model constructed in the previous version still has some flaws, there is still a large laminar flow indication area in the turbulent flow region, which will reduce the simulation accuracy of the turbulent flow after transition. In this paper, the hybrid function is improved, which is examined in the simulation of the high-subsonic flow around the V103 compressor blade. Our analysis shows that the new hybrid model operates in the flow around the V103 compressor blade. The predictions of laminar separation bubble-induced transition, separation evolution, and reattachment are in accord with measurements and the LES results over both smooth and rough surfaces, indicating that the present transitional VLES method can achieve the simulation accuracy close to the LES method while reducing the computational cost.

Investigation of Very Large Eddy Simulation Method for Applications of Supersonic Turbulent Combustion

The very large eddy simulation (VLES) method was investigated for supersonic reacting flows in the present work. The advantages and characteristics of the VLES model and the widely used improved delayed detached eddy simulation (IDDES) method were revealed through a supersonic ramped-cavity cold flow. Compared to the IDDES model, the VLES model transformed from RANS mode to LES mode faster, resulting in a smaller gray region caused by the mode transition. However, the original volume-averaging truncation length scale could lead to poor predictions of the velocity profiles and wall pressure distribution. By introducing a hybrid truncation length scale combining the maximum grid length and the shear layer adaptive (SLA) length with different coefficients, the accuracy of the VLES method was significantly improved, and the issue of the low shear layer position was solved. Moreover, owing to the resolution control function, the VLES method could adaptively model more turbulent kinetic energy and maintain a good accuracy in a coarser mesh. Finally, the modified VLES method was applied in conjunction with a hybrid combustion model constructed by the partially stirred reactor (PaSR) model and the Ingenito supersonic combustion model (ISCM) in simulations of the supersonic flame in the DLR scramjet combustor. After introducing the correction of the molecular collision frequency by the ISCM, the results obtained by the hybrid combustion model were more consistent with the experimental results, especially for the time-averaging temperature profile in the ignition zone.

Reducing Particle Exposure and SARS-CoV-2 Risk in Built Environments through Accurate Virtual Twins and Computational Fluid Dynamics

The World Health Organization has pointed out that airborne transmission via aerosol particles can be a strong vector for the spread of SARS-CoV-2. Protecting occupants from infectious diseases or harmful particulate matter (PM) in general can be challenging. While experimentally outlining the detailed flow of PM in rooms may require complex setups, computational fluid dynamics (CFD) simulations can provide insights into improving the safety of the built environment and the most effective positioning of air-purifying devices. While previous studies have typically leveraged Reynolds-averaged Navier–Stokes (RANS) approaches for predicting particle propagation, the turbulence length scales accurately captured in these simulations may not be sufficient to provide a realistic spread and the mixing of particles under the effects of forced convection. In this paper, we experimentally validate a Lattice Boltzmann very large eddy simulation (VLES) approach including particle modeling. We also demonstrate how this simulation approach can be used to improve the effectiveness of air filtration devices in realistic office environments.

Numerical Simulation of the Effect of a Single Gust on the Flow Past a Square Cylinder

The flow past a square cylinder under the influence of a one dimensional gust was investigated using computational fluid dynamics (CFD). The effect of upstream wind gusts of the same amplitude but different duration was investigated with respect to their effect on the flow, the vortex-shedding, and the pressure distribution around the square cylinder. For the computations, a very large eddy simulation (VLES) model was implemented in an in-house code and validated against numerical and experimental results from the literature. The gusts of different duration were found to have a distinctly different effect. The short-duration gust causes a lock-on behavior with cessation of the alternating vortex shedding, and a symmetric pair-vortex was created above and below the square cylinder. It was observed that the pressure distribution on the lateral sides of the cylinder has the same magnitude and phase, which resulted in a zero total lift coefficient. In terms of a free-standing structures, such as a building, this would lead to zero instantaneous forces and pressure difference in the lateral direction with obvious implications for dynamic response and cross ventilation.

Assessment of turbulence and cavitation models in prediction of vortex induced cavitating flow in fuel injector nozzles

Vortex-induced cavitation develops in high pressure fuel injector equipment in areas where despite the very high surrounding pressures (>100 MPa), the rotational motion of the fluid causes large pressure drops in the core of a well-organized longitudinal vortex structure leading to cavitation with normally string type correspondingly also called string cavitation ; typically, the size of the string cavitation is small compared to the overall dimensions while they develop in a transient mode linked with the local flow turbulence. To address these complex interaction processes, some turbulence models have been tested in a diesel injector nozzle for which quantitative experimental data are available and are utilized to assess the predictive capability of relevant models; the flow has been considered compressible, turbulent and isothermal. The closure models considered include the RNG k−ε, SST k−ω, RSM, WALE LES as well as a developed very-large eddy simulation model (VLES). The employed mass-transfer cavitation models are the Schnerr and Sauer (SS), Zwart-Gerber-Belamri (ZGB), HEM and a modified ZGB model with a self-adaptive adjustment of condensation coefficient. The results indicate that the RNG k−ε model and SST k−ω model fail to predict the string cavitation induced by vortex flow in the nozzle due to the drawback of the isotropic vortex-viscosity hypothesis. The RSM model could capture an acceptable string cavitation morphology. The numerical results utilizing the VLES model and LES model are in considerably good agreement with the experimental data, while the VLES model can provide the predictions of cavitating flow as precise as that using the LES model but on a much coarser mesh. The distributions of tangential velocity are similar to the velocity characteristics of Rankin vortex or Taylor vortex, which can help understand the coherent vortex structures in string cavitating flow. It is also shown that the RSM turbulence model with modified ZGB cavitation model improves the accuracy of prediction in the vortex-induced cavitation compared to the other three cavitation models.

Very-Large-Eddy Simulation of Nonreactive Turbulent Flow for Annular Trapped Vortex Combustor

A hybrid Reynolds-averaged Navier-Stokes (RANS) large eddy simulation (LES) method is applied in this work. It called very-large-eddy simulation (VLES) turbulence closure model. The aim of this present study is firstly to validate the accuracy of this method for a specific engineering application (a trapped vortex combustor) and secondly to describe its flow characteristics. The trapped vortex combustor is a new concept that utilizes a large recirculation vortex to stabilize the flame. An accurate prediction of the turbulent flow is meaningful for the trapped vortex combustor. The time-averaged velocity, root-mean-square (rms) velocity, and flow pattern are compared with the experimental data. And the LES model, RANS BSL k-ω model, and RANS k-ɛ model are also applied for the simulation with different mesh resolutions. The results show that the VLES BSL k-ω model provides improved accuracy for velocity prediction. The classical large vortex structure for the trapped vortex combustor is captured qualitatively by the VLES BSL k-ω model also. In addition, the vortex breakdown and processing vortex cone are visualized using the Q-criterion. Furthermore, the VLES BSL k-ω model is not sensitive to the gird resolution. The VLES method is able to predict the turbulent flow of trapped vortex combustor relatively well‎.

Very Large Eddy Simulation of Aero-Thermal Performance in Squealer Tip Gap

Abstract To improve the resolution accuracy and get deep insight into the flow structures in squealer tip gap, the very large Eddy simulation (VLES) method was implemented into the commercial computational fluid dynamics (CFD) solver with the user-defined function (UDF). Based on the published experimental data, the numerical accuracy of VLES method was validated. With the VLES method, the unsteady heat transfer coefficient distributions on the squealer tip and total pressure loss in the blade passage were computed. The influences of coherent vortex structures on aero-thermal performance in the squealer tip gap were analyzed. The results show that the Brown-Roshko vortices are the main driver for the formation of cavity vortex system. The direct impingement of pass-over leakage into the cavity is the main cause of high heat transfer area on the cavity floor near leading edge. The unsteady fluctuations of leakage rate through the tip gap reach about ±8% of the time-averaged value. The development of leakage vortex accounts for the major contribution of total pressure loss in the squealer tipped blade. Due to flow unsteadiness, the fluctuation of pitch-averaged total pressure loss coefficient induced by leakage vortex system reaches about ±30% of the time-averaged value. The unsteady fluctuation of pitch-averaged heat transfer coefficient on the cavity floor reaches about ±35% of the time-averaged value, while on the shroud surface it is only fluctuated by about ±10%.

An Arbitrary Hybrid Turbulence Modeling Approach for Efficient and Accurate Automotive Aerodynamic Analysis and Design Optimization

The demand in solving complex turbulent fluid flows has been growing rapidly in the automotive industry for the last decade as engineers strive to design better vehicles to improve drag coefficients, noise levels and drivability. This paper presents the implementation of an arbitrary hybrid turbulence modeling (AHTM) approach in OpenFOAM for the efficient simulation of common automotive aerodynamics with unsteady turbulent separated flows such as the Kelvin–Helmholtz effect, which can also be used as an efficient part of aerodynamic design optimization (ADO) tools. This AHTM approach is based on the concept of Very Large Eddy Simulation (VLES), which can arbitrarily combine RANS, URANS, LES and DNS turbulence models in a single flow field depending on the local mesh refinement. As a result, the design engineer can take advantage of this unique and highly flexible approach to tailor his grid according to his design and resolution requirements in different areas of the flow field over the car body without sacrificing accuracy and efficiency at the same time. This paper presents the details of the implementation and careful validation of the AHTM method using the standard benchmark case of the Ahmed body, in comparison with some other existing models, such as RANS, URANS, DES and LES, which shows VLES to be the most accurate among the five examined. Furthermore, the results of this study demonstrate that the AHTM approach has the flexibility, efficiency and accuracy to be integrated with ADO tools for engineering design in the automotive industry. The approach can also be used for the detailed study of highly complex turbulent phenomena such as the Kelvin–Helmholtz instability commonly found in automotive aerodynamics. Currently, the AHTM implementation is being integrated with the DAFoam for gradient-based multi-point ADO using an efficient adjoint solver based on a Sparse Nonlinear optimizer (SNOPT).

Numerical study of the impact of glottis properties on the airflow field in the human trachea using V-LES

The influences of the profiles and cross-sectional areas of glottal aperture on the upper respiratory airway are investigated using an idealized cast-based mouth-throat model and three dimensional computational fluid dynamics (CFD). The open source CFD code OpenFOAM is employed. The transient flows are modeled using the very-large eddy simulation with the Smagorinsky sub-grid scale (SGS) model. Five different shapes of glottis are considered, including circular glottis with 100 %, 75 % and 50 % cross-sectional area and elliptic glottis with 75 % and 50 % cross-sectional area. Both instantaneous and averaged flow fields are analyzed. It is found that the variations of glottis have great impacts on the properties of downstream flow fields such as the secondary flow, laryngeal jet, recirculation zone, turbulent kinetic energy, and vortex. Evident impacts are observed in the region within 6 tracheal diameters downstream of the glottis. The profile of the glottis has more impacts on the laryngeal shape, while the cross-sectional area has more impacts on velocity of the laryngeal jet and turbulent intensity. It is concluded that both the glottal areas and profiles are critical for an idealized geometrical mouth-throat model.

Numerical investigation of flat-plate film cooling using Very-Large Eddy Simulation method

Film cooling is an important cooling method to protect the hot components in aero-engines and it is a classical research topic for decades. However, accurate simulation of the film cooling is still challenging. The present study aims to study film cooling with a newly developed Very-Large Eddy Simulation (VLES) method. State-of-the-art turbulence modelling methods are also included for comprehensive comparisons of film cooling simulations, i.e., VLES and DES (Detached Eddy Simulation) of hybrid RANS-LES method and the classical LES (Large Eddy Simulation) method are applied. Numerical simulations are performed for a single row film cooling benchmark test case (Sinha et al., 1991), in which the turbulence intensity of the freestream flow is about 0.2%, at two density ratios and two blowing ratios, corresponding to four momentum flux ratios. The results predicted by different turbulence methods are compared in detail, including the film cooling effectiveness, the velocity flow fields, as well as the relevant turbulent flow structures. The predicted results are also compared with available experimental data. It is found that for the three cases with low momentum flux ratios (smaller than 0.3), the three turbulence methods produce quite different results, while for the case with higher momentum flux ratio of 0.5, the differences between the results of three turbulence methods become less significant. Overall, the present VLES method performs best among the three selected methods, and its predictions agree well with the experimental data. While for the DES method, it severely delays the Kelvin-Helmholtz instability of the turbulence mixing process in film cooling and thus fails to accurately predict the cooling effectiveness at low momentum flux ratio. The study demonstrates the potentials of the newly developed VLES method for accurate prediction of complex film cooling problems.

Very-large eddy simulation of the rotational effects on turbulent flow in a ribbed channel

Very-large eddy simulation of the rotational effects on turbulent flow in a ribbed channel

Predicting high-speed feedback mechanisms in rectangular cavities using lattice-Boltzmann very-large eddy simulations

Aeroacoustic feedback mechanisms in rectangular cavities are strongly dependent on length-to-depth ratio and free stream velocity. The capability to predict the effects of these parameters using a scale-resolving transonic lattice-Boltzmann flow simulations is assessed, for the first time, by considering the M219 benchmark configuration for two different length-to-depth ratios (10:1 and 5:1) and two free-stream Mach numbers (0.85 and 1.35). Numerical results are compared and validated using unsteady wall pressure measured in wind tunnel tests. Tonal and broadband components of the pressure fluctuations are properly captured. For the shallow cavity configuration, an increase in Mach number is found to enhance the development of Rossiter modes inside the cavity. A more detailed analysis of these Rossiter modes is carried out for the shallow cavity at supersonic speed, for which physical insight is gained through the usage of spectral proper-orthogonal decomposition and wavelet/Fourier analyses.

Very Large Eddy Simulation of Lean Premixed Flames to Imposed Inlet Velocity Oscillations

ABSTRACT The present work describes an assessment and validation of the newly developed Very-Large Eddy Simulation (VLES) method for the numerical study of complex non-linear response of a lean premixed flame to acoustic velocity fluctuations. The VLES method is a self-adaptive turbulence method which can achieve seamless switching between RANS, LES, and finally approaching DNS. It is extended to the turbulent combustion simulation in the present study. The selected test case is a bluff-body stabilized lean premixed turbulent flame with acoustic forcing. Turbulence modeling is achieved using the VLES method and the premixed turbulent combustion is modeled using the Thickened Flame Model (TFM) combining a reduced two-step chemical reaction mechanism. Harmonically varying velocity fluctuation is imposed on the inlet to introduce the acoustic forcing. Three types of flow including the cold flow, unforced reactive flow, and forced reactive flow are used to validate the simulation method. The mean velocity, dynamic response of the flame to inlet velocity forcing and the quantitative response of the heat release rate are captured by the present calculations with very reasonable accuracy compared with the experimental data. The present VLES predictions are comparable or even better than the present LES results using the combustion model and the same computational grid. This work thus confirms that the VLES methodology is capable of successfully predicting the response of lean premixed turbulent flame to acoustic forcing.

The effects of tip gap variation on transonic turbine blade tip leakage flow based on VLES approach

Transonic turbine blade Over-Tip Leakage (OTL) flow has significant effects on turbine efficiency and blade thermal load. At present, most of the numerical studies on OTL flow are based on Reynolds-Averaged Navier-Stokes (RANS) method. However, it is difficult to characterize the physical mechanisms of transonic OTL flow accurately due to the inherent shortcomings of RANS method, especially for the unsteady characteristics. To understand the underlying loss mechanisms and aerothermal characteristics of the transonic OTL flow with tip gap variations, the present study applies a Very Large Eddy Simulation (VLES) method based on Wilcox k–ω model to study a transonic cascade OTL flow. The feasibility and accuracy of the VLES turbulence model in OTL flow simulations are confirmed by numerical assessments comparison with the experimental results. The detailed flow structure and its effect on the losses of tip leakage flow are studied for different cases with varying tip gap heights. It is observed that the flow structures including separation vortex and shock waves in the gap become more complex with increasing the tip gap. The mass flow rate, pressure loss, entropy generation, and aerothermal performance for different tip gap heights are further discussed.

Corner Stall Prediction in a Compressor Linear Cascade Using Very Large Eddy Simulation Lattice-Boltzmann Method

AbstractCompressor corner stall is a phenomenon difficult to predict with numerical tools but essential to the design of axial compressors. Predictive methods are beneficial early in the design process to understand design and off-design limitations. Prior numerical work using Navier–Stokes computational methods has assessed the prediction capability for corner stall. Reynolds-averaged Navier–Stokes (RANS) simulations using several turbulence models have shown to over-predict the region of corner hub stall where large eddy simulations (LES) and detached eddy simulations (DES) approaches improved the airfoil surface and wake pressure loss prediction. A linear compressor cascade designed and tested at Ecole Centrale de Lyon provides a good benchmark for the evaluation of the accuracy of numerical methods for corner stall. This paper presents results obtained with Lattice-Boltzmann method (LBM) coupled with very large-eddy simulations (VLES) approach of PowerFLOW and compares them with the experimental and numerical work from Ecole Centrale de Lyon. The ability to achieve equivalent accuracy at a lower computational cost compared to LES scale resolving methods can enable multi-stage design and off-design compressor predictions. A methodical approach is taken by first accurately simulating the upstream flow conditions. Geometric trips are modeled upstream on the endwalls to match both the mean and fluctuating inflow boundary layer conditions. These conditions were then applied to the simulation of the linear compressor cascade. The benchline experimental study includes trips on both the pressure and suction of the airfoil. These trips are also included for the current simulation. The significance of capturing both inflow conditions and including trips on the airfoil is assessed. Detailed comparisons are then made to airfoil loading and downstream losses between experiment and previous RANS and LES simulations. LBM-VLES corner stall results of pitchwise averaged total pressure match LES agreement relative to experimental data at 50 times lower computational cost.

VLES turbulence modelling for separated flow simulation with OpenFOAM

In the present study, a detailed assessment investigation of Very-Large Eddy Simulation (VLES) method is performed using the open-source toolbox OpenFOAM for the simulation of turbulent separated flow. The VLES method is a unified simulation approach enabling a seamless evolution from Reynolds-Averaged Navier-Stokes (RANS) to quasi Direct Numerical Simulation (quasi-DNS) depending on the numerical resolution. Two VLES models, i.e. VLES-k-ε and VLES-k-ω, are implemented in OpenFOAM and applied in the present study. In order to assess the performance of VLES models based on the framework of OpenFOAM, four classic separated flow are selected and investigated in details, including the flow past a triangular bluff body at Re ​= ​45,000, the flow past a backward facing step at Re ​= ​40,000, the periodic hill flow at Re ​= ​10,595, and the flow past a circular cylinder at Re ​= ​41,300. The comparisons between VLES simulation results and experiments give an indication on the adequacy and accuracy of the VLES modelling method based on the OpenFOAM toolbox. The VLES-k-ω model performs slightly better than the VLES-k-ε model on a relatively low mesh resolution, especially in predicting the turbulent separation flow induced by the pressure gradient on a smooth surface. The results also demonstrate the reliability and accuracy of the VLES modelling based on OpenFOAM platform for complex separated flow simulations.

Analysis of fan-stage gap-flow data to inform simulation of fan broadband noise.

Time-resolved simulations present a new opportunity for studying the disturbances responsible for the broadband interaction noise created by the fan stage. In this paper, two vane configurations from the source diagnostic test at the approach rotor speed were computed with PowerFLOW's very large-eddy simulation (VLES) method using two solution strategies: a coarser mesh near the rotor and a trip to trigger turbulent transition on the rotor; and a much finer mesh near the rotor with no trip. The simulated data allow for an investigation of the potential effect from the vane configuration and an in-depth study of the mean and turbulent flow in the interstage gap. A challenge related to post-processing of high-resolution simulations is discussed. Comparison of the flow quantities with previously obtained Reynolds Averaged Navier-Stokes simulation results indicates that little advantage is gained by running a lattice Boltmann method (LBM)/VLES to simply recover the gap flow parameters for use with a lower-order fan broadband interaction noise calculation method. The true benefit of the LBM/VLES is that the noise calculation can be directly and simultaneously completed with the flow simulation. This article is part of the theme issue 'Frontiers of aeroacoustics research: theory, computation and experiment'.