SGS Model Research Articles

Large-Eddy simulation of flow and thermal behavior in jet impingement on a flat plate under rotating conditions

As an effective method for enhancing local heat transfer, impingement cooling is widely used in various applications. The present study focuses on the impingement jets in rotating channels and investigates the influence of additional force on the flow structure and heat transfer with large eddy simulations (LES). The second-order statistics of impinging jets give a deeper understanding of the turbulent dynamics in impinging jets and provide practical guidance for optimizing jet designs. In this paper, a model of three equally spaced impingement holes with a jet spacing between adjacent impingement holes of 5.25D and the jet to the target distance of 3.75D is simulated. The jet Reynolds number is kept fixed at 16000 and different rotation numbers of 0, 0.0042,0.0083, and 0.0125 are investigated. The LES results are compared with experimental results and show the WALE SGS model is the most suitable for impingement jet simulation. The numerical results reveal that rotation has an unignorable influence on the flow structure and heat transfer. Rotation induced the centrifugal buoyancy force and the Coriolis force has a large effect on the flow structure of the jet. Compared with the stationary condition, the jet shifts towards the -X direction and + Y direction causing an uneven velocity distribution in the impingement chamber under rotating conditions. The uneven velocity induces more turbulence in the jet and triggers additional vortices in the impingement chamber, especially on the +y side of the jet. The shifting of the jet also weakens the impingement cooling performance. The Nusselt number decreases with the increase of the Rotation number.

Key ingredients for wall-modeled LES with the Lattice Boltzmann method: Systematic comparison of collision schemes, SGS models, and wall functions on simulation accuracy and efficiency for turbulent channel flow

Key ingredients for wall-modeled LES with the Lattice Boltzmann method: Systematic comparison of collision schemes, SGS models, and wall functions on simulation accuracy and efficiency for turbulent channel flow

Application of a Local Dynamic Model of Large Eddy Simulation to a Marine Propeller Wake

With recent development of computer technology, the use of large eddy simulation method to solve industrial problems is gaining acceptance. From a theoretical and applied perspective, a local dynamic k-equation subgrid-scale model is applied to study the flow over a marine propeller. This local dynamic SGS model of LES have already been used in simple flows such as classical Taylor-Green vortex flow to investigate its robustness and superior than other dynamic SGS models. In this paper it will be applied to a more complex flows, i.e., simulation of a marine propeller wake, to further evaluate its ability range. 42 intentionally selected numerical experiments were conducted. The results of this local dynamic model of LES shows some superior than the dynamic Smagorinsky model, and well captures the wake evolution mechanism of a propeller, although which actually depends on its geometry.

Heat transfer characteristics of jet impingement on a surface mounted with ribs using LES

The present study investigates flow and heat transfer features of jet impingement on a surface fitted with detached ribs at Re = 20,000 using OpenFOAM 4.1 based large eddy simulation (LES). The WALE SGS model is applied. A wall resolved simulation is performed to understand the flow behaviour and heat transfer enhancement in the impingement regions and ribs. The WALE model is observed to be suitable for accurate calculations of complex application-based flow configurations. A comprehensive discussion on flow physics is carried out to understand heat transfer in the rib regions and the effects of flow and turbulence parameters. The investigation reveals that augmentations in the velocity and turbulence are directly linked to the local heat transfer enhancement in the clearance between the ribs and heated plate. A dual peak exists for turbulence kinetic energy profiles. The local heat transfer is observed to be maximum in the clearance region between the second rib and plate. The region of maximum local heat transfer was observed to be related to the peak velocity and turbulence kinetic energy in the same region. The present computation is capable of capturing small-scale dynamics and turbulence structures. The present results may be utilized in testing RANS-based turbulence models and the findings are useful for researchers and engineers working on EV, HPC and related cooling industries where high localized cooling is required.

Aerodynamic-induced Effects of Artificial Subglottic Stenosis on Vocal Fold Model Phonatory Response

ObjectiveSubglottic stenosis (SGS) is characterized by a narrowing of the trachea near the cricotracheal junction and impairs breathing. SGS may also adversely affect voice quality, but for reasons that are not fully understood. The purpose of this study is to provide experiment-based data concerning the effects on phonation of airway obstruction due to SGS. Study DesignBasic science MethodsA device simulating a SGS of adjustable severity ranging from 36% to 99.8% obstruction was created. Self-oscillating synthetic VF models were mounted downstream of the device and data were acquired to evaluate the effects of the obstruction on phonatory response. ResultsOnset pressures were relatively insensitive to obstructions of up to approximately 80% to 90% reductions in subglottic airway area and sharply increased thereafter. Flow rate (under conditions of constant pressure), flow resistance, and fundamental frequency all exhibited similar degrees of sensitivity to SGS obstruction as onset pressure. High-frequency noise became significant by 80% obstruction. Glottal area appeared to be less sensitive, not being affected until approximately 90% obstruction. ConclusionsConsistent with previous computational studies, this study found that aerodynamic, acoustic, and vibratory responses of self-oscillating VF models were largely unaffected by SGS until approximately 80% to 90% obstruction, and significantly affected at higher obstructions. This suggests that Grades I and II stenoses are unlikely to introduce subglottic airway aerodynamic disturbances that are sufficient in and of themselves to significantly alter phonatory output. The SGS model introduces a framework for future benchtop studies involving subglottic and supraglottic airway constrictions.

Non-Premixed Filtered Tabulated Chemistry for LES: Evaluation on Sandia Flames D and E

The non-premixed filtered tabulated chemistry for large eddy simulations employs numerical filtering to resolve a thin flame front on practical LES numerical grids. The flame structure is modified to be coherent with the domain discretization. The first turbulent combustion application of the non-premixed filtered tabulated chemistry approach is presented. A keen comparison of the flamelet filtering transformation in the premixed and non-premixed regimes is carried out. Three distinctive features are outlined: the flame thickness variation, the filtered manifold transformation, and the model activation dependence on the chosen diffusion flamelet configuration for a non-premixed filtered approach. The model performance is assessed on two real turbulent flame configurations, Sandia flames D and E, employing a three-dimensional tabulation strategy, where the numerical grid is coupled with the model by the third parameter, i.e., the computational cell size. The repercussions of the above cited aspects are carefully assessed. The results demonstrate that the formalism coupling with an SGS modeling function can adequately describe wrinkled flame front effects. The predictions for both the major stable species and the minor ones accurately correspond with the underlying physics. It turns out that there is a substantial variation of the filter effect as a function of the strain rate of the flame and the considered species. The varying filter sensitivity along the manifold influences the response of the model correction terms and the retrieved variables. The non-premixed FTACLES formalism possibilities and conditions for the model’s utilization and optimal performance are clearly stated, to confirm the idea that SGS closure in diffusive combustion can be derived based on filtering arguments, and not only based on statistical approaches.

하이브리드 난류 모델을 이용한 전류고정덕트 후류의 고정도 수치 해석

A hybrid turbulence model has developed by combining a sub-grid scale model using dynamic k equation in LES with k-ω SST model of RANS equation. To ascertain potential applicability of the hybrid turbulence model, fully developed turbulent channel flows at ReSUBτ/SUB=180 have been simulated of which computational domain has a top wall with coarse cells and a bottom wall with fine cells. The streamwise mean velocity and turbulent intensity profiles showed a good agreement with DNS data when using the hybrid model rather than using a single model in k-ω SST or dynamic k equation models. Computational simulations of turbulent flows around KVLCC2 with a pre-swirl duct have been mainly performed using the hybrid turbulence model. Compared to the results obtained from RANS simulation with k-ω SST model as well as LES with dynamic k equation SGS model, turbulent wakes of the duct in the present simulation using the hybrid turbulence model were very similar to that of LES. Also, the resistances acting on hull, rudder and duct in hybrid turbulence model were similar to those in RANS simulation whereas the viscous forces acting on the hull in LES had a significant error due to coarse cells inappropriate to the sub-grid scale model.

Large-eddy simulation of the OECD-NEA cold-leg mixing benchmark

Large Eddy Simulation (LES) was conducted to study the mixing of the two miscible fluids in the cold-leg mixing benchmark organized by OECD-NEA. A simulation set was implemented using different formulations of the large-eddy simulation Subgrid-scale models, i.e., Smagorinsky model, Dynamic model, and WALE model, to identify the influence of the SGS models on the simulation results. The estimation of the minimal mesh resolution for LES was studied. Spectral analysis of the velocity fluctuations in the cold-leg was examined. The time-average mean and root-mean-square (RMS) velocity of the three Subgrid-scale models were validated with PIV data and benchmark participant data in the cold-leg and downcomer. The mixing behavior of the fluid front was studied at different instants of time in the cold-leg and downcomer. Therefore, the strong turbulent mixing of the heavy fluid in the downcomer can cause a large temperature gradient in a loss-of-coolant accident (LOCA).

A derivation of temperature-based energy equation for LES of isochoric turbulent combustion with FDSGS model

In this study, with the aim of deriving a temperature-based energy equation and evaluating the fractal dynamic SGS (FDSGS) combustion model under an isochoric condition, modelling of the filtered energy conservation equation in the temperature form is considered and is tested in the contexts of a priori and a posteriori, comparing with a corresponding DNS of turbulent combustion in a constant volume vessel. Conditional averages of various terms in the filtered energy conservation equation in the temperature form show that three unclosed terms, the SGS scalar flux, the SGS pressure-dilatation and the heat release rate, are leading terms. New SGS models for the SGS pressure-dilatation and the heat release rate are proposed and are tested in a priori manner. The zeroth-order models are also compared. The heat release rate models are proposed for reaction zones and post-flame zones separately, since the present analysis revealed a substantial contribution of heat generated in the post-flame zones. In addition to the tests of the SGS models for the energy conservation equation, isochoric treatments for various flame quantities under varying mean pressure and unburnt temperature due to the isochoric condition are also proposed and are tested. LES using the FDSGS combustion model with the new SGS models and isochoric treatments is performed considering the same combustion conditions and configuration as the DNS. The LES with several other SGS combustion models are also performed and are compared. The performed LES show reasonable prediction of flame propagation for any of the SGS combustion models tested, although the prediction by the FDSGS combustion model shows closest to the DNS. Regardless of the SGS combustion models, the peak values of the mean pressure are close to the DNS value, which suggests that the proposed SGS models and isochoric treatments for the energy conservation equation work well.

Assessment of anisotropic minimum-dissipation (AMD) subgrid-scale model: Gently-curved backward-facing step flow

The impetus of this study is to evaluate the performance of the anisotropic minimum-dissipation (AMD) subgrid-scale model (SGS) for flow over a gently-curved backward-facing step (BFS) at a Reynolds number of 13 700. Minimum-dissipation sub-grid models were developed as simple alternatives to the dynamic eddy-viscosity SGS models. AMD model is a static type of eddy-viscosity SGS model incorporating anisotropic SGS effects into numerical predictions through the large-eddy simulation (LES) approach. The open-source CFD package of OpenFOAM was used to implement the AMD model. Before focusing on the BFS flow, we investigated the impact of the AMD model coefficient magnitude on the numerical predictions of the decaying isotropic turbulence flow. In the next step, numerical solutions were obtained for the curved backward-facing step using the AMD model and Dynamic Smagorinsky model (DSM). The curved backward-facing step was considered here for the evaluation of the SGS model predictions due to its weak adverse pressure gradient and high sensitive flow mechanism. The rescaling/recycling method was employed as a turbulent inflow generation technique. The AMD model results were compared with the prediction of the DSM and Vreman model. Moreover, AMD model predictions were compared with the reported solutions obtained using different turbulent inflow generation methods. The assessments revealed the high capability of the AMD model to capture decaying turbulence and predict velocity profiles and resolved flow statistics turbulent parameters in the gently-curved backward step flow.

An Anisotropic Subgrid-Scale Parameterization for Large-Eddy Simulations of Stratified Turbulence

AbstractSubgrid-scale (SGS) parameterizations in atmosphere and ocean models are often defined independently in the horizontal and vertical directions because the grid spacing is not the same in these directions (anisotropic grids). In this paper, we introduce a new anisotropic SGS model in large-eddy simulations (LES) of stratified turbulence based on horizontal filtering of the equations of motion. Unlike the common horizontal SGS parameterizations in atmosphere and ocean models, the vertical derivatives of the horizontal SGS fluxes are included in our anisotropic SGS scheme, and therefore the horizontal and vertical SGS dissipation mechanisms are not disconnected in the newly developed model. Our model is tested with two vertical grid spacings and various horizontal resolutions, where the horizontal grid spacing is comparatively larger than that in the vertical. Our anisotropic LES model can successfully reproduce the results of direct numerical simulations, while the computational cost is significantly reduced in the LES. We suggest the new anisotropic SGS model as an alternative to current SGS parameterizations in atmosphere and ocean models, in which the schemes for horizontal and vertical scales are often decoupled. The new SGS scheme may improve the dissipative performance of atmosphere and ocean models without adding any backscatter or other energizing terms at small horizontal scales.

Numerical Study of the Turbulent Flow from a Steam Dumping Pressurizer Relief Tank

Due to the complex geometry and turbulent flow characteristics, it is hard to simulate the process of steam dumping of the pressurizer relief tank (PRT). In this study, we develop a compressible fluid solver PRTFOAM to numerically study the turbulent flow dynamics from a PRT. The PRTFOAM is implemented based on the OpenFOAM and designed to be capable of integrating various turbulence models. Two representative Reynolds-averaged Navier–Stokes (RANS) models and a Smagorinsky–Lilly SGS model based on Large Eddy Simulation (LES) are coupled and tested with PRTFOAM. The case of a flow past a circular cylinder (Re = 3900) is tested and analyzed comprehensively as a benchmark case. Then, the turbulent steam dumping process in the full geometry of a PRT is analyzed and compared with ANSYS CFX and literature reports. In addition, we tested the WALE model based on the PRT steam dumping process. The results show that SST k-ω model and Smagorinsky–Lilly SGS model-based LES approach are more appropriate than the LRR model for PRT simulations. Moreover, it shows that the simulation results of Smagorinsky–Lilly SGS model and WALE model are basically consistent under the condition of PRT steam dumping process. Under this condition, the drawbacks of Smagorinsky–Lilly SGS model are not obvious. Furthermore, the comparison with CFX showed that our open source solver could be used to obtain better results in complex engineering cases. The design and testing results would provide guidance for further analysis of thermal-hydraulics in reactors based on open source codes.

Large eddy simulations of wind loads on an external floating-roof tank

Wind load of an external floating-roof tank is analyzed using large eddy simulation (LES). First, the governing equations and a LES solution procedure employing FLUENT software are presented. Subsequently, using two benchmark tank models, the accuracy of LES is examined and the advantages of LES over wind Reynolds-averaged Navier-Stokes (RANS) method in computing the unsteady wind loads and vortex structures around a tank are validated. Finally, using LES, the turbulent flows and wind loads around an external floating-roof tank at varying Reynolds numbers and liquid levels are analyzed. The DSM SGS model was found to have better accuracy when LES is applied to the tank flow. For an external floating-roof tank, the Reynolds number has little influence on the wind load at Re > 106, while the liquid level affects significantly both the wind loads on the tank and the vortex structures in the flow, and the wind pressure on the internal wall distributes more evenly than that on the external wall, due to the recirculation of the flow in the cavity above the roof. In all cases, the pressure on the external floating-roof tank oscillates apparently with time, which indicates the necessity of applying LES to the wind load analysis of tanks.

An Improved Surface Boundary Condition for Large-Eddy Simulations Based on Monin\u2013Obukhov Similarity Theory: Evaluation and Consequences for Grid Convergence in Neutral and Stable Conditions

Monin–Obukhov similarity theory is used in large-eddy simulation (LES) models as a surface boundary condition to predict the surface shear stress and scalar fluxes based on the gradients between the surface and the first grid level above the surface. We outline deficiencies of this methodology, such as the systematical underestimation of the surface shear stress, and propose a modified boundary condition to correct for this issue. The proposed boundary condition is applied to a set of LES for both neutral and stable boundary layers with successively decreasing grid spacing. The results indicate that the proposed boundary condition reliably corrects the surface shear stress and the sensible heat flux, and improves grid convergence of these quantities. The LES data indicate improved grid convergence for the surface shear stress, more so than for the surface heat flux. This is either due to a limited performance of the Monin–Obukhov similarity functions or due to problems in the LES model in representing stable conditions. Furthermore, we find that the correction achieved using the proposed boundary condition does not lead to improved grid convergence of the wind-speed and temperature profiles. From this we conclude that the sensitivity of the wind-speed and temperature profiles in the LES model to the grid spacing is more likely related to under-resolved near-surface gradients and turbulent mixing at the boundary-layer top, to the SGS model formulation, and/or to numerical issues, and not to deficiencies due to the use of improper surface boundary conditions.

LES/FMDF of premixed methane/air flow in a backward-facing step combustor

In the present study, a hybrid Eulerian-Lagrangian methodology is utilized for large eddy simulation (LES) of premixed fuel/air flow over a three-dimensional backward facing step (BFS). The fluid dynamic features are obtained by solving the Eulerian filtered compressible transport equations while the species are predicted by using the filtered mass density function method (FMDF). Some scalar fields are duplicated in FD and MC solvers to examine the numerical consistency between them. A good agreement is achieved by comparing the essential characteristics of the BFS flow (such as the mean and RMS values of the velocity and temperature fields and also the reattachment length) obtained from numerical results with the measurements. This ensures that the proposed hybrid method is reliable for studying the reacting flow in relatively complex combustion systems. Additionally, the performance of several SGS models are assessed, and the results indicate that the dynamic Smagorinsky and WALE models are superior to standard Smagorinsky and MKEV models.

Large-Eddy Simulation of a Laminar Separation Bubble

Large-eddy simulation of a laminar separation bubble on a flat plate has been performed and compared with the data in the literature. Suitability of different subgrid-scale models has been examined for simulation of transition. Comparison of various parameters and three-dimensional visualization of instantaneous flow fields indicate that standard Smagorinsky model, being too dissipative, is not suitable for this kind of problem and fails to properly resolve transition. With the application of low Reynolds number correction and a reduced model constant, a good agreement with the dynamic model is obtained at a lower computational cost. Of the three SGS models investigated, dynamic model gives the most physically accurate description of transition. The simulations illustrate that the appearance of Λ-vortices, vortex stretching and break down of longitudinal streaks characterize the transition process. Low values of reverse flow make it clear that a convective instability is involved. It is concluded that the initial amplification of disturbances is due to Tollmien -Schlichting mechanism while the roll-up of the shear layer takes place due to Kelvin-Helmholtz instability. It is observed that the universal log-law profile is not reached by the velocity profiles even far downstream.

Effect of grid sensitivity on the performance of wall adapting SGS models for LES of swirling and separating–reattaching flows

The present study assesses the performance of the Wall Adapting SGS models along with the Dynamic Smagorinsky model for flows involving separation, reattachment and swirl. Due to the simple geometry and wide application in a variety of engineering systems, the Backward-Facing Step (BFS) geometry and Confined Swirling Flow (CSF) geometry are invoked in the present case. The calculation of the SGS stresses employs three models, namely, the Dynamic Smagorinsky model, the Wall Adapting Local Eddy viscosity (WALE) model and the Dynamic WALE model. For studying the effect of the grid sensitivity, the simulations are performed over two sets of grids with different resolutions based on the non-dimensional wall distance parameter ( y+ ). Grids corresponding to y+= 70 and y+= 20 are employed for the subsonic flow over the BFS while grids corresponding to y+= 40 and y+= 20 are employed for supersonic flow over the BFS and for confined swirling flow geometry. The validation against the experimental results includes the mean flow fields and the turbulent stresses obtained for each case. The results reveal that for the fine grid (y+= 20), the near wall eddy viscosity profile for the WALE model is better than both the Dynamic WALE and the Dynamic Smagorinsky model. The difference between the predictions of the coarse and fine grids for Dynamic Smagorinsky and the WALE model is high whereas, the Dynamic WALE model is almost insensitive to the grid resolutions considered for the present case. The mean velocity and pressure values as well as the turbulent quantities predicted by the Dynamic WALE model are closest to the experimental values for all the cases.

Qualitative and Quantitative Investigation of Multiple Large Eddy Simulation Aspects for Pollutant Dispersion in Street Canyons Using OpenFOAM

Air pollution is probably the single largest environment risk to health and urban streets are the localized, relevant hotspots. Numerous studies reviewed the state-of-the-art models, proposed best-practice guidelines and explored, using various software, how different approaches (e.g., Reynolds-averaged Navier–Stokes (RANS), large eddy simulations (LES)) inter-compare. Open source tools are continuously attracting interest but lack of similar, extensive and comprehensive investigations. At the same time, their configuration varies significantly among the related studies leading to non-reproducible results. Therefore, the typical quasi-2D street canyon geometry was selected to employ the well-known open-source software OpenFOAM and to investigate and validate the main parameters affecting LES transient simulation of a pollutant dispersion. In brief, domain height slightly affected street level concentration but source height had a major impact. All sub-grid scale models predicted the velocity profiles adequately, but the k-equation SGS model best-resolved pollutant dispersion. Finally, an easily reproducible LES configuration is proposed that provided a satisfactory compromise between computational demands and accuracy.

Study of the internal characteristics of the stall in a centrifugal pump with a cubic non-linear SGS model

The stall in a centrifugal pump impeller under a quarter-load condition is investigated by using a third-order SGS model named the DCNM, for a better understanding of the rotation effect on the stall phenomenon. The study of the distributions of the Reynolds stresses, the production term and the rotation term reveals that the production and the rotation jointly result in the non-uniform Reynolds stress distribution. Further study of the two components of the production and the rotation shows that they jointly transport a certain energy from the Reynolds component Rvv to Ruu.

Anisotropic minimum-dissipation (AMD) subgrid-scale model implemented in OpenFOAM: Verification and assessment in single-phase and multi-phase flows

Minimum-dissipation sub-grid models represent simple alternatives to the Smagorinsky-type approaches to the implementation of sub-grid scales’ (SGS) effects in the large-eddy simulation (LES) approach. Recently, the anisotropic minimum-dissipation (AMD) model has been introduced, which is a static type of eddy-viscosity SGS model and is a new addition to this family. This model is easy to implement; furthermore, not only can it consider the effect of various directions in computing sub-grid stress, it can also operate transitional flows from laminar to turbulent. For the first time, we implemented AMD in the open source package OpenFOAM. Foremost, we verified the OpenFOAM implementation of the AMD model for the prediction of isotropic decaying turbulence, a temporal mixing layer and an internal channel flow. To achieve this, decaying turbulence was considered for the isotropic turbulence produced by a grid with a mesh size of 5.08 × 10–2 m in a flow of mean velocity of 10 m/s. The temporal mixing layer was simulated at a Reynolds number based on half the initial vorticity thickness of 105. Channel flow was investigated at the three frictional Reynolds numbers of 180, 395 and 590. The verification results revealed that the implemented AMD model in OpenFOAM offered satisfactory accuracy to capture decaying turbulence and temporal mixing layer and calculate boundary layer velocity profiles and first and second-order turbulent parameters in the channel flow on anisotropic grids. Following this, for the first time, we evaluated AMD's performance in predicting external non-cavitating and cavitating flows over a 3D sphere. Non-cavitating and cavitating flow over the sphere were considered at Re = 2 × 104 and Re = 1.5 × 106, respectively. The AMD results were then compared with the direct numerical simulation (DNS) data and the numerical results obtained from the established SGS models such as dynamic Smagorinsky (DS), one equation eddy viscosity model (OEEVM), detached eddy simulation (DES), and delayed detached eddy simulation (DDES), where applicable. In treating sphere flow, in spite of the use of a coarse grid rather than the alternative SGS models, the AMD model predicted accurate results for pressure coefficient, shear stress, and separation point location over the sphere as well as the velocity profile in the wake region. It was detected that the simulation time of the AMD model was lower than that of DS in all the considered test cases by approximately 10–16%.