SGS Model Research Articles

Three-Dimensional Simulation of SOFC Anode Polarization Characteristics Based on Sub-Grid Scale Model

Three-dimensional numerical simulation of SOFC anode polarization is conducted with a structure obtained by a focused ion beam and scanning electron microscope (FIB-SEM). Electronic, ionic and gaseous transports with electrochemical reaction are considered. A sub-grid scale model is newly developed and effectively used to evaluate the transport flux in the porous structure. The proposed SGS model shows its potential to reasonably evaluate the transport flux considering the microstructure smaller than the grid size.

Effects of Transverse Magnetic Field on Thermal Fluctuations in the Melt of a Cz-Si Crystal Growth

Three dimensional (3D) unsteady computations were carried out to understand the effects of transverse magnetic field (TMF) on thermal fluctuations in the melt of a 300 mm Cz-Si crystal growth. A developed LES program with the dynamic SGS model was employed to predict the melt turbulence with or without TMF. The effects of TMF on statistical behaviors of the melt convection were studied. It was found that the thermal fluctuations in the crystallization zone decreased significantly when the TMF was applied. This indicates that the TMF can suppress the flow instability in the melt of the Cz-Si crystal growth.

A fluid-dynamical subgrid scale model for highly compressible astrophysical turbulence

We formulate and implement the Euler equations with SGS dynamics and provide numerical tests of an SGS turbulence energy model that predicts the turbulent pressure of unresolved velocity fluctuations and the rate of dissipation for highly compressible turbulence. We test closures for the turbulence energy cascade by filtering data from high-resolution simulations of forced isothermal and adiabatic turbulence. Optimal properties and an excellent correlation are found for a linear combination of the eddy-viscosity closure that is employed in LES of weakly compressible turbulence and a term that is non-linear in the Jacobian matrix of the velocity. Using this mixed closure, the SGS turbulence energy model is validated in LES of turbulence with stochastic forcing. It is found that the SGS model satisfies several important requirements: 1. The mean SGS turbulence energy follows a power law for varying grid scale. 2. The root mean square (RMS) Mach number of the unresolved velocity fluctuations is proportional to the RMS Mach number of the resolved turbulence, independent of the forcing. 3. The rate of dissipation and the turbulence energy flux are constant. Moreover, we discuss difficulties with direct estimates of the turbulent pressure and the dissipation rate on the basis of resolved flow quantities that have recently been proposed. In combination with the energy injection by stellar feedback and other unresolved processes, the proposed SGS model is applicable to a variety of problems in computational astrophysics. Computing the SGS turbulence energy, the treatment of star formation and stellar feedback in galaxy simulations can be improved. Further, we expect that the turbulent pressure on the grid scale affects the stability of gas against gravitational collapse.

Large eddy simulation of wind effects on a super-tall building

A new inflow turbulence generation method and a combined dynamic SGS model recently developed by the authors were applied to evaluate the wind effects on 508 m high Taipei 101 Tower. Unlike the majority of the past studies on large eddy simulation (LES) of wind effects on tall buildings, the present numerical simulations were conducted for the full-scale tall building with Reynolds number greater than <TEX>$10^8$</TEX>. The inflow turbulent flow field was generated based on the new method called discretizing and synthesizing of random flow generation technique (DSRFG) with a prominent feature that the generated wind velocity fluctuations satisfy any target spectrum and target profiles of turbulence intensity and turbulence integral length scale. The new dynamic SGS model takes both advantages of one-equation SGS model and a dynamic production term without test-filtering operation, which is particular suitable to relative coarse grid situations and high Reynolds number flows. The results of comparative investigations with and without generation of inflow turbulence show that: (1) proper simulation of an inflow turbulent field is essential in accurate evaluation of dynamic wind loads on a tall building and the prescribed inflow turbulence characteristics can be adequately imposed on the inflow boundary by the DSRFG method; (2) the DSRFG can generate a large number of random vortex-like patterns in oncoming flow, leading to good agreements of both mean and dynamic forces with wind tunnel test results; (3) The dynamic mechanism of the adopted SGS model behaves adequately in the present LES and its integration with the DSRFG technique can provide satisfactory predictions of the wind effects on the super-tall building.

난류 경계층에 놓인 공동 내부유동에 관한 수치해석적 연구

본 논문은 공동 주위 난류유동특성을 LES 기법으로 수치해석을 수행하여 알아보았다. 본 연구에 적용된 레이놀즈수는 공동 깊이만큼의 높이 h 에서의 유속을 기준으로 1.0×105 이며 3 차원 공동에서의 유동특성을 알아보았다. 적절한 비압축성 Filtered Navier-Stokes 방정식을 적용하기 위해, 계산격자를 공동 표면 근처에는 조밀하게 멀어질수록 성기게 생성하였으며, 이는 계산시간을 단축시키며 빠른 수렴을 도와준다. 또한, Boussinesq 가설을 subgrid-scale 난류모델에 적용하였고, Subgrid-scale 난류점성을 얻기 위해 smagorinsky-Lilly SGS 모델을 적용하였으며, 그 때의 CFL 수는 1.0 이다. 또한, 본 논문은 서로 다른 4 가지 형상의 공동의 및 입구조건의 변화에 따른 유동 특성도 함께 연구되었다.

LES of Impingement Heat Transfer on a Concave Surface

Wall-resolved and zonal numerical large eddy type simulations are performed for a round jet impinging on a concave hemisphere at Re = 23,000. The zonal method uses a near-wall k–l model and a Hamilton-Jacobi equation to match this to the large eddy simulation zone. To minimize numerical dissipation, a self-adaptive discretization (SDS) scheme is examined. Both second- (n = 2) and sixth- (n = 6) order-based central discretization schemes are tested. The characteristics of the schemes is assessed using two test cases: the development of a subcritical Tollmien-Schlichting (T-S) stability wave in a plane channel and the decay of homogenous, isotropic turbulence (DHIT). It is found, that Smagorinsky LES simulations tend to be too dissipative in the high wave-number region, even with the SDS scheme; hence, the SGS model is omitted. Significant flow feedback is observed for the hemisphere case. Both shear-layer excitation and stabilization is observed. Computed wall pressure coefficients for the zonal NLES method are encouraging; for the wall-resolved case the stagnation region value is overpredicted. Heat transfer for the wall-resolved and zonal large eddy simulations are encouraging. For both quantities the difference between the n = 2 and n = 6 schemes is small, and the modeling approach used appears to be more influential. It is concluded that the presence of feedback mechanisms should be considered when designing experiments and/or numerical simulations for this case, and that the importance of boundary conditions for LES should not be neglected.

Multiphase flow and mixing in dilute bubble swarms

AbstractHigh‐fidelity three‐dimensional (3‐D) simulations of multiphase flow and mixing in dilute bubble swarms were performed using the Euler‐Lagrange simulation approach. Included was species transport, as well as complex chemical reactions in the simulations. It was found that the algebraic SGS model satisfactory predicts experimental data for the mean flow field. A detailed description of multiphase flow was used and developed to simulate the time evolution of scalar and reactive mixing in a bubble column. An analysis involving the scale of segregation ϕ, a metric that characterizes the mean driving force for mixing, is applied for the first time to multiphase flow. The study shows that ϕ is inversely proportional to the bubble diameter at constant gas‐feed rate, but only a weak function of the gas‐feed rate. Also, we observed significant differences of mixing metrics in reactive and nonreactive systems. © 2010 American Institute of Chemical Engineers AIChE J, 2010

Assessment of Implicit Large-Eddy Simulation with a Conservative Immersed Interface Method for turbulent cylinder flow

The success of Large-Eddy Simulations (LES) of wall-bounded turbulence depends strongly on an accurate representation of the flow near the boundaries. Since in implicit LES the truncation error of the numerical discretization itself functions as SGS model, the order of accuracy of the discretization should be maintained near the boundary. In this paper, we analyze the performance of implicit LES for predicting turbulent flows along complex geometries. For the representation of wall-boundaries two different approaches are compared: (A) the case of body-fitted grids, and (B) a novel second-order accurate Conservative Immersed Interface Method (CIIM) for representing boundaries of arbitrary shape on Cartesian grids. The need for mapping domain boundaries on the grid raises the question whether a comparable accuracy with body-fitted grids can be achieved. In both (A) and (B) we employ the Adaptive Local Deconvolution Method (ALDM) for implicit LES. The assessment is carried out on the example of the flow over a circular cylinder at Re = 3900. Two simulations with comparable numerical resolution are performed and compared to experimental reference data. Both methods result in similar accuracy for turbulent statistics and mean integral quantities. This demonstrates that ALDM on Cartesian grids is a feasible and efficient approach to simulate turbulent flows on complex domains.

Sub-grid scale combustion models for large eddy simulation of unsteady premixed flame propagation around obstacles

In this work, an assessment of different sub-grid scale (sgs) combustion models proposed for large eddy simulation (LES) of steady turbulent premixed combustion (Colin et al., Phys. Fluids 12 (2000) 1843–1863; Flohr and Pitsch, Proc. CTR Summer Program, 2000, pp. 61–82; Kim and Menon, Combust. Sci. Technol. 160 (2000) 119–150; Charlette et al., Combust. Flame 131 (2002) 159–180; Pitsch and Duchamp de Lageneste, Proc. Combust. Inst. 29 (2002) 2001–2008) was performed to identify the model that best predicts unsteady flame propagation in gas explosions. Numerical results were compared to the experimental data by Patel et al. (Proc. Combust. Inst. 29 (2002) 1849–1854) for premixed deflagrating flame in a vented chamber in the presence of three sequential obstacles. It is found that all sgs combustion models are able to reproduce qualitatively the experiment in terms of step of flame acceleration and deceleration around each obstacle, and shape of the propagating flame. Without adjusting any constants and parameters, the sgs model by Charlette et al. also provides satisfactory quantitative predictions for flame speed and pressure peak. Conversely, the sgs combustion models other than Charlette et al. give correct predictions only after an ad hoc tuning of constants and parameters.

A new simple h-mesh adaptation algorithm for standard Smagorinsky LES: a first step of Taylor scale as a refinement variable

The interaction between discretization error and modeling error has led to some doubts in adopting Solution Adaptive Grid (SAG) strategies with LES. Existing SAG approaches contain undesired aspects making the use of one complicated and less convenient to apply to real engineering applications. In this work, a new refinement algorithm is proposed aiming to enhance the efficiency of SAG methodology in terms of simplicity in defining, less user’s judgment, designed especially for standard Smagorinsky LES and computational affordability. The construction of a new refinement variable as a function of the Taylor scale, corresponding to the kinetic energy balance requirement of the Smagorinsky SGS model is presented. The numerical study has been tested out with a turbulent plane jet in two dimensions. It is found that the result quality can be effectively improved as well as a significant reduction in CPU time compared to fixed grid cases.

Unified formulation for URANS and LES in DxUNSp code

The aim of this work is to find a unified and efficient implementation of a LES turbulence model in an existing URANS CFD code, initially based on unsteady RANS equations with a k-e turbulence model. This code has the capability to be developed for nonreacting/reacting multifluid flows in research applications. The paper intends to present mainly three aspects of this implementation for unstructured mesh based solvers, for high Reynolds compressible flows: the influence of the numerical reconstruction scheme on the results for compressible LES, the influence of the compressible SGS modeling and the efficient implementation of a wall-law based approach for complex geometry. The results will be presented for a test case (3D flow over a square cylinder at Reynolds = 22.000 [0]) and compared with experimental data and other simulations. Some details for the computational efficiency and implementation on parallel computer cluster at INCAS will also be presented [0].

Prediction of Melt-Crystal Interface Shape and Melt Convection in a Large-scale CZ-Si Growth System Using RANS and LES Methods in Global Simulation.

Global simulations were performed for a large-scale CZ-Si growth system, in which three models were tested to predict the turbulent melt flow. The three turbulence models are the standard turbulence model using wall functions at solid boundaries, a modified two-layer turbulence model in RANS method and the standard Smagorinsky SGS model in LES method, respectively. The comparison of the simulation results obtained with these different models showed that each turbulence model has its own features in prediction of the melt convection and the melt-crystal interface shape in a large-scale CZ-Si growth system.

Computation of Inviscid Supersonic Flows Around Cylinders and Spheres With the V-SGS Stabilization and YZβ Shock-Capturing

The YZβ shock-capturing technique was introduced originally for use in combination with the streamline-upwind/Petrov–Galerkin (SUPG) formulation of compressible flows in conservation variables. It is a simple residual-based shock-capturing technique. Later it was also combined with the variable subgrid scale (V-SGS) formulation of compressible flows in conservation variables and tested on standard 2D test problems. The V-SGS method is based on an approximation of the class of SGS models derived from the Hughes variational multiscale method. In this paper, we carry out numerical experiments with inviscid supersonic flows around cylinders and spheres to evaluate the performance of the YZβ shock-capturing combined with the V-SGS method. The cylinder computations are carried out at Mach numbers 3 and 8, and the sphere computations are carried out at Mach number 3. The results compare well to those obtained with the YZβ shock-capturing combined with the SUPG formulation, which were shown earlier to compare very favorably to those obtained with the well established OVERFLOW code.

Design and analysis of a new filter for LES and DES

Spatial filters have been rigorously analyzed here for stabilization and dispersion relation preservation properties using spectral-matrix theory. Traditional LES faces problems of instability, aliasing and additional complexity of SGS modeling. Here, an alternative is proposed where unfiltered governing equation is solved, followed by spatial upwind filtering, without requiring any SGS models. This filter retains the resolution of high accuracy methods and removes energy, like the action of hyper-viscosity used in spectral methods. As examples, solutions for (a) accelerated flow past a NACA-0015 airfoil held normal to the flow and (b) transitional flow past a NLF airfoil, have been reported.

Stochastic modelling of aerosol deposition for LES of 90° bend turbulent flow

Aerosols deposition in turbulent bend flows is a major concern that is critical to many industrial, environmental and biomedical applications. In this work, a well-resolved LES was performed to compute the deposition efficiency of aerosols in turbulent circular cross-section bend flow of Dean number De = 4 , 225 . The numerical predictions were compared to the experimental work of Pui et al. [Pui, D.Y.H., Romay-Novas, F., Liu, B.Y.H., 1987. Experimental study of particle deposition in bend of circular cross-section. Aerosol Sci. Technol. 7, 301–315] and the fully-resolved LES of Breuer et al. [Breuer, M., Baytekin, H.T., Matida, E.A., 2006. Prediction of aerosol deposition in 90° bends using LES and an efficient Lagrangian tracking method. J. Aerosol Sci. 37, 1407–1428]. In the present LES, a slightly coarser but unstructured-grid numerical description was adopted, entailing that a portion of the small scales’ contribution to particle dispersion to be discarded. Thus, a Langevin-type stochastic model was used to model the effect of the discarded sub-grid motion on aerosol deposition. This stochastic model was shown to perform well in previous studies [Berrouk, A.S., Laurence, D., Riley, J.J., Stock, D.E., 2007. Stochastic modelling of inertial particle dispersion by subgrid motion for LES of high Reynolds number pipe flow. J. Turbulence, 8, 50]. Good care was taken to ensure that the main dynamical features of the continuous phase were captured by the present LES. An estimation of the filtered-out kinetic energy was provided. Results of the present LES with SGS model for particles were found to compare well with the experimental work and the fully-resolved LES (near-wall DNS) of Breuer for all the range of the Stokes number considered, 0.001 < St < 1.5 . Influence of the SGS model for particles was visible for the deposition efficiency of aerosols with Stokes number St < 0.3 .

A velocity-estimation subgrid model constrained by subgrid scale dissipation

Purely dissipative eddy-viscosity subgrid models have proven very successful in large-eddy simulations (LES) at moderate resolution. Simulations at coarse resolutions where the underlying assumption of small-scale universality is not valid, warrant more advanced models. However, non-eddy viscosity models are often unstable due to the lack of sufficient dissipation. This paper proposes a simple modeling approach which incorporates the dissipative nature of existing eddy viscosity models into more physically appealing non-eddy viscosity SGS models. The key idea is to impose the SGS dissipation of the eddy viscosity model as a constraint on the non-eddy viscosity model when determining the coefficients in the non-eddy viscosity model. We propose a new subgrid scale model (RSEM), which is based on estimation of the unresolved velocity field. RSEM is developed in physical space and does not require the use of finer grids to estimate the subgrid velocity field. The model coefficient is determined such that total SGS dissipation matches that from a target SGS model in the mean or least-squares sense. The dynamic Smagorinsky model is used to provide the target dissipation. Results are shown for LES of decaying isotropic turbulence and turbulent channel flow. For isotropic turbulence, RSEM displays some level of backward dissipation, while yielding as good results as the dynamic Smagorinsky model. For channel flow, the results from RSEM are better than those from the dynamic Smagorinsky model for both statistics and instantaneous flow structures.

Effects of subgrid-scale modeling on Lagrangian statistics in large-eddy simulation

The application of large-eddy simulation (LES) to turbulent transport processes requires accurate prediction of the Lagrangian statistics of flow fields. However, in most existing SGS models, no explicit consideration is given to Lagrangian statistics. In this paper, we focus on the effects of SGS modeling on Lagrangian statistics in LES ranging from statistics determining single-particle dispersion to those of pair dispersion and multiparticle dispersion. Lagrangian statistics in homogeneous isotropic turbulence are extracted from direct numerical simulation (DNS) and the LES with a spectral eddy-viscosity model. For the case of longtime single-particle dispersion, it is shown that, compared to DNS, LES overpredicts the time scale of the Lagrangian velocity correlation but underpredicts the Lagrangian velocity fluctuation. These two effects tend to cancel one another leading to an accurate prediction of the longtime turbulent dispersion coefficient. Unlike the single-particle dispersion, LES tends to underestimate significantly the rate of relative dispersion of particle pairs and multiple-particles, when initial separation distances are less than the minimum resolved scale due to the lack of subgrid fluctuations. The overprediction of LES on the time scale of the Lagrangian velocity correlation is further confirmed by a theoretical analysis using a turbulence closure theory.

Lagrangian dynamic SGS model for stochastic coherent adaptive large eddy simulation

Stochastic coherent adaptive large eddy simulation (SCALES) is an extension of large eddy simulation that uses a wavelet filter-based dynamic grid adaptation strategy to solve for the most energetic coherent structures in a turbulent flow field, while modeling the effect of the less energetic ones. A localized dynamic subgrid scale model is needed to fully exploit the ability of the method to track coherent structures. In this paper, new local Lagrangian models based on a modified Germano dynamic procedure, redefined in terms of wavelet thresholding filters, are proposed. These models extend the original path-line formulation of Meneveau et al. [J. Fluid Mech. 319 (1996)] in two ways: as Lagrangian path-line diffusive and Lagrangian path-tube averaging procedures. The proposed models are tested for freely decaying homogeneous turbulence with initial Re λ = 72. It is shown that the SCALES results, obtained with less than 0.4% of the total non-adaptive nodes required for a DNS with the same wavelet solver, closely match reference DNS data. In contrast to classical LES, this agreement holds not only for large scale global statistical quantities, but also for energy and, more importantly, enstrophy spectra up to the dissipative wavenumber range.

A study of Large-Eddy Simulation for similarity theory of Atmospheric Boundary Layer with Urban Geometry

In this study, a Large-Eddy Simulation model that is capable of resolving urban buildings was used to investigate the influence over turbulence statistics caused by the existence of urban geometry.In the convective boundary layer, the horizontal temperature variances from both flat and urban geometry are smaller than the field observation results. And the horizontal velocity variances in the urban ABL are slightly smaller than those for a flat throughout the whole range of height. In the surface layer, the results of the urban simulation are not in agreement with the results from observations. And that is partly caused by the disability of current SGS model.

A dynamic scale-similarity model for dynamo simulations in a rotating plane layer

We test a dynamic version of the scale-similarity model in plane-layer dynamo simulations. Previous work has shown that the scale-similarity model reproduces the spatial pattern of the SGS heat flux, even near the boundaries, when a correction is added for the commutation error. However, the average amplitude of the SGS terms deviates from the estimate inferred from a fully resolved calculation. In this study, we attempt to improve the average amplitude of the SGS terms by applying the dynamic scheme of Germano et al. (1991) to the scale-similarity model. This procedure yields estimates for the coefficients in the SGS model, which we evaluate as a function of vertical position in the plane layer. The resulting SGS heat flux is compared with the value determined from a resolved dynamo solution on a finer grid. The results show that the dynamic scheme improves the amplitude of the SGS heat flux. Furthermore, the results show that the coefficients for each SGS term in the dynamo simulation (heat flux, momentum flux, Maxwell stress, and magnetic induction) should be evaluated independently because the predicted coefficients for each SGS term have different values.