Shear improved Smagorinsky model for large eddy simulation of flow in a stirred tank with a Rushton disk turbine

In this study, three-dimensional computational fluid dynamics (CFD) simulations of particle deposition from turbulent flows in a vertical straight pipe were carried out, using the STAR-CCM+ v5.04 CFD package. The highlight of the study is the development of a post-processing approach for quantitatively assessing the accuracy of the Lagrangian particle tracking scheme. Three Reynolds-averaged Navier–Stokes (RANS) models and a large eddy simulation (LES) model were employed in the simulations, in conjunction with two wall treatment schemes and several near-wall mesh conditions. The particle deposition velocity was obtained based on the CFD simulation results, and compared to the experimental results. In post-processing, a “particle responsiveness factor,” defined as the ratio of particle mean square velocity fluctuation to fluid mean square velocity fluctuation for the wall-adjacent cells, was quantified using an Eulerian particle transport formulation. The particle responsiveness factor of the CFD simulations was then compared with that obtained using an empirical equation. The most accurate aerosol deposition results were obtained with a fine near-wall mesh (y+≈1) and resolved near-wall flow (no wall function). The LES model and the Reynolds stress model (RSM) produced the most accurate deposition velocity results, but the computation time of the former was up to ten times longer than that of the latter. The lower accuracy of the isotropic RANS models was attributed to their overprediction of the near-wall turbulence intensity gradient, and less accurate particle tracking as suggested by the particle responsiveness factor. The particle responsiveness factor, introduced for the first time in this study, was shown to be a useful index for evaluating accuracy of the Lagrangian particle tracking scheme due to its independence of specific knowledge of CFD algorithm and coding.

Flow-Induced Vibration (FIV) caused by turbulent flow inside a pipe could lead to fatigue failure with shell mode vibration. Our previous study investigated the excitation source of the FIV for tee junctions experimentally to understand the FIV mechanism and provided Power Spectral Density (PSD) profiles of pressure fluctuation. In the present study, experiments were performed with more extensive measurement points for both 90- and 45-degree tees to understand a more detailed mechanism. PSD plots were provided, featuring different pressure fluctuation characteristics at each measurement point among both angle tees. It also emerged that the PSD level declined with increasing distance from the impingement point. Unsteady Computational Fluid Dynamics (CFD) simulations with the Large Eddy Simulation (LES) model were also performed to understand the turbulent structure for the tee junctions. The frequency characteristic of the simulated pressure fluctuation effectively matched those of the experiments at each measurement point, which implies that CFD simulation with an LES model could reveal reasonable predictions of the FIV excitation source for tee junctions. Simulation results showed that the relatively large vortex shed from the branch pipe impinged periodically on the main pipe bottom and the large vortex was dissipated downstream. These vortex behaviors would be the main mechanism generating the FIV excitation source.

The LES model's role in jet noise

Fluid flow plays a significant role in the continuous casting of molten steel. In this paper, two Reynolds Averaged Naiver-Stokes (RANS) models and a Large Eddy Simulation (LES) model were comparatively employed to characterize the fluid flow inside a dissipative ladle shroud and a tundish. LES model was proved to be powerful to characterize the turbulence structure inside the ladle shroud. The effect of meshing density on the computational accuracy was considered using the LES model. The fine-mesh model can capture multiscale vortices inside the ladle shroud; while, the coarse-mesh model disables the LES to obtain the detailed flow information. The experiment of particle image velocimetry (PIV) was used to verify the flow field obtained by the LES model inside the tundish. The PIV and the LES results agree well in terms of flow pattern and velocity vector.

Standard Computational Fluid Dynamic (CFD) techniques are widely used in the design of hydraulic valves for optimising the valve performance and reducing the production effort. They calculate the turbulent flow and predict cavitation. Unfortunately, the currently used models are often inadequate and out of date to catch the complexity of these phenomena such as the transient interaction between cavitation and turbulence. Advanced computational methods have been developed and applied to other engineering branches. Despite this fact, they face many difficulties to be employed in hydraulics. In this paper, a first step is taken towards the usage of these cutting edge CFD methods for hydraulic valves. At first, the different challenges for a CFD code to simulate valve flows are highlighted. A novel computational approach is then presented. It combines a Large Eddy Simulation (LES) model for the turbulence modelling as well as a Full Cavitation Model (FCM). The LES technique explicitly resolves the large turbulence scales while the smaller ones are modelled. The FCM not only predicts vapour but also gas cavitation, which plays a vital role in hydraulic fluids. This method is tested to simulate the flow in a pilot stage of a jet-pipe servo-valve. The test case is presented and the different boundary conditions used for the simulations are given. The results of the simulation are compared with experimental results showing a good agreement. A comparison between the LES model and the standard two-equation turbulence model shows the advantages of the LES approach. Finally, the transient features of the flow are highlighted in terms of velocity oscillation.

Computational fluid dynamics (CFD) simulations of steel flow in an Rheinsahl–Heraeus (RH) process are realized by a discrete phase model (DPM) for the driving bubble plumes, a volume of fluid (VoF) method for the free surface in the vacuum chamber (VC), and a large eddy simulations (LES) model for the transport and mixing of steel alloys. CFD simulations are opposed to particle image velocimetry (PIV) analyses of flow pattern at the bath surface in the VC. While simple Reynolds averaged turbulence models fail to reproduce these plant observations, LES agrees fairly well. Furthermore, the steel recirculation rate is compared with empirical correlations from the literature, yielding good agreement with respect to the dependency of the recirculation rate on the gas injection rate. The absolute value of the recirculation rate increases by 15%, in case (realistic) eroded edges are considered instead of a (unrealistic) sharp‐edged geometry. Data‐assisted recurrence CFD (rCFD) is applied to accelerate conventional CFD. The rCFD simulations yield a computational speed‐up of four orders of magnitude, enabling real‐time LES at full grid resolution of three million cells. Titanium homogenization in the steel ladle is addressed by means of rCFD and compared with corresponding plant trials yielding good agreement.

A shear-improved Smagorinsky model is introduced based on results concerning mean-shear effects in wall-bounded turbulence. The Smagorinsky eddy-viscosity is modified asvT=(Csδ)2(|S|—|〈S〉|): the magnitude of the mean shear |〈S〉|is subtracted from the magnitude of the instantaneous resolved rate-of-strain tensor |S|;CSis the standard Smagorinsky constant and Δ denotes the grid spacing. This subgrid-scale model is tested in large-eddy simulations of plane-channel flows at Reynolds numbersReτ= 395 andReτ= 590. First comparisons with the dynamic Smagorinsky model and direct numerical simulations for mean velocity, turbulent kinetic energy and Reynolds stress profiles, are shown to be extremely satisfactory. The proposed model, in addition to being physically sound and consistent with the scale-by-scale energy budget of locally homogeneous shear turbulence, has a low computational cost and possesses a high potential for generalization to complex non-homogeneous turbulent flows.

Large eddy simulation of flow through a periodic array of urban-like obstacles using a canopy stress method

Assessment of a recently proposed subgrid-scale(SGS) eddy viscosity model (Horiuti 1993) in large eddy simulation(LES) is made utilizing the direct numerical simulation(DNS) database for fully developed turbulent channel and mixing layer flows. In this new model (generalized normal stress model), the SGS normal shear stress is adopted as for its representative velocity scale in place of the total SGS energy adopted in the conventional Smagorinsky model(Smagorinsky 1963), and the eddy viscosity coefficients are made non-isotropic to preserve the tensorial invariance of the SGS Reynolds stresses. Evaluation of the proposed model is also done in the actual LES of channel and mixing layer flows, and it is shown that the proposed model is equally useful as the Smagorinsky model containing the Van Driest damping functions. As an alternative SGS modeling approach for the conventional eddy viscosity models, a possibility of the use of the scale similarity model to carry out LES computations is investigated.

Application of modified eddy dissipation concept with large eddy simulation for numerical investigation of internal combustion engines

Present-day demands on combustion equipment are increasing the need for improved understanding and prediction of turbulent combustion. Large eddy simulation (LES), in which the large-scale flow is resolved on the grid, leaving only the small-scale flow to be modeled, provides a natural framework for combustion simulations, as the transient nature of the flow is resolved. In most situations, however, the flame is thinner than the LES grid, and subgrid modeling is required to also handle the turbulence-chemistry interactions. Here, we examine the predictive capabilities of the flamelet LES models, such as the Flamelet Progress Variable LES (LES-FPV) models, and the finite rate chemistry LES models, such as the LES-Thickened Flame Model (LES-TFM), the partially stirred reactor model (LES-PaSR) and the Eddy Dissipation Concept (LES-EDC) model. These different combustion LES models are used here to study the reacting flow in an axisymmetric dump combustor at a Reynolds number of 55,800, the Damköhler number of 167 and a Karlowitz number of 0.15, placing the flame in the corrugated flame regime. The computational results are compared to experimental data of velocity and temperature to examine predictive capabilities of the different models.

A large eddy simulation (LES) model, with ice phase included, has been used to study the marine convective boundary layer filled with snow. Extensions to Moeng's LES model include the diagnosis of cloud ice mixing ratio, snow precipitation, and the parameterization of detailed microphysical processes. Model simulations are compared with cold air outbreak field observations over Lake Michigan, as well as with the liquid phase LES results for the same atmospheric conditions. The buoyancy flux and vertical velocity variance profiles generated by the ice phase LES are found to be more consistent with the observations than those generated by the liquid phase LES results. The incorporation of the ice phase into the LES model has also improved the agreement of vertical velocity skewness (Sw) between observations and LES model results. It has also been found that the presence of precipitation, and the associated microphysical processes, has a significant effect on the structure of the convective boundary...

of thesis entitled Large-Eddy Simulation of Wind Flow and Air Pollutant Transport inside Urban Street Canyons of Different Aspect Ratios submitted by Li, Xianxiang for the degree of Doctor of Philosophy at the University of Hong Kong in June 2008 The characteristics of the wind flow and air pollutant transport inside urban street canyons are of fundamental importance to the air quality monitoring and improvement. An investigation of these characteristics was performed in this study using both experimental and numerical techniques. The focus is on the mechanisms of pollutant transport and removal inside urban street canyons of high aspect ratios (AR, ratio of the building height to the street width). A physical model in water channel was first developed to study the wind flow in street canyons of different ARs of 0.5, 1.0, and 2.0. The velocity and turbulent fluctuation were measured by a Laser-Doppler Anemometer (LDA). The measured velocity and turbulent fluctuation at various locations were validated with several experimental datasets available in literature. The measured results in most locations were also in good agreement with previous numerical results. The comprehensive measurement data can provide a validation database for the numerical model development. To take into account the detailed transient turbulent processes, a large-eddy simulation (LES) model was developed based on a one-equation subgrid-scale (SGS) model and finite element method (FEM). This model was validated and fine-tuned by applying to an open channel flow at Reτ = 180. By comparing the calculated velocity and fluctuations with those obtained from experiment and direct numerical simulation (DNS), a set of model constants was determined for the LES model. A 1/7th wall model was further incorporated into this LES model to mitigate the strict near-wall resolution requirement. To validate the newly developed LES model for street canyons, the LES results for the street canyons of AR 1 and 2 were compared extensively with the waterchannel experimental data and previous LES results. The good agreement showed that the newly developed LES model was capable of predicting the complicated flow patterns and pollutant dispersion in street canyons. The validated LES model was then employed to simulate the street canyons of AR 3, 5, and 10. Three, five, and eight vertically aligned primary recirculations were found for the three cases, respectively, which showed decreasing strength with decreasing height. The very small ground-level wind speeds made the ground-level pollutants extremely difficult to disperse. Local maxima of the turbulence intensities were found at the interfaces between the primary recirculations and the free surface layer. The pollutant followed the trajectories of the primary recirculations. High pollutant concentration and variance were found near the buildings where wind flowed upward. Large gradients of pollutant concentration and variance were also observed at the interfaces between the primary recirculations and the free surface layer. Detailed analyses of concentration budget terms showed that the advection terms were responsible for pollutant redistribution within primary recirculations, while the turbulent transport terms were responsible for pollutant penetration between primary recirculations and pollutant removal from the street canyon. Based on the LES results, several quantities were introduced to compare the pollutant removal capability of different street canyon configurations. It was found that these quantities were all non-linear functions of the street canyon AR. Large-Eddy Simulation of Wind Flow and Air Pollutant Transport inside Urban Street Canyons of Different Aspect Ratios

With the rapid development of computational fluid dynamics (CFD) technology, it has been widely used to study the wind field characteristics of downbursts in mountainous areas. However, there is little guidance on the selection of different turbulence models for simulating downburst wind fields over hills using CFD, and few comparative studies have been conducted. This paper used nine turbulence models to simulate the wind field of a downburst over a 3D quadratic ideal hill. The simulated values of average and transient winds were compared with wind tunnel test data, and the flow characteristics at different moments under a downburst were analyzed. The flow characteristics in the wake region of the downburst over the hill are also quantitatively analyzed using the proper orthogonal decomposition (POD) method. The results show that approximately 85% of the results from the LES and REA models fall within a 30% error range, so the large eddy simulation (LES) model and the realizable k-ε model (REA) are more accurate in simulating the mean wind field, and the transient wind field simulated by the LES model is also in good agreement with the experimental data. In addition, this paper reveals the evolution mechanism of the transient wind field structure over a hill model under a downburst and finds that the first-order mode obtained by POD may be related to the acceleration effect on the hilltop.

This paper presents a study on the effectiveness of two turbulence models, the large eddy simulation (LES) model and the k-ε turbulence model, in predicting mixing time within a ladle furnace using the computational fluid dynamics (CFD) technique. The CFD model was developed based on a downscaled water ladle from an industrial ladle. Corresponding experiments were conducted to provide insights into the flow field, which were used for the validation of CFD simulations. The correlation between the flow structure and turbulence kinetic energy in relation to mixing time was investigated. Flow field results indicated that both turbulence models aligned well with time-averaged velocity data from the experiments. However, the LES model not only offered a closer match in magnitude but also provided a more detailed representation of turbulence eddies. With respect to predicting mixing time, increased flow rates resulted in extended mixing times in both turbulence models. However, the LES model consistently projected longer mixing times due to its capability to capture a more intricate distribution of turbulence eddies.