Dynamic Smagorinsky Subgrid-scale Model Research Articles

Large eddy simulation of a turbulent channel flow using dynamic Smagorinsky subgrid scale model and differential equation wall model

A large eddy simulation (LES) is conducted in a plane channel flow which is turbulent. For understanding the mechanism of the turbulent motion more efficiently, the present study is an attempt to conduct the LES using a Dynamic Smagorinsky Model (DSM) and a Differential Equation Wall Model (DEWM). DSM is a subgrid-scale model and DEWM is for approximating the near wall layer. The simulation is performed in a domain of 2 π δ × 2 δ × π δwhich is discretized by 32 × 20 × 32grid points where δ is the channel half width. The discretization techniques have been used in the simulation, are the 3rd order low storage R-K method in time and 2nd order finite difference formulation in space. To assess the performance of the LES with DSM and DEWM, the simulation results are compared with direct numerical simulation (DNS) data and LES data using Standard Smagorinsky Model (SSM). Comparing the results in the essential turbulence statistical field it has been found that DSM performs better than SSM in most of the cases and shows better agreement with the DNS results. Flow structures in the turbulent flow field are compared in isosurfaces and contour plots, and found that tube-like vortical structures are randomly distributed over the entire flow field, and the number of the vortical structures are more for the DSM case than that of SSM.

Large-eddy simulation of airflow dynamics around a cluster of buildings

The wind flow around the buildings with different rooftops is studied numerically using large-eddy simulation (LES). The filtered Navier-Stokes equations in LES are used to compute the large eddies, whereas the dynamic Smagorinsky subgrid-scale (SGS) model calculates the small eddies. A turbulent spot method is applied for the synthesis of artificial turbulent fields at the inlet. In this separated and reattached flow, our aim is to analyze the wakes’ vortical structure along with the buildings of different heights and shapes. A large number of engineering applications involve precise predictions of the airflow around buildings to ensure performance and safety. To validate the numerical solver, a comparison is performed with the earlier reported numerical and experimental data. The turbulent flow characteristics are discussed in terms of instantaneous flow structure and time-averaged statistical flow quantities. All the cases are simulated for a Reynolds number [Formula: see text] to understand the turbulent airflow patterns. The flow shows a separable bubble at the leading edge of the rooftop of the building that leads to recirculation at the lee side of the first row of buildings. Due to the recirculation, the suction arises near the leading edge of the building to assure a continuous flow of air around the obstacles. Further, Reynold stresses show high momentum fluxes in the frontal region of the buildings.

Turbulence suppression and regeneration in a magnetohydrodynamic duct flow due to influence of arbitrary electrically conductive walls

In the current study, we present a series of numerical simulations describing a turbulent magnetohydrodynamic flow subjected to a transverse magnetic field in a square duct with arbitrary electrically conductive walls. The characteristic flow and electromagnetic non-dimensional parameters (Reynolds and Hartmann number, respectively) were fixed, while the wall conductivity ratio (Cd) was varied from the perfectly electrically insulated (Cd = 0) to perfectly electrically conducting (Cd→∞). We have assumed the one-way coupling between the flow of an electrically conducting fluid and the imposed magnetic field. The influence of the electrically conducting walls was imposed by simulating additional finite solid domains coupled with the channel interior. The turbulence was simulated by applying the large eddy simulation approach with the dynamic Smagorinsky sub-grid scale model. The obtained results confirmed a significant impact of the conductivity of the surrounding walls on the flow and turbulence reorganization. We have observed that the initially fully developed turbulence regime was gradually suppressed in the 0≤Cd<0.15 range, while the fully laminarized state was obtained at Cd=0.15. We found that the process of turbulence suppression was accompanied by the appearance of the patterned turbulence phenomenon in the proximity of the walls parallel to the magnetic field direction. With a further increase in the wall conductivity parameter (0.15<Cd<∞), we have observed a complete turbulence regeneration. We found that this turbulence regeneration was caused by the local reorganization of the total current density loops near the electrically conducting walls.

Dynamic LES of the magnetohydrodynamic flow in a square duct with the varied wall conductance parameters

The current study is focused on the magnetohydrodynamics and demonstrates how electrical conductivity of the wall can affect the turbulent flow in the square duct. Different variations of the boundary walls have been considered including arbitrary conductive walls. The Large Eddy Simulations method with the dynamic Smagorinsky sub-grid scale model have been used for the turbulent structures resolving. Results show the significant impact of the wall conductance parameters for both Hartmann and side walls.

Experimental and Numerical Study for Improved Understanding of Mixed-Convection Type of Flows in Turbine Casing Cavities During Shut-Down Regimes

Abstract Due to an increase in the power generation from renewable sources, steam and gas turbines will be required to adapt for more flexible operations with frequent start-ups and shut-downs to provide load leveling capacity. During shut-down regimes, mixed convection takes place with natural convection dominance depending on the operating conditions in turbine cavities. Buoyant flows inside the turbine that are responsible for nonuniform cooling leading to thermal stresses and compromise clearances directly limits the operational flexibility. Computational fluid dynamics (CFD) tools are required to predict the flow field during these regimes since direct measurements are extremely difficult to conduct due to the harsh operating conditions. Natural convection with the presence of cross-flow -mixed convection has not been extensively studied to provide detailed measurements. Since the literature lacks of research on such flows with real engine representative operating conditions for CFD validation, the confidence in numerical predictions is rather inadequate. This paper presents a novel experimental facility that has been designed and commissioned to perform very accurate unsteady temperature and flow field measurements in a simplified turbine casing geometry. The facility is capable of reproducing a wide range of Richardson, Grashof, and Reynolds numbers which are representative of engine realistic operating conditions. In addition, high fidelity, wall resolved large eddy simulation (LES) with dynamic Smagorinsky subgrid scale model has been performed. The flow field as well as heat transfer characteristics have been accurately captured with LES. Finally, the inadequacy of Reynolds-averaged Navier–Stokes (RANS) for the mixed type of flows has been highlighted.

A fluid–structure interaction simulation on the wake-induced vibration of tandem cylinders with pivoted rotational motion

This paper predicts the wake-induced vibration of tandem cylinders with pivoted rotational or translational motion using the fluid–structure interaction technique. Large eddy simulation using a dynamic Smagorinsky subgrid-scale model is applied for the turbulent flow. Structural computation is applied to obtain the displacement of the downstream cylinder in the cross-flow direction. Conditions such as damping ratio, natural frequency, and mass ratio of pivoted rotational motion are the same as those of translational motion. The computational conditions are Re=3790, 8470 and 15 160, corresponding to U*=5.0, 11.2 and 20.0, respectively. The unsteady characteristics of lift, drag, and cross-flow motion of tandem cylinders are investigated. Distributions of pressure, λ2 criterion, vorticity, and streakline are also investigated. Finally, the energy harvesting capability of the pendulum system is compared with the translation system.

Studying the flow dynamics and heat transfer of stranded conductor cables using large eddy simulations

Stranded cables are widely used in applications where their heat transfer and fluid dynamics are important, but they have not been extensively studied. This paper investigates, using large eddy simulations with the dynamic Smagorinsky sub-grid scale model, a helically wound stranded conductor cable in comparison to a circular cylinder at a Reynolds number of 1000 and Prandtl number of 0.7. The cylinder and the cable were normal to the flow. The triply decomposed heat transport equations were derived, and proper orthogonal decomposition was applied to the fluctuating vorticity and temperature fields to determine the total, coherent, and incoherent terms in the heat transport equations. The results showed that the stranded cable, relative to circular cylinder, has (i) three-dimensional mean flow and heat transfer, especially within and around recirculation region, (ii) 9% higher drag and 8% higher base pressure magnitude, (iii) near-stagnant flow in the gaps between the strands, which results in a significant variation in the local Nusselt number, (iv) ∼15% lower span-wise averaged local Nusselt number in the attached boundary layer, suggesting that surface modifications should be addressed to enhance heat transfer, (v) ∼36° variation in the separation angle along the span, (vi) 12% higher turbulent kinetic energy and 39% higher spanwise normal Reynolds stresses, (vii) insignificant difference in shedding frequency, suggesting similar flow induced vibrations to the cylinder, (viii) asymmetry in the flow and heat fields around the x axis, (ix) significantly different coherent temperature fields and dynamics, and (x) in general, high heat energy transport close to the cable rear side.

Large Eddy Simulation of Wake Formation around a Free Surface–Piercing Circular Cylinder

Guo, H.; Bahrian, F.; Zhang, W.; Sun, C., and Hu, J., 2021. Large eddy simulation of wake formation around a free surface–piercing circular cylinder. Journal of Coastal Research, 37(1), 132–142. Coconut Creek (Florida), ISSN 0749-0208.Numerical simulation of flow past a free surface–piercing circular cylinder using large eddy simulation was performed at Reynolds number Re = 2.7 × 104 and Froude number Fr = 0.8. Commercially available code in ANSYS Fluent was used to investigate the free surface and underlying vortical structures. The dynamic Smagorinsky subgrid-scale model and volume of fluid method for capturing the free surface were used in the simulation. The mean velocities and interface height from the simulation are in good agreement with the available experimental data. The organized Karman vortex generation in deep flow was found to attenuate near the free surface. The overall force developed on the cylinder also decreased due to the presence of the free surface. The simulation shows that the vortical structures present in the flow correlate with the mean parameters of the free surface. Vorticity magnitude and Reynolds shear stress were found to show significantly different behavior close to the free surface as compared to that in deep flow.

A Cartesian cut-cell based multiphase flow model for large-eddy simulation of three-dimensional wave-structure interaction

A multiphase flow numerical approach for performing large-eddy simulations of three-dimensional (3D) wave-structure interaction is presented in this study. The approach combines a volume-of-fluid method to capture the air-water interface and a Cartesian cut-cell method to deal with complex geometries. The filtered Navier–Stokes equations are discretised by the finite volume method with the PISO algorithm for velocity-pressure coupling and the dynamic Smagorinsky subgrid-scale model is used to compute the unresolved (subgrid) scales of turbulence. The versatility and robustness of the presented numerical approach are illustrated by applying it to solve various three-dimensional wave-structure interaction problems featuring complex geometries, such as a 3D travelling wave in a closed channel, a 3D solitary wave interacting with a vertical circular cylinder, a 3D solitary wave interacting with a horizontal thin plate, and a 3D focusing wave impacting on an FPSO-like structure. For all cases, convincing agreement between the numerical predictions and the corresponding experimental data and/or analytical or numerical solutions is obtained. In addition, for all cases, water surface profiles and turbulent vortical structures are presented and discussed.

A three-dimensional Cartesian cut-cell/volume-of-fluid method for two-phase flows with moving bodies

A three-dimensional Cartesian cut-cell method for the large-eddy simulation of two-phase flows with moving bodies is presented in this study, which combines a volume-of-fluid method to capture the air-water interface and a moving body algorithm on a stationary, non-uniform, staggered, Cartesian grid. The filtered Navier–Stokes equations are discretised using the finite volume method with the PISO algorithm for velocity-pressure coupling and the dynamic Smagorinsky subgrid-scale model is employed to compute the effect of the unresolved (subgrid) scales of turbulence on the large scales. In the present study, the small cut-cells are unmodified and due to the use of an implicit time integration no instabilities occur during the computations. The versatility and robustness of the present two-phase flow model is illustrated via various two- and three-dimensional flow problems with fixed/moving bodies, such as dambreak flows with and without a square cylinder, a moving cylinder in a quiescent fluid, dambreak flow over a wet bed with a moving gate, water entry and exist of a circular cylinder, and landside-generated waves. Good agreement is obtained between the numerical results and the corresponding experimental measurements.

Turbulence modeling approaches on unsteady flow structures around a semi-circular cylinder

This paper presents the flow field around a Semi-Circular Cylinder (SCC), a basic geometry for fluid mechanics, obtained numerically by Computational Fluid Dynamics (CFD) at Reynolds number of 3.2 × 104. Time-averaged flow characteristics such as vorticity, velocity fields with streamline patterns, Reynolds stress correlations, Turbulence Kinetic Energy (TKE), pressure and drag coefficients have been examined by using Reynolds-averaged Navier-Stokes (RANS), Detached Eddy Simulation (DES) and Large Eddy Simulations (LES) turbulence models. Several RANS approaches with turbulence modeling including SST k-ω, k-ε and their derivatives were used, while in the LES simulations, the dynamic Smagorinsky subgrid-scale model was employed. The obtained results of the present study have been compared with an experimental study from the open literature. The results of the investigation showed that when numerical findings are compared to experimental ones, the results of LES and DES turbulence models were found to be more consistent than those obtained by using k-ε and SST k-ω models. Time-averaged flow patterns have been found to be considerably symmetric with respect to the axis passing through the end of the curved surface of the SCC. The deviation of LES and DES results, and experimental results was less than 7% in terms of the drag coefficient.

Thermal-hydraulic study on rib and dimple structures for cooling the First Wall of DEMO

The thermal hydraulics of helium-gas cooled channels for the First Wall of DEMO structured by fully attached and semi-detached upward directed V-shaped ribs and spherical dimples in a staggered array were investigated. Large-Eddy-Simulations with the dynamic Smagorinsky sub-grid scale model were performed. The global and local heat transfer coefficient, pressure drop and cooling performance were analyzed. Compared to smooth channel flows, the global heat transfer coefficient was increased by a factor of 2.6–2.9 and the corresponding pressure drop was increased by a factor of 5.3–5.7 for the V-shaped ribs. Here, the semi-detached ribs provided the highest heat transfer and the best cooling performance. The dimpled structures increased the heat transfer by the factor of 1.1–1.4 without a significant pressure drop rise when compared to smooth channel flows, but regions of local heat transfer deterioration occur over each upstream dimple half.

Investigation of Numerical Dissipation in Classical and Implicit Large Eddy Simulations

The quantitative measure of dissipative properties of different numerical schemes is crucial to computational methods in the field of aerospace applications. Therefore, the objective of the present study is to examine the resolving power of Monotonic Upwind Scheme for Conservation Laws (MUSCL) scheme with three different slope limiters: one second-order and two third-order used within the framework of Implicit Large Eddy Simulations (ILES). The performance of the dynamic Smagorinsky subgrid-scale model used in the classical Large Eddy Simulation (LES) approach is examined. The assessment of these schemes is of significant importance to understand the numerical dissipation that could affect the accuracy of the numerical solution. A modified equation analysis has been employed to the convective term of the fully-compressible Navier–Stokes equations to formulate an analytical expression of truncation error for the second-order upwind scheme. The contribution of second-order partial derivatives in the expression of truncation error showed that the effect of this numerical error could not be neglected compared to the total kinetic energy dissipation rate. Transitions from laminar to turbulent flow are visualized considering the inviscid Taylor–Green Vortex (TGV) test-case. The evolution in time of volumetrically-averaged kinetic energy and kinetic energy dissipation rate have been monitored for all numerical schemes and all grid levels. The dissipation mechanism has been compared to Direct Numerical Simulation (DNS) data found in the literature at different Reynolds numbers. We found that the resolving power and the symmetry breaking property are enhanced with finer grid resolutions. The production of vorticity has been observed in terms of enstrophy and effective viscosity. The instantaneous kinetic energy spectrum has been computed using a three-dimensional Fast Fourier Transform (FFT). All combinations of numerical methods produce a k − 4 spectrum at t * = 4 , and near the dissipation peak, all methods were capable of predicting the k − 5 / 3 slope accurately when refining the mesh.

Impact of windward inlet-opening positions on fluctuation characteristics of wind-driven natural cross ventilation in an isolated house using LES

ABSTRACTThis paper presents a computational fluid dynamics (CFD) simulation of coupled outdoor wind flow and indoor airflow in the investigation on the horizontal positions of openings on the fluctuation of cross ventilation and flow-field inside an isolated family house. Two inlet-opening positions located at the same height are used to investigate the impact of the position of windward inlet openings on the ventilation rate and flow-field inside the building. Due to unsteady flow and high turbulence near the openings, the study employed the large eddy simulation with the dynamic Smagorinsky subgrid-scale model techniques, with the CFD simulations being validated against data from available wind tunnel experiments. The study showed that the rate of ventilation through openings located near the centre of the building is higher and more steady than the flow rate of openings located near the sides of the building.

Comparative study of different subgrid-scale models for large eddy simulations of magnetohydrodynamic turbulent duct flow in OpenFOAM

Abstract Magnetohydrodynamic (MHD) turbulent duct flow has been numerically investigated using large eddy simulation (LES) method with different subgrid-scale models in OpenFOAM environment. Smagorinsky, dynamic Smagorinsky, and dynamic one equation eddy viscosity subgrid-scale models have been used to simulate MHD square duct flow with R e = 5681 . The results demonstrate that the average velocity and the turbulent kinetic energy obtained from the dynamic Smagorinsky subgrid-scale model agree well with those from direct numerical simulations (DNS). Furthermore, a MHD duct flow with R e = 29000 has been modeled using dynamic Smagorinsky subgrid-scale model. The results are compared with the results of coherent structure model. A good agreement in mean velocity values is observed. The agreement confirms that the dynamic Smagorinsky subgrid-scale model suitable for modeling MHD turbulent duct flows. Finally, LES with dynamic Smagorinsky subgrid-scale model has been used to simulate MHD flow in a square duct with electrically insulated walls with the same Reynolds number and different Hartmann number. The results for the mean velocity, velocity fluctuations and skin friction coefficient are presented. The laminar-turbulent transition parameters agree with the value given by Chaudhary's DNS results.

Impact of an external boundary wall on indoor flow field and natural cross-ventilation in an isolated family house using numerical simulations

The external boundary wall is a main architectural feature of a typical residential building in Iraq, which is expected to decrease the rate of airflow entering the openings of the building. In this study, the impact of an external boundary wall on natural cross-ventilation and flow patterns inside an isolated family house was analyzed using computational fluid dynamics (CFD) simulations. The wall was located in front of the building and three different conditions were tested: basic case (without a wall) and two cases using walls of different heights. The study employed the techniques of large eddy simulation (LES) with the dynamic Smagorinsky subgrid-scale model because of the unsteady flow and high turbulence around the building. The CFD simulations were validated against the available wind tunnel experiments. It was observed that the external boundary wall created well distributed indoor air flow and improved the indoor environment regarding the mean velocity inside the building. Also, increasing the height of the wall by 20% did not offer noticeable improvement on the mean velocity distribution, whereas the ventilation airflow rate was reduced significantly to less than half when the wall was present. The results of this study are expected to inform building designers of the impact of an external boundary wall on the flow patterns in relation to the rate of ventilation and indoor mean velocity.

On the accuracy of CFD simulations of cross-ventilation flows for a generic isolated building: Comparison of RANS, LES and experiments

Accurate and reliable computational fluid dynamics (CFD) simulations are essential for the assessment of cross-ventilation of buildings. To determine which CFD models are most suitable, validation studies are required. A detailed review of the literature indicates that most CFD validation studies only employed the 3D steady Reynolds-averaged Navier-Stokes (RANS) approach and/or focused on a limited set of flow parameters. Therefore, the objective of this paper is the validation of both 3D steady RANS simulations and large eddy simulation (LES) of cross-ventilation in a generic isolated enclosure with wind-tunnel measurements. The evaluation is based on five parameters: mean velocity, turbulent kinetic energy, ventilation flow rate, incoming jet angle and incoming jet spreading width. The RANS simulations are conducted with the standard k-ε (SKE), RNG k-ε, realizable k-ε (RLZ), SST k-ω and RSM turbulence models, whereas the LES is performed with the dynamic Smagorinsky subgrid-scale model. SST/RNG/RSM reproduce the experimentally observed direction of the incoming jet, but all RANS models fail in reproducing the turbulent kinetic energy, which is too low especially above and below the jet, because steady RANS does not capture the vertical flapping of the jet. This transient feature is reproduced by LES, resulting in a better reproduction of all three measured parameters (velocity, turbulent kinetic energy, volume flow rate). It is concluded that choice of the model (RANS vs. LES) actually depends on which parameter is the target parameter, noting that the use of LES entails an increase in computational demand with a factor of ≈80–100.

Modelling of pollutant dispersion with atmospheric instabilities in an industrial park

Large Eddy Simulation (LES) combined with a dynamic Smagorinsky subgrid scale model was used to simulate the wind field and particulate pollutant dispersion considering the effects of plant canopies and weather conditions. The model of the plant canopy was adopted to present the effects of drag force and buoyancy force and validated by bench mark results from previous reference. Then, using the model, four study cases in the Nanjing Sample Industrial Park were simulated under different weather conditions to consider the momentum effect of plant canopy, energy effect of plant canopy and the effect of atmospheric instabilities, respectively. The numerical results were also compared with those of idealized street canyon models, and it was proven that the accurate landform model was more practical to be applied in the study. The wind field and pollutant distribution were analyzed in each case to investigate the influence of plant canopy. It was shown that the wind velocity was lower near the canopy regions, especially in unstable conditions due to the entrainment effect of the buoyancy force. Meanwhile, the streamline could be changed by the buoyancy force in an upward trend. The distribution of the turbulent kinetic energy indicated that the turbulent flow in the atmosphere was weakened through the plant canopy, especially in unstable conditions. Considering the effect of buoyancy force, the direction of pollutant dispersion in horizontal profiles was different to the neutral condition, which proved that plant canopy can protect the downstream regions in unstable atmospheric conditions.

Numerical analysis of blood flow through an elliptic stenosis using large eddy simulation.

The presence of a stenosis caused by the abnormal narrowing of the lumen in the artery tree can cause significant variations in flow parameters of blood. The original flow, which is believed to be laminar in most situations, may turn out to turbulent by the geometric perturbation created by the stenosis. Flow may evolve to fully turbulent or it may relaminarise back according to the intensity of the perturbation. This article reports the numerical simulation of flow through an eccentrically located asymmetric stenosis having elliptical cross section using computational fluid dynamics. Large eddy simulation technique using dynamic Smagorinsky sub-grid scale model is applied to capture the turbulent features of flow. Analysis is carried out for two situations: steady inflow as ideal condition and pulsatile inflow corresponding to the actual physiological condition in common carotid artery. The spatially varying pulsatile inflow waveforms are mathematically derived from instantaneous mass flow measurements available in the literature. Carreau viscosity model is used to estimate the effect of non-Newtonian nature of blood. The present simulations for steady and pulsatile conditions show that post-stenotic flow field undergoes transition to turbulence in all cases. The characteristics of mean and turbulent flow fields have been presented and discussed in detail.

Analysis of Turbulence Modelling Approaches to Simulate Single-phase Buoyancy Driven Counter-current Flow in a Tilted Tube

In the present paper, both Large Eddy Simulation (LES) and unsteady-RANS (uRANS) CFD studies of single-phase buoyancy-driven counter-current flow in a pipe are presented for an Atwood number of 1.15×10−2 and an inclination angle of 15 degrees from the vertical. The basic flow phenomena involved are fundamentally the same as those encountered in various industrial applications including passive coolant flow in the loops of a nuclear reactor. Earlier work on this type of flow was focused on its physical aspects using both experimental measurements and Direct Numerical Simulation (DNS). The present work investigates the performance of several commonly used LES eddy viscosity subgrid-scale models. The results show that LES is able to reproduce accurately the experimental results and that the dynamic Smagorinsky subgrid-scale model gives the best predictions. The results of those calculations were then used to obtain more information on the physics of such flow. As regards uRANS simulations, a range of classic two-equation linear eddy viscosity models, based on low-y + formulation at the wall and the single gradient diffusion hypothesis for the turbulent density fluxes were compared against experimental data available in the literature. An elliptic blending Reynolds stresses model (EBRSM) with generalised gradient diffusion hypothesis (GGDH) for the scalar fluxes was then implemented and compared with experiments. The latter showed very good agreement for the first order moment with the experimental data whereas all the two-equation eddy viscosity based models showed rather high levels of discrepancy. The main cause of discrepancy was found to be the underprediction of the axial turbulent buoyancy production effect, which has a more detrimental impact on the eddy viscosity models than the second moment closure one.