Dynamic Smagorinsky Subgrid Scale Research Articles

Wake interference of tandem wind turbines considering pitch strategy based on the AL-LDS-Ωnew coupling method

This paper establishes a high-fidelity and efficient Computational Fluid Dynamics (CFD) numerical method (AL-LDS-Ωnew) for wind turbine wake by combining the actuator line (AL), the localized dynamic Smagorinsky (LDS) sub-grid scale (SGS), and the new generation Ωnew vortex identification method under the framework of large eddy simulation. The model advantages are encouraging: 1) In terms of turbine modeling, the AL model is adopted to replace the traditional three-dimensional solid model, which avoids solving the boundary layer on the blade surface and improves computational efficiency; 2) In terms of wake simulation, the LDS SGS model is used to model turbulence, reducing vortex dissipation and further improving the refinement of turbine wake; 3) In terms of vortex identification, the new generation Ωnew vortex identification method avoids the difficult threshold selection in previous vortex identification and captures more refined vortex structures. The accuracy of the model is validated against published data of a NREL 5 MW wind turbine, and then extended to simulate the wake interference of tandem twin-rotor turbines by changing the pitch angle of the upstream wind turbine (WT1). The influence mechanisms between array wake interference and energy conversion efficiency under the pitch strategy are explored, demonstrating the AL-LDS-Ωnew coupling method is computationally accurate and efficient for simulating the complex wake interference. From analyses, the pitch strategy can effectively suppress the wake effect of the upstream turbine (WT1) and increase the power output of the downstream turbine (WT2), thus improving the overall output power of the array farm. Compared with the non-pitch condition (0 pitch angle), a pitch angle of (2°) maximizes the global energy conversion efficiency of the twin-rotor array: power augmentation by 0.29%, and thrust reduction by 5%. This optimal state reduces the fatigue load of the turbine and is more conducive to long-term operation. The findings, whilst preliminary, encourage the use of turbine pitch strategies in the wind farm planning and operation.

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.

New criterion for characterization of thermal-saline jets discharged from thermal desalination plants

• The hydrodynamic pattern of inclined thermal-saline jets was examined using LES. • Jets pattern depended only on the ratio of salinity to thermal Froude numbers ( R ρ ). • Jets with R ρ lower than 1.2 behaved similar to the negatively buoyant jet. • Jets with R ρ in a range of 1.2-1.3 achieved better mixing characteristics. • Geometrical characteristics of jets became asymptotic for R ρ less than 0.7. More than 80% of desalination plants use the multi-stage flash (MSF) technology in the Persian Gulf and hence may have a severe irreversible impact on the marine environment. In the present study, geometrical, mixing, and turbulence characteristics of thermal-saline jets, similar to MSF effluents, are numerically investigated using the dynamic Smagorinsky sub-grid scale (SGS) model and UNESCO equation of state. For this purpose, two affecting dimensionless parameters are considered: (1) density ratio, which is thermal flux to salinity flux ratio, and (2) salinity Froude number. Examining the effects of density ratio reveals that the jet flow pattern only depends on the density ratio with a separating range of 1.2-1.3. In addition, increasing the density ratio increases the geometrical values of descending jets, reduces the geometrical values of ascending ones, and increases the descending jet's dilution before returning to the bed. Furthermore, geometrical values of descending jets show asymptotic behavior for density ratios less than 0.7. Eventually, turbulence characteristics of different jet flow patterns show that large turbulence structures are more explicit for the quasi-neutral scenario. The current results recommend that thermal desalination discharges similar to quasi-neutral jets with density ratios in a range of 1.2-1.3 achieve better mixing characteristics.

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.

A novel actuator line-immersed boundary (AL-IB) hybrid approach for wake characteristics prediction of a horizontal-axis wind turbine

The core idea of the actuator line (AL) model is to simplify the horizontal-axis wind turbine (HAWT) into a line with force distributed along the radial direction. The body force on the AL is employed to simulate the effect of the HAWT on the flow field, and the Navier-Stokes (N-S) equation is solved to obtain the aerodynamic and wake characteristics of the HAWT. Recently, the AL model has been comprehensively implemented in the investigation of wind turbine wakes. However, the existing standard AL model ignores or inaccurately describes the nacelle effects, which have been proven to considerably affect the accuracy of the near wake simulation. This paper develops an actuator line-immersed boundary (AL-IB) hybrid approach for the HAWT with high-accuracy and low-computational cost to address the physics problem underlying here. The wind turbine adopts the AL model instead of the body-fitted mesh method, and the nacelle is realized by the immersed boundary (IB) method. The large eddy simulation (LES) with a new localized dynamic Smagorinsky (LDS) sub-grid scale (SGS) model is adopted to model the turbulence to improve simulation accuracy of the near wake, thereby forming a set of high-fidelity numerical simulation framework of the HAWT wake. Firstly, the simulation accuracy of the AL-IB hybrid approach is validated by comparing the mean wake velocity profiles and turbulent kinetic energy (TKE) profiles of the HAWT with the experimental measurement results of the NTNU “Blind test 1”. Furthermore, the research is extended to the wake difference of the HAWT under two different numerical methods (AL and AL-IB). The results show that the solver developed based on AL-IB hybrid approach has higher accuracy than the standard AL model, which provides a feasible way for the refined numerical simulation of the large-scale wind farm.

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.

High-fidelity simulations of gravity currents using a high-order finite-difference spectral vanishing viscosity approach

This numerical work investigates the potential of a high-order finite-difference spectral vanishing viscosity approach to simulate gravity currents at high Reynolds numbers. The method introduces targeted numerical dissipation at small scales through altering the discretisation of the second derivatives of the viscous terms in the incompressible Navier-Stokes equations to mimic the spectral vanishing viscosity (SVV) operator, originally designed for the regularisation of spectral element method (SEM) solutions of pure advection problems. Using a sixth-order accurate finite-difference scheme, the adoption of the SVV method is straightforward and comes with a negligible additional computational cost. In order to assess the ability of this high-order finite-difference spectral vanishing viscosity approach, we performed large-eddy simulations (LES) of a gravity current in a channelised lock-exchange set-up with our SVV model and with the well-known explicit static and dynamic Smagorinsky sub-grid scale (SGS) models. The obtained data are compared with a direct numerical simulation (DNS) based on more than 800 million mesh nodes, and with experimental measurements. A framework for the energy budget is introduced to investigate the behaviour of the gravity current. First, it is found that the DNS is in good agreement with the experimental data for the evolution of the front location and velocity field as well as for the stirring and mixing inside the gravity current. Secondly, the LES performed with less than 0.4% of the total number of mesh nodes compared to the DNS, can reproduce the main features of the gravity currents, with the SVV model yielding slightly more accurate results. It is also found that the dynamic Smagorinsky model performs better than its static version. For the present study, the static and dynamic Smagorinsky models are 1.8 and 2.5 times more expensive than the SVV model, because the latter does not require the calculation of explicit SGS terms in the Navier-Stokes equations nor spatial filtering operations.

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.

An Acoustic Source Model for Applications in Low Mach Number Turbulent Flows, Such as a Large-Scale Wind Turbine Blade

Aerodynamic noise from wind turbine blades is one of the major hindrances for the widespread use of large-scale wind turbines generating green energy. In order to more accurately guide wind turbine blade manufacturers to optimize the blade geometry for aerodynamic noise reduction, an acoustic model that not only understands the relation between the behavior of the sound source and the sound generation, but also accounts for the compressibility effect, was derived by rearranging the continuity and Navier–Stokes equations as a wave equation with a lump of source terms, including the material derivative and square of the velocity divergence. Our acoustic model was applied to low Mach number, weakly compressible turbulent flows around NACA0012 airfoil. For the computation of flow fields, a large-eddy simulation (LES) with the dynamic Smagorinsky subgrid scale (SGS) model and the cubic interpolated pseudo particle (CIP)-combined unified numerical procedure method were conducted. The reproduced turbulent flow around NACA0012 airfoil was in good agreement with the experimental data. For the estimation of acoustic fields, our acoustic model and classical sound source models, such as Lighthill and Powell, were performed using our LES database. The investigation suggested that the derived material derivative of the velocity divergence plays a dominant role as sound source. The distribution of the sources in our acoustic model was consistent with that of the classical sound source models. The sound pressure level (SPL) predicted based on the above-mentioned LES and our newly derived acoustic model was in reasonable agreement with the experimental data. The influence of the increase of Mach number on the acoustic field was investigated. Our acoustic source model was verified to be capable of treating the influence of Mach numbers on the acoustic field.

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.

Large eddy simulation of a transitional, thermal Blasius flow at low Reynolds number

Large eddy simulation (LES) of a transitional, thermal Blasius flowfield at a sufficiently low Reynolds number along an isothermal, flat plate is presented herein. Prior experiments observed that boundary layer transition is induced by the presence of streamwise vortex instability caused by the complex interaction between thermal buoyancy and forced convection dynamics. The maximum Grashof and Reynolds numbers employed in the LES were approximately 1.05×1011 and 1.18×105, respectively. To further enhance the accuracy, computational efficiency, and numerical stability, the LES solved the low-Mach number compressible flow governing equations, which included fluctuating density effects and pressure-density decoupling. For the subgrid scale (SGS) closure, a locally dynamic Smagorinsky SGS model was implemented into the LES solver to enable the backscatter phenomenon intrinsic to transitional boundary layer flows. The LES accurately predicted the onset of streamwise vortex instability and the eventual three-dimensional vortex breakdown of the underlying mean flow into a fully developed turbulent boundary layer, when compared to previously measured data. In the developed turbulence region, quadrant analyses indicated Q2 and Q4 events dominated the contribution to the Reynolds shear stress in the near-wall region, whereas Q1 and Q3 events contributed considerably to the wall-normal turbulent heat flux. And, as a result of the instability within the conduction layer, the layer erupts and intermittently releases buoyant thermal plumes. These recurring intermittent events inside the conduction layer deflect and deform the flowfield quantities near the wall, resulting in a plurality of peculiar peaks and shear layers inside the boundary layer. Furthermore, these buoyant thermal plumes are the dominant turbulence production mechanism farther away from the wall in the downstream region of the developed turbulent boundary layer flow. Near the wall, however, forced convection turbulent flow effects were observed, in which the shear production term was the primary contributor to the generation of turbulent kinetic energy, and quasi-streamwise and horseshoe-like vortex structures were observed in the developed turbulence region.

Experimental and Computational Analysis of the Swirling Flow Generated by an Axial Counter-Rotating Swirler in a Rectangular Model Chamber Using Water Test Rig

This paper reports the particle image velocimetry (PIV) measurement of swirling flow in a confined rectangular-shaped model combustion chamber. Water is used as the working fluid, and the average profiles of axial, radial, and magnitudes of velocity are given. Flow behavior is investigated and a rebound angle term is defined to investigate the direct effects of the noncircular chamber shape. Flow behavior near the walls is discussed in detail, as are other important swirling flow features such as the appearance of corner and central toroidal recirculation zones. Additionally, experimental data are compared with simulation results. Analyses were performed via commercial software STAR-CCM+ version 9.0. The large eddy simulation (LES) dynamic Smagorinsky subgrid scale, realizable k–ε model, and k–ω shear-stress transport (SST) detached eddy version were used as simulation tools. Three different test filters of 1.0, 2.2, and 3.0 were applied to the LES to identify improvements in accuracy. The overall best turbulence model is compared to the experimental result and reliability of such model is evaluated. The ability of such model was profound within the upstream and to some extent unreliable in downstream.

Wall-resolved large eddy simulation of turbulent mixed-convection heat transfer along a heated vertical flat plate

The present study aims to assess the predictive capabilities of wall-resolved large eddy simulation (LES) in computing the fluid flow and heat transfer characteristics in a mixed-convection turbulent boundary layer that developed along a large flat plate vertically mounted in air. The maximum Rayleigh number was approximately 3×1011, which resulted in fully developed turbulence conditions. To enhance the accuracy, computational efficiency, and numerical stability, the LES solved the low-Mach number compressible flow governing equations, which included fluctuating density effects and pressure-density decoupling. For the subgrid scale (SGS) closure, a locally dynamic Smagorinsky SGS model was implemented into the LES solver to enable the backscatter phenomenon intrinsic to transitioning boundary layer flows. The LES illustrated exceptional agreement with the statistics profiles in the boundary layer obtained with the experiments of previous studies, which showed sensitivity to freestream conditions. In particular, the LES correctly predicted the turbulent heat flux in the streamwise direction near the heated surface. Furthermore, the LES captured the rapid changes in spectra of fluctuating temperature and velocities due to the delay of transition with increasing freestream velocity.

Numerical investigation of the interaction of the turbulent dual-jet and acoustic propagation**Project supported by the Fundamental Research Funds for the Central Universities, China (Grant No. N150204003).

In order to study the interaction between two independent jets, a three-dimensional (3D) transient mathematical model is developed to investigate the flow field and acoustic properties of the two-stream jets. The results are compared with those of the single-stream jet at Mach number 0.9 and Reynolds number 3600. The large eddy simulation (LES) with dynamic Smagorinsky sub-grid scale (SGS) approach is used to simulate the turbulent jet flow structure. The acoustic field is evaluated by the Ffowcs Williams–Hawkings (FW-H) integral equation. Considering the compressibility of high-speed gas jets, the density-based explicit formulation is adopted to solve the governing equations. Meanwhile, the viscosity is approximated by using the Sutherland kinetic theory. The predicted flow characteristics as well as the acoustic properties show that they are in good agreement with the existing experimental and numerical results under the same flow conditions available in the literature. The results indicate that the merging phenomenon of the dual-jet is triggered by the deflection mechanism of the Coanda effect, which sequentially introduces additional complexity and instability of flow structure. One of the main factors affecting the dual-jet merging is the aperture ratio, which has a direct influence on the potential core and surrounding flow fluctuation. The analysis on the noise pollution reveals that the potential core plays a fundamental role in noise emission while the additional mixing noise makes less contribution than the single jet noise. The overall sound pressure level (OASPL) profiles have a directive property, suggesting an approximate 25° deflection from the streamwise direction, however, shifting toward lateral direction of about 10° to 15° in the dual-jet. The conclusion obtained in this study can provide valuable data to guide the development of manufacturing-green technology in the multi-jet applications.

Large eddy simulation of two isothermal and reacting turbulent separated oxy-fuel jets

In this work, a Large eddy simulation (LES) method and a tabulated chemistry approach according to the Flamelet Generated Manifold (FGM) strategy are coupled to numerically study the interactions of turbulent isothermal and reacting flows stemming from two aligned jets providing alternately fuel (natural gas) and oxidant (pure oxygen gas).The jets feature different geometries and deliver unequal momentums at the boundaries. The effect of oxygen in comparison to air environment on the FGM tabulation and results is pointed out. In addition, the impact of combustion on the flow and mixing field evolvement is analyzed. The LES relies on a dynamic Smagorinsky subgrid scale (SGS) model and a linear eddy diffusivity ansatz to close the SGS stresses and the SGS scalar fluxes for describing the turbulent flow field and the turbulent scalar field, respectively. For model assessment, available laser-based experimental data are used for model validation. In particular the numerical results are compared with available experimental data for the flow field. The latter are gained experimentally by the Particle Image Velocimetry (PIV) and laser tomography, respectively. In the first part of this paper, the jets interaction process is studied for the isothermal case while the oxy-fuel combustion in the reacting case is analyzed in the second part. The analysis is achieved in terms of statistical quantities for the flow velocity, mixture fraction, chemical species and temperature. An overall satisfactory agreement is reported.

Wall-resolved Large Eddy Simulation of a flow through a square-edged orifice in a round pipe at Re = 25,000

The orifice plate is a pressure differential device frequently used for flow measurements in pipes across different industries. The present study demonstrates the accuracy obtainable using a wall-resolved Large Eddy Simulation (LES) approach to predict the velocity, the Reynolds stresses, the pressure loss and the discharge coefficient for a flow through a square-edged orifice in a round pipe at a Reynolds number of 25,000. The ratio of the orifice diameter to the pipe diameter is β=0.62, and the ratio of the orifice thickness to the pipe diameter is 0.11. The mesh is sized using refinement criteria at the wall and preliminary RANS results to ensure that the solution is resolved beyond an estimated Taylor micro-scale. The inlet condition is simulated using a recycling method, and the LES is run with a dynamic Smagorinsky sub-grid scale (SGS) model. The sensitivity to the SGS model and to the pressure–velocity coupling is shown to be small in the present study. The LES is compared with the available experimental data and ISO 5167-2. In general, the LES shows good agreement with the velocity from the experimental data. The profiles of the Reynolds stresses are similar, but an offset is observed in the diagonal stresses. The pressure loss and discharge coefficients are shown to be in very good agreement with the predictions of ISO 5167-2. Therefore, the wall-resolved LES is shown to be highly accurate in simulating the flow across a square-edged orifice.

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.