Wall-Adapting Local Eddy-viscosity Research Articles

Evaluation of flow and flame characteristics of turbulent bluff-body CH4-H2 flame using LES-FPV approach

ABSTRACT This study investigated the flow and flame structure of a bluff-body stabilised CH4-H2 flame using the large eddy simulation (LES) and the flamelet progress variable (FPV) model. The performance of three subgrid-scale (SGS) LES models, i.e. the standard Smagorinsky model (SSM), the wall-adapting local eddy viscosity (WALE) model, and the dynamic Smagorinsky model (DSM) in predicting turbulent and reactive flow characteristics was assessed. Parameters, namely y+ , Pope index (M), vorticity field, and resolved turbulent kinetic energy (kt ), were analysed to determine the predictive capability of the three models. The results demonstrated that DSM and WALE successfully resolved 80% of kt for chosen grid configurations, whereas SSM exhibited discrepancies. The overall trends captured by all three models matched well with the experimental trend with DSM and WALE outperforming SSM in predicting reacting flow fields in the recirculation zone, while SSM produced better predictions in the regions away from the recirculating zone.

Flow field and particle dispersion in a naturally ventilated model building: Application of large eddy simulation based on parallel lattice Boltzmann method

Flow field analysis and pollutant particles tracking in a natural ventilation flow are computationally intensive due to the coupled outdoor and indoor airflow, as well as the existence of different turbulence scales, and large numbers of particles. As a breakthrough approach, an explicit and parallel algorithm based on lattice Boltzmann method (LBM) solver is developed to simulate transient particulate flow indoor and outdoor of a model building. Since flow analysis in natural ventilation requires high-order CFD methods, large eddy simulation (LES) by the wall-adapting local eddy-viscosity (WALE) sub-grid scales model is employed. Also, one-step collision for reduction of memory usage, domain decomposition method for parallel processing of both flow and particles, and Lagrangian tracking for following particles are implemented into the developed algorithm. Investigation of grid independency shows that medium lattice resolution provides an average mean deviation equal to 18% and 34% for horizontal and vertical centerline of model building, respectively. In addition, when compared to finite volume method (FVM), LBM requires 2.7 times higher lattice resolution to achieve acceptable accuracy. However, it approximately speeds simulation duration up to 3 times. Moreover, 100 seconds of parallel particle tracking lasts almost 800 minutes on a simple desktop.

A mathematical‐boundary‐recognition domain‐decomposition Lattice Boltzmann method combined with large eddy simulation applied to airfoil aeroacoustics simulation

AbstractBeing a direct computational aeroacoustics method, Lattice Boltzmann method (LBM) has great potential and broad application perspective in the field of numerical simulation of aerodynamic noise due to its low dispersion and low dissipation. A series of numerical algorithms and the related improvements based on the standard LBM method are proposed and developed in this paper to adapt to the airfoil noise calculation with complex grid at middle‐high Reynolds number. First, a new mathematical‐boundary‐recognition algorithm based on Green's formula is proposed to deal with complex curved geometric models, which is validated by three‐element airfoil 30P30N benchmark. Then, in order to reduce grid redundancy and improve computing efficiency, the grid refinement technique of domain decomposition model (DDM) is adopted and also improved, which is verified by calculating the flow and sound fields around 2D and 3D cylinders at Reynolds number equal to 90,000. Finally, three different LES turbulence models are combined with the standard MRT‐LBM method, where different finite difference schemes are used to solve Reynolds stress tensor which is different from the traditional one. Through the direct acoustic numerical simulation of NACA0012 airfoil at Reynolds number equal to 200,000, the effects of Smagorinsky models and Wall‐adapting local eddy‐viscosity (WALE) model on aerodynamic noise prediction are compared and analyzed. Overall, the proposed methodology is shown to be appropriate for predicting the aerodynamic noise at low Mach number and can successfully simulate the generation and propagation of far field acoustics.

Calibrating sub-grid scale models for high-order wall-modeled large eddy simulation

AbstractHigh-order methods have demonstrated orders of magnitude reduction in computational cost for large eddy simulation (LES) over low-order methods in the past decade. Most such simulations are wall-resolved implicit LES (ILES) without an explicit sub-grid scale (SGS) model. The use of high-order ILES for severely under-resolved LES such as wall-modeled LES (WMLES) often runs into robustness and accuracy issues due to the low dissipation embedded in these methods. In the present study, we investigate the performance of several popular SGS models, the static Smagorinsky model, the wall-adapting local eddy-viscosity (WALE) model and the Vreman model, to improve the robustness and accuracy of under-resolved LES using high-order methods. The models are implemented in the high-order unstructured grid LES solver called hpMusic based on the discontinuous flux reconstruction method. The length scales in these SGS models are calibrated using the direct numerical simulation (DNS) database for the turbulent channel flow problem. The Vreman model has been found to produce the most accurate and consistent results with a proper choice of the length scale for WMLES.

Large Eddy Simulation of a Turbulent Polydisperse Spray Flow: a Comparative Study of Subgrid Scale Models and Droplet Injection Models

Abstract This study performs an investigation of the effects of the subgrid-scale and droplet injection models in the large eddy simulation (LES) of turbulent two-phase spray flows. Three LES subgrid-scale (SGS) models (Smagorinsky, wall-adapting local eddy viscosity, and dynamic Smagorinsky) and two droplet injection models (cone nozzle injection and conditional droplet injection) are validated to the experimental measurements. For both gaseous and liquid phases, all SGS models provide comparable results, indicating that the current two-phase flow field does not exhibit a pronounced sensitivity to the LES SGS model. As for different droplet injection models and spray dispersion angles, minimal differences are observed in the prediction of the gaseous mean and RMS velocity profiles. However, for the result of liquid phase, CDIM (conditional droplet injection model) predictions of the droplet mean diameter and velocity are in better agreement with experiments, and less sensitive to spray dispersion angle settings. While the CNIM (cone nozzle injection model) prediction of droplet diameter is less accurate when increasing the dispersion angle. The study suggests that turbulent two-phase spray flows are more influenced by the spray boundary conditions rather than the LES SGS models.

Flow over a 25°Ahmed body at a Reynolds number characteristic of suburban driving speeds

Four turbulence models were used to predict the flow over a 25°Ahmed body at a Reynolds number characteristic of suburban driving speeds in the United States: the k-ω shear stress transport (SST) model, large eddy simulation (LES) with the Smagorinsky-Lilly subgrid scale model, LES with the wall-adapting local eddy viscosity (WALE) subgrid scale model, and improved delayed detached eddy simulation (IDDES). The Reynolds number based on body height was ReH=2.65×105 - this corresponds to a freestream velocity of 13.44m/s, or approximately 30mph. The k-ω SST model only showed promise in predicting the drag coefficient; otherwise, it significantly underpredicted the lift coefficient and predicted an incorrect wake structure in terms of flow separation and the size of the separation bubble behind the body. All three scale-resolving models predicted a wake structure in good agreement with experimental data, with the WALE model and IDDES providing the best agreement in terms of separation bubble size, streamwise velocity magnitude, and turbulent kinetic energy in the wake. The best predictions of the aerodynamic forces were those of the WALE model, with overpredictions of only 5.45% for drag and 5.65% for lift. Overall, both the WALE model and IDDES demonstrated promise in predicting the wake structure and aerodynamic forces, but the best agreements were achieved with the WALE model. Therefore, the WALE model showed the lowest dependence on Reynolds number when comparing lower Reynolds number simulations to higher Reynolds number experiments for this situation.

On the effectiveness of Reynolds-averaged and subgrid scale models in predicting flows inside car cabins

The aim of the present study is to analyze the performances of unsteady Reynolds-averaged Navier–Stokes (URANS) and large eddy simulation (LES) approaches in predicting the airflow patterns inside car cabins and to give insight in the design of computational fluid dynamics simulations of a real car cabin. For this purpose, one eddy viscosity-based turbulence model (shear stress transport k–ω) and two subgrid scale models (wall-adapting local eddy-viscosity and dynamic kinetic energy) were tested, and numerical results were compared with particle image velocimetry measurements carried out on a commercial car. The URANS model exhibited great accuracy in predicting the mean flow behavior and was appreciably outperformed by the LES models only far from the inlet sections. For this reason, it was deemed suitable for conducting further analyses, aimed at characterizing the airflow patterns in winter and summer conditions and performing a thermal comfort analysis. The thermal regime was found to have a very little effect on the air flow patterns, once the quasi-steady state regime is achieved; in fact, both in winter and in summer, the temperature field is fairly uniform within the car cabin, making the contribution of buoyancy negligible and velocity fields to be very similar in the two seasons. Findings also reveal that thermal comfort sensation can be different for passengers sharing the same car but sitting on different seats; this aspect should be considered when designing and operating the ventilation system, since the minimum comfort requirements should be met for all the occupants.

Analysis of the local equivalence ratio under fuel-feeding excitations in a lean gas turbine model combustor: Large eddy simulation approach

The synchronization process between the fuel-feeding excitation (ff) and the local equivalence ratio (ϕlocal) is a key issue that has not been well studied numerically under lean conditions. Therefore, in this study, a large eddy simulation (LES) with a wall-adapting local eddy-viscosity (WALE) subgrid model was utilized to analyze the interaction between various fuel-feeding excitations and the local equivalence ratio in a lean partially premixed gas turbine model combustor. The model combustor was simulated using five values of fuel-feeding excitation: 0 Hz, 15 Hz, 50 Hz, 100 Hz and 200 Hz. The results demonstrated that the complex interaction between the swirling flow and the fuel-feeding excitation affected the flow structure. These changes enhanced the excitation of toroidal counter-rotating vortices in the region between the fuel jet exit and shear layers, which improved the homogeneity of ϕlocal inside the inner recirculation zone (IRZ). Furthermore, the fuel-feeding excitation modulated the shear layer thickness by increasing the rolling-up of the instantaneous counter-rotating vortices, leading to spatial regularity for ϕlocal inside the IRZ. The coupling process between the fuel-feeding excitation and the precessing vortex core (PVC) significantly increased the fluctuations of ϕlocal around the fuel jet exit and the shear layer region. An analysis of the probability distribution showed a nonlinear relationship between ff and ϕlocal, where ϕlocal was high when ff = 50 Hz, enhancing the uniformity of ϕlocal inside the IRZ. However, ϕlocal was more spread radially when ff = 0 Hz in the downstream direction. The merging of vortex shedding and the PVC increasingly excited the self-oscillation of ϕlocal, while the fuel-feeding excitation dominated the oscillation of the counter-rotating vortices by increasing the subharmonic vortices. Moreover, the fuel-feeding excitation controlled and organized the small vortical structures, leading to stabilization of ϕlocal when ff = 50 Hz, 100 Hz, and 200 Hz.

Numerical simulation of bluff body turbulent flows using hybrid RANS/LES turbulence models

Many engineering applications involve turbulent flows around bluff bodies. Because of their intrinsically unsteady dynamics, bluff body characteristic flows feature unique turbulence-related phenomena, which makes their numerical modeling challenging. Accordingly, accounting for a circular bluff body flow configuration, three different turbulence modeling approaches are investigated in this work, (i) Reynolds-averaged Navier–Stokes (RANS), (ii) large eddy simulation (LES), and (iii) hybrid RANS/LES. Regarding the hybrid approaches, two variants of the detached eddy simulation (DES) one, delayed DES (DDES) and improved delayed DES (IDDES), are studied. As RANS model, the $$\mathrm{k}-\mathrm{\omega SST}$$ is utilized here. This RANS model is also used as the background one for both DDES and IDDES. Wall-adaptive local eddy viscosity (WALE) is used in turn as the sub-grid scale (SGS) model for LES. The velocity two-point correlation function is used to assess the mesh size requirements. When compared to experimental data, the obtained numerical results indicate that RANS overestimates the recirculating bubble length by over 18% and is not capable of describing the turbulent kinetic energy and the flow anisotropy in agreement with the experimental data. In contrast, LES, DDES, and IDDES are all within 1% of the recirculating bubble length while predicting both the Reynolds stress tensor components and the corresponding flow anisotropy in agreement with the measurements. Besides, normalized anisotropy tensor invariants maxima in the shear layer were reproduced by all scale resolving models studied here, but they failed to yield the local extrema measured within the wake recirculation region. A comparative analysis of the anisotropic Reynolds stress tensor invariances underscores the adequacy of the scale resolving models.

CFD wind tunnel investigation for wind loading on angle members in lattice tower structures

Wind loading on lattice structures is studied locally by considering the applied forces on individual angle members. This work represents a comprehensive Computational Fluid Dynamics (CFD) analysis of wind loading on angle members, which includes a quantitative evaluation of the aerodynamic drag and lift loading and also a qualitative investigation of the wake characteristics behind the angle member. The focus is on the flow around the angle member cross-section which has two equal length sides at a right angle to each other. CFD has been used to investigate the flow around a single angle member characterized by its length D, thickness T, and angle of incidence to the oncoming flow. Both Reynolds-Averaged Navier–Stokes (RANS) and Large-Eddy Simulation (LES) are evaluated for a Reynolds number of 37,700, where the k-ϵ RANS and Wall-Adapting Local Eddy-viscosity (WALE) LES models are respectively used. The results are compared with the wind tunnel experiments reported in the literature (Wardlaw, 1967; Slater, 1969; Prud’homme et al., 2014) and the National Building Code of Canada (Canadian Commission on Building and Fire Codes and National Building Code of Canada, 2005), where the drag and lift coefficients at a range of incidence angles were reported. Quantitatively, the RANS performs well when compared to experiments for the aerodynamic coefficients at a range of incidence angles and less well for angles where a cavity style flow is formed and the separation becomes influential. This is in contrast to other bluff body cases where RANS does not perform so well. The LES is thus run at selected incidence angles for comparison and to better understand the flow where the RANS performs well and less well. Details of the turbulence in the wake are difficult to interpret when using RANS modeling, but this behavior is well illustrated with the LES model and details are reported here.

Impact of Surrounding Gas Density on the Turbulent Liquid Jet

This numerical study aims to investigate the impact of surrounding gas density on liquid turbulent jet. Incompressible large eddy simulations (LES) with Wall-Adapting Local Eddy-Viscosity (WALE) sub-grid scale model in ANSYS-FLUENT were performed to capture the morphology of the breakup as well as the important flow field characteristics. A volume of fluid (VOF) approach was used to track the unsteady evolution and breakup of the liquid jet. Different variables have been examined to assess this impact. These variables are instantaneous velocity, liquid volume fraction, and turbulent kinetic energy. Also, the centerline values have been investigated to obtain the jet half-width and spray dispersion angle. The jet diameter and exit velocity are 0.2 mm and 100 m/s, respectively. Three different liquid-to-gas density ratios of 54.03, 32.42 and 23.49 are considered, corresponding to ambient gas density of 15, 25 and 34.5 kg/m3, respectively. It has been found that the jet speed and the liquid core length are inversely proportional to the density of the ambient gas. Regarding the liquid core length, it was noted that the effect of gas density on the liquid core length is considerably stronger in the gas density range between 15 - 25 kg/m³ (corresponding to density ratios of 54.03 - 32.42) than that in the range of 25 - 34.5 kg/m³ (corresponding to density ratios of 32.42 - 23.49). The results, also, showed that increasing the gas density leads to an increase in Weber number, which in turn causes a high instability, that makes the breakup initiates closer to the nozzle exit and occurs with higher rate. Likewise, it has been found that the jet width and the spray dispersion angle increase with increasing the gas density. HIGHLIGHTS Spray jets are included in many applications because its important in increasing the heat and mass transfer between the liquid and the surrounding fluid Large eddy simulations (LES) and volume of fluid (VOF) models are used to investigate the characteristics of turbulent liquid jet The investigation was characterized as a function of man variables such as turbulent kinetic energy, liquid potential core, the jet dispersion angle and more The investigation showed that the density of surrounding gas has an influence on the liquid jet characteristics GRAPHICAL ABSTRACT

Hydrodynamics of a rotor–stator spinning disk reactor: Investigations by large-eddy simulation

In this work, computational fluid dynamics are used to study the hydrodynamics in a complete rotor–stator spinning disk reactor with throughflow. Large-eddy simulations of OpenFOAM 9 were used to capture the turbulent structures of the flow in combination with the wall-adapting local eddy viscosity sub-grid-scale model. The method was validated based on residence time distributions (RTDs) for a range of rotational Reynolds numbers (Re = ωRD2ν−1 = 3.2–52 × 104) and a dimensionless flow rate (Cw = Q ν−1RD−1) of 150 and G = 0.0303 (G = h RD−1). The experimental RTD was obtained from tracer experiments with UV/VIS flow cells. From the RTD, the plug flow (PFR) volume fraction, the Péclet number, and the radial position (rtrans) where the flow changes from PFR into ideally mixed were determined by using an engineering model based on axial dispersion. For the turbulent cases, good agreement based on the RTD curve, PFR volume, the Péclet number, and rtrans was found. Furthermore, the boundary layer thickness on the rotor and stator and the entrainment coefficient were in good agreement with the literature. Finally, the turbulent intensity was analyzed illustrating a high intensity at the rim of the rotor and was 10% larger in centripetal flow compared to centrifugal flow.

Dynamics of an oscillating wave surge converter: an analysis on the influence of the bottom slope

The Oscillating Wave Surge Converter represents a technology with great potential for generating electricity from ocean waves, promoting energy continuously throughout a day. Its development is on a pre-commercial scale and has been encouraging several companies and institutions to invest in the improvement and study of this conversion system. Simulations in Computational Fluid Dynamics are very important tools for the development of these converters, since they provide essential details for the analysis of the variables that influence the system. In this context, the present work uses the Large Eddy Simulation modeling to study the influence of the bottom slope on the device dynamics, which is performed using the OpenFOAM v.4.1 computational code and its extension OlaFlow. The model used in the analysis is two-dimensional and based on the Wall-Adapting Local Eddy-Viscosity methodology, while a structured mesh is applied. Through a detailed analysis, it was observed that the converter hydrodynamics is strongly influenced by the bottom slope, which is responsible for the intensification of the captured power in regions with slopes lower than 5º, and for the power decrease in cases where the slope exceeds 15º. Significant increases in slope can cause a decrease up to 28% in the converter excitation torque, significantly affecting its performance in electric power generation.

Assessment of the Thermodynamic and Numerical Modeling of LES of Multi-Component Jet Mixing at High Pressure

Mixing under high pressure conditions plays a central role in several engineering applications, such as direct-injection engines and liquid rocket engines. Numerical flow simulations have become a complementary tool to study the mixing process under these conditions but require complex thermodynamic modeling as well as validation with accurate experimental data. For this reason, we use experiments of supercritical single-phase jet mixing from the literature, where the mixing is quantified by the mixture speed of sound, as a reference for our work. We here focus on the thermodynamic modeling of multi-component flows under high pressure conditions and the analytical calculation of the mixture speed of sound. Our thermodynamic model is based on cubic equations of state extended for multi-components. Using an extension of OpenFOAM, we perform large-eddy simulations of hexane and pentane injections and compare our results with the experimentally measured mixture speed of sound at specific positions. The simulation results show the same characteristic trends, indicating that the mixing effects are well reproduced in the simulations. Additionally, the effect of the sub-grid scale modeling is assessed by comparing results using different models (Smagorinsky, Vreman, and Wall-Adapting Local Eddy-viscosity). The comprehensive simulation data presented here, in combination with the experimental data, provide a benchmark for numerical simulations of jet mixing in high pressure conditions.

Impact of Submerged Entry Nozzle (SEN) Immersion Depth on Meniscus Flow in Continuous Casting Mold under Electromagnetic Brake (EMBr)

Complex multi-phase phenomena, including turbulent flow, solidification, and magnetohydrodynamics (MHD) forces, occur during the continuous casting (CC) under the applied electromagnetic brake (EMBr). The results of the small-scale experiment of the liquid metal model for continuous casting (mini-LIMMCAST) at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), investigating MHD flow with a deep immersion depth of 100 mm, are supplemented by newly presented numerical studies with the shallow position of the submerged entry nozzle (SEN) at 50 mm below the meniscus. Herein, the focus is on the MHD effects at the meniscus level considering (i) a fully insulating domain boundary, (ii) a perfectly conductive mold, or (iii) the presence of the solid shell. The volume-of-fluid (VOF) approach is utilized to model a Galinstan flow, including free surface behavior. A multiphase solver is developed using conservative MHD formulations in the framework of the open-source computational fluid dynamics (CFD) package OpenFOAM®. The wall-adapting local eddy-viscosity (WALE) subgrid-scale (SGS) model is employed to model the turbulent effects on the free surface flow. We found that, for the deep immersion depth, the meniscus remains calm under the EMBr for the conductive and semi-conductive domain. For the insulated mold disregarding the SEN position, the self-inducing MHD vortices, aligned with the magnetic field, cause strong waving of the meniscus and air bubble entrapment for shallow immersion depth. Secondary MHD structures can form close to the meniscus under specific conditions. The influence of the EMBr and immersion depth on the flow energy characteristics is analyzed using power spectral density (PSD).

HPC performance study of different collision models using the Lattice Boltzmann solver Musubi

Over the past decades, the lattice Boltzmann method (LBM) has become increasingly popular thanks to its capabilities in the domain of Large-Eddy Simulations (LES). Different collision schemes have been proposed to extend the scope of application to higher Reynolds number flows. This study compares the accuracy and the performance of some of these schemes on a D3Q27 lattice, including the original Multiple Relaxation Times (MRT) model, the Hybrid Recursive Regularized Bhatnagar–Gross–Krook (HRR) operator, as well as the Projected Recursive Regularized Bhatnagar–Gross–Krook (PRR) operator and the parametrized Cumulant collision scheme. For this purpose, the above-mentioned schemes are implemented in the HPC LBM solver Musubi and tested on a well-documented test-case describing the flow past a circular cylinder at a Reynolds number of 3900. Three different subgrid scale (SGS) models are used to account for the unresolved turbulence, i.e. the Smagorinsky model, the Wall-Adapting Local Eddy-viscosity (WALE) model, and the Vreman model. The Cumulant scheme uses an Implicit LES (ILES) subgrid scale model and shows the best agreement with the experimental data followed by MRT with WALE, and HRR with Vreman. The examined collision models are able to capture the second peak at f=3fvs of the power spectra density of the y velocity component first discovered in experiments.With respect to performance, the collision models are compared in terms of MLUPs/node and parallel efficiency for a strong scaling analysis. Again the Cumulant scheme outperforms the other collision models even when they are run on the reduced D3Q19 stencil. All the collision schemes show a strong scaling parallel efficiency above 60% on up to 16384 cores in our implementation.

Active flow control of the airflow of a ship at yaw

This paper implements the steady Coanda effect active flow control (AFC) on the Chalmers ship model (CSM) to study its influence on the ship’s side force and airwake under the yaw effect. The study is conducted numerically using Large Eddy Simulation (LES) with Wall-Adapting Local-Eddy Viscosity (WALE) model. Numerical methods are validated by the experimental data acquired from the baseline CSM under 10∘ port-side wind. The model with AFC is created by modifying the square-shaped hanger base to the Coanda surface and added with injection slots along the base’s roof edge and two side edges. The results show that the base-shape modification significantly alters the vortex structure on deck from z-direction vortex (ZV) to streamwise vortex (SV), and the steady Coanda effect with a momentum coefficient (Cμ) of 0.02 further enhances the SV with the removal of port-side vortex (PV). The side force and yaw moment are reduced by 5.27% and 7.97%, respectively in the AFC case due to the reduction of port-side (windward) ship-surface pressure. Furthermore, the current AFC can suppress the low-speed region and alleviate the velocity gradient in the lateral direction, which mitigates the regions of high TKE (turbulent kinetic energy) and high shear stress along the port-side deck.

Large eddy simulation of ship airflow control with steady Coanda effect

This paper numerically studies the steady Coanda effect for drag reduction and airwake manipulations on the Chalmers ship model (CSM) using large eddy simulation with wall-adapting local-eddy viscosity model. Numerical methods are validated by experimental data acquired from the baseline CSM. In creating the flow control model, the hanger base of the baseline CSM is modified with Coanda surfaces and injection slots along its roof edge and two side edges. Four representative cases are studied: a no-jet case and three cases with the same momentum coefficient of the jet flow activated at different locations (roof, sides, and combined). The results show that the four cases have various performances in drag reduction and vortex structures on the deck. They are also different in mean and turbulent quantities as well as POD (proper orthogonal decomposition) modes in their airwake. It is found that the roof-jet has a stronger Coanda effect and is more vectored toward the low-speed area (LSA) on the deck than the side-jets that detach earlier from the Coanda surface. The energization process is, therefore, different where the roof-jet is more effective that directly brings high momentum to LSA and side-jets manipulate shear layers for mixing enhancement. The cases with roof-jet achieve better mitigation of flow re-circulation and higher recovery of streamwise velocity with lower turbulent fluctuation in the airwake. POD analysis suggests that the roof-jet can stabilize the wake.

Analysis of numerical factors affecting large eddy simulation of pollutant diffusion around buildings

In recent years, Large Eddy Simulation (LES) has been often used for predicting the pollutant diffusion around buildings. However, due to the numerical discretization error and the sub-grid scale modeling error, some empirical factors related to LES have not yet formed with completion for the best practice. This study focused on LES simulation of the airflow and pollutant diffusion around building with a synthetic inflow turbulence method and Wall-Adapting Local Eddy-Viscosity (WALE) model, discuss LES empirical factors, such as effects of grid resolution, sub-grid scale (SGS) WALE model with coefficient Cw and the sub-grid scale Schmidt number ScSGS in details. The results indicate that the numerical wind tunnel created by LES can better reproduce the characteristics of the airflow and concentration field around buildings while keeping predicted physical fields are in line with the evaluation index. In addition, the results also indicate that the solution with the finest grid resolution may not have the best agreement with the experimental data, the flow field is more sensitive to the change of SGS model coefficient Cw when the grid resolution is lower, which shows a trend that the larger coefficient Cw has better simulation in the windward sides of the building, while the value of the coefficient Cw shows an opposite trend in the recycling area at the rear of the building.

Drag reduction of ship airflow using steady Coanda effect

This paper studies the steady Coanda effect for reducing the aerodynamic drag of the Chalmers ship model (CSM) using Large Eddy Simulation (LES) with Wall-Adapting Local-Eddy Viscosity (WALE) model. The flow control mechanism is explored, and the analysis of energy efficiency is conducted to evaluate the net benefit of the flow control. Validating the numerical methods, the predicted aerodynamic drag of the ship and pressure coefficients distribution on the baseline CSM agree well with the experimental measurements and the maximum discrepancy is 4.2%. In creating the flow control models, the hanger base of the baseline CSM is modified with a Coanda surface and two different sizes of jet-blowing slots, 1%h (hanger height) and 2%h, respectively. A drag reduction of 5.34% is achieved by the 1%h slot-size case. The 2%h slot-size case further increases the drag reduction to 6.22% but has doubled power consumption. It is found that vectoring vorticity towards the low-speed area on deck is effective for enhancing the energization. Finally, the analysis of energy efficiency indicates that the net benefit is achieved in both flow control cases, and the case with the 1%h slot size is 11.9% more efficient due to a stronger Coanda effect.