Papers by Keyword: Large Eddy Simulation (LES)

Abstract: The present work exhibits a numerical study comparing the fluid dynamic and thermal fields of turbulent, three-dimensional forced convective cylindrical cavity flows obtained with Large Eddy Simulation (LES) and Reynolds-Averaged Navier Stokes (RANS). In the latter approach, three different closure models are employed: Reynolds Stress Model (RSM), standard k – ε and standard k - ω. It is considered a three-dimensional, incompressible, turbulent fluid flow at the steady state with Re_D = 22,000 and Pr = 0.71. The main purpose is to investigate whether discrepancies are noticed in time-averaged and statistics of turbulent flows between LES and RANS predictions. Differences in time-averaged and statistical fields can be important for evaluation of convective fluxes in turbulent flows and combined convective and radiative transfer in participant media, i.e., for study of Turbulence-Radiation Interactions (TRI). The spatially-filtered and time-averaged conservation equations of mass, momentum and energy are solved with the Finite Volume Method (FVM). Results showed that time-averaged and RMS thermal fields obtained with LES and RANS presented reasonable discrepancies in regions near the cavity surfaces, which affects the convective fluxes in this region. For the highest temperature region of the cavity (near its inlet) the predictions obtained with LES and RANS are similar, which can led to similar predictions in heat exchange when thermal radiation is taken into account in optically thin participant media. For optically thick media, where local differences increase their importance, the employment of RANS is not recommended.

Abstract: This paper presents the calculation of hydrodynamic characteristics of two side-by-side cylinders of different diameters in three dimensional incompressible uniform cross flow by using Large-eddy simulation method based on dynamical Smagorinsky-Lilly sub-grid scale model. Solution of the three dimensional N-S equations were obtained by the finite volume method. The numerical simulation focused on investigating the characteristic of the pressure distribution (drag and lift force), vorticity field and turbulence Re=. Results shows that, the asymmetry of the time –averaged velocity distribution in the flow direction behind the two cylinders is very obvious; the frequency of eddy shedding of the small cylinder is about twice of the large one. The turbulence of cylinders is more obvious.

Abstract: Various methods –direct methods, structure function methods and dimensional analysis-have been proposed to estimate the dissipate rate of turbulent kinetic energy. In this study these methods were appraised using LES data of a turbulent flow in the straight pipe at Re=5300. The fact that the corrected second-order structure function method is robust can be found. Note that when approaching to the wall at low Reynolds number, structure function methods and dimensional analysis method usually become inaccurate since they can not reflect information about the radial gradients. Direct computation methods now give acceptable trend and good estimation. Methods mentioned above lead to the same order of magnitude as values from DNS data most times.

Abstract: In the premise of parallel computing, large eddy simulation (LES) model was adopted to set up math formers for the characteristics of airflow origination in CRH₂ Electric Motor Train Units(EMU) in this paper. The Smagorinsky-Lilly dynamic sub-grid model was adopted for fitting local turbulent structure and confirming benchmark model coefficient. Amended balance layer model was used to the wall functions. The temperature and velocity fields in the compartments were numerically simulated. Furthermore, area discretization adopted finite volume method. The results showed, adopting the method of enlarging calculation region, the instantaneous and continuity of turbulence were reflected truly and accurately. The analysis provided a reference for LES of airflow organization in high-speed train. It was also a new reference for the evaluation of comfort environment in the train.

Abstract: Indoor air quality in a public transport interchange station in rush hour has been studied. Carbon monoxide is selected as the main pollutant for description of air quality. Ventilation systems, bus traffic and passenger flow, air quality have been investigated by on-site survey. Large eddy simulation technology has been used to analyze indoor air quality of public transport interchange station. The boundary conditions are determined according to the measured date. Indoor air quality results at heights of 0.8m and 1.6m in rush hour and two operating conditions of public transport interchange station are calculated. Results have shown carbon monoxide concentrations at height of 0.8m are higher than those at height of 1.6m. Air quality would reach the harmful degree within 5min to 10min if the ventilation system is not operating. The ventilation system should be operated continuously during the peak hour in order to meet the requirement of indoor air quality standards.

Abstract: This paper aims to study the impact of the rearview mirror shape on aerodynamic performance. Two typical rearview mirrors were selected to conduct the wind tunnel test, and the test result showed that the noise on the rear monitoring point of the mirror 1 was lower than that of the mirror 2. This paper then conducted simulating computation through computational fluid mechanics (CFD) theory and Fluent software, and obtained the size of the monitoring points of the two typical rearview mirrors, static pressure chart, motion pattern and turbulent kinetic energy distribution diagram, and sequentially analyzed the reason for more noise of the mirror 2. The study shows that different mirror cover structures have a great influence on the flow line flowing through the rearview mirror cover, and significantly influenced the rear flow field of the rearview mirror and the static pressure and the turbulent kinetic energy of the monitoring point.

Abstract: To overcome the deficiency that model experiments are unable to take accurate measurements without damaging the structure of the fine flow fields, a large eddy simulation is employed to simulate the three dimensional structure of the flow passed a pervious cubic-blunt body at Re=2.2×10⁴. A comparative analysis have been taken qualitatively and quantitatively between the flow passed a pervious cubic-blunt body and the flow passed a non-pervious cubic-blunt body from the aspects of the flow structure (mainly including separation and reattachment), unsteady vortex shedding, distribution of static pressure and drag coefficient, etc. Therefore, characteristics of this kind of flow field are concluded and along with a better understanding of concrete effects they bring, which can give guidance to engineering.

Abstract: The large eddy simulation (LES) method was employed to compute the flow past three types of obstacles on slope surface. The predictions give profile of mean velocity and flow reattachment length. The calculations in present paper are in good agreement with the experimental data. Wake characteristics are analyzed. The computational results show that the horseshoe vortex are formatted at upstream base of obstacles. The wake flow is controlled by the downwash flow on the free end, as well as, the upwash flow in the wake disappears completely. With the angle between the leeward side and the bottom becomes larger, the length of the vortex separation becomes larger, and the scope of vortices greater. The large eddy simulation can obtain almost all the flow pattern observed by the experiments.

Abstract: The remit of mechanical engineering extends well beyond machines. Mechanical engineering theories have diverse applications the transport and distribution of particulate matter into a turbulent boundary layer involves a systematic application of fluid mechanics. In this paper, we have used fluid mechanical models accompanied by experimental observations to study how turbulent eddies transport and distribute cook stove emissions over slums in the megacity Chennai - the Detroit of India. It is home to 8 million people and is a manufacturing hub with a propensity to release a variety of suspended particulate matter mixed with marine aerosols transported from the Bay of Bengal. Of these 8 million, about 2 million live in slums-these impoverished slum dwellers comprise of Chennais huge unskilled work force. Our study revealed that they primarily use cow dung cakes and unseasoned wood to cook, releasing a huge amount of particulate matter up to radii of 10 microns (PM₁₀). A scanning electron micro-graphy ascertained the drop size distribution ensuring that the majority of these particles were small enough for them to be easily transported by boundary layer eddies the latter are simulated using the United Kingdom Met Office Large Eddy Simulation Model. It is observed that the large eddies are spread over alternating up and down draughts with maximum vertical velocity perturbations of the order of 1 ms^-1, which is significantly higher than the Stokesian still air settling rates of suspended particles. Whilst remaining aloft, they act as cloud condensation nuclei and thence grow into cloud droplets. The large cloud covers over Chennai city can be linked to this latter effect. Chennais hot and humid atmosphere ensures that these droplets collide within the cloud topped turbulent boundary layer to trigger a process of stochastic coalescence and subsequent rain showers.

Abstract: Three-dimensional flow around a horizontal-axis wind turbine has been investigated with LES method coupled with sliding mesh and experimental measurement. The boundary conditions are set as the same as those of the experiment. The images of the pressure distribution, flow rate distribution, turbulent intensity, velocity vector and vortices of the wind turbine are presented to show the three-dimensional flow characteristics around the wind turbine. The relationship between flow and sound is further studied by analyzing the flow parameters pulsation spectrum to get the sound pressure level. LES results are compared with the wind-tunnel measurements collected with PIV, and good agreement is observed. The results serve as a reference for optimum design of the wind turbine with high functional performance and low level noise generation.