Mass and momentum exchange in lateral bank cavities with increasing aspect ratio using large-eddy simulation

2020 ◽  
pp. 1284-1292
Author(s):  
Pablo Ouro ◽  
Carmelo Juez ◽  
Mário J. Franca
Keyword(s):  
Aspect Ratio ◽  
Large Eddy Simulation ◽  
Momentum Exchange ◽  
Eddy Simulation ◽  
Large Eddy

Large-Eddy Simulation of Reactive Pollutant Dispersion Over Street Canyons of Different Aspect Ratios

2014 ◽  
Author(s):  
T. Z. Du ◽  
Chun-Ho Liu ◽  
Y. B. Zhao
Keyword(s):  
Aspect Ratio ◽  
Large Eddy Simulation ◽  
Chemical Reactions ◽  
Pollutant Dispersion ◽  
Pollutant Removal ◽  
Street Canyons ◽  
Eddy Simulation ◽  
Aspect Ratios ◽  
Large Eddy ◽  
Reactive Pollutant

In urban areas, pollutants are emitted from vehicles then disperse from the ground level to the downstream urban canopy layer (UCL) under the effect of the prevailing wind. For a hypothetical urban area in the form of idealized street canyons, the building-height-to-street-width (aspect) ratio (AR) changes the ground roughness which in turn leads to different turbulent airflow features. Turbulence is considered an important factor for the removal of reactive pollutants by means of dispersion/dilution and chemical reactions. Three values of aspect ratio, covering most flow scenarios of urban street canyons, are employed in this study. The pollutant dispersion and reaction are calculated using large-eddy simulation (LES) with chemical reactions. Turbulence timescale and reaction timescale at every single point of the UCL domain are calculated to examine the pollutant removal. The characteristic mechanism of reactive pollutant dispersion over street canyons will be reported in the conference.

scholarly journals Comparison of In-Canopy Flux Footprints between Large-Eddy Simulation and the Lagrangian Simulation

2008 ◽  
Vol 47 (8) ◽  
pp. 2115-2128 ◽  
Author(s):  
T. V. Prabha ◽  
M. Y. Leclerc ◽  
D. Baldocchi
Keyword(s):  
Large Eddy Simulation ◽  
Stochastic Model ◽  
Time Scale ◽  
Lagrangian Model ◽  
Lagrangian Stochastic Model ◽  
Momentum Exchange ◽  
Lagrangian Simulation ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Flux Footprints

Abstract Flux footprints for neutral shear-driven canopy flows are evaluated using large-eddy simulation (LES) and a Lagrangian stochastic (LS) model. The Lagrangian stochastic model is driven by flow statistics derived from the large-eddy simulation. LES results suggest that both surface and elevated sources inside the canopy contribute equally to the cumulative flux from an upwind distance of 4 times the canopy height. LES flux footprints are more contracted than those obtained using the Lagrangian stochastic model. This is attributed to an enhanced vertical diffusion and reduced horizontal diffusion. The ejection and sweep contributions to momentum exchange in the Lagrangian stochastic model are weaker than those in the large-eddy simulation. Ejections of low-momentum air dominate at all levels in the canopy modeled by the LES. In contrast, high-momentum sweep events are dominant within the LES canopy and low-momentum ejection events are dominant above the canopy. Dispersion parameters for the first- and second-order statistics of concentration from both LES and LS for three line sources representing the canopy crown, midcanopy, and surface sources are also investigated. Lagrangian model results are sensitive to the choice of the time scale. A time scale based on the dissipation rate agrees well with the LS and LES plume heights of surface source. However, flux footprints from LS are closer to those from the LES, while an intermediate time scale (0.15z/σw) was used inside the canopy.

Effects of Fuel Physical Properties and Breakup Model Constants on Large-Eddy Simulation of Diesel Sprays

2015 ◽  
Author(s):  
Chi-Wei Tsang ◽  
Christopher J. Rutland
Keyword(s):  
Molecular Weight ◽  
Large Eddy Simulation ◽  
Physical Properties ◽  
Time Constant ◽  
Momentum Exchange ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Momentum Fluxes ◽  
Length Constant ◽  
Breakup Model

The Kelvin-Helmholtz/Rayleigh-Taylor (KH-RT) wave breakup model is a commonly used model in predicting primary and secondary atomization and breakup processes in Lagrangian-Eulerian Diesel spray simulations. Droplet sizes predicted by this model are dependent on several parameters. The parameters include fuel physical properties, such as density, viscosity, and surface tension, and a number of adjustable model constants, such as KH and RT time constants, KH and RT size constants, and the breakup length constant. The purpose of this study is to investigate the effects of these parameters on predicting spray motions using large-eddy simulation with the dynamic structure sub-grid stress model. The code used in this study is OpenFOAM. This study has three major parts. Firstly, effects of the model constants on the prediction of momentum exchange process were examined by comparing liquid and gas momentum fluxes. Drag Forces exerted on liquid spray by gas phase can be determined from the slopes of gas and liquid momentum fluxes plotted against axial distance. We found that the prediction of momentum exchange between gas and liquid is most sensitive to the KH time constant, B1, among the other model constants. Secondly, effects of fuel physical properties were investigated by using four different fuels in the simulations of non-vaporizing and vaporizing sprays. The four fuels used were n-dodecane, F76 fuel, n-hexadecane, and methyl tetradecanoate. The F76 fuel is a multi-component fuel containing twenty-one hydrocarbons. Global spray quantities such as liquid and vapor penetrations, Sauter mean diameter, total liquid mass, number of parcels, and breakup model quantities such as Ohnesorge number and KH wave speed were compared. The key finding is that not all of these quantities monotonically increase or decrease with fuel molecular weight. Lastly, effects of fuel physical properties on sensitivities of the breakup model constants were studied. We compared liquid penetration and vapor penetration for each fuel using different values of the model constants. We found that the prediction of vapor penetrations is more sensitive to the KH time constant B1 when a fuel with lighter molecular weight was used, and the prediction of liquid penetrations is sensitive to the breakup length constant, Cb, in all of the four fuels. The computational investigations in this study reveal some limitations of the current spray breakup model, and motivate us to develop more advanced models to overcome these limitations.

Large Eddy Simulation of Flow and Heat Transfer in a Rectangular Channel With Crossed Angled Ribs

2004 ◽  
Author(s):  
Toshihiko Takahashi ◽  
Kazunori Watanabe
Keyword(s):  
Heat Transfer ◽  
Aspect Ratio ◽  
Large Eddy Simulation ◽  
Rectangular Channel ◽  
Hydraulic Diameter ◽  
Channel Cross Section ◽  
Eddy Simulation ◽  
Flow And Heat Transfer ◽  
Rectangular Channels ◽  
Large Eddy

Characteristics of fully developed flow and heat transfer in stationary rectangular channels with two opposite ribbed walls, adopted for internal cooling of a gas turbine blade, were investigated by using Large Eddy Simulation (LES). Effect of aspect ratio of the channel cross section on the flow and thermal fields in the channels with crossed, 60-deg. angle ribs was studied. The thermo-flow field in the stationary rectangular channels with the crossed angled ribs has a single secondary swirl in the cross sections, which exhibits a basic pattern for convection in the channels with the angled ribs. The aspect ratio was varied from 0.5 to 2.0. The rib height-to-hydraulic diameter and the rib pitch-to-height ratio were kept at 0.075 and 11.547, respectively. Calculations were conducted for Reynolds number based on the hydraulic diameter at around 50,000 and 120,000. The present calculations showed that formation of rib induced separation vortex varies with the channel aspect ratio. The change in the vortex formation has an influence on augmentation of friction and heat transfer. Current correlations for friction and heat transfer agreed reasonably with previous experiments.

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