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.

A stochastic model for particle motion in large-eddy simulation

2006 ◽  
Vol 7 ◽  
pp. N18 ◽  
Author(s):  
Babak Shotorban ◽  
Farzad Mashayek
Keyword(s):  
Large Eddy Simulation ◽  
Stochastic Model ◽  
Particle Motion ◽  
Eddy Simulation ◽  
Large Eddy

scholarly journals Numerical modelling of microburst with Large-Eddy Simulation

2010 ◽  
Vol 10 (10) ◽  
pp. 24345-24370
Author(s):  
V. Anabor ◽  
U. Rizza ◽  
G. A. Degrazia ◽  
E. de Lima Nascimento
Keyword(s):  
Large Eddy Simulation ◽  
Time Scale ◽  
Doppler Radar ◽  
Leading Edge ◽  
Ring Vortex ◽  
Short Time Scale ◽  
Eddy Simulation ◽  
Cloud Models ◽  
Large Eddy ◽  
The Impact

Abstract. An isolated and stationary microburst is simulated using a 3-D time-dependent, high resolution Large-Eddy Simulation (LES) model. The microburst downdraft is initiated by specifying a simplified cooling source at the top of the domain near 2 km. The modelled time scale for this damaging wind (30 m/s) is of order of few min with a spatial scale enclosing a region with 500 m radius around the impact point. These features are comparable with results obtained from full-cloud models. The simulated flow shows the principal features observed by Doppler radar and others observational full-scale downburst events. In particular are observed the expansion of the primary and secondary cores, the presence of the ring vortex at the leading edge of the cool outflow, and finally an accelerating outburst of surface winds. This result evidences the capability of LES to reproduce complexes phenomena like a Microburst and indicates the potential of LES for utilization in atmospheric phenomena situated below the storm scale and above the microscale, which generally involves high velocities in a short time scale.

scholarly journals Connecting the Failure of K Theory inside and above Vegetation Canopies and Ejection–Sweep Cycles by a Large-Eddy Simulation

2017 ◽  
Vol 56 (12) ◽  
pp. 3119-3131 ◽  
Author(s):  
Tirtha Banerjee ◽  
Frederik De Roo ◽  
Matthias Mauder
Keyword(s):  
Large Eddy Simulation ◽  
Time Scale ◽  
Turbulent Flux ◽  
Turbulent Transport ◽  
Sensible Heat Flux ◽  
Quadrant Analysis ◽  
Local Concentration ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Vegetation Canopies

AbstractParameterizations of biosphere–atmosphere interaction processes in climate models and other hydrological applications require characterization of turbulent transport of momentum and scalars between vegetation canopies and the atmosphere, which is often modeled using a turbulent analogy to molecular diffusion processes. Simple flux–gradient approaches (K theory) fail for canopy turbulence, however. One cause is turbulent transport by large coherent eddies at the canopy scale, which can be linked to sweep–ejection events and bear signatures of nonlocal organized eddy motions. The K theory, which parameterizes the turbulent flux or stress proportional to the local concentration or velocity gradient, fails to account for these nonlocal organized motions. The connection to sweep–ejection cycles and the local turbulent flux can be traced back to the turbulence triple moment . In this work, large-eddy simulation is used to investigate the diagnostic connection between the failure of K theory and sweep–ejection motions. Analyzed schemes are quadrant analysis and complete and incomplete cumulant expansion methods. The latter approaches introduce a turbulence time scale in the modeling. Furthermore, it is found that the momentum flux and sensible heat flux need different formulations for the turbulence time scale. Accounting for buoyancy in stratified conditions is also deemed important in addition to accounting for nonlocal events to predict the correct momentum or scalar fluxes.

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