6C.2
Conditional sampling and compositing of coherent structures in a large-eddy simulation of canopy turbulence
Li Wang, University of California, Davis, CA; and R. H. Shaw, K. T. Paw U, and E. G. Patton
Turbulent coherent structures associated with scalar microfronts and sweep/ejection cycles were studied with a large-eddy simulation (LES) of flow inside and immediately above a vegetation canopy. Their common features were obtained by applying a conditional sampling and compositing method to a 30-minute time series. We identified structures on the basis of the high-pressure region associated with a scalar microfront and sweep/ejection cycle. The methodology was designed to ensure optimal phase alignment during the compositing process. The resulting coherent structure is four-dimensional, and thus includes time evolution. We found that the size of a coherent structure is larger in the streamwise direction than in the lateral direction. Flow divergence exists in the lateral direction as a consequence of convergence between the sweep and ejection, and the zone of lateral flow divergence tilts downstream like the microfront. The tilt with height of a scalar microfront, and of associated velocity and pressure, changes with time during the lifecycle. Our results also demonstrate that pressure and vortical patterns are associated with a scalar microfront and sweep/ejection cycle. A broad high-pressure region is centered at the intersection of a scalar microfront and the canopy top as a consequence of velocity deformation. Streamwise and vertical vortices as well as transverse vortices are associated with a scalar microfront and sweep/ejection cycle. In this presentation, computer visualization will be used to demonstrate the essential features of coherent structures revealed by the conditional sampling and compositing method.
Session 6C, Vegetative Canopies
Friday, 11 August 2000, 2:15 PM-4:45 PM
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