Conditional sampling and compositing of coherent structures in a large-eddy simulation of canopy turbulence

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.