LES study on organized turbulence structures over plant canopies

In our experience of tower flux measurements, it is very common to see ramp patterns in time records of scalars (e.g., CO2, temperature). After a good deal of field and wind tunnel studies, we now know that the ramp patterns are traces of organized turbulence structures that passed the tower, and that much fraction of the fluxes is driven by these organized structures.

As in actual field data, clear ramp patterns were seen in the scalar time records produced by a LES of canopy flow. An average figure of the three-dimensional structure of organized turbulence that causes the ramp patterns was captured by the conditional sampling technique.

The conditionally averaged results clearly illustrated that the scalar ramp is created by the passage of the scalar’s microfrontal structure, which is associated with the ejection–sweep structure in the streamwise and vertical velocities, a laterally divergent flow, and a positive, vertically-coherent pressure perturbation. These vertical structures were consistent with previous measurements made in fields or wind tunnels. However, the horizontal slice at the canopy top of the same structure revealed that this structure represents the moment when a streamwise-elongated region of high-speed streamwise velocity impacts on another elongated region of low-speed velocity, indicating that eddies originating from the inflection instability near the canopy top are not involved directly in the formation of the scalar’s microfront. These elongated structures resemble the so-called streak structures that are commonly observed in near-wall shear layers. Since elongated structures of essentially similar spatial scales were observed independent of simulation setups (domain size, etc.), these streak structures are phenomena that are inherent in near-canopy turbulence. Presumably, a strong wind shear formed just above the canopy is involved in their formation.