Lagrangian-based Visualization Techniques for the Observation of Instantaneous Coherent Turbulence Structures in Plant Canopy Flows

One of the most significant discoveries in the field of plant canopy turbulence was the idea that the structure of the flow is not dominated by small leaf-scale motions, but rather larger canopy-scale coherent structures. This notion later led to the well-known “mixing-layer analogy”, which hypothesizes that near-canopy flow is similar to that of a planar mixing layer, which is dominated by quasi-two-dimensional roller-like structures. This hypothesis was primarily supported indirectly by scaling arguments, as direct observation of such phenomena is extremely difficult in real flows with a virtually infinite range of turbulent scales. As available computational power increased, workers began performing high-resolution simulations that allowed for a more detailed investigation of turbulence structure. Even with high-resolution data, the structure of the flow can still only be observed indirectly, such as by using conditional averages. These studies suggested that the quasi-two-dimensional structure is not able to survive in a highly perturbed, nearly infinite Reynolds number environment, and instead that the flow is dominated by highly three-dimensional structures continuously undergoing localized vortex pairings. These observations are seemingly supported by direct experimental evidence such as the observation that turbulent length scales show little spanwise coherence.

Recent work by Bailey and Stoll (2016) [The creation and evolution of coherent structures in plant canopy flows and their role in turbulent transport. J. Fluid Mech. 789:425-460] presented evidence that the quasi-two-dimensional structure originally associated with the mixing-layer analogy exists in typical canopy flows, and dominates turbulent fluxes. However, this structure contains irregularities that make them impossible to detect using, e.g., two-point spanwise correlation calculations. The study concluded that the quasi-two-dimensional roller structures exist in the flow, but that they are much more difficult to detect than the smaller-scale three-dimensional vortex structures that also evolve concurrently in the flow. Despite this conclusion, the study called for future work that focuses on direct, instantaneous observation of the structure of the flow that does not rely on elaborate averaging schemes.

This presentation focuses on new visualization techniques that are able to show the instantaneous evolution of the turbulent flow structure in simulated plant canopy flows. The premise behind the techniques is the use of Lagrangian visualization techniques. When we observe real turbulent flows around us, we generally do so by following Lagrangian tracers as they move through the flow. Because Lagrangian tracers are integrators, they smooth turbulent perturbations and often provide a much clearer picture than observations using Eulerian data. Using these techniques combined with large-eddy simulation data, we were able to visualize the instantaneous quasi-two-dimensional wave-like structure of the flow, thus verifying their existence. These results have important implications for future work and can explain canopy phenomena such as scalar ramp structures or the unexpected behavior of advection velocity scales.