PIV measurements of flow structure and turbulence within and above a corn canopy and a wind tunnel model canopy

Particle Image Velocimetry (PIV) measurements just within and above a mature corn canopy have been performed to clarify the small-scale spatial structure of the turbulence. Frequency spectra are combined with instantaneous spatial spectra to resolve more than five orders of magnitudes of length scales which display an inertial range spanning three decades. The small scale turbulence in the dissipation range exhibits anisotropy at all measurement heights. Directly calculated subgrid scale (SGS) energy flux increases significantly with decreasing filter size, providing support for existence of spectral shortcut that bypasses the cascading process. As a result, the cascading energy flux is overestimated by a -5/3 line fit to the spectra, but not by orders of magnitude. Quadrant-Hole (Q-H) analysis shows that small scale flow structures that dominate the vorticity and cascading dissipation rate, and large scale structures that dominate the Reynolds stresses and production rate seem to have very little correlation with each other. However sweep events are the largest contributors to both at least close to canopy height. Although the outward interaction events are rare, they are highly dissipative and significantly contribute to the overall dissipation and (negative) production rates.; Turbulence characteristics measured in the filed are compared with those of a model canopy setup in a wind tunnel. The laboratory normalized mean velocity profile is adjusted using variable mesh screens to match the normalized mean shear of the corn field data. However the profiles of normalized Reynolds shear stress in the field and the laboratory differ. A comparison of results from Q-H analysis for both data sets shows that in quadrants 1 to 3, the wind tunnel and field conditionally sampled stresses show similar trends. However, a conflicting trend is found in the sweep quadrant, presumable due to the difference in roughness density. The analysis confirms that sweeps and ejections dominate the momentum flux and dissipation rate.; Furthermore, quadrant analysis of the wind tunnel data reveals fundamental differences in flow structure, especially between sweep and ejection events, which dominate the flow in terms of duration. The differences in flow structure affect the distributions of turbulent kinetic energy (TKE), Reynolds shear stress, turbulence production, turbulent diffusion, and dissipation rates. During sweeps the TKE and Reynolds shear stress have distinct peaks just below canopy height, whereas during ejections they have broad maxima well above the canopy. A distinct large peak in production just below canopy height during sweep events is the main contributor to the high overall production there. At z/h > 1.2, ejections take over. Since eddies generated within the wakes behind canopy elements are already within the dissipation range, the extent of dissipation below canopy height increases with velocity magnitude. Consequently, the dissipation there has the highest values during sweep and quadrant 1 events, and is significantly lower during ejection and quadrant 3 events. Well above the canopy, ejections are the most dissipative. Turbulent diffusion during sweep events acts as source below narrow shear layer within the canopy and a sink above it. Diffusion during ejection events is a source only well above the canopy.