A Comparison of PIV Measurements of Canopy Turbulence Performed in the Field and in a Wind Tunnel Model

W. Zhu, R. van Hout, L. Luznik, H.S. Kang, J. Katz, Joseph, & C. Meneveau
Department of Mechanical Engineerin & Center for Environmental and Applied Fluid Mechanics The Johns Hopkins University, Baltimore MD 21218

ABSTRACT: Particle Image Velocimetry (PIV) has been used to compare between turbulence characteristics just within and above a mature corn canopy and those of a model canopy set up 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 (CF) data. The smallest resolved scale in the field is about 15 times the Kolmogorov length scale (ηCF ≈ 0.4mm), whereas in the wind tunnel (WT) it is 5 times ηWT (ηWT ≈ 0.15mm). In both cases, the mean velocity and turbulence statistics are consistent with those measured using single point sensors. However the profiles of normalized Reynolds shear stress in the field and the laboratory differ. Turbulent spectral densities calculated from PIV spatial and time series in the field display an inertial range spanning three decades. In the laboratory due to lower Reynolds numbers, the inertial range shrinks to two decades. Quadrant-Hole analysis is applied to Reynolds shear stress, vorticity magnitude and dissipation rates. 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. The analysis confirms that sweep and ejections dominate the momentum flux and dissipation rate.

(2006), Exp. Fluids 41, 309-318.

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