Surface layer turbulence response to spanwise heterogeneous vegetative canopies *

Using combined wind-tunnel measurements and large-eddy simulations (LES), the influence of spanwise heterogeneity on secondary circulations and on evaporative fluxes is investigated. The experiments were performed at the Technion environmental wind-tunnel. A 5m long canopy, modeled after available leaf-area index data for corn, consisted of triangular perforated sheet elements (h=20cm, in-line setup). Measurements were performed by hot-wire anemometry and stereo-PIV enabling to spatially and temporally resolve the turbulent flow. Measurements were performed at a bulk flow velocity of 1.5m/s (Re_h=19200). Several stereo-PIV data sets were acquired in wall-parallel planes positioned between 0.7h to 1.3h in steps of 0.05h. Evaporation rates at various locations within the canopy were measured using in-house made sensors. Measurements were performed for a homogeneous canopy and one that exhibited spanwise heterogeneity under the same bulk flow conditions. Turbulent flow characteristics, including turbulence production terms, will be presented and compared to the LES. The effect of heterogeneity on coherent structures near the canopy top was analyzed using quadrant and spectral analyses.