Turbulent flows on forested hilly terrain: the recirculation region

A number of analytical and numerical studies employing first-order closure principles have suggested that canopy flows on gentle sinusoidal hills feature a recirculation region, situated on the lee side, that can dramatically affect scalar transfer between the biosphere and the atmosphere. To date, the onset of this region, and its effects on bulk flow properties, have not been experimentally investigated. We study the applicability of first-order closure schemes jointly with the properties of this recirculation region, using detailed laser Doppler anemometry (LDA) measurements. These experiments are conducted in a neutrally stratified boundary-layer flow within a large flume over a train of gentle and narrow hills. The canopy is composed of an array of vertical cylinders with a frontal-area index concentrated in the upper third, to resemble a tall hardwood forest at maximum leaf area. The LDA measurements are recorded for both sparse and dense canopies. We find that, while the onset of a recirculation region is ambiguous in the sparse-canopy case, it is well delineated in the dense-canopy case. This finding constitutes the first experimental evidence confirming the analytical and numerical model predictions concerning this region in dense canopies on gentle hills. Moreover, we show that the presence of the recirculation region can explain the anomalous pressure variation across the hill (first reported in numerical simulations) using an 'effective hill shape' function. Detailed momentum-flux measurements show, surprisingly, that the effective mixing length l(eff) within the canopy and in the inner layer is not significantly affected by the recirculation region. We expected l(eff) to be comparable to the size of the vortex responsible for the recirculation zone, but the measurements show that l(eff) maintains its canonical canopy turbulence shape. Using laser-induced fluorescence (LIF) measurements, we find that the recirculation region is not characterized by a classical 'rotor', but by a highly intermittent zone with alternating positive and negative velocity values in the lower layers of the canopy. These LIF measurements may explain why l(eff) maintains its canonical canopy turbulence shape in the recirculation region. The LIF measurements also show that the main mechanism for scalar transfer within the recirculation region is a sequence of accumulation-ejection episodes that are quasi-periodic in nature.