Turbulent Flow within and above sparse canopy

The problem of turbulent flow-canopies interaction has been extensively studied in literature. When fully submerged canopies are dense and homogeneous, the mixing layer analogy is generally used. In case of sparse canopies this analogy cannot be applied anymore and the flow behaviour is analogous to a boundary layer perturbed by element wakes. In case of transition from the perturbed boundary layer to the mixing layer, the effects of the canopy density and of the measurements location are not yet well characterized. The aim of the paper is to experimentally study the effects of vegetation density and measurements positions on the structure of an uniform turbulent flow above and inside sparse canopies. Water flume experiments were conducted in a channel 8m long and with a square section 0.40 × 0.40m2. The model of canopies consisted of sparse rigid cylinders of the same heights but set in two different aligned allocation (squares or rectangles), with two different densities. Two different measurement locations were considered, in any case in a reach of the channel where the uniform flow was attained. In order to measure instantaneous velocities an LDA system was used. Experiments were carried out by varying the flow rate and the slope of the channel. Starting from instantaneous velocity data collected, distributions of turbulent statistical quantities as mean velocities, variances and longitudinal integral length scales were evaluated and analyzed.