Author(s): D. Poggi; G. G. Katul; L. Ridolfi
Linked Author(s): Davide Poggi
Keywords: Turbulence; Aquatic vegetation; Complex bed surface; Laser doppler measurements
Abstract: Simplifications and scaling arguments employed by analytical models to link bed morphology to mean velocity perturbations within canopies were explored using flume experiments. Laser doppler anemometry (LDA) measurements were conducted in a large recirculating open channel over a train of gentle dunes covered by a dense canopy (H c /L c << 1, where H c is the canopy height and L c is the adjustment length scale, respectively). The dune and canopy configuration were such that the mean dune slope was sufficiently gentle (H/L << 1) but narrow (L c /L ∼ 1, where H and L are the hill height and half-length, respectively). The LDA data revealed a recirculation region within the lower canopy on the lee slope. We showed that adjusting the outer-layer pressure perturbations by a virtual ground that accounts for the mean streamline distortions induced by this recirculation zone improved the model performance. Moreover, we developed a new analytical model for the velocity perturbations in the deeper layers of the canopy that retained a balance between mean horizontal advection, the mean pressure gradient, and the mean drag force but neglected the turbulent shear stress gradient. The proposed model reproduced the LDA measurements significantly better than a recent analytical model that neglected advection but retained the turbulent shear stress gradient in the lower layers of the canopy and near the hill top. In essence, this simplified model assumed that in the deeper layers of the canopy, the spatial features of the mean flow perturbations around their background state are set by the inviscid mean momentum equation.
Year: 2007