Integral length scales in vegetated open channel flows

Abstract

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.

The problem of turbulent flow-canopies interaction has been extensively studied in literature. Interaction between flow and aquatic riparian vegetation is an extremely complex problem. Although many studies have attempted to explore it, flow through vegetation phenomenon has not yet been fully described quantitatively. At the beginning, effects of vegetation on turbulent air flow have been studied through wind tunnel and fields measurements. Afterwards researcher’s attention has been aimed at turbulent characteristics of a water flows developing in experimental flumes with vegetated bottom. Generally, turbulence characteristics whose changes were described were mean velocity, standards deviation, skewness, kurtosis. In their studies some researchers deepened the effect of vegetation on more specific characteristics of the turbulent flow as the integral length scales. The aim of the paper is to experimentally study the effects of vegetation density on the longitudinal integral length scales of a 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 three different aligned allocations (squares or rectangles), with three 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 longitudinal integral length scales were determined and analyzed. Finally more correct methodologies in order to evaluate integral length scales and to normalize them were proposed.