An Lda Study of Turbulent Flow behind a Submerged Stick in a Channel

The study of the effects that vegetation in natural river beds and artificial channels has on the characteristics of the flow has become at the present time much more important than once, due to present ecological reasons. A kind of vegetation frequently considered is the rigid submerged one, which can be modelled through the use of vertical fully submerged cylindrical sticks, generally disposed in regular arrays. In international literature about this topic, some authors investigated the influence on the current of single stick or of a few sparse sticks, and other ones deepened the flow resistance of vegetated beds. In a previous work the authors of this paper experimentally obtained the resistance coefficients of currents flowing over vegetated beds modelled through sparse brass sticks. These values showed that, doubling the density of vegetation, the resistance coefficients do not double. In this paper the hypothesis is made that this behaviour depends on the interaction among the wakes of the different sticks. This hypothesis has been experimentally verified. In fact, experiments have been carried out on a current where only one stick is placed, measuring through an LDA device the instantaneous velocity values in different verticals within its wake and computing the distributions of mean velocities, and of the variance, skewness and kurtosis of the fluctuating velocities, and integral length scales. The obtained distributions clearly show that the influence zone of the stick spreads till a distance much greater than the distance of the subsequent sticks in the previous work. Consequently, in the previously considered vegetated bed, each sticks’ wake would attain many other sticks behind. The results of this paper confirm the wakes interaction hypothesis.