Influence field of a single rigid submerged stick in a liquid current and its relations with a sticks array

The study of the effects that vegetation in natural river beds and artificial channels has on the characteristics of the stream flowing down in them 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 metallic fully submerged cylindrical sticks, generally disposed in regular arrays. Within the many topics in international literature about this subject, some authors investigated the influence on the current of single sticks or of a few sparse sticks, and other ones deepened the resistance properties of vegetated surfaces. 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 done that this behaviour depends on the interaction among the wakes of the different sticks and this hypothesis is experimentally verified. Experiments are 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: these distributions clearly show that the influence zone of the stick spreads, and, therefore, it means that in the previously considered vegetated bed, it would attain many other sticks behind. These results confirm the wakes interaction hypothesis.