عنوان مقاله [English]
Background and objectives: It is important to study the coefficient of resistance in streams, especially in open channels, canals and rivers. One of the factors influencing the flow resistance is bed-forms. Cluster microforms are types of bed-forms in mountainous rivers, which are important both in biological and in hydraulic as well as in the secondary currents. Study on the recognition and influence of clusters on flow resistance is novel. The purpose of this study is to investigate the effect of cluster shape and particle size of clusters on the flow roughness coefficients.
Materials and methods: In order to investigate the effect of the shape and size of the particles that forming clusters, experiments were carried out in a laboratory channel of 20 meters in length, 0.6 m in width and 0.6 m in height. Using gravel particles with three different sizes of 9.5, 12.5, 15.5 mm, cluster three cluster types (linear, heap and rings) were constructed in laboratory flume. Two roughness coefficients namely Darcy-Wiesbach and Manning were calculated using the water surface slope measurements.
Results: The results of this study showed that the linear cluster has the least effect on the rate of flow resistance coefficient. The ring and heap clusters have a roughness coefficient greater than the linear cluster, but both have almost same impact on the flow resistance. The results of these two forms of the cluster are very close for particles of 9.5 and 12.5 mm, but for a particle of 15.5 mm, the heap cluster coefficient of roughness is higher than the roughness coefficient of another. The manning’s roughness coefficient is also increased by increasing the diameter of the gravel particles of the cluster builder. By performing experiments with gravel particles of different diameters, the percentage of change in roughness coefficient relative to the non-cluster state for cluster clumps in particles of 9.5, 12.5 and 15.5 mm was 47, 52 and 75, respectively. For rings and particles of 9.5, 12.5 and 15.5 mm, the percentage changes were 48, 49 and 75 percent, respectively, and the percentage for linear clusters for particles of 9.5 and 19 percent, and particles 5 / 12 and 15.5 mm were observed at 37 and 67% respectively, indicating an increase in the flow resistance with an increase in the particle diameter. Also, the results showed that increase in Froude number would decrease the roughness coefficient.
Conclusion: The results of the experiments made it clear that the cluster affect the flow resistance by increasing it. The results of this study showed that the heap cluster has the most effect on flow resistance.