Lateral distribution of turbulence and secondary currents in non-uniform open channel flow

摘要

This research was motivated by previous field observations in gravel bed rivers characterized by macro-scale bedforms such as pools, riffles and bars. Hydrodynamic interpretation of the field results was limited by lack of knowledge of the effect of convective deceleration and acceleration on secondary flows and the lateral distribution of Reynolds stress. To address this, we conducted experiments in a 1.5 m wide flume with gravel sediment (D50 = 0.99 cm). A straight pool was constructed with entry and exit slopes of 5%, a maximum depth of 0.25 m, and an overall length of 7.29 m. Three-dimensional velocity measurements were recorded at 50 Hz using an array of three Vectrino ADVs. Three to eight profiles of ten points per profile were measured at one upstream section, nine sections through the pool, and one downstream section. A total of five runs demonstrate the effects of Reynolds number at a constant flow depth and flow depth at a constant Reynolds number. Velocity and Reynolds stress profiles in the channel centerline qualitatively agree with previous results in nonuniform flow and include increased Reynolds stress as a result of deceleration and high velocity near the bed as a result of acceleration. Strong lateral effects include secondary circulation that reverses between the decelerating and accelerating sections, an accentuation of the effect of the non-uniform boundary on both velocity and Reynolds stress profiles near the wall, and flow convergence and divergence. Lateral convergence increases with Reynolds number and decreases with flow depth. These results show that a simple, two-dimensional morphology is sufficient to induce a number of hydrodynamic effects observed in field studies of macro-bedforms and suggests that these effects are components of a larger process of flow deceleration and acceleration.