Velocity measurements in a developed open channel flow in the presence of an upstream perturbation

The effect of a near-free surface disturbance imposed on an open channel flow in the presence of low and high background turbulence is studied using a two-component laser Doppler anemometer. A bluff body located near the entrance of the flume was used to generate the free surface disturbance and the velocity characteristics were measured at a downstream location where the flow is fully developed. Experiments were conducted in two sets of flow fields. In the first set, the basic condition was an undisturbed open channel flow on a smooth bed, which is then compared with a flow with free surface disturbance. In this set, the turbulence levels are relatively lower and comparable with typical laboratory flume conditions. In the second set, the basic condition is a flow over a train of fixed two-dimensional bed forms, which is then compared with a flow with both bed forms and free surface disturbance. In the presence of the bed forms, there is continuous generation and sustenance of turbulence resulting in significantly higher background turbulence levels. The turbulence levels in the second set are comparable to natural open channel flows. Within the first set, though the mean profiles in inner coordinates are nearly identical, differences can be noticed in the mean defect profiles, while the Coles' parameter and the normalized turbulence levels are reduced in the presence of the free surface disturbance. Furthermore, quadrant decomposition indicates that in the presence of the free surface disturbance, three of the four quadrants are significantly altered. In the second set, the increased background turbulence due to the existing shear layer dynamics has an overwhelming influence on the flow characteristics while the free surface disturbance has a secondary role.