Author(s): Alex Bey; M.A.A. Faruque; Ram Balachandar
Keywords: Asymptotic state; channel width; cohesionless soil; plane jet; scour; submergence; tailwater ratio; turbulence; velocity-time history
Abstract: An experimental program was carried out to understand the role of channel width and varying tailwater depth on the scour caused by a plane wall jet. To this end, four groups of tests, denoted as A, B, C and D, were conducted. In group A, during each test the tailwater depth was gradually increased from 2bo to 20bo in a systematic manner. Here, bo is the nozzle height at the exit. In group B, the tailwater depth was gradually decreased from 20bo to 2bo. Group C tests were conducted at fixed tailwater depths based on inferences made from groups A and B tests but correspond to low tailwater conditions. Group D tests were conducted at fixed tailwater conditions corresponding to a large submergence.Within each group, the jet exit velocity was varied from 0.75 to 1.16 m while the channel width was varied from 100 to 400 mm. A two-dimensional laser Doppler anemometer was used to characterize the velocity field. Present observations indicate that the velocity-time history can be used to characterize various regimes of flow. Distinct velocity signature patterns have been identified for the digging and refilling phases. These patterns also serve to distinguish between high and low submergence flow. The complete absence of the refilling phase indicates high submergence flow. Most previous studies made the distinction between high and low submergence based on the tailwater ratio. However, the present results show that the distinction also depends on the jet exit velocity. Though the channel width does not have a significant effect on distinguishing the flow as high or low submergence, the scour pattern is affected by the channel width. Narrower channels tend to provide for a two-dimensional (2D) scour pattern in a nominally 2D flow, whereas, wider channels provide for a three-dimensional mound region. These aspects should be borne in mind while conducting future scour experiments and in developing empirical equations that are commonly used to describe scour geometry.