Local scour by submerged offset jets Available to Purchase

In this paper an effort is made to understand the scour caused by submerged offset jets impinging on a cohesionless sand bed. A square cross-section nozzle and a two-dimensional (2D) nozzle spanning the width of the flume were used to generate the jets. The 2D offset jet experiments were conducted at two values of the densimetric Froude number (F₀ = 5·5 and 10) and two receiving water depths corresponding to shallow and deep tailwater conditions. The square offset jet tests were conducted at F₀ = 10 and correspond to deep tailwater conditions. Velocity measurements were conducted using a laser Doppler anemometer. For both jet types, at a given densimetric Froude number, progressing from the start of the test towards asymptotic conditions, the geometric parameters used to describe scour were found to be sensitive to the offset distance. Offset refers to the vertical distance between the nozzle invert and the initial sand bed and an asymptotic state refers to the situation where no significant change in the scour geometry was noticed with increasing time. For the 2D zero-offset jet condition, a backward-facing step was formed at the invert of the nozzle and the general flow features resembled those of an offset jet. At the lower value of the relative tailwater depth, the flow and the corresponding scour pattern was different from that found at the deep tailwater condition. For the square jet, the shape of the scour hole and the corresponding mound were elongated longitudinally and decreased laterally as the offset distance was increased. The distance between the nozzle and the point of the beginning of the scour hole was found to increase with increasing offset distance. Turbulent bursts were noted to occur and to influence the scour process. By proper choice of scaling variables, the centreline scour profiles in the hole region were collapsed onto a single curve.