Effects of spoiler and piggyback on local scour under single and twin submerged pipes

Abstract

A series of laboratory experiments was carried out to investigate the effects of spoiler and piggyback on the flow field and local scour formation under single and twin submerged pipes. The shape and orientation of spoiler and piggyback were tested in single and twin pipes with different distances from each other. Three piggyback shapes of rectangular, triangular, and circular were designed and the local scour profiles were measured for five orientation angles of θ = 90, 135, 180, 225, and 270°. Flow visualization was employed to study the effects of spoiler and piggyback on the rear vortex field. The velocity field was measured with an Acoustic Doppler Velocimeter (ADV) probe and the magnitudes of Turbulent Kinetic Energy (TKE) around pipelines were extracted from the time series of velocity data. Quadrant analysis was performed based on velocity data to study the effect of spoilers on turbulent momentum transfer between twin pipes. The energy and momentum coefficients were extracted from velocity profiles indicating significant disruption of velocity field and causing additional drag in the rear pipe. A comparison of the equilibrium bed profiles indicated that the minimum and maximum peak scour depths occurred when a rectangular spoiler with θ = 180° and a circular piggyback with θ = 270° were installed, respectively. The self-burial mechanism was examined by alternating the positions of piggybacks and spoilers in twin pipes. The downstream pipe was buried when either spoiler or piggyback is in the upstream pipe. It was found that the rate of self-burial reduced as the distance between two pipes increased.