On increasing the scour funnel size upstream of a bottom orifice by using a cuboid pile

Scour funnels formed before bottom orifices are of vital importance regarding intaking water with low-concentration and small-sized sediment to protect the water turbine. Via flume experiments, this work investigates the effect of a bottom-mounted cuboid pile on the formation of a scour funnel upstream of a bottom orifice. A phenomenon we call as “reverse sediment collapse” is observed for the first time, which causes a rapid increase in the scour depth upstream of the orifice. Instantaneous velocity measurements show that the turbulence kinetic energy near the cuboid pile is significantly higher compared with that in a case without a pile or with a cylinder under a same waterhead condition, indicating that large-scale vortex motions are enhanced due to the use of a cuboid pile. The equilibrium scour funnel depth upstream of the orifice is found to be increased by as much as 124.6% when a cuboid pile exists compared with that in a case without a pile under a same waterhead condition. The scour process is highly accelerated when a cuboid pile exists, and it is found that the time required when a cuboid pile exists to reach for the first time the same equilibrium scour depth without a pile is dramatically reduced by 97.2%–99.7%. A formula was established to calculate the equilibrium scour depth. The findings provide guidance for future optimization of the form and placement of structures upstream of an orifice to efficiently achieve a larger-sized scour funnel.