Formation of a scour funnel upstream of an orifice affected by a bottom-mounted cylinder

Abstract

A scour funnel before a bottom orifice on a dam is able to help intaking water with low sediment concentration and thus protect the water turbine. Appropriate placement of pier-like structures in front of a bottom orifice may increase the size of a scour funnel. This work experimentally investigates the formation and development of a scour funnel upstream of a bottom orifice affected by a bottom-mounted cylinder. It is found that the cylinder blocks the flow so as to reduce the flow rate by as much as 8.27%. Nevertheless, the sizes of equilibrium scour funnel generally increase due to the use of a cylinder; for instance, the equilibrium scour depth may increase by as much as 36.1%. The enlargement of scour funnel is closely related to the motion of large-scale turbulent vortices caused by the cylinder. We found that the turbulent kinetic energy around the cylinder is significantly increased due to the presence of a cylinder with a maximum relative increase in 192.5%. The placement of a cylinder leads to a drastical increment of sediment erosion rate. For cases with a cylinder, it may take only 1.1% of the time (for the first time) to reach the same level of equilibrium scour depth in a same case without a cylinder. Therefore, by appropriately placing a cylinder before a bottom orifice, it is possible to achieve a larger-sized scour funnel in a very shorter time with smaller abandoned water. A formula is established to calculate the maximum equilibrium scour depth.