PERFORMANCE OF PIANO KEY WEIR WITH CURVILINEAR KEYS

Abstract

A substantial number of dams around the globe are considerably old and require restoration of their lost storage capacity due to siltation. Also, studies on impact of climate change and availability of greater data for statistics around the world suggest for re evaluation of the maximal flood discharges thus leading to the rehabilitation of existing dams. This strong motivation to maximize the discharging capacity of spillways have resulted in development of various non-linear crest geometries. The Piano Key Weir (PKW) is a modified labyrinth-type weir that makes use of inclined apexes in order to maximize the allowable weir length that can fit in a given channel width, while simultaneously shrinking the required structural footprint. Owing to these cantilevered apexes, they are more practical solutions than a standard labyrinth weir for placement on a dam crest. This innovative weir has emerged as a cost effective and viable solution for dam rehabilitation and new projects with high level of space constraints. The geometrical dimensions of PKW are main factors affecting their discharging capacity. Since, it is a new type of weir that has only been developed in recent years, the description of the effect of its geometrical parameters is still immature. The geometric designs of these weirs presented in the literatures incorporate only linear slope of the inlet and outlet keys. To better understand the influence of curvilinear shapes of outlet key slopes on discharge capacity, one model with linear slope and three models with curvilinear slope were tested through experimentations and numerical simulations. All tested models were based on Type-A PKW with three cycles. The curvilinear profiles were derived from the standard equation of downstream ogee shape for different design heads. This shape was selected as it is known for its ability to pass flows efficiently and safely. The laboratory scaled weir models were installed in the midway of a sufficiently long rectangular flume consisting of glass walls. The discharge was measured using ultrasonic flow meter and the pointer gauge system was used to measure the upstream water level for varying flows. Ansys Fluent was used to perform numerical analyses and the results were validated using the data from the experimental model tests. The results from numerical simulations showed a good agreement with experimental head discharge data. The mean relative deviation between observed and simulated discharge values for all PKW models were less that 8.45%. In general, the experimental results showed that PKWs have high discharge efficiency at low hydraulic heads. The efficiency of these weirs decreases substantially with the increase in head due to interference jets flowing into the outlet key. The PKW models with ogee shaped outlet key are more efficient than the linear shaped counterpart for heads higher than its design head. The separation of flow near the outlet crest of model with linear slope leads to formation of vortices and lower velocity in this region. On the other hand, ogee shapes allowed for relatively smooth, homogenous and rapid flow near the outlet crest region. Thus, the limit of nappe interference in case of PKW models with curvilinear slopes moves less towards downstream than model with linear slope for high heads.