Experimental investigation of flow splitters' impact on the hydraulics of Piano Key Weirs

Piano Key Weirs (PKWs) have gained significant attention due to due to challenges associated with nappe oscillation in their flow dynamics. This experimental study explores the effectiveness of flow splitters—specifically circular, square, and rectangular designs—in enhancing the hydraulic performance of various PKW shapes: triangular, rectangular, and trapezoidal, under a range of hydraulic conditions, including both free and submerged flow. Experiments were conducted in a dedicated channel, 10 m long, 0.75 m wide, and 0.80 m high. The results indicate that under submerged flow conditions, discharge coefficients decreased by 61 % for rectangular, 59 % for triangular, and 55 % for trapezoidal PKWs compared to free flow, reflecting a reduction in efficiency. Notably, the trapezoidal PKWs maintained the highest discharge coefficient, improving efficiency by 2 % over triangular PKWs and by 12 % over rectangular PKWs. Flow splitters facilitate flow separation by linking entrapped air beneath the flow to the free surface, thereby mitigating nappe oscillation. In free flow conditions, rectangular and square splitters are more effective than circular ones for flow separation and energy dissipation, although geometric variations did not significantly influence the upstream water head in submerged flow scenarios. While flow splitters did not affect the discharge coefficient or efficiency of the various weir shapes under submerged conditions, they were found to decrease discharge by 10 % for triangular PKWs in free flow conditions. Overall, flow splitters demonstrated greater efficiency in submerged flow scenarios. In free flow, triangular PKWs were observed to dissipate approximately 4.4 % more energy than rectangular PKWs and 6 % more than trapezoidal shapes, with minimal differences in energy dissipation during submerged flow. This research also introduces a new equation for estimating the discharge coefficient in free flow, incorporating a correction factor yielding R² = 0.967, RMSE = 0.217, and MRPE = 5.94 %, expanding upon the work of Zarei et al. [19] for submerged flows. The study enhances understanding of hydraulic behaviors and discharge coefficients of PKWs equipped with flow splitters, contributing to improved aeration performance.