CFD-Based Comparative Analysis of Conventional and Modified Piano Key Weirs for Improved Discharge Efficiency

The Piano Key Weir (PKW) has earned recognition for its adaptability for large discharges across weir types of varying heights and with small footprints. Therefore, it has the potential to be a substitute for linear weirs (space being a factor), Ouamane and Lempérière, (2006). Even with the above-mentioned advantages of PKWs, other geometries leave much to be desired. The rectangular PKW and the trapezoidal PKW illustrate a most common inefficiency example. Standard literature describes construction and operational shortfalls such as flowing separation at the inlet key, varying discharge and uneven velocities along the crest, vortex shedding and formation at the key intersections, dead zones in the inlet-outlet, zones of intensified energy dissipation, and lowering weir versatility at high flows. These challenges are combined to mean loss of efficiency in weir discharge capability. In response to these challenges, the present study introduces the Modified Piano Key Weir (MPKW) to assess its performance using 3D computational hydraulic modeling. The Volume of Fluid (VOF) methodology for free surface tracking and the Reynolds-Averaged Navier Stokes (RANS) for turbulence closure modeling characterize pressure gradients, flow accelerations in the several dimensions, and eddies. A systematic numerical investigation was conducted to compare the discharge efficiency of RPKW, TPKW, and MPKW across a range of steady inflow discharges: 0.030, 0.060, 0.090, 0.120, and 0.160 m³·s⁻¹. The MPKW demonstrated consistently superior discharge efficiency over both RPKW and TPKW for all tested cases, without requiring an increase in structural footprint or crest length. The highest relative improvement was observed at 0.060 m³·s⁻¹, which was therefore selected as a representative discharge for in-depth flow diagnostics. Discharge at 0.060 m³·s⁻¹ was applied to determine vorticity structures, turbulent kinetic energy (TKE), and energy dissipation to better understand the flow mechanisms that explain the efficiency of the weir. The MPKW design, with refined geometry and improved inlet–outlet design, rounded key transitions, and adjustable wall skew, was successful in mitigating flow separation at the key inlets and reducing the large-scale vortex formation at the key junctions. The modified sidewall skewed the internal recirculation, and as a consequence, TKE in the stagnation zones was less, and recirculation was more along the crests of the weir, thereby nullifying turbulent structures. While the breakdown of turbulence resulted in localized energy dissipation, the stabilization of the approach flow was improved because the process converted rotational energy of large eddies with a low energy loss to rapidly decaying eddies which do not sustain and produce a recycling of energy. Thus, less energy was concentrated in the vortex cells at the key junctions, the loss due to flow contraction was less, and the nappe cohesion over the crests was improved. MPKW, relative to other configurations, was characterized by a lower level of turbulence and vorticity at the junctions, a greater effective utilization of the crest, and improved pressure recovery. The results confirm MPKW as a hydraulically efficient and economically feasible solution for both new installations and retrofit applications under head or footprint constraints.