A Study of Nappe Oscillations and Effects of Aeration on Environmental Noise of Piano Key Weirs with Different Shapes

Under certain conditions, free-falling jet downstream side of weirs, known as “nappe,” oscillates. In addition to vibration of weir’s structure, nappe oscillation may produce excessive acoustic energies (noise) that can disturb local residents and dam operators. Despite a plethora of studies on nappe oscillation, researchers have yet to determine the exact cause of nappe oscillation and factors affecting it, especially in Piano Key Weir (PKW). To date, a few studies have addressed the sound level produced by flow over PKW. In this study, the behavior of oscillating nappe is studied, using a Computational Fluid Dynamics tool and the underlying cause of nappe oscillation is identified based on numerical results. In addition, the environmental noise of rectangular and trapezoidal PKWs are investigated. As one way to reduce the noise is nappe aeration, effect of aeration on this sound level is assessed. The results show that the generated sounds do not cause hearing damage. However, an optimum aerator area in PKW is proposed to achieve the lowest noise. Rectangular and trapezoidal PKW’s responses to the same aeration ratio are compared. Finally, pressurized aeration is studied and necessity of aerator system in PKW is discussed according to the environmental health criteria.