The influence of projection angle on nappe stability at low energy heads over the weir crest

Abstract

Under certain low head hydraulic conditions, weir discharge under gravity flow can produce an oscillating nappe characterized by horizontal waves and audible low-frequency acoustic energy, which in some cases can have a significant impact on the local environment. In an effort to better understand the mechanism behind nappe oscillation, an experimental investigation was conducted using a linear weir (1.83 m wide, 1.10 m in height, and 0.076 m in thickness) with four different crest shapes (rectangular, quarter-round upstream, half-round, and quarter-round downstream) at a wide range of unit discharge ranging from 8×10−3 m2/s to 36×10−3 m2/s. Using Particle Image Velocimetry (PIV) techniques, differential pressure measurements, and water surface profiles, the influence of Weber number, angle at which the nappe hits the downstream pool (angle of impact), and nappe projection angle on the nappe oscillation are reported. No significant relationship was found between the Weber number and nappe instability. The results suggest that the nappe is stable for unit discharge between 20×10−3 and 24×10−3m2/s regardless the shape of the weir crest. Also, the stable projection angle is −17° for rectangular and quarter-round upstream weirs and −35° for half-round and quarter-round downstream weirs for the specific tested weir. HIGHLIGHTS Explain the mechanism behind the nappe oscillation precisely.; Discover different types of nappe oscillation.; Propose some mitigation solutions for nappe oscillation.; Help designers choose the appropriate crest shape for nappe stability.; Provide a solution for the existing dams that suffer from nappe oscillation.;

Keywords

• nappe
• particle image velocimetry (piv)
• pressure
• unaerated
• vibration
• weber number