PIV measurements for oscillating liquid nappe

Under certain low-head conditions, weir flow can produce a vibrating or oscillatory nappe behavior, a condition referred to as nappe oscillation. An oscillating nappe describes a nappe featuring waves in the longitudinal cross section profile, which start small near the crest and grow in amplitude along the trajectory. Oscillating nappes can also produce significant sound energy. Previous studies have suggested that nappe instabilities or oscillations originate at the crest. In an effort to develop a better understanding of the flow characteristics near the crest and throughout the nappe for oscillating and non-oscillating flows, an experimental investigation using the Particle Image Velocimetry was performed. The study found that velocity field deviations between a non-oscillating and oscillating nappe vary with location along the nappe trajectory. No significant velocity profile differences were found near the weir crest. Reduced local nappe velocities in the horizontal direction and increased velocities in the vertical direction were found further downstream along the nappe trajectory.