Frequencies of Nappe Vibration for Free-overfall Structures

Under relatively low-head discharges, the occurrence of nappe oscillation, otherwise known as nappe vibration, may be observed on hydraulic structures with a free overfall, such as weirs, crest gates and fountains. This phenomenon, which has been early identified as undesirable and potentially dangerous on gates, is characterized by oscillations in the thin flow nappe cascading downstream of the crest. In addition, these oscillations produce a significant level of noise and acoustic pressure waves that increase the environmental and societal impacts of the structure. A review of the scientific literature shows a lack of consensus regarding the causes and source of the oscillations development. In this context of relatively poor understanding of the dominant processes, a detailed investigation has been undertaken to identify and quantify the nappe vibration mechanism. The research is being performed with a prototype-scale linear weir located at the Engineering Hydraulics laboratory of the University of Liège. The study employs high-speed cameras and audio equipment to characterize the nappe vibration. This paper presents first characteristics of the nappe vibrations gained from images and sound analysis, especially in terms of vibration frequency, for a quarter round and a half round weir crest. This study shows that frequencies measured by sound and image analyses are identical and depend on the crest shape, the fall height and the unit discharge.