Nappe Oscillations on Free-Overfall Structures: Experimental Analysis

Abstract

Under relatively low heads, the occurrence of nappe oscillation, also known as nappe vibration, may be observed on hydraulic structures with a free overfall, such as weirs, crest gates, and fountains. This phenomenon is characterized by oscillations of the thin flow nappe cascading downstream of the crest and results in a disturbing noise production that increases the environmental and societal impacts of the hydraulic structure. Given limited information available regarding the physical processes involved in this phenomenon, a detailed investigation has been undertaken to characterize the flow for free-overfall structures where nappe oscillation may be of concern. The research is conducted on a prototype-scale linear weir model (weir length of 3.5 m and a fall height of 3 m) using high-speed cameras and audio equipment to characterize the nappe oscillation. The paper presents the quantitative characteristics of the nappe oscillation gained from images and sound analysis, especially in terms of frequency, for aerated and confined nappe configurations.