Investigation of weir nappe vibration: causes and countermeasures

Nappes, the free-falling jet on the downstream side of a weir, are known to oscillate under certain hydraulic conditions. Characteristics of this dynamic behavior include excessive acoustic energy (sound pressure waves) and horizontal waves in the nappe. The resultant acoustic energy can reach nuisance levels locally. The instability of water jets moving through air has been a topic of study for over a century. This study, however, focuses specifically on instabilities in curvilinear weir nappe flow, including causes and mitigation techniques
Research was conducted at the Utah Water Research Laboratory (UWRL) using a large physical model (15.3-ft wide and 11-ft tall). While nappe oscillation most commonly occurs under confined or suppressed nappe conditions (enclosed air pocket behind the nappe), this study investigates the occurrence of nappe oscillation under unconfined conditions. Oscillation frequencies were recorded for a variety of weir configurations and analyzed using a microphone and audio recording software.