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Experimental Analysis of Overflow Free-Falling Jets

Author(s): Patricio Ortega Lara; Jose Maria Carrillo Sanchez; Luis Gerardo Castillo Elsitdie; Juan Tomas Garcia Bermejo

Linked Author(s): Patricio Ortega Lara, Juan Tomás García Bermejo

Keywords: Ir-water flow; Free-falling jet; Air entrainment; Detection probe; Rectangular jets

Abstract: Hydraulic structures must be designed to operate in safety conditions and to avoid any damage of the hydraulic structure in different loading scenarios. During extreme events, several dams in service may overflow due to the higher design flows of the new design recommendations, so creating new scenarios of operation in the spillways of large dams. The knowledge of air-water flow characteristics in free-falling jets is essential to analyze the energy dissipation in plunge pools and dam’s stability. In turbulent water jets, the air entrainment processes generate energy dissipation during the fall. The turbulence effect around the free-surface jet is the principal mechanism of air entrainment and self-aeration. Turbulence fluctuations change the original flow properties. Furthermore, self-aeration processes in rectangular jets reduce the falling velocity, increment the jet spread, and causes undulations and instabilities in the free-surface. Air-water interface and turbulent shear stress affect the nappe during the falling jet; being the behavior slightly different between the upper and lower nappe of the jet. To date, information regarding the air entrainment processes in rectangular free-falling jets and the analysis of the variation of the solid inner jet core is scarce. This study analyzes the air-water properties and its interactions in rectangular free-falling jets measured in a fairly large-scale experimental device. Three different specific flows were considered (0.034, 0.072 and 0.096 m³/s/m). Air-water flow properties were measured with a phase-detection probe in different sections from the sharp-crested weir, with falling distances up to 2.0 m. Each measurement was sampled at 20 kHz for 45 s. For measuring the velocities profile, a back-flushing Pitot-Prandtl probe was used in the same locations. Several hydrodynamics variables were analyzed: phase-change count rate, mean velocity, bubble chord length and Sauter mean diameter. The jet thickness and the angle spread with the 50% void fraction were also analyzed during the fall. The results showed differences between the behavior the upper and lower nappe. This modifies the cross-section profile. There is a reduction of the inner jet core an increment in the aeration during the fall. The jet thickness considering the 50% void fraction seems be similar to the jet thickness calculated by only gravitational effect. The new measurements will allow to improve the knowledge of the air-water properties in rectangular free-falling jets.


Year: 2022

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