Author(s): Francisca Marco Cutillas; Pedro Manso; Giovanni De Cesare; Luis Gerardo Castillo Elsitdie

Linked Author(s): Francisca Marco Cutillas, Pedro Manso, Giovanni De Cesare

Keywords: Energy dissipation; Hydrodynamic pressure; Piezoresistive sensors; Plunge pool; Spillway

Abstract: Nappe flow impingement on downstream lined pools or rocky riverbeds constitutes one of the methods to dissipate the energy of excess water released during overflow spillway operation in concrete dams. The study of the hydrodynamic pressures at impact is paramount to assess the stability of concrete lined pools and the potential for rock scour, and ultimately assess the risk of dam instability during and after overflow spillway operation. A better knowledge of the pressures in the plunge pool is important for engineering practitioners, safety experts and dam operators as it may affect the safe operation of the hydraulic structures. This study presents an experimental work on dynamic pressures at the surface of a stilling basin that receives overflow rectangular impinging jets. The experiments were conducted in a fairly large-scale facility, designed for the analysis of turbulent jets and the energy dissipation in overflow structures. The vertical distance between the sharp-crested weir and the bottom of the plunge pool is 2.20 m. The study considered eight different flow rates with twelve different downstream water depths. These conditions allow to study developed jets (i.e. vertical distance H between the energy head over the weir crest and the jet entrance on plunge pool basin to the break-up length Lb ratio H/Lb > 1) and non-developed jets (H/Lb < 1.0) with effective water cushion depths (i.e. water depth Y at the end of the plunge pool to impingement jet thickness Bj ratio Y/Bj > 5.5) and non-effective water cushion depths (Y/Bj < 5.5). The study presents a range of specific flow rates from 0.024 m3/s/m to 0.141 m3/s/m, impingement velocities from 6.12 to 6.33 m/s, H/Lb values varying from 0.47 to 1.26 and Y/Bj from 1.17 to 51.48. Piezoresistive pressure transducers recorded instantaneous pressures created by the impact of high-velocity turbulent jets plunging in the plunge pools. The fourteen sensors used were located on the bottom surface of the flat plunge pool, longitudinally distributed along the symmetry plane of the turbulent jet device and equally spaced at 5 cm intervals. The sampling acquisition frequency was 20 Hz during 360 s, obtaining a total of 7200 data per measurement. The paper present new insights on the pressure distribution generated by overflow plunging jets, with various trajectories and stagnation points. Different longitudinal pressure statistics show how the interplay between the overflow rate and downstream tailwater levels leads to different flow features and dynamic load conditions at the pool surface. The outcome of this research aims at improving the design criteria for pool slabs and construction joints.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022178

Year: 2022