Plunging Circular Jets: Experimental Characterization of Dynamic Pressures near the Stagnation Zone

Abstract

Spillways are a requirement for dams’ safety, mainly preventing overtopping during floods. A common spillway solution involves plunging jets, which dissipate a considerable flow energy in the plunge pool. Energy dissipation has to occur in a controlled manner to avoid endangering the dam foundation and river slopes. Indeed, a scouring process in the downstream riverbed will inevitably develop until equilibrium is reached, otherwise a suitable pre-excavated or concrete lined plunge pool has to be provided. This paper focuses on experimental studies in which particular attention was paid to the dynamic pressures in the plunge pool floor at the vicinity of the jet stagnation zone sampled at 2.4 kHz. A rectangular experimental facility, 4.00 m long and 2.65 m wide, was used as plunge pool. Tests involved a vertical circular plunging jet with velocity ranging from 5 to 18 m/s and plunge pool depth ranging from 4.2 to 12.5 jet diameters. Differences in dynamic pressure measurements are highlighted between transducers located in the inner and outer regions of the jet diameter footprint. Several parameters characterizing the dynamic pressures evidence trends tied with the jet velocity that, to the authors’ knowledge, were not dealt in previous research. These can derive from the coupling effects of consequent recirculating motions and air entrainment in the limited-size plunge pool. Both effects, increasing with velocity, cause an reduction in the efficiency of the diffusing jet shear layer. This aspect deserves further investigation to achieve a better understanding and more complete characterization.

Analysis of the actions for the design of energy dissipation basins in the overflow of concrete dams

“…Por otro lado, en la bibliografía existen también trabajos exhaustivos de registros de presiones en chorros circulares, obtenidos en modelos con elevadas velocidades e intensidades de turbulencia de hasta 8% (Ervine et al, 1997, Manso, 2006, Manso et al, 2008, Federspiel, 2011, Duarte, 2014, Jamet et al, 2022. Sin embargo, el rango de ensayos en general se corresponde con relaciones de H/Lb < 0.50.…”

“…El punto de inicio de la zona de disipación, que conduce a la cascada de energía (donde se distribuye la energía de los remolinos a gran escala a los pequeños vórtices con frecuencias más altas), se da para una frecuencia de 10 Hz para colchones efectivos (grandes calados) y 20 Hz para colchones no efectivos (pequeños calados). Jamet et al (2022) observó que, para chorros circulares, este punto de transición estaba influenciado por la velocidad y se desplazaba de aproximadamente 15 Hz (V = 5 m/s) a 100 Hz (V = 18 m/s) para pequeñas profundidades (Y/D = 4.2) y de 3 Hz (V = 5 m/s) a 25 Hz (V = 18 m/s) para grandes profundidades (Y/D = 12.5).…”