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

Grégoire Jamet,Pedro A Manso,Giovanni De Cesare,José F Melo,António Muralha

doi:10.3390/w14020173

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.

Highlights

Scour due to high-velocity jets has been a phenomenon of considerable interest for engineers and researchers since the beginning of the 20th century, during which a major increase was observed in dams’ height
The results of this study showed an important decrease in the mean dynamic pressure between the sensor at r/D = 0.00 and sensors at r/D = 0.25 to 0.69, an important change in terms of mean, fluctuating pressures and spectra between sensors inside the jet diameter footprint r/D < 0.50 and outside r/D > 0.50
Impact pressures of plunging jets were investigated with velocities ranging from 5 to 18 m/s under an acquisition frequency of 2400 Hz

Summary

Introduction

Scour due to high-velocity jets has been a phenomenon of considerable interest for engineers and researchers since the beginning of the 20th century, during which a major increase was observed in dams’ height. Fédérale de Lausanne (EPFL) on rock scour and especially on near-prototype velocity circular jets of up to 30 m/s (Bollaert [2], Manso [3], Federspiel [4], Duarte [5]) This pioneer project aimed to assess pressure fluctuations in joints, the influence of scour hole geometry, 3D rock mass response and the influence of air entrainement. The present experimental study involves turbulent high-velocity circular jets, and allows for an exploration of the spatial distribution of dynamic pressures on a flat plunge pool bottom in terms of mean dynamic and fluctuating pressures, as well as power spectra recorded at a higher frequency than in the previous cited works.

Experimental Arrangement and Test Program

About Scale Effects

Impact Pressures at the Stagnation Zone—Results and Discussion

Findings

Conclusions