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.
