Distribution of mean flow and turbulence statistics in plunge pools

Abstract

When the capacity of the spillway of a dam is exceeded for a given flood, overtopping occurs; in such cases potentially dangerous hydrodynamic actions and scour downstream of the dam need to be foreseen. Detailed studies of jets impinging in plunge pools from overflow nappe flows are scarce. This work addresses plunge pool flows, and compares numerical results against our own experiments. The energy dissipation is larger than 75% of the impingement jet energy. Instantaneous velocities and air entrainment were obtained with the use of an Acoustic Doppler Velocimeter and optical fibre probe, respectively. Mean velocity field and turbulence kinetic energy profiles were determined. To identify the level of reliability of models, numerical simulations were carried out by using the 'homogeneous' model of ANSYS CFX, together with different turbulence closures. The numerical results fall fairly close to the values measured in the laboratory, and with expressions for submerged hydraulic jumps and horizontal wall jets. The observations can be well predicted for characterized profiles at a minimum distance of 0.40 m downstream from the stagnation point, horizontal velocities greater than 40% of the maximum velocity in each profile, and when the ratio of the water cushion depth to the jet thickness is lower than 20.