Experimental and numerical analysis of velocities in plunge pools of free falling spillways

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Luis G. Castillo, José M. Carrillo

Monday 29 june 2015

16:00 - 16:15h at Africa (level 0)

Themes: (T) Water engineering, (ST) Experimental facilities and instrumentation

Parallel session: 3F. Engineering instrumentation

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The increasing magnitude of design floods has prompted reevaluations of spillway capacity for large dams throughout the world. Current capacity of many spillways is inadequate. Creating additional spillway capacity is often expensive and sometimes technically infeasible, and in these cases, dam owners sometimes consider accepting overtopping as a planned operation during extreme events. This creates new loading scenarios for the dam and raises questions about erosion and scour downstream from the dam. Rectangular jet or nappe flow constitutes one of the energy dissipation methods in the overtopping of dams. The pressure fluctuations are the main mechanism in the incipient movement of the particles. In order to obtain the right pool depth, the designer needs to know the magnitude, frequency and extent of the dynamic pressures on the pool floor as a function of the jet characteristics. The hydraulics laboratory at the Universidad Politécnica de Cartagena in Spain has a turbulent jet experimental facility in which the energy dissipation of turbulent rectangular jets is being studied. The mobile mechanism allows researchers to vary the discharge heights between 1.70 and 4.00 m and flows from 10 to 150 l/s. The plunge pool, in which different water cushions may be regulated, is a 1.60 m high and 1.05 m wide box made of methacrylate. To advance the understanding of the phenomenon of free falling jets, instantaneous pressure measurements were registered with piezoresistive transducers located on the plunge pool bottom, kinetic energy at the inlet channel and instantaneous velocity measurements in the plunge pool with Acoustic Doppler Velocimeter (ADV) equipment, mean velocities and air entrainment rate in different sections of the falling jet with optical fibre instrumentation. At the same time, transient numerical simulations are being carried out with computational fluid dynamics programs. Two-equation turbulence models have been selected to solve the closure problem of the Reynolds-Averaged Navier-Stokes equations. This work analyzes and compares the laboratory data of velocities in the plunge pool with the numerical simulations carried out with ANSYS CFX.