Author(s): Erik Bollaert; Anton Schleiss
Linked Author(s): Erik Bollaert, Anton J. Schleiss
Keywords: High velocity jets; Scour in fissured rock mass; Transientpressurized air-water flow; Air concentration; Pressure wave celerity; Damspillways
Abstract: High velocity plunging jets, appearing for example at the downstream end of spillways of dams, create local erosion of the rock bed. The appropriate prediction of these scours is important for the safety evaluation of the dam and its abutments. Traditionally, scour formation is estimated with empirical or semi-empirical formulae, mostly developed from physical models. Nevertheless, these approaches are often unreliable and not representative because neglecting some of the basic physics involved. The full understanding of the scour process and the interaction of the various parameters is still incomplete and a more reliable theoretical approach to the problem is actually missing. Above all, the relationship between dynamic water pressures in rock fissures and jet characteristics in the plunge pool is unknown for prototype jet velocities. Experimental pressure measurements in simulated rock fissures have been performed which revealed the appearance of violent transient flow phenomena inside the fissures, such as oscillations, resonance conditions, e.g. These phenomena propagate at specific celerities, depending on the air content of the two-phase air-water mixture inside the fissures. It was found that pressure fluctuations in rock fissures are highly dependent on the mean pressure value inside and on the air concentration of the two-phase mixture. Furthermore, the air content in the fissure can be related to jet and plunge pool characteristics. Numerical simulation of the unsteady two-phase flow conditions inside rock fissures will finally provide the framework for an improved scouring evaluation method.
Year: 2001