Response of an embedded block impacted by high-velocity jets: Communication 47 (Laboratoire de constructions hydrauliques, Ecole polytechnique fédérale de Lausanne)
The safety of dams is very often endangered by the formation of scour near the foundation as a result of high-velocity jet impact. These jets issue from appurtenant structures such as spillways and may impact in the immediate vicinity of the dam toe. The process of rock scour is very complex and full physical understanding of the water-air-rock interaction is still lacking. With his PhD research Dr. Matteo Federspiel filled up a gap towards a better scientific understanding of the scouring process. For the first time the response of an artificial rock block under high-velocity plunging jets was studied in a systematic way by physical modelling and numerical analysis. By means of an experimental set-up producing near-prototype scaled jet velocities impacting on a highly instrumented artificial rock block embedded in a cylindrical plunge pool, the dynamic pressure field acting inside the fissures surrounding the block was recorded. Beside pressure fluctuations, both block accelerations and displacements could be measured independently at very high acquisition frequency. This allowed sound analysis of the response of an embedded rock block due to high velocity jet impact. Also, the influence of the degree of freedom of the block as well as the air entrainment on block displacements was studied for the first time. This unique and highly sophisticated experimental facility allowed systematic recording of the interaction of the artificial rock block with the high-velocity turbulent flow in the cylindrical plunge pool for several jet impact positions (symmetrical and asymmetrical) and jet characteristics (core, transition and developed jets). Furthermore, the water depth in the plunge pool and the jet velocity were varied. Finally, both active and passive additional air entrainment added to the jet before issuance have been studied. Dr. Matteo Federspiel presented the practical relevance of his experimental work by discussing the net force, net impulsion and uplift as well as the time evolution of the pressures acting on the block. A net block uplift coefficient could be defined which may be useful for real-life scour problems and predictions.