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The Influence of Added Mass on Rock Block Uplift in Plunge Pools

Author(s): Erik F. R. Bollaert; Matteo P. E. A. Federspiel; Anton J. Schleiss

Linked Author(s): Erik Bollaert, Anton J. Schleiss

Keywords: Dded mass; Rock block uplift; Vibration; Stiffness; Damping; Pressure fluctuations

Abstract: This paper presents the influence of added mass, stiffness and damping on vibrational movements of rock blocks in plunge pools impacted by high-velocity jets. Turbulent flows impacting a pool generate severe pressure fluctuations at the water-rock interface. These pressures are transmitted through the joints of the rock foundation. Hence, during significant pressure differentials over and under a single rock block, the block may start vibrating or even be ejected from the surrounding mass. Large-scale laboratory measurements performed at the Laboratory of Hydraulic Constructions of the Swiss Federal Institute of Technology in Lausanne, Switzerland, allowed simultaneous measurements of pressure fluctuations and detailed vibrational movements and displacements of an artificialrock block. The 3D cubical shaped steel-made laboratory block has a side length of 200 mm and contains a large number of pressure transducers and displacement sensors. The circular shaped laboratory jet has an outlet diameter of 72 mm generating impact velocities of up to 30 m/s, i. e. at near-prototype scale. Turbulent pressure fluctuations and rock block displacements have been recorded simultaneously and at high frequencies. Sound comparison of these measurements with the equation of motion for a one-degree of freedom, spring-supported and damped rigid body vibrating in a still fluid, allow detailed assessment of the block movements. As such, for a wide range of jet impact velocities, plunge pool depths and jet impact positions, the added mass of the fluid during block movement as well as the stiffness and damping generated by the fluid arebeing determined. The paper presents a detailed comparison of measured and computed block vibrational movements for a single test run involving a jet impacting one of the vertical sides of the block. Sound calibration of the block vibration equation needed the introductionof significant added mass and viscous damping forces. Also, stiffness of the system was adapted to correctly reproduce the natural frequency of the block. As a conclusion, for one single test case, vibrational movements of a rock block impacted by a high-velocity turbulent flow have been assessed and described by a soundmathematical equation, including effects of added mass, stiffness and viscous fluid damping.


Year: 2013

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