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One-Degree-of-Freedom Rigid Body Dynamics of Rock Blocks in Turbulent Flows

Author(s): Erik F. R. Bollaert

Linked Author(s): Erik Bollaert

Keywords: High-velocity turbulent flows; Vibrations; Added mass; Block uplift computations

Abstract: This paper presents an overview of aone-degree-of-freedom dynamic behavior typical for rock blocks in turbulent flows. Turbulent flows such as jets impacting a plunge pool or plunging river flows around bridge piers may generate severe dynamic pressures that may be transmitted through the joints separatingthe blocksfrom their foundations. In here, the main solicitations of turbulent jet flow impinging a block are outlined, from time-averaged pressures to high-frequency pressure fluctuations. Furthermore, the related potential movements of the block are being assessed. The considered movements are based on the principle of single degree-of-freedom vertical displacements. These movements are strongly affected by the time duration of the positive or negative pressure differences. As such, for very short-lived pressure differences, the impulsion given to the block will remain moderate and so will be the vertical acceleration and final displacement. For longer time durations, related impulsions become increasingly important. It is shown that, for a block that is free to move following a net uplift impulsion, the final displacement may be roughly estimated based on a transformation of the kinetic energy of the final impulse velocity into an equivalent uplift height. Nevertheless, positive (uplift) pressure differences may be affected by the presence of other vertical forces. For example, an uplift pressure difference on a rock block may be followed by a negative (downwards oriented) pressure difference, or may be influenced by suction of damping effects during block movements. In such cases, the upwards movement is not only influenced by inertia but also by an additional (counteracting) downward force, attempting to get the block back into its initial position. This results in block vibrations, governed by the added mass, stiffness and damping of the surrounding fluid. The paper compares different computational methods to determine rock block uplift for plunge pool scour computations. These methods have mainly been developed based on large-scale laboratory measurements of net uplift forcesand block displacements generated by high-velocity turbulent flows impacting artificial rock blocks. The practical applicability of the different methodsisbriefly pointed out.


Year: 2013

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