Flow-induced vibration of shell-type long-span gates

Abstract

This study describes the fundamental dynamic characteristics of the flow-induced vibrations of shell-type long-span gates, in which the upstream gate face consists of vertical and inclined skin plates (also referred to as weir plates). Shell-type gates possess two degrees-of-freedom, one each in the streamwise (horizontal) and vertical directions, due to the gate bending flexibility in these two directions. The streamwise and vertical vibrations can become closely coupled with each other through hydrodynamic forces acting on the skin plates, resulting in severe self-excited vibrations. A two-dimensional model study of a shell-type long-span gate under small gate opening was performed to measure the vibration frequency, the excitation ratio † † The excitation ratio is defined as the logarithmic increment divided by 2π. The logarithmic increment is defined for an exponentially growing signal in the same manner as the classical logarithmic decrement for an exponentially decaying signal. The logarithmic increment is equal to minus the logarithmic decrement and the excitation ratio is equal to minus the damping factor. By defining the excitation ratio in this manner, it is always a positive number and one avoids the confusion concerning whether an increase in the magnitude of negative damping is referred to as an increase in negative damping or a decrease in negative damping. and the trajectories of gate motion. The measured trajectories of gate motion revealed that the coupling of the vertical and streamwise vibrations made the gate behave as a press-shut device; press-shut devices are known to undergo self-excited vibrations. In addition, fundamental dynamic characteristics, such as the ratios of horizontal to vertical frequencies, the ratios of in-water to in-air frequency, a reduced fluid-excitation coefficient, a reduced added mass, and the phase difference between vertical and horizontal vibratory motions, were determined. The experimental results for the horizontal in-water to in-air vibration frequency ratio and the reduced mass are carefully compared with the theoretical results for a slender long-span gate undergoing streamwise vibration. The trajectories of gate motion are carefully analysed by introducing theoretical results for the hydrodynamic pressure acting on the gate. Finally, the possibility of flow-induced vibration of shell-type long-span gates in practice is examined and criteria for dynamic stability are suggested.