Flow-Induced Vibrations of Fibre-Reinforced Polymer Hydraulic Gates

Abstract

Hydraulic structures that are surrounded by flowing water can experience undesirable Flow-Induced Vibrations. These vibrations have a record of negative consequences. The most important cause of vibrations is self-excitation. This is a mechanism in which the forces acting on the hydraulic gate are amplified by the movement of the hydraulic gate itself which then further increases the vibration intensity. This mechanism potentially results in structural failure of hydraulic gates. Therefore, prevention of this type of excitations should be priority number one in the design stage of hydraulic gates One of the vertical self-excitation mechanisms is the galloping-type vibration and is addressed in this thesis. In this research the possible occurrence of galloping-type vibrations of a dynamic one degree of freedom system is simulated.

CFD computations were used to obtain a force matrix that served as an input for the simulation of vertical vibrations of the one degree of freedom system. This research has shown that the force matrix, obtained using CFD software, can used to derive the hydrodynamic damping coefficients due to flow and hydrodynamic stiffness coefficients due to flow and buoyancy for a range of combinations of opening heights and accompanying vertical velocities of the hydraulic gate.

The damping coefficient that was required to obtain a fully stable situation for all initial opening heights complied with the results found by determining the negative hydrodynamic damping coefficients due to flow. Therefore, this research has shown that the stability to galloping-type vibrations and the required external damping coefficient can be derived from the force matrix obtained using CFD.

The research performed has shown that the hydraulic gate, designed as an FRP laminate experienced roughly twice as high negative hydrodynamic damping values than a typical steel hydraulic gate consisting of a plate stiffened with ribs. This means that the FRP laminate requires twice as much external damping to neutralise the significantly higher negative hydrodynamic damping due to flow. The steel hydraulic gate experiences a larger range of negative hydrodynamic damping albeit with much lower negative hydrodynamic damping coefficients than the FRP laminate hydraulic gate.

Concluding this research, results show that FRP hydraulic gates are more susceptible to galloping-type vibrations than traditional steel hydraulic gates, meaning that either the design of the hydraulic gate has to be altered or much more external damping, in this case twice as much, is required to compensate for the negative hydrodynamic damping due to flow, with respect to steel hydraulic gates.