In order to obtain the optimum condition for designing a vibration absorber used in water, a two-degree-of-freedom system composed of a main vibrating-system and a vibration absorber in water has been considered as a first step. Then, the system has been assumed to vibrate vertically due to the forced displacement applied at its top, and the optimum condition for the absorber has been determined so as to minimize the amplitude of the main system. The result indicates that there are six design-parameters affecting the optimum conditions of the absorber in water instead of four parameters in air. Among them, the parameters inherent in the absorber in water are the dimensionless hydrodynamic-damping coefficient for the main system and cross-sectional area ratio of the absorber's to the main system's. Moreover, the optimum conditions for designing absorbers in water are found for various cases by equalizing the double peak amplitudes of each response-curve. Finally. the performance of absorber in water is found to be more affected by the mass ratio and the above-mentioned dimensionless hydrodynamic-damping coefficient than the others.
With progress of the ocean development, controls of vibrating bodies in water have been required in many cases, and a vibration absorber could be considered as one of the control systems. Up to the present, the optimum condition for designing a vibration absorber used in air has been extensively studied by many researchers (for example, Den Hartog, 1947; Reed, 1961) and now it is well established. However, the condition for an absorber used in water has not been found yet in any publications. Using an absorber, the authors (Aso et aI., I 992, 1994a, 1994b, 1995) have studied the longitudinal vibration-control of the pipe string for mining manganese nodules from deep sea floors and have been feeling the necessity of fundamental studies on that condition.
