Vibration of Tethered Microstructure Profilers

Abstract

Although loosely tethered turbulence profilers have many advantages, they are prone to resonant vibrations at frequencies in the dissipation range when they are falling rapidly or when the tether is strummed. Using the Advanced Microstructure Profiler as a case study, we have found that the major vibration in the dissipation range corresponds to the natural frequency of the first bending mode of the pressure tube. This resonance is excited when the AMP falls faster than about 0.5 m s?1 and when the tether is drawn taut and strummed, either by rapid drift of the ship or by large current shear in the upper ocean. Knowing the source of the resonance, we demonstrate with a simple beam model that it can be moved to frequencies higher than the dissipation range by using a shorter, larger-diameter pressure tube. Accurate estimation of the natural frequency, however, requires a finite-element numerical model since the frequency is sensitive to joints and to the position of heavy objects, such as batteries and internal bulkheads. We verified our analyses by designing and testing a prototype hull whose vibration level at 60?70 Hz is 1/300 that of our operational vehicle.