Damping Estimation Methods Applied to in Service Measurements of a 350 Metre Container Ship

Large modern container ships feature an open cross section which results in a low stiffness of the vessel to global hull excitation. The contribution of whipping is a significant portion of the overall acceleration and stress response of such a vessel. Whipping results in an increase of the maximum hull girder bending moment experienced by the vessel as well as an increase in the fatigue accumulation of critical details. In this research, we present damping estimates based on in-service measurements from a container ship. An array of accelerometers was used to derive the first three vertical and first twist vibration modes. The flexural vibrations were isolated using two different methods: first, the Enhanced Frequency Domain Decomposition and secondly, the time domain Stochastic Subspace Identification. Both methods were applied to four representative voyages of the vessel covering a variety of environmental and loading conditions. On the two- and three-node vertical vibration modes, consistent results between both methods have been obtained. Both the four-node vertical vibration mode and the twist mode showed larger deviations between the two methods. On the vertical vibration modes, the damping is between 0.5 and 2%. On the twist modes, the damping is significantly larger at 5% and up. A correlation between the combined damping and the significant wave height was observed for the different flexural modes.

Synchronization of hydrodynamic and orientational modes during electroconvection in a nematic liquid crystal

Spatiotemporal dynamics of oscillating electroconvective structures in a nematic liquid crystal during the combined action of AC and DC electric voltages were experimentally studied. It was found that the increase of DC component of the applied voltage leads to the synchronization of the hydrodynamic modes with the orientational twist mode. The results indicate the flexoelectric mechanism of synchronization.

Electrode Size Effect on the Resonance Spectrum of PZT Piezoceramic Resonator

The resonance spectrums of disc type piezoceramic resonators with different electrode sizes were measured in the frequency range from 100 kHz to 1 MHz. Besides the fundamental mode, some overtone and other vibration modes existed in this frequency range, depending on the electrode size. The series resonance frequency of fundamental modes of the resonators increased with the reduction of the electrode size. When the electrode size was decreased down to 80% of size, some small spurious signals will exist; these spurious signals may be caused by another vibration mode such as twist mode or bending mode. The planar electromechanical coupling factor increased first with the decreasing of the electrode size, and reached a maximum when the diameter of electrode was decreased down to 80% of size, and then decreased with the decreasing of the electrode size.