Wet Damping Estimation of the Scaled Segmented Hull Model Using the Random Decrement Technique

摘要

Damping is one of the most uncertain factors in structural dynamicrnproblems, and plays a very important role in resonance phenomena suchrnas springing. This paper presents the wet damping estimation of arnsegmented hull model using a random decrement technique togetherrnwith continuous wavelet transform. The 16 sea states were groupedrntogether based on the speed of the ship to determine the possiblerninfluence of the ship speed on the damping ratio. The measured timernhistories of the vertical bending moment for each tested sea state werernprocessed using the random decrement technique to derive the freerndecay signal, from which the damping ratios were estimated. In addition,rnthe autocorrelation functions of the filtered signal were calculated and arncomparison was made with the free decay signal obtained from thernrandom decrement technique. The wet damping ratios for each sea staterngroup, as well as precise wet natural frequencies, were estimated using arncontinuous wavelet transform. The wet natural frequencies derived fromrnthe measured signal did not show any significant discrepancy comparedrnto those obtained using the wet hammering test, whereas a significantrndiscrepancy was observed with the damping ratio. The discrepancy ofrnthe damping ratio between in calm and moving water might be due tornthe viscous effects caused by the dramatically different flow patternsrnand relative velocity between the vibrating structure and surroundingrnfluid particles.