10 – Experimental measurement and time series acquisition

This chapter explains the recovery of a particular time series and its analysis in the context of observing a moored model structure excited by waves in a tank. The experiment demonstrates that the important point about the method of analysis, whether the analysis is related to the recovery of the desired signal or the postprocessing of the sought signal, is an integral part of the design of the experiment. The motions in each degree of freedom were predefined and determined theoretically the resultant motion at the would-be locations of the LEDs on the vessel. Hence, the motions could be unscrambled and compared with the known constituent motions with the LED-related time series, which are generated theoretically. Comparison of predicted and measured wave damping in this case was difficult to make and so a new theoretical method was devised to account the finiteness of the tank and shallowness of the water depth. The in-built redundancy and the careful production and examination of the mooring lines allow confidence levels in the measurement technique. The lack of confidence in the quality of the waves therefore, puts comparison of theory and measurement into perspective.