Evaluation of turbulence-related high-frequency tidal current velocity fluctuation

Abstract

Within the development needed for economy viability of tidal stream energy, adaptability of laboratory converters to sea flow conditions is a milestone. The objective of this work is to investigate the high frequency fluctuations in current velocity magnitude and direction related to the turbulent nature of the flow and present a new method for their prediction. With this purpose, high frequency data measured by two ADV (32 Hz) and two ADCP (8 Hz) at four different points in the sea area surrounding Goto Islands (Japan) are analyzed. The data were divided in short-time samples (3-min data for ADV and 5-min data for ADCP) and treated separately. Velocity magnitude fits a normal distribution, with prediction levels higher than 95% for a margin of error of 0.25 m/s when comparing different percentiles between 0.1 and 99.9. Flow direction is analyzed in terms of opening angle between two representative percentiles equidistant from the median (99.9-0.1, 95-5, …), giving as a result a leptokurtic distribution, more outlier-prone than normal. Empirically, for opening angles 99.9-0.1, 97.7-2.3 and 95-5, slopes of 6.79 (6 in normal distribution), 4.17 (4) and 3.38 (3.29) were found, with results similar to a theoretical normal distribution for narrower angles. The new prediction method for high frequency fluctuations is based in this direct correlation between velocity magnitude and direction fluctuations with turbulence intensity and transverse turbulence intensity, respectively. These two parameters can be estimated indirectly by numerical models, giving rise to a tool for the prediction of turbulence-related high frequency fluctuation