Effect of sea states on the wind velocity fluctuations near the surface under near-neutral conditions

Abstract

This study examines the effect of sea states on wind velocity fluctuations in the longitudinal direction near the sea surface under near-neutral stratification, characterized by inverse wave age. The analysis is based on wind and wave data collected from a discus buoy measurement off the coast of Taiwan in the Taiwan Strait. It is observed that wind waves and swells modify the overlying turbulence structure differently. In particular, wind-swell coupling results in greater variations and values of the scaled velocity standard deviation and turbulence intensity with increasing wind speed, as well as a more pronounced peak in the scaled wind velocity spectrum compared to pure wind sea conditions. The spectral shapes averaged for pure wind sea and swell conditions are described by a plateau followed by a wave-induced peak in the intermediate-frequency range. These features suggest that near-surface turbulence is driven by eddy impingement on the sea surface, associated with wave-induced fluctuations in the lower part of the eddy surface layer. The study has practical applications in evaluating the wind loads on offshore structures, particularly in the context of offshore wind energy development.