Spectral measurement of water particle velocities under waves
Bordy, Michael William
Thornton, Edward B.
Naval Postgraduate School
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Simultaneous measurements of the instantaneous sea surface elevation and water particle velocities were made using a wave staff and an electromagnetic current meter. Measurements were made at three elevations in 18 meters of water at an open ocean site during moderate wave and wind conditions . Coherence of the wave height and orbital velocities was computed to be greater than 0.90 through the range of significant energy-density between 0.05 and 0.22 Hertz. This range contained greater than 90 percent of the total spectral energy-density which indicated that the water particle velocities were almost totally waveinduced. Measured velocity energy-density spectra were compared to theoretically computed spectra using linear wave theory formulation. The measured spectra were 27 to 45 percent greater than theoretical spectra indicating that measured velocities were 5 to 7 percent greater than the theoretical value. Phase spectra were computed for the measured wave heights and orbital velocities. They compared reasonably with first order wave theory.
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