Experimental Study on Turbulent Flow Field, Bubble Entrainment, and Scale Effects in Plunging Breaking Waves
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First, the kinematics and dynamics of deep-water plunging breakers of 0.2 m wave height were investigated. Flow properties were measured using modified particle image velocimetry (PIV) and bubble image velocimetry (BIV); void fraction was measured using the fiber optic reflectometer (FOR) in the aerated crest region. A similarity profile of void fraction was found in the successive impinging and splash-up rollers. In the highly foamy splashing roller, the increase of turbulent level and vorticity level were strongly correlated with the increase of void fraction when the range of void fraction was between 0 and 0.4. When including the density variation due to the air bubbles, the wave energy dissipated exponentially a short distance after breaking about 54% and 85% of the total energy dissipated within one and two wavelengths beyond the breaking wave impingement point, respectively. Then, turbulent flow field and bubble size distributions were discussed as an extension of the first part. The most excited mode of the local intermittency measure of the turbulent flow and its corresponding length scale were obtained using a wavelet-based method and found to correlate with the swirling strength and vorticity. The number of bubbles with a chord length less than 2 mm demonstrated good correlation with the swirling strength. The power law scaling and the Hinze scale of the bubbles determined from the bubble chord length distribution compared favorably with existing measurements. The turbulent dissipation rate, accounting for void fraction, was estimated using mixture theory. A significant discrepancy of approximately 67% between the total energy dissipation rate and the turbulence dissipation rate was found. Of this uncounted dissipation, 23% was caused by bubble-induced dissipation. Finally, the kinematics and the bubble entrainment in a 1-m height of plunging breaking wave in a very large wave tank of 5 m in width and depth and 300 m in length were discussed. To investigate the scale effects, velocity fields, turbulence, and void fraction were compared with the study of small scale plunging breakers (of 0.2-m wave height) discussed in the first two chapters. The temporal evolutions of the maximum surface horizontal velocity in the two scales are comparable, with a maximum difference only about 0.2CwhereC is the phase velocity. The measured vertical profile of the wave-averaged turbulent kinetic energy can be fitted to an exponential curve, consistent with the previous studies in surf zone breaking waves. The temporal contours of void fraction are similar between the two scales.
Na, Byoungjoon (2016). Experimental Study on Turbulent Flow Field, Bubble Entrainment, and Scale Effects in Plunging Breaking Waves. Doctoral dissertation, Texas A & M University. Available electronically from