Void fraction and bubble size distribution measurements underneath laboratory breaking waves

This thesis describes a series of laboratory experiments that were conducted to study the air entrainment produced underneath mechanical generated deep-water breaking waves.;Two fiber-optic probes were calibrated for making void fraction and bubble size measurements underneath breaking waves. Tests showed that the normalized RMS error in the void fraction measurements was ∼10%. It was also determined that if a minimum of ∼15 individual bubble velocities were averaged, the mean bubble velocities were accurate to +/-10%. The bubble size distribution measured with the probes was compared to the size distribution measured from digital video recordings, and it was found that these two distributions agreed closely with each other.;Three significant events were identified during the breaking process of a plunging wave: the plunging water jet impacting the forward face of the wave; the air cavity collapsing and evolving into a dense bubble cloud: and the splash-up impacting the water surface. Numerical models must be able to accurately predict the timing and nature of these events. There were 13 measurement positions along the plunging wave and the peak void fractions measured inside the bubble cloud varied from 0.024 to 0.97; and the mean void fractions varied from 0.012 to 0.37. For the spilling wave case, there were 4 measurement positions and the mean void fractions at these positions varied from 0.17 to 0.29. Based on ensemble averaged time series of <alpha> it was deduced that, for the spilling wave case, the void fraction contours run parallel to the free surface. The speed of advance of the air cavity and the splash-up cloud beneath the plunging wave were estimated to be ∼75 and ∼90% of the phase speed, respectively. For the spilling wave, the speed of advance of the bubble cloud was estimated to be ∼100% of the phase speed.