Experiments investigating the use of fiber-optic probes for measuring bubble-size distributions

An experimental investigation was conducted in the laboratory in order to evaluate the use of fiber-optic probes for measuring the size distribution of large transient bubbles entrained beneath breaking ocean waves. Measurements were made in a unidirectional how cell using two-fiber Optoflow fiberoptic probes (Photonetics, Inc.). It was found that the rise times of the signal pulses treated when bubbles crossed the probe tip were closely correlated with the bubble velocities. Therefore, bubble velocities and hence bubble sizes could be estimated using a single probe. It was observed that the correlation between the rise time and the bubble velocity varied significantly between probes but was only a weak function of water type (i.e., freshwater or seawater) and the bubble impaction angle. Estimates of the bubble-size distributions and void fraction made using the fiber-optic probes and video image data were compared. It was found that the fiberoptic probes could be used to obtain fairly accurate estimates of the void fraction. The bubble-size distributions estimated from the probe signals compared favorably with those estimated from video images, It was concluded that the probes could be used to make reasonably accurate measurements of the bubble-size distributions beneath breaking ocean waves.