Breaking Wave Measurement by a Void Fraction Techniqueby Ming-Yang Su, Naval Research Lab, Stennis Space Center, United States,
John Cartmill, Naval Research Lab, Stennis Space Center, United States,
Abstract: Breaking of steep surface gravity waves in seas under moderate to strong wind forcing always involves the entrainment of air into the water column below the air-sea interface. As such, the presence of air in the near-surface zone may be a positive indicator for occurrence of wave breaking. This simple idea is the physical basis for our measurement of breaking waves by means of a new technique that directly measures the percentage of air content (void fraction) under a pre-fixed depth below the sea surface. The data from such void fraction measurements conducted in the open sea under moderate to high sea state were processed to obtain several breaking wave statistics. A comparison of these statistics with five previously-published statistics employing different techniques shows that the new void fraction technique is a better technique both in its easier execution and its more quantitative discrimination of various sizes of breaking waves. We shall also present the space-time variations in void fraction of a large breaking wave in a laboratory tank, acquired in comparison to other previous results.
Subject Headings: Breaking waves | Wave measurement | Voids | Surface waves | Seas and oceans | Statistics | Gravity waves
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