Surface tension effects on energy dissipation by small scale, experimental breaking waves

Much of the existing knowledge about breaking waves comes from physical model experiments scaled using Froude's law. A widely held assumption is that surface tension effects are not significant at typical laboratory scales and specifically for waves longer than 2 m. Since, however, smaller wavelengths are not untypical in small to medium scale laboratory facilities, a consideration of surface tension effects is indeed important. Although some emphasis has been given in the past, little is known regarding the importance of surface tension following impingement of the breaking-wave crest and especially on the overall energy dissipation by laboratory breaking waves. To answer this question a laboratory study was conducted comparing the energy dissipation rate by waves breaking in fresh-water and in a 10% isopropyl-alcohol and distilled water solution of lower surface tension. It was found that when waves shorter than 4 m and smaller than 0.11 m break under the weakened influence of surface tension they dissipate up to 65% more wave energy. Visual observations link the increased wave energy dissipation with greater breaking intensity, increased air entrainment and longer bubble lifetime. Overall, the results presented here indicate that the inability to maintain Weber similarity (ratio of fluid inertia to its surface tension) has a more significant effect on the intensity of wave energy dissipation than previously assumed.