Short wave damping in the simultaneous presence of a surface film and turbulence

Abstract

Spatial decay rates of short gravity waves and gravity‐capillary waves in the frequency range of 3 to 8 Hz were measured in a laboratory wave tank equipped with a submerged oscillating grid to generate turbulence. Experiments were conducted with: clean fresh water, fresh water with an oleyl alcohol insoluble surfactant surface film, fresh water with a Triton X‐100 soluble surfactant surface film, and with sea water having a surface film of its own natural surfactants. With each material, decay rate tests were conducted with and without turbulence. The principal purpose of the study was to determine the extent to which turbulence modified the wave decay due to the presence of surface films. This is of importance for modeling the energy balance of short sea waves which influence microwave backscattering used for remote sensing of the ocean. The results show that the wave decay rate is equal to the sum of the rates due to surfactant alone and with turbulence alone. It is concluded that the statistically stationary component of the turbulence that exists near the free surface does not significantly modify the decay mechanism associated with the presence of the surfactant.