Surfactant Effects on the Free Surface Thermal Structure and Subsurface Flow in a Wind-Wave Tunnel

Abstract

In this study, the dynamic effects of surfactant (oleyl alcohol) on the surface temperature and the near surface velocity field of a wind driven free surface are investigated. Different surfactant concentrations and wind speeds were examined to elucidate the flow physics. The water surface was imaged with an infrared (IR) detector and the subsurface flow was interrogated utilizing digital particle image velocimetry (DPIV). The IR imagery reveals the presence of a Reynolds ridge that demarcates the boundary between clean (hot) fluid and contaminated (cold) fluid. The clean region was found to be composed of laminae structures known as fishscales. A “wake region” which is an intermediate temperature region resulting from mixing of the near surface fluid layers develops behind the ridge. Experimental results from infrared imagery indicate that the fishscales in the clean region become elongated and narrowed as the wind speed increases. In addition, the results reveal that higher wind speed is required to form a Reynolds ridge in the presence of higher surfactant concentration. The plots of the surface temperature probability density functions reveal that these thermal structures undergo the same evaporative process while the increase in wind speed enhances this process. DPIV results reveal that the growth of a subsurface boundary layer for the contaminated case is more pronounced than that for the clean case.