Abstract

The study of airside flow structure and its interaction with water at the air-water interface is important in order to understand the exchange of momentum, heat and mass fluxes between the two mediums. The present dissertation deals with the quantitative investigation of the near-surface flow above wind-sheared water surface through a series of laboratory experiments conducted over a wind speed range of 1.5 m s -1 to 4.4 m s-1 and at a fetch of 2.1 m. The two-dimensional velocity fields were measured using particle image velocimetry (PIV). To compare the airflow structure over the water surface with that over solid wall, the measurements were also made over the smooth and wavy walls at the same location, under identical conditions. The results show a reduction in the mean velocity magnitudes and the tangential stresses when gravity waves appear on the water surface. An enhanced vorticity layer was observed immediately above the water surface that extended to a height of approximately two times of the significant wave height. A novel approach is used to separate the wave-induced component from the instantaneous velocity fields. The flow structure was analyzed as a function of wave phase. The phase-averaged profiles of wave-induced velocity, vorticity and Reynolds stress showed different behaviour on the windward and leeward sides of the wave in the near-surface region. The results also show that the turbulent Reynolds stress mainly supports downward momentum transfer whereas the wave-induced Reynolds stress is responsible for the upward momentum transfer from wave to wind. This dissertation also provides first quantitative comparison of the mean, wave-induced and turbulent properties for the separated and non-separated flows over wind generated water waves. The maximum difference between the flow characteristics of the separated and non-separated flows is observed on the leeward side, within core of the separation region, where, higher magnitudes of the vorticity and turbulent properties were observed, indicating that the turbulence is significantly enhanced within the separation region. The comparison of the flow over smooth and wavy water and solid surfaces showed that although the trends in profiles over water and solid surfaces are mostly similar, the relative magnitudes of turbulent properties and their level of enhancement towards the surface are different over water and solid surfaces. Thus, the models for the flow over solid surfaces may not accurately predict the flow properties over the water surface especially in the near-surface region.