A preliminary study on the occurrence of air flow separation over mechanically generated water waves under following wind conditions is presented. Separated air flows over both non-breaking and breaking waves are observed in the flow visualization. A first attempt to identify an air flow separation criterion based on both wind speed and wave steepness is made. It was believed that, in the case of water waves propagating in the following wind condition, air flow separation will occur only in the presence of breaking waves. However, some laboratory experiments and field measurements suggested the occurrence of air flow separation over nonbreaking waves. Therefore, we conducted lab experiments to observe the air flow over mechanically generated waves. In the experiments, the air is seeded with water droplets generated with a high-pressure spray gun and is illuminated with a thin laser light sheet. A high-speed imaging system is used to record and observe the air flow over the mechanically generated wave waves. Our observations show that the separation of air flow occurs above both breaking and non-breaking wave crests, implying that wave breaking is sufficient, but not necessary for air flow separation. In addition, as compared to the separation over breaking waves, a higher wind speed is necessary for the separation over non-breaking ones, indicating that a robust air flow separation criterion likely depends on both the wave crest geometry and the wind speed above the crest. Our preliminary results support, to a certain degree, such a criterion. To the best of our knowledge, this criterion has not been reported previously in laboratory studies.

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