Structure of air flow separation over wind wave crests
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Air flow over wind waves generated in a wind-wave tunnel was visualized by numerous tiny suspended particles (zinc stearate), and instantaneous air flow fields over about one wavelength of wind waves were obtained. Air flow separation was detected over the wave crest in about a half of the samples. In such cases, the separation started near the crest about half of the time, with a vortex trapped over the convergence point of the surface flow which appeared at the leeward face of the crest. This structure was much different from a previously imagined picture in which the separation started at the convergence point. The high frequency of its occurrence suggested the stability of this structure. However, even when this structure was clearly seen, the structure behind the vortex to the next wave crest had various patterns. This variety seems to be related to an instability of the high-shear layer accompanied by separation. Other varieties were also seen, such as the occurrence of separation without the above mentioned structure, as well as the existence of non-separated air flow structures. These varieties seem to be related to the variability of individual wind wave crests. An analysis of correlation between the wave form and the air flow structure over it shows that there is a critical value of local gradient of wave form, above which the air flow always separates. This fact suggests a strong coupling between the air and the water, i.e., the local stress exerted on the water surface changes the nature of a wave crest, especially its form, and as a result, the air flow structure over it changes drastically.
KeywordsVortex Crest Stearate Suspended Particle Wave Form
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- Aihara, T. and Saito, E.: 1972, ‘Measurement of Free Convection Velocity Field around the Periphery of a Horizontal Torus’, Trans. Amer. Soc. Mech. Engr., J. Heat Transf. 94, 95–98.Google Scholar
- Aihara, T., Yamada, Y., and Endo, S.: 1972, ‘Free Convection along the Downward-Facing Surface of a Heated Horizontal Plate’, Int. J. Heat and Mass Transf. 15, 2535–2549.Google Scholar
- Banner, M. L. and Melville, W. K.: 1976, ‘On the Separation of Air Flow over Water Waves’, J. Fluid Mech. 77, 825–842.Google Scholar
- Kawai, S.: 1981, ‘Visualization of Airflow Separation over Wind-Wave Crests under Moderate Wind’, Boundary-Layer Meteorol. 21, 93–104.Google Scholar
- Kawamura, H., Okuda, K., Kawai, S., and Toba, Y.: 1981, ‘Structure of Turbulent Boundary Layer over Wind Waves in a Wind-Wave Tunnel’, Tohoku Geoph. Journ. (Sci. Rep. Tohoku Univ., Ser. 5, Geophys.) 28, 69–86.Google Scholar
- Koga, M.: 1981, ‘Direct Production of Droplets from Breaking Wind Waves — Its Observation by Multi-Colored Overlapping Exposure Photographing Technique’, Tellus 33, 552–563.Google Scholar
- Okuda, K.: 1980, ‘Study on the Internal Structure of Wind Waves’, Dr. Sci. Thesis, Tohoku University, 145 pp.Google Scholar