심해 풍파 아래에서의 응집 구조
Coherent Structures beneath Wind-Generated Deepwater Waves
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초록
이 연구에서는 심해 풍파 아래에서의 응집구조(coherent structures)에 대한 실험실 실험 결과를 제시하였다. 풍속과 취송거리가 서로 다른 실험 조건에서 입자화상유속계를 이용하여 취득된 심해 풍파의 속도장으로부터 와도장을 가시화하고 응집구조가 시ㆍ공간적으로 변천하는 양상 및 그에 따른 와도의 연직 분포 변화를 정성적으로 분석하였다. 파봉 아래에서는 파와 같은 위상으로 함께 진행하는 응집구조가 존재함을 확인하였다. 이 응집구조의 회전 방향은 바람이 10 m/s 이하인 실험 조건에서는 파 내부의 입자 궤도 운동과 반대 방향인 반면, 바람의 세기가 약 13 m/s이고 파봉 근처에서 쇄파가 발생하는 경우에는 같은 방향이었다. 수면 근처에서는 응집구조의 진행에 따른 복잡한 소용돌이 상호 작용이 나타나는 반면, 수면으로부터 깊은 수심에서는 파 궤도 운동의 영향이 미미하여 응집구조의 시ㆍ공간적 변화가 거의 없었다.
Abstract
The results of experimental investigation of coherent structures beneath wind-generated waves in deep water are presented. Vorticity fields of deepwater wind waves were visualized by analyzing the velocity fields obtained by PIV measurements under different wind and fetch conditions. In addition, spatio-temporal evolution of the coherent structures and subsequent changes in vertical profiles of the instantaneous vorticity were qualitatively examined. It was found that a coherent structure is formed right underneath the wave crest and traveled in phase with the surface wave. The direction of rotation of the coherent structure was contrary to the wave orbital motion when wind speed is less than 10 m/s, while was same as the wave orbital motion when wind speed is approximately 13 m/s and wave breaking occurs at the wave crest. In the near-surface region, complex vortex-vortex interactions were observed according to the traveling of the coherent structure. In contrast, coherent structures far below the water surface changed little due to weak influence of orbital motion by the surface waves.
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