보오텍스에 의해 발생한 원통형 실린더의2자유도 운동에 관한 실험연구

Abstract

This paper presents results of an experimental investigation of vortex-induced vibration(VIV) of a flexibly mounted and rigid cylinder with two-degrees-of-freedom with respect to variable in-line to cross-flow natural frequency ratio at fixed low mass ratio. Combined motion between in-line and cross-flow motion was observed in a sub-critical Reynolds number range. Three-dimensional displacement meter and tension meter were used to measure dynamic responses of the model. To validate the results and the experiment system, x and y response amplitudes and ratio of oscillation frequency to cross-flow natural frequency were compared with other experimental results. It has been found that the higher harmonics, such as third and more than third vibration components, can occur on a certain part of steel catenary riser under a condition of dual resonance. In this present work, however, due to the limitation of a size of circulating water channel, it was not sufficient to conduct the VIV experiment with a whole configuration of the riser at an adequate scale. Therefore, we have modelled a rigid cylinder and assumed that the cylinder is a part of steel catenary riser where the higher harmonic motions could occur. Through the experiment, we have found that even though the cylinder was assumed to be rigid, the occurrence of the higher harmonic motions was observed in a small reduced velocity () range, where the influence of the in-line response is relatively large. Transition of the vortex shedding mode from one to another mode was examined by using x and y displacement time history over all experimental cases. We also observed the influence of in-line restoring force power spectral density.

본 논문은 2자유도 실린더의 vortex-induced vibration 응답에 관한 내용이며, 실린더의 응답에 영향을 주는 주요 변수에 따른 실린더 응답의 영향을 관측하였다. 관측 대상인 실린더는 길이와 직경의 비가 10인 rigid 구조물이며 질량-스프링 시스템의 응답 특성을 보인다. 실린더의 VIV응답은 레이놀즈수의 범위에 따라 sub-critical, critical, super-critical, 그리고 trans-critical로 분류할 수 있는데, 이번 실험은 sub-critical 범위내의 VIV 응답에 관한 실험 결과를 분석하였다. 실린더의 x와 y 방향의 변위는 실린더에 부착된 반사테이프의 움직임을 3차원 변위계를 이용하여 계측하였으며, 스프링에 직접 연결된 장력계를 통해 실린더의 복원력 데이터를 x와 y 방향 별로 얻었다. 본 실험케이스와 유사한 조건으로 수행 된 영국 Strathclyde 대학의 Kelvin Hydrodynamics Laboratory (KHL)의 연구결과와의 비교를 통해 실린더 진폭응답의 전체적인 경향이 비슷함을 알 수 있었고, 이에 더하여 각 실험 조건에 따른 여러 물리적 현상을 관측하였다.