주상체의 모서리 형상이 동적 불안정성에 미치는 영향 전산해석
Numerical Analysis for Effects of Corner Shape of Cylindrical Body on Dynamic Instability
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초록
해상 교량, 고층건물이나 라이저와 같은 세장 주상체의 설계 및 시공이 늘어남에 따라 높은 유속에 대한 동적 안정성 검토의 중요성이 부각되고 있다. 특히 교량구조물은 장대화와 함께 경량화가 진행되었고 지진, 바람 등과 같은 동적 하중에 대해서 진동하기 쉬운 구조물의 특징을 가진다. 구조물 주위를 흐르는 공기에 의해 추가적인 공기력들이 발생하게 되고 이로 인해 공기 흐름장과 물체의 운동 사이에 상호작용을 가져오게 되고 부가적으로 발생된 공기력이 음(-)의 감쇠력으로 유입 풍향에 직각방향인 진동에 작용하여 갤로핑(galloping)등과 같은 자발진동을 발생시킨다. 본 논문에서는 가상경계법을 이용하여 사각 단면, 팔각단면, 모서리가 둥근 사각단면 및 양쪽 모서리가 절단된 형태의 사각단면에 대하여 유동해석을 실시하고 Glauert-Den Hartog가 제시한 방법으로 단면에 대한 갤로핑 불안정성의 발생 가능성을 평가하였다.
Abstract
The slender structures such as long-span bridge, high-rise building and riser are very susceptible to dynamic loading due to their flexibility. The safety as well as serviceability of the slender cylindrical structures can be threatened by excessive vibration, because the structures should endure harsh condition. Due to development in material, the cylindrical structures such as long-span bridges have become lighter and easy to vibrate against the dynamic load. Fluid flowing around the body generates an additional force related to fluid dynamics and structural displacement. Under the dynamic forces, the cylindrical bodies are designed to act in the direction to suppress the vibration with a positive damping. However, under certain conditions, induce dynamic forces negative damping to increase vibration, which is called self-excitation like a galloping and often observed in pylon with sharp edge. In this paper, Immersed Boundary Method(IBM), one of the computational fluid dynamics methods, is used for analyzing the flow around rectangular body, rounded corners rectangular body, rectangle with corners cut off and octagonal body. IBM is based on a finite-volume approach on a staggered grid with a fractional-step method to solve the Navier-Stokes equation and continuity equation. Aerodynamic force coefficients are computed and show good agreement with previous study. And the dynamic instability of galloping in also examined with Glauert-Den Hartog’s method.
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