각진 장애물 주위 유동의 2차 불안정성

초록

Floquet stability analysis has been carried out to detect the onset of the secondary instability in the laminar flow past a two-dimensional angulated obstacle. The shape of the obstacles considered here includes a normal flat plate up to square cylinder as the aspect ratio(AR) varies. As AR decreases, mode B and mode QP vanish and mode A2 and mode QP2 emerge. This has not been observed in case of flow past a square cylinder. Mode A2 has an odd RT(Reflection Translation) symmetry, and the dominant spanwise wavelength of mode A2 is much shorter than that of mode A. On the other hand, the dominant spanwise wavelength of mode QP2 is about twice longer than that of mode QP. In a partial range of AR, mode A instability becomes unstable and then stable again as Re increases. This distinctive phenomenon can be seen via a closed curve on the neutral stability diagram. Three dimensional calculations of some selected cases were performed for validation.