周期的かく乱に対する乱流はく離泡の応答 : かく乱周波数,かく乱強さおよびレイノルズ数の効果

The leading-edge separation bubble of a blunt circular cylinder of diameter d was forced by sinusoidal disturbances with single frequency components at Reynolds numbers of the order of l05. The disturbance was introduced by a woofer installed inside the cylinder through a thin slit along the separation edge. The level of forcing was defined in terms of the rms value of the velocity fluctuation of the forcing frequency at a position immediately upstream of the separation edge q'f, being in a range of q'f/U∞=0.005-0.1, where U∞ is the main-flow velocity. The reattachment length plotted against the forcing frequency attained a minimum for low and high forcing levels, and two minima for intermediate forcing levels, at frequencies in the range of (1.6- 2.1) U∞/d. The minimum value decreases with increasing Reynolds number. A maximum of the reattachment length, which was greater than that without forcing, appeared at a forcing frequency of approximately one-half of the frequency of the Kelvin-Helmholtz instability immediately downstream of the separation edge. The reattachment length was independent of the forcing frequencies if they were greater than twice the Kelvin-Helmholtz frequency.