チャンネル乱流における低速ストリークに発生した二次不安定性モードについて

抄録

It is estimated that the low-velocity streaks in a wall-bounded turbulence break down by the secondary instability creating smaller vortices. As seen sinuous and varicose modes on the Goertler vortex growing on a concave-wall boundary layer,¨ the two modes of the secondary instability of the streaks are expected. In the recent years, the secondary instability is extracted by inserting low and high-frequency artificial disturbances into wall bounded turbulent flows and using an ensemble-average technique for velocity data. In this study, we attempt to control the secondary instability mode by changing the symmetry of the high-frequency disturbances aligned in the spanwise direction. With the in-phase high-frequency disturbances inserted from the two disturbance holes, the extracted perturbation forms of the spanwise symmetry that seems to be the varicose mode. On the other hand, high-frequency disturbances inserted out of phase extract the axisymmetric perturbation that suggests that the secondary instability of sinuous mode also exists in the actual wall-bounded turbulence. These results show that the secondary instability mode of the streak is successfully controlled by the symmetry of the initial disturbances.