広範囲の強さとスケールを持つ主流乱れによる境界層遷移の実験

Abstract

It is known that in a boundary layer subjected to a few percent of free stream turbulence intensity, streaks appear and eventually break down to turbulence after wavy motions. However, the boundary layer transition subjected to lower free stream turbulence intensity is still not been fully elucidated. The present paper focused on revealing effects on intensity and scales of free stream turbulence on boundary layer transition, especially on how it effects the boundary layer transition as well as the growth rate of disturbance energy in the streamwise direction. In this study we performed flow visualization and hot-wire measurements in order to investigate transition points and process with wide-range of free stream turbulence intensity and scales. From the flow visualizations at intensity 1.42%<Tu<3.99% indicate that, the streak structures elongated in streamwise direction and collapse to turbulent downstream. This is similar to that at a few percent of turbulent intensity. Even though the turbulence intensity is the same, the Transition Reynolds number increases with free stream velocity. So, it is strongly suggests that, the Transition Reynolds number is depends on the ration between the boundary layer thickness and turbulence scales.