Investigation of natural transition in a flat plate boundary layer subjected to free stream turbulence has been done using the PIV (Particle Image Velocimetry) technique for hydrogen bubbles. The disturbances of the free stream turbulence generate streamwise elongated regions of low streamwise velocity, so called streaky structures, in the boundary layer and this structures develop downstream and breakdown to turbulence with spanwise waviness motions. This study is focused on the investigation of the lift-up mechanism that explains in the non-modal theory that the wall-normal velocity disturbance in a shear flow induces the large streamwise velocity disturbance. A new method that includes image processing and algorithm to detect three-dimensional positions of bubbles from the stereo images has been developed to obtain instantaneous time-spanwise distributions of the streamwise velocity and the wall-normal velocity. This information makes the streaky structure visible in the velocity fields and reveals the lift-up mechanism in the natural transition process.
水素気泡法による平板境界層の自然遷移の観察
nalsp0047016.pdf
(1.6MB)
