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ISSN オンライン: 2642-0554

DIRECT OBSERVATION OF THE LIFT-UP EFFECT IN BOUNDARY LAYER TRANSITION USING HYDROGEN-BUBBLE METHOD

Ayumu Inasawa
Department of Aerospace Engineering Tokyo Metropolitan University 6-6 Asahigaoka, Hino, Tokyo 191-0065, JAPAN

Fredrik Lundell
Dept. of Mech., KTH, S-100 44 Stockholm, Sweden

Masaharu Matsubara
Dept. of Mech. Eng., Shinshu Univ. Wakasato 4-17-1, Nagano 380-8553, Japan

Yasuaki P. Kohama
Institute of Fluid Science, Tohoku Univ., Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan

要約

Transition of a boundary layer subjected to free-stream turbulence has been studied in a water-channel. A stereo PIV-system (Particle Image Velocimetry) using hydrogen bubbles as particles has been developed and validated against ID LDV (Laser Doppler Velocimetry) measurements. It is well known that the disturbances of the free-stream turbulence generate streamwise elongated regions of low streamwise velocity in the boundary layer. The elongated regions, or streaks, develop downstream and eventually breaks down to turbulence after the appearance of wavy secondary instabilities, seen in flow visualizations. In the present study, the lift-up mechanism, creating large amplitude streamwise velocity disturbances, is studied. Lift-up is the transport of low velocity fluid from close to the wall to higher regions of the boundary layer by a small wall-normal velocity disturbance, induced by the free-stream turbulence. Stereo PIV using two synchronized video cameras gives instantaneous spanwise distributions of the streamwise and the wall-normal velocities, showing a strong negative correlation between the wall-normal and streamwise velocities in the centre of the boundary layer. This demonstrates the lift-up effect from direct velocity measurements.