INITIAL-STAGE OF A 3-DIMENSIONAL VORTEX STRUCTURE EXISTING IN A 2-DIMENSIONAL BOUNDARY-LAYER SEPARATION FLOW - (OBSERVATION OF LAMINAR BOUNDARY-LAYER SEPARATION OVER A CIRCULAR-CYLINDER BY FLOW VISUALIZATION)

Although it is generally known that the Karman vortex street consists of large-scale vortex structures with three-dimensional characteristics, the causes of the three-dimensional instability have not yet been elucidated. In this study, the three-dimensional characteristics of boundary layer separation around a circular cylinder were observed in water flow by using the laser light sheet technique in the range of Reynolds numbers from 170 to 1 340 in a closed-circuit water channel and a towing water tank. As a result, it was determined that the three-dimensional instability of wake flow is based on the three-dimensional characteristics of boundary layer separation around a circular cylinder, which varies with the Reynolds number of the flow.