The initial stage of a three-dimensional vortex structure existing in a two-dimensional boundary layer separation flow. The 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 understood. 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 number from 170 to 1340 in a closed-circuit water tunnel and a towing water tank. As a result, it was known that the three-dimensional instability of the wake flow is based on the three-dimensional characteristics of boundary layer separation around a circular cylinder, which vary with Reynolds number of the flow.