Breakdown Structure of Longitudinal Vortices along a Concave Wall

The breakdown structure of longitudinal vortices in a boundary layer along a concave wall of 1m in radius of curvature is studied experimentally under main air flow velocity of 2.5 m/s. The flow patterns are observed by lightoil vapour with stroboscope, and the mean velocity and the turbulence intensity are measured by a hot-wire anemometer. In the upstream reagion, every couple of the vortices forms periodic high-frequency horseshoe-type vortices, which bring about dominant behaviour in the outer layer. In the downstream reagion, it is observed that high-intensity irregular fluctuations are generated beneath the horseshoe-type vortices in the inner layer. Therefore, the longitudinal vortices are broken down, and the boundary layer transition is also hastened downstream.