PIV EXPERIMENTAL INVESTIGATION OF VORTICES ON LARGE-SPAN ROOFS

Through Particle Image Velocimetry (PIV), flow visualization of separation bubble and conical vortices on large-span flat and saddle roofs was conducted in a wind tunnel. The streamline and vorticity field of visual planes on large-span roofs were derived by PIV. The influence of wind direction and roof curvature on the appearance of conical vortices was discussed. The results indicate that separated bubble occurs when the flow is normal to the leading edge of the flat roof. The location of the vortex core is corresponding to the negative peak vorticity. For cornering flow, conical vortices are observed near the leading edge of the flat roof and the saddle roofs whose high points are windward. When the wind direction changes from along the diagonal to deviating from the diagonal of the roofs, the conical vortex close to the approaching flow changes to be more oblong shaped. The negative vorticity is concentrated on the roof surface, while the peak is near the leading edge and around the vortices. At the same wind direction, a greater roof curvature leads to a larger angle between the vortex axis of conical vortices and the leading edge of the saddle roof. The instantaneous streamline of the visual planes illustrates that conical vortices are resizing and swaying constantly, which reveals its intermittent characteristic.