3D CHARACTERISTICS OF SEPARATION BUBBLES AROUND FLAT ROOFS BY PIV TECHNIQUE

Flow visualization of separation bubbles around flat roofs was conducted in a wind tunnel through the Particle Image Velocimetry (PIV) technique. The streamlines and vorticity field with multiple visual planes on the roof surface are performed for different flow conditions. The results indicate that the phenomenon of separation bubbles occurs when the flow approaches normal to the roof's leading edge. The location of front windward corresponds to the negative peak vorticity, and the wind pressure generally decreases with the distance from the leading edge. The flow first separates at the leading edge, and then attaches to the roof surface. The reattachment length is almost across the whole roof in a uniform flow field. The shear layer is prompted to reattach earlier by small scale turbulence in the turbulent flow field, and the shorter the reattachment length is, the greater is the turbulence intensity. Compared with the results of the PIV experiment, the reliability and accuracy of Fluent numerical simulation are validated. Three dimensional morphological characteristics are investigated with comprehensive arrangement of the multiple visual planes. The inner link is established for the evolution of vortices, the vortex core position and pressure distribution. Some valuable engineering conclusions are obtained.