Unsteady decay behaviour of secondary flow generated in 90-degee bends

Decay processes of secondary flow generated in 90-degree bends are visualized by using a PIV measurement system. The main parameters are a Re number and a curvature ratio of the bend, and three kinds of the 90-degree bend are prepared. It is confirmed that symmetrical and apparent secondary flow, which is a twin-vortex, is formed in a cross section of the pipe shortly after the bend outlet for each curvature ratio where the flow field is analyzed in the time average. However, it is clarified that this secondary flow unsteadily and intensely fluctuates in the cross section regardless of the curvature ratio. Especially, where the curvature ratio is the lowest of 1.0, it is confirmed that the flow field is even more complex and its flow pattern rapidly disappears in more downstream area by the interaction between the secondary flow formed by a centrifugal force and newly generated separation vortex.