Spatial structure of the flow through an axisymmetric sudden expansion

Spatio-temporal velocity fields of an axisymmetric sudden expansion were measured using an ultrasonic velocity profiler and analyzed to investigate the transitional scheme of the spatial structure using two-dimensional Fourier transform and proper orthogonal decomposition techniques. The variation of the zero-crossing point, the fluctuation energy directed upstream and the eigenmode spectrum all have the same transitional scheme as a function of the Reynolds number. The transitional scheme can be classified Re-d<1,000 for the laminar regime, Re-d=1,000-3,000 for the transitional regime and Re-d>3,000 for the turbulent regime. Especially, in the transitional regime, we found large changes in the flow structure at Re-d=1,500 and 2,000. The jump at Re-d=2,000 is caused by the change in the flow condition upstream. The jump at Re-d=1,500 clearly shows a change in the spatial structure of the flow.