A DPIV study of a starting flow downstream of a backward-facing step

In this paper an experimental investigation of a starting water flow downstream of a backward-facing step is described. The Reynolds number of the asymptotic steady flow is Re approximate to 4300 based on the step height of s = 2 cm and the free stream velocity of U = 21.4 cm/s. Velocity measurements were performed with video-based DPIV (Digital Particle Image Velocimetry) at a sampling frequency of 25 Hz. The main purpose of this study is to reveal the temporal development of global structures which could not be analyzed with single-point probes. It was found that at initialization of the flow a regular vorticity street is formed, which collapses at a normalized time t* = U t/s approximate to 17 due to vorticity interactions. After this the flow is dominated by complicated vorticity roll-up and shedding dynamics in the recirculation region. The starting phase is terminated for t* > 40. Prior to the collapse of the vorticity street values of 9 times the steady state asymptotic wall normal stress and of twice the steady state negative wall shear stress were observed. The early increasing slope of the reattachment length is constant over a time of approximately t* = 8. The collapse of the vorticity street and the vorticity interactions thereafter cause fluctuations both in the velocity in the free shear layer and in the reattachment length. The fully developed flow has a dominant frequency corresponding to a Strouhal number St = fs/U approximate to 0.04.