Vortex formation and evolution process in an impulsively starting jet from long pipe

Abstract

This paper presents some results of particle image velocimetry (PIV) measurements on an impulsively starting isothermal jet from a fully-developed circular pipe with Re = 4880. The temporal evolution of the starting jet from initiation to steady state is clearly revealed by time sequence of instantaneous velocity/vorticity fields obtained by PIV, in terms of the formation, evolution and interactions of the vortex rings (leading and trailing). The current jet exhibits a drastically different vortex topology as compared to that generated by conventional piston-driven starting jets from sharp-edged orifices or short pipes/nozzles in the literature. According to the recorded time series of centerline velocity near the jet exit, the evolution of the starting jet can be divided into two stages with different speeds of acceleration, and each stage is characterized by distinct vortex patterns. One-cycle catching-up and merging process occurs between each pair of consecutively pinched-off vortex rings, such as the leading ring and the first trailing ring, the second and third trailing rings, and so on. As a result of the merging with the trailing ring, the leading vortex ring increases in circulation, but its propagation speed is essentially unaffected.