3-D flow visualization of axisymmetric jets at Reynolds number 6,700 and 10,200

Abstract

A recently developed 3-D flow visualization system is used to capture instantaneous 3-D images in the near-field of an axisymmetric jet at Reynolds number 6,700 and 10,200. The 3-D images, which are acquired through the scanning of a pulsed MHz-rate laser sheet, have spatial resolution of up to 312 × 260 × 100 voxels and time resolution of 100 μs, producing effectively instantaneous, high-resolution 3-D images of the jet. These images represent the first truly volumetric flow visualization images to be captured in the near-field of a jet over this range of Reynolds numbers. The images show the formation of large-scale axisymmetric vortex rings along with the formation of counter-rotating streamwise vortex pairs that surround the periphery of the jet in a periodic manner and co-exist with the ring vortices. The azimuthal frequency of the streamwise vortices increases with Reynolds number from 10 to 14 structures around the circumference at Re = 6,700 to 18–22 structures at Re = 10,200. In some instances, the streamwise vortices are observed to maintain their phase and span across several vortex rings in the streamwise direction. In other instances, however, the streamwise vortices display abrupt changes in the downstream direction. The images also display a complex interaction between the vortex rings and streamwise structures in the transition region near the end of the potential core illustrating the importance of the streamwise vortices on the evolution of the jet flow.