VORTEX DYNAMICS ANALYSIS IN JET FLOWS USING 2DPLANAR PIV AND HIGH-SPEED LASER TOMOGRAPHY IMAGE PROCESSING

摘要

This study associates PIV measurements and high speed visualizations enriched by low-level image processing, for the analysis of the vortical phenomena in the near field of jet flows. We show that in the circular jet, the entrainment is produced in the braid region where the streamwise structures develop. In the Kelvin-Helmholtz ring the entrainment is dramatically reduced due to the attenuation of the streamwise structures. This undesired effect of the periodical dynamics of the Kelvin-Helmholtz rings may be avoided by using lobed shaped nozzles or orifices. In the present study, the daisy-shaped geometry introduces a local transverse shear which conducts to a "cutting" of the Kelvin-Helmholtz structures into "ring segments". These discontinuities of the azimuthal structures allow the development of large scale streamwise vortices which may continuously develop. In this way they dominate the entrainment independently of the Kelvin-Helmholtz vortex presence.