Experimental investigation of helicity in turbulent swirling jet using dual-plane dye laser PIV technique

This paper reports a new method of generating two light sheets using a dye laser system and the use of this dual-plane dye laser system to analyse average helicity and energy dissipation in a turbulent swirling flow. The dual-plane PIV system that was used in this study consisted of three cameras and a single frequency Nd:YAG laser, which was used to generate two parallel light sheet planes with differing wavelengths(colour). The method of generating two different light sheet wavelengths using a single laser source is an innovative and new technique. Stereoscopic PIV measurements were obtained in one plane with the use of two CCD cameras, and standard PIV measurements were obtained in the other plane with the use of one CCD camera. The light scattered by the particles on two different light sheets were separated using appropriate optical filters. The measurements obtained were used to estimate the components of the velocity gradient tensor. The tensor components were then used to determine the average vorticity components and helicity quantities of the fluid that was investigated. To determine the average turbulent kinetic energy dissipation, the continuity equation was used to infer the out-of-plane gradient of the out-of-plane velocity. From the analysis of the results, it was found that regions with high helicity were correlated with regions of high turbulent kinetic energy dissipation.