An experimental study of turbulent vortex rings during their early development

In this paper, the early development of turbulent vortex rings at two Reynolds numbers is studied using two-dimensional and stereoscopic particle image velocimetry (PIV). In the late 1980s, a similarity theory of turbulent vortex rings was proposed and this theory was tested primarily using laser Doppler velocimetry (LDV). However, because of limitations of the experimental technique, the tests were inconclusive and important assumptions could not be checked. Because single-point measurements were used, vortex ring structures could only be inferred using a complex signal analysis technique. In this study, the PIV technique provides spatial measurements of the full field of the cross-section of a ring from which a more rigorous investigation of the similarity theory is possible. Because the region over which the similarity theory appears to hold starts at about 2.5 orifice diameters downstream, this study focusses on the early development region from this point to 8 diameters downstream. Finally, Reynolds stresses and turbulence production contours are presented. The effects of ring dispersion on the measurements is also studied and quantified.