Flow, Turbulence and Combustion 68: 359–377, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. 359 Organizational Modes of Large-Scale Vortices in an Axisymmetric Turbulent Jet AMIT AGRAWAL and AJAY K. PRASAD Department of Mechanical Engineering, University of Delaware, Newark, DE 19716, U.S.A.; E-mail: {agrawaa,prasad}@me.udel.edu Received 9 April 2002; accepted in revised form 19 September 2002 Abstract. Large vortices occurring in the axial plane of a self-similar axisymmetric turbulent jet are educed by spatially filtering PIV data. First, the instantaneous PIV frame is convolved with a Gaussian kernel to obtain a smooth (low-pass) field. Next, the low-pass field is Galilean transformed to expose the large vortices residing near the edges of the jet. Large vortices tend to organize them- selves in preferred modes; evidence of ring and helical modes is revealed by Galilean transformation of the low-pass filtered field. Both modes seem to occur prominently in jets, with the helical mode being the more frequent. The overall diameter of both ring and helical modes is comparable with the local jet width. The low-pass field occasionally exhibits arrowhead shaped structures with large entrainment at their downstream tips. Stochastic estimates computed from the Galilean-transformed low-pass filtered field indicate that jet meander and a sweep-in of ambient fluid are sufficient to reconstruct large vortices. The frequency of occurrence of modes agrees with previously quoted results. Key words: arrowhead structure, Galilean transformation, linear stochastic estimate, low-pass filter, PIV, ring and helical modes. 1. Introduction The ubiquitous presence of coherent structures in turbulent flows was realized after visualization studies in planar mixing layers by Brown and Roshko [6]. Since then, coherent structures have been a subject of great interest in fluid mechanics. It has been established that they affect important practical processes such as dispersion and mixing of fluids in turbulent flows, noise production in the exhaust of an air- craft, and combustion. It is believed that understanding them, and their interaction with themselves and the surrounding fluid will provide new insights into the flow physics. Although coherent structures in mixing layers are quite well documented, the nature of coherent structures in turbulent jets is not fully established. It is partly be- cause the energy content of coherent motion in jets is lower (about 10%) compared to 20% for mixing layers [9] that makes recognition of structures more difficult in jets. Extensive measurements in the far-field of jets along with better eduction methods are therefore warranted.