The growth characteristics of a starting axisymmetric turbulent jet

This paper characterizes the growth of the initial unexcited vortical structure produced by a rapidly started axisymmetric turbulent air jet. Although previous studies have been performed primarily with water, the flows were artificially excited in order to increase the size of the vortical structures which ultimately allows for easier capture. A major finding of these studies has indicated that the leading vortex eventually ‘pinches off’ from the trailing fluid. In order to more accurately simulate wingtip vortices, smoke visualization was used on a continuous, axisymmetric turbulent jet at a Reynolds number based on diameter of 7500. The growth of the initial vortical structure diameter and overall vortical growth were measured and compared to the large-scale structures that develop in the flow trailing these initial vortical structures. The results show that when the flow is unexcited, the initial vortex does not completely ‘pinch off’. Further, the initial vortical structure reaches a size over twice that of the trailing large-scale structures and has a spread rate 1.7 times that of the trailing large-scale structures. In addition, the initial structure was observed to survive approximately 2 pipe diameters further downstream before breaking down.