Dynamic eduction of coherent structures in turbulent jet flow imagery by wavelet techniques: part I

Abstract

The use of two-dimensional wavelet techniques enables us to discern, at appropriate wavelet scales, certain features of a coherent structure from a single flow visualization image. By analysing a large number of planar laser-induced fluorescence images of chiefly the diametral section of a turbulent jet, with some supplementary axial sections, we show that it is possible to describe the passage of whole coherent structures through a specified section of the flow, and thereby acquire useful information on their life cycle. In particular it is found that a typical cycle is characterized by a lobed ring-like structure about 8% of the time, a dye-filled core a similar fraction of the time, with a mixed regime in between. Based on comparisons with dye-concentration data and DNS results, it is suggested that the lobed ring seen in the wavelet transform of the raw image may represent a vortex ring exhibiting the Widnall instability.

Keywords:

• Coherent structures
• Wavelet techniques
• Turbulent jet

Acknowledgements

We thank the referees for several useful comments. RN thanks the Defence Research and Development Organization, Government of India, in particular Dr S. Vathsal and Dr V. Siddhartha, for support of this work through a contract. He is also grateful to the Centre for Atmospheric and Oceanic Sciences at the Indian Institute of Science for their continued hospitality. AS is deeply thankful to JNCASR for awarding him a summer fellowship and to Dr N. R. Panchapakesan, Department of Applied Mechanics, IIT Madras for providing invaluable support and encouragement.