Coherent structures in plumes with and without off-source heating using wavelet analysis of flow imagery

Abstract

Laser flow visualization using fluorescent dye provides instantaneous snapshots of dye concentration across selected sections of the flow. We demonstrate the potential power of wavelet analysis in revealing the nature of organization present in such pictures of turbulent shear flows, in a case where it is difficult to discern any such organization in the raw image. The technique consists of examining the contours of the transform coefficients using two-dimensional wavelets on the raw image. It is applied here to diametral cross sections of a plume with and without off-source heating. It is found that in the unheated plume there is definite evidence for a sinuous large-scale ring-like organization, which, however, is absent when the off-source heating is switched on. The proposed technique not only enables eduction of coherent structures in flow-visualization pictures, but can also provide a wealth of scale-specific information on flow structure.