Investigation of coherent structure in the similarity region of the planar turbulent jet using POD and wavelet analysis

The coherent structure in the self-similar region of a turbulent planar jet at moderate Reynolds number was experimentally investigated by application of wavelet-based techniques and the Proper Orthogonal Decomposition (POD). Three rakes containing a total of 24 X-wire probes were used to acquire the required u′, v ′ and w′ velocity fluctuation time-series data. The measurements were performed for different spanwise separations between the rakes (i.e. different spanwise wavenumbers) at several streamwise locations. The POD technique was used to extract average coherent structures and their associated energy. Coherent structures are defined as spatial POD modes. The results show that the POD modes exhibit self-similar behavior for x/D > 60 (D is the nozzle width), which proves that the dynamics of the jet is self-similar. Projection of the POD modes onto instantaneous realizations of the flow field provides instantaneous shape of the coherent structure in the spatio-temporal domain. Two different kinds of structure were found in the jet: planar spanwise vortices and more complex spanwise periodic structures, topologically similar to an array of spanwise vortices. Among other things, a distinct intermittent behavior of these structures is observed. The results provide important and unique information about the topology of structures in the planar jet and lead to a better understanding of the jet physics.