Organized Structure Dynamics in a Turbulent Round Jet

Abstract

The dynamics of large scale azimuthally-coherent structures obtained in a high Reynolds-number axisymmetric mixing layer are presented. The structure dynamics are obtained by application of the Proper Orthogonal Decomposition (POD) to an ensemble of realizations of the streamwise velocity field obtained from 138 probes located 3 diameters downstream of the nozzle exit. The velocity field is measured at all 138 positions simultaneously, making it possible to obtain the instantaneous coefficients of the POD modes as well as to extract the large scale structure.Pictures showing the dynamics of the structure interaction in the mixing layer demonstrate the importance of the mode-0, 3, 4, 5 and 6 azimuthal modes in both the entrainment and advection of fluid in the layer. Azimuthally coherent structures, which are remnants of circular vortex rings produced in the shear layer, appear near the potential core at regular intervals corresponding to the Strouhal frequency. The region between rings is dominated by a high azimuthal mode structures which appear near the outside of the layer. These structures do not appear to be singular but rather are made up of pairs of counter-rotating streamwise vortices that are very similar to the ribs seen in simulations of the axisymmetric layer and in plane mixing layers. The vortices advect high-momentum fluid toward the outside of the layer and low-momentum fluid toward the potential core.KeywordsShear LayerCoherent StructureProper Orthogonal DecompositionPotential CoreProper Orthogonal Decomposition ModeThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.