An investigation of the Reynolds-number dependence of the axisymmetric jet mixing layer using a 138 hot-wire probe and the POD

The Proper Orthogonal Decomposition (POD) technique is applied to find the effects of Reynolds number and the characteristics of the organized motions or coherent structures as a function of downstream position in a turbulent axisymmetric shear layer. In this experiment measurements were made at positions of x/D = 2 to 6 spaced 0.5_x/ D apart along downstream direction for the three different jet Reynolds number of 78 400, 117 600, and 156 800. Data were taken simultaneously at all measuring positions using 138 hot-wire probes.; One of the goals of this investigation was to quantify how the energy distribution among the various modes varied as a function of Reynolds number and to determine when or if ever the asymptotic trends become Reynolds number independent. Also, since the earlier experiments were performed at only a single downstream position, another goal was to investigate whether the modal character of the flow changed with downstream position.; In brief, the distributions of POD mode energy have strong dependence on x/D. Even more surprising, mode-0 behaves in a manner entirely different than the higher modes. Also, the results are very nearly independent of Reynolds number. The lowest azimuthal mode (mode-0) for all POD Modes, which dominate the dynamics at x/ D = 2 dies off rapidly downstream. On the other hand, for the higher azimuthal modes, the peaks shift to lower mode numbers and actually increase with downstream distance. The eigenvalues from the first POD mode vary as a function of mode azimuthal number for (m ≥ 1), downstream distance and Reynolds number. With scaling by x/ D, they collapse for all Reynolds numbers onto a single curve.; The full-field streamwise instantaneous fluctuating velocity was reconstructed from the POD modes. Modes constructed from these show clearly that the “volcano”-like events around x/D = 2 to 3 evolve into a more “propeller”-like pattern where the number of “blades” diminishes downstream. It will be argued that this behavior is similar to that predicted from inviscid instability theory.