Downstream evolution of the most energetic modes in a turbulent axisymmetric jet at high Reynolds number. Part 1. The near-field region

Abstract

Experiments were carried out in an axisymmetric turbulent jet from 2 to 6 diameters downstream at exit Reynolds numbers of 78 400, 117 600, and 156 800. Data were collected using the 138 hot-wire probe constructed by Citriniti & George (2000). The proper orthogonal decomposition (POD) was then applied to a double Fourier transform in time and azimuthal direction of the two-point velocity correlation tensor. Azimuthal mode-0, which dominated the dynamics at in the previous experiments, dies off rapidly downstream towards a non-zero value. For the higher azimuthal modes, the peak shifts from mode-6 towards lower azimuthal mode numbers with increasing downstream distance, until the peak is at mode-2 by the end of the potential core. The POD eigenspectra collapse in similarity variables for the mixing layer at all downstream positions, and are nearly independent of Reynolds number. Reconstruction of the full-field streamwise velocity component using the dominant POD modes shows clearly the evolution of the flow with downstream position, from ‘volcano-type’ eruptions at 2 to 3 diameters downstream to a ‘propeller-like’ pattern where the number of blades diminishes downstream.