Downstream evolution of the most energetic modes in a turbulent axisymmetric jet at high Reynolds number. Part 2. The far-field region

Abstract

The evolution of the most energetic features in the far-field region of a turbulent axisymmetric jet are investigated by applying the proper orthogonal decomposition (POD) to the double Fourier transform in time and azimuthal direction of the two-point velocity correlations of the streamwise velocity component. Measurements were obtained from 15 to 69 diameters downstream at jet exit Reynolds numbers ranging from 40 000 to 84 700 using two different experimental techniques, an array of 139 long hot wires and an articulated array of 15 hot wires.

The POD eigenspectra for the far jet were found to follow equilibrium similarity scaling sooner than the single-point statistics. The first POD eigenspectrum accounts for more than 50% of the resolved streamwise energy, and has three major peaks: a dominant one at azimuthal mode-2 for near-zero frequency, another at mode-1 at a constant local Strouhal number, $f \delta_{1/2}/U_c$, and a third smaller one for azimuthal mode-0 at near-zero frequency. The eigenspectra were independent of downstream position, but similar to those obtained as early as $x/D\,{=}\,6$ in Part 1 of this paper. All the eigenspectra integrated over frequency peaked at azimuthal mode-2.