A scale-dependent coherence analysis of turbulent round jets including the effects of shear layer manipulation

Abstract

Two-point hot-wire measurements were conducted to assess the scaling of coherent structures in turbulent round jets. A scale-dependent coherence analysis was applied to datasets consisting of two-point synchronized axial velocity measurements with varying radial separations between the hot-wire positions while the reference hot-wire was fixed at the centreline and two off centreline positions in different datasets. Measurements were carried out at ReD=10000, 20000, and 50000 and a range of axial locations (3 ≤ x/D ≤ 25). The scale-dependent coherence analysis reveals a hierarchical structure of eddies over the axial range 15 ≤ x/D ≤ 25 for λx/y0.5 < 3, where λx is the axial wavelength and y0.5 is the jet half width. While these eddy structures are not geometrically self-similar at the centreline, away from the centreline they approach self-similarity status. In the jet shear-layer where the eddy structures are self-similar, a stochastic aspect ratio of ▪ ≈ 3.4 was found at ReD=10000. The aspect ratio increases to ▪ ≈ 3.9 at ReD=20000, and ▪ ≈ 4.7 at ReD=50000. These values are significantly smaller than the aspect ratio of ▪ =14 observed in turbulent boundary layers. The eddy structure aspect ratio is not affected by the axial distance from the nozzle (within x/D=15−25) or shear layer manipulation by control rings. However, the stochastic radial span of the largest eddies was found to change due to shear layer manipulation and change in the axial location.