Experimental investigation of the fine scale structure in turbulent gas-phase jet flows

The fine scale structure of gas-phase axisymmetric turbulent jets was investigated using two-component particle image velocimetry and planar laser-induced fluorescence. Measurements were conducted in a flow in which the classical Kolmogorov scale (h) was 0.6 mm and was fully resolved by the optical diagnostics. Statistics of vorticity, strain rate, kinetic energy dissipation, and scalar dissipation were computed, and show trends previously identified in experimental and numerical studies. In particular, good correlation was found between areas of high principal compressive strain, scalar dissipation, and kinetic energy dissipation. The effect of resolution on these statistics was explored by processing the data at three progressively worse resolutions. No difference in statistics was observed in the vorticity field; however, the coarser resolutions underestimated high magnitudes of principal compressive strain and kinetic energy dissipation. An analysis of the shape of scalar and kinetic energy dissipative structures showed that scalar dissipation layers tend to be sheet-like, while the shape of kinetic energy dissipative structures was seen to be more complex. Structures in the scalar dissipation field had thicknesses ranging from 1.5h to 6h with a mean thickness of 3.4h. The structures in the kinetic energy dissipation field exhibit more variation in thickness, ranging in size from 1h to 8h, with a mean thickness of 4h. These values are significantly lower than those observed in previous studies. The discrepancy may be due to resolution limitations in those studies. Additional data were acquired in a flow with higher Reynolds number and a Kolmogorov scale of 0.3 mm. The same analyses were performed for this flow condition, and the results showed essentially the same trends as the fully-resolved data. However, noticeable differences were now observed in all statistics as the resolution was worsened. In addition, the dissipative structure thickness analyses indicated that previous scaling laws for structure size may not be universal, but rather depend on the flow geometry and Reynolds number. The data suggest that resolution requirements for imaging of the fine scales might be relaxed to coarser than 1h depending on an investigation's particular purpose.