Turbulence structure of neutral and negatively buoyant jets

High-fidelity measurements of velocity and concentration are carried out in a neutral jet (NJ) and a negatively buoyant jet (NBJ) by injecting a jet of fresh water vertically downwards into ambient fresh and saline water, respectively. The Reynolds number (Re) based on the pipe inlet diameter (d) and the source velocity (W-o) is approximately 5900 in all the experiments, while the source Froude number based on density difference is approximately 30 in the NBJ experiments. Velocity and concentration measurements are obtained in the region 17 <= z/d <= 40 (z being the axial coordinate) using particle image velocimetry and planar laser induced fluorescence techniques, respectively. Consistent with the literature on jets, the centreline velocity (W-c) decays as z(-1) in the NJ, but in the NBJ, Wc decays faster along z due to the action of negative buoyancy. Nonetheless, the mean velocity (W) and concentration (C) profiles in both the flows exhibit self-similar Gaussian form, when scaled by the local centreline parameters (W-c, C-c) and the jet half-widths (r*(W), r*(C)). On the other hand, the turbulence statistics and Reynolds stress in the NBJ do not scale with W-c. The results of autocorrelation functions, integral length scales and two-dimensional correlation maps show the similarity of turbulence structure in the NJ and the NBJ when the axial and radial distances are normalised by the local jet half-width. Further, the spectra and probability density functions are similar on the axis and only minor differences are seen near the jet interface. The above findings are fundamentally consistent with our recent analysis (Milton-McGurk et al., J. Fluid Mech., 2020b), where we observed that the mean and turbulence statistics in the NBJ have different development characteristics. Overall, we find that the turbulence structure of the NBJ (when scaled by local velocity and length scales) is very similar to the momentum-driven NJ, and the differences (e.g. spreading rate, scaling of turbulence intensities, etc.) between the NJ and the NBJ seem to be of secondary importance.