Effects of passive control rings positioned in the shear layer and potential core of a turbulent round jet

Experimental measurements were carried out to study the effect of flow modification near the jet exit on a turbulent round free jet. To achieve this, two different fine rings, with square cross-sections, were designed and placed very close to the jet exit (x/D = 0.03). The first ring was placed in the middle of the shear layer while the second ring covered a small region of the potential core. Flying and stationary hot wire measurements were carried out to study the near- and intermediate-field development of the jet. Three Reynolds numbers (based on the jet exit mean velocity and the nozzle diameter) were used: 10 000, 30 000, and 50 000. The results show a considerable reduction in the jet spread rate and turbulence intensity when the passive rings are employed. This is more obvious for the ring placed in the shear layer. The radial profiles of the mean and rms axial velocity and third and fourth moments of fluctuations at several axial positions are considered. The power density spectra reveal the suppression of the initial shear layer instability (shear layer mode), while the jet preferred instability (preferred mode) remains active as the shear layer is modified. In the case when potential core modifications are used, the suppression of the initial shear layer instability is still considerable although there is an evidence of another vortical structure behind the ring, which, it is speculated, is due to the vortex shedding behind the ring. In this case, too, the jet preferred instability remains active. Therefore, the current paper confirms the separation of these two modes. In addition, the current contribution provides a comprehensive study of the characteristics of different length scales in the development region of the jet (e.g., integral scale, Taylor microscale, and Kolmogorov scale) for both the modified and unmodified jets.