The effect of internal square cross-section grooves in a circular convergent-divergent nozzle as vortex generators on the spreading characteristics of supersonic jets ejected from the nozzle has been studied experimentally. The area ratio of the nozzle was equal to 1.44 and the outlet Mach number for the optimum expansion level was equal to 1.8. The experiments were carried out at three nozzle pressure ratios equal to 3.6, 5.5, and 7.2 corresponding to the over expansion, near optimum, and under expansion conditions, respectively, using both a plain circular nozzle and a nozzle with two diametrically opposite grooves. The radial Pitot pressure distributions were obtained at various axial distances from the nozzle exit. At overexpansion and near optimum expansion, the spread was lowered by introduction of grooves. At underexpansion conditions, the grooves increased the jet spread. The shear layer width of the jets from both plain and grooved nozzles increased with the growth of the axial distance but the variations revealed that the effect of grooves was to reduce the width at all axial locations at the over expansion and near optimum expansion levels. At the under expansion conditions, the jet from the grooved nozzle exhibited the higher shear layer width. The variation in the jet full width of both the jets exhibited a similar behaviour as that of the shear layer width. From the experimental data, it can be inferred that the grooves increased the spread at the underexpansion conditions and decreased the spread at the overexpansion and near optimum expansion conditions.
Ilakkiya et al. (Sun,) studied this question.