Influence of tab geometry and its orientation on under-expanded sonic jets

Abstract

The effect of tab geometry and its orientation placed at the nozzle exit on the evolution of an axi-symmetric sonic under-expanded and fully expanded jet was investigated experimentally. The tab used was a hollow semi-circular tube. The near jet flow field was studied for two configurations of the tab, namely, the concave surface facing the flow exiting nozzle (arc-tab facing-in) and convex surface facing the flow (arc-tab facing-out) for three different protrusions of the tab corresponding to blockages of 3.82, 7.64, and 11.46 per cent ( z1, z2, and z3, respectively). The results were compared with that obtained for a plain rectangular tab of the same blockage and a plain circular nozzle. The jet was found to decay at a faster rate in the case of arc-tab facing-in configuration at all blockage levels as compared with the other two configurations. With arc-tab facing-in, the core length was reduced by 80 per cent and the corresponding reduction was 40 per cent for the jet with arc-tab facing-out and rectangular tabs. For the under-expanded sonic jet at the nozzle pressure ratio (NPR) 2, 3, and 5, the arc-tab facing-in configuration was found to reduce the potential core length and weaken the shock structure significantly as compared with the other two configurations as indicated by the lesser amplitude in pitot pressure variations. The shadowgraph pictures showed that the shock-cell structure was significantly perturbed and length of the cells was reduced considerably. This effect was found to be more pronounced at higher blockages. The iso-baric contours indicate that the jet spread is faster and wider in the direction normal to the tab revealing that the streamwise vortices shed by the tabs cause an inward indentation of the ambient air into the jet core and outward ejection of the jet core to the ambient. The geometry and orientation of the tab along with the level of blockage seem to have a profound influence on the development of the jet in the near field.