Elliptic supersonic jet control using C-D injections

Purpose This study aims to investigate the mixing and decay behaviour of a Mach 2 elliptic jet (aspect ratio 2) in an over-expanded state (nozzle pressure ratio 4), controlled by two convergent-divergent (C-D) injections at the nozzle exit. The objective is to evaluate the effectiveness of these injections, in two different configurations [injection along the major axis (IMA) and injection along the minor axis (IMI)], on jet mixing/decay, axis-switching and entrainment characteristics at three different injection pressure ratios (IPRs) of 4, 5 and 6. Design/methodology/approach Numerical simulations were carried out using the commercial software ANSYS-CFX. The study analysed the jet centreline Mach decay, density gradients, jet half-width and normalised mass flow rate to assess the impact of C-D injections on mixing/decay of the elliptic jet. The results were also compared with that of an equivalent circular jet of diameter 13 mm (Deq), to assess the superiority of mixing/decay characteristics of the elliptic jet. Findings The results indicate that the IMA configuration enhances axis-switching in the elliptic jet, whereas the IMI configuration significantly reduces the supersonic core (Lc*) of the jet at all the IPRs studied. In particular, for the IMI case at IPR 6, where the mass flow fraction (Cm) was 3%, a remarkable 78% reduction in the supersonic core was obtained when compared to the elliptic free jet. Practical implications The findings offer valuable insights into the jet mixing enhancement strategies using C-D injections, which can be applied in aerospace propulsion systems, fuel-air mixing in scramjets and noise reduction in high-speed jets. The ability to control axis-switching and reduce the supersonic core has potential applications in optimising the performance of aerospace vehicles. Originality/value Passive control techniques such as tabs, grooves, etc. have been employed on both circular as well as elliptic supersonic nozzles, while active control strategies on supersonic jets were predominantly carried out for axi-symmetric nozzles. Hence, the present study focusses to address this gap by numerically investigating the effect of C-D injections in two different orientations on an elliptic supersonic jet.