An experimental investigation on the mixing characteristics of Mach 1.5 rectangular jet emanating from aspect ratios (ARs) 2 and 3 convergent-divergent nozzles is carried out at nozzle pressure ratios (NPRs) 3, 3.69, 4 and 5, respectively. At overexpanded (NPR 3), correctly expanded (NPR 3.69) and underexpanded (NPRs 4, 5) levels of Mach 1.5 jet, the Pitot pressure measured along the centerline of AR2 and AR3 rectangular jets showed that the influence of aspect ratio on the jet mixing is not as strong as expected. The effect of aspect ratio is noticeable only in the core of both the jets. The core length of AR2 jet is shorter than AR3 jet at all the NPRs, which indicates faster mixing of AR2 jet with ambient fluid as compared to AR3 jet. But after core, the decay of AR2 and AR3 jets in the characteristic decay region remained almost same at all the NPRs except at correctly expanded NPR of 3.69, the AR3 jet decays slightly faster than AR2 jet. For all the cases, the jets have become fully developed together at far downstream location. The pressure profiles taken along major axis and minor axis of Mach 1.5 rectangular jets confirm the faster spreading of AR2 and AR3 jets along minor axis than along major axis. Both the jets are found to switch axes between 8D e to 10D e . The shadowgraph visualization reveals the complex structure of waves present in the major axis and minor axis planes of AR2 and AR3 rectangular jets.

The jet mixing caused by two rectangular and triangular tabs of geometric blockage 2.5% each, separated by 90⁰ at the nozzle exit plane of a Mach 2 circular jet, in the presence of varying levels of expansion, corresponding to nozzle pressure ratio (NPR) 4–8, was investigated experimentally. The relative mixing promoting capability of these tabs was assessed. It is found that the rectangular tabs at 90⁰ interval retards mixing for NPRs 4, 5, and 6, and promotes mixing at NPRs 7 and 8. Comparison of the present result for rectangular tabs at 90⁰ interval with that of Arun Kumar and Rathakrishnan 8 for 180⁰ interval shows that the mixing promoting capability of 90⁰ and 180⁰ intervals are comparable, at NPR 7. But at NPR 8, the mixing performance of rectangular tabs at 90⁰ interval is better than 180⁰ interval. For NPRs 7 and 8, the core length reduction due to rectangular tabs at 90⁰ and 180⁰ are 26% and 36%, and 28% and 26%, respectively. On the contrary, the triangular tabs at 90⁰ interval promote mixing at all NPRs of the present study. The mixing caused by triangular tabs at 180⁰ interval reported in literature 8 is only marginally better than triangular tabs at 90⁰ interval studied in this work. For NPR 4, 5, 6, 7, and 8, the core length reduction caused by triangular tabs, at 90⁰ and 180⁰ intervals, are 24%, 58%, 69%, 80%, and 76%; and 38%, 65%, 77%, 87%, and 85%, respectively.

The mixing characteristics of a Mach 1.9 jet at three levels of overexpansion, corresponding to nozzle pressure ratio (NPR) 3, 4 and 5, in the presence of a sonic co-flow (secondary flow), which was submerged in a subsonic co-flow (tertiary flow) was studied experimentally. For these NPRs the secondary co-flow is sonic with underexpanded levels and the tertiary flow Mach number was found to be 0.41, 0.71 and 0.85, respectively. The centerline decay results of the primary jet show that the jet mixing is abated by the co-flow, at all levels of expansion. However, in spite of the reduced mixing encountered by the supersonic primary jet, the waves in the jet core are found to be weaker in the presence of co-flows. This may be regarded as an advantage from the shock associated noise point of view, in accordance with Tam’s theory; which states weaker the waves in the core, the lesser is the shock associated noise. The results show that the reduced mixing environment caused by the sonic co-flow alone leads to the jet core elongation of about 20%, 23% and 49%, at NPRs 3, 4 and 5, respectively. The core length of the jet is found to increase by 29%, 46% and 62%, respectively, at NPRs 3, 4 and 5, when both sonic and subsonic co-flow streams are present.

A core Mach 2 jet from the convergent-divergent nozzle in the presence of two coaxial sonic streams from convergent nozzles was investigated experimentally at different nozzle pressure ratios using visualization technique. The jet field was visualized using shadowgraph for nozzle pressure ratio (NPR) 3 to 6. From the visualization results, it was observed that the core length of Mach 2 jet for NPRs 3 to 6 increased with increasing coflow NPRs, indicating reduced mixing of coflow three stream jets as compared to Mach 2 jet (without coflows). From the present study, it was clear that the use of two sonic coflows on Mach 2 jet would result in core elongation for all the NPRs thus acting as a mixing inhibitor. But the sonic coflows were found to weaken the waves in Mach 2 jet, which is advantageous from the aeroacoustic point of view.

Stable 200-400-MeV quasi-monoenergetic electron bunches (ΔE/E<10%), ~ 10-pC charge, and no dark-current are produced when a self-injected laser plasma accelerator is optimized. PIC simulations demonstrate these beams are produced near the threshold for selfinjection.