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This study aims to present the numerical study on supersonic jet mixing characteristics of the co-flow jet by varying lip thickness (LT). The LT chosen for the study is 2 mm, 7.75 mm and 15 mm.

The primary nozzle is designed for delivering Mach 2.0 jet, whereas the secondary nozzle is designed for delivering Mach 1.6 jet. The Nozzle pressure ratio chosen for the study is 3 and 5. To study the mixing characteristics of the co-flow jet, total pressure and Mach number measurements were taken along and normal to the jet axis. To validate the numerical results, the numerical total pressure values were also compared with the experimental result and it is proven to have a good agreement.

The results exhibit that, the 2 mm lip is shear dominant. The 7.75 mm and 15 mm lip is wake dominant. The jet interaction along the jet axis was also studied using the contours of total pressure, Mach number, turbulent kinetic energy and density gradient. The radial Mach number contours at the various axial location of the jet was also studied.

The effect of varying LT in exhaust nozzle plays a vital role in supersonic turbofan aircraft.

Supersonic co-flowing jet mixing effectiveness by varying the LT between the primary supersonic nozzle and the secondary supersonic nozzle has not been analyzed in the past.

The purpose of this research is to study and investigate the flow control of 0.8 Mach jet using three tab configurations. The tabs with the slots will eventually lead to generation of vortices and thus enhances the mixing characteristics.

The jet flow control is achieved by the usage of three tabs, namely, Tab A, Tab B and Tab C that are placed at the exit plane of the convergent nozzle at 180 degrees apart. Three tabs with different slot profile are designed with the same constant blockage ratio of 7.3%. The tabs produce vortices of varying sizes that directly influence and modify the jet structure, thereby enhancing the efficiency in mass entrainment and mixing. The tabs are studied numerically first and then are compared with the results of the experiments.

The results are compared with that of the results of the uncontrolled jet. For Mach 0.8 jet, Tab C is found to reduce the core length and gives reduction of 90.23%, in comparison to Tab A and Tab B, which provides 84.1% and 87.79%, respectively. The results of numerical are then compared with the centerline results obtained via experiments. With the engagement of Tabs A, B and C, the jet structure is seen to have been modified at Mach 0.8 with Tab C performing better.

The tabs are a passive control device that can be practically enabled in the aircraft nozzles to control the flow and even suppress the noise emanated by the jet. Tabs can be effectively used for better thrust vector control and assist in jet noise suppression. Thus, this study on tabs and its uses are important and essential in aerospace technology.

This particular study on mechanical slotted tabs is innovatively carried out by designing the tabs in such a way that one such has not been designed before. The slots run through the adjacent sides of the tabs which is a novelty in itself, whereas perforations made only through the opposite sides of the tabs are studied by various researchers till now. The slots in the adjacent faces modify the flow physics in such a way that it enhances mixing by the creation of turbulence because of the interaction between the main stream and the secondary jet exactly at the core. So far, such slots and profiles are not investigated. By the usage of such tabs, the flow to mix faster is much closer to the core of the jet by creating mixed size vortices and thus has higher efficiency.

In comparison to a nozzle with a larger/finite separation distance (Thanigaiarasu et al., 2019), a thin-lip nozzle (Srinivasarao et al., 2017) minimizes drag. Coaxial nozzles with thin lips are an appropriate tool for studying high subsonic jets because it does not create a dominant re-circulation zone. This study aims to analyze the characteristic of separation distances, between primary and secondary nozzles, within the range of 0.7–3.2 mm which can be considered a thin lip.

A separation distance of 0.7  (Papamoschou, 2004), 1.7  and 2.65 mm (Lovaraju and Rathakrishnan, 2011) is considered for the present study. The main nozzle exit Mach number is maintained at a subsonic condition of Mach 0.6, and the co-flowing nozzle exit Mach number is varied from 0% (secondary jet stopped/single jet) to 100% (Mach 0.6) in steps of 20% with respect to the main nozzle exit Mach number. A comparison was made between these velocity ratios for all three lip thicknesses in the present study. Design mesh and analysis were done by using Gambit 2.6.4 and Fluent 6.12. Velocity contours and turbulence contours were studied for qualitative analysis.

When lip thickness increases from 0.7 to 2.65 mm, the potential core length (PCL) of the primary jet decreases marginally. Additionally, the PCL of the primary jet elongates significantly as the velocity ratio increases. The primary shear layer is dominant at 20% co-flow (20 PCF), less dominant at 60% co-flow (60 PCF) and almost disappeared at 100% co-flow (100 PCF). Concurrently, the secondary shear layer almost disappeared in 20 PCF, dominant in 60 PCF and more dominant in 100 PCF. Different zones such as initial merging, intermediate and fully merged zones are quantitatively and qualitatively analyzed.

Co-flow nozzle is used in turbofan engine exhaust. The scaled-down model of a turbofan engine has been analyzed. Core length is directly proportional to the jet noise. The PCL signifies the jet noise reduction in a high-speed jet. For a low-velocity ratio, the potential core is reduced and hence can reduce the jet noise. At the same time, as the velocity ratio increases, the mass flow rate of the coaxial increases. The increase in the mass flow increases the thrust of the engine. The aircraft engine designer should analyze the requirement of the aircraft and choose the optimal velocity ratio coaxial nozzle for the engine exhaust (Papamoschou, 2004).

There have been many research studies carried out previously at various lip thickness such as 0.4  (Georgiadis, 2003), 0.7  (Papamoschou, 2004), 1.5  (Srinivasarao et al., 2014a), 1.7  (Sharma et al., 2008), 2  (Naren, Thanigaiarasu and Rathakrishnan, 2016), 2.65  (Lovaraju and Rathakrishnan, 2011), 3  (Inturiet al., 2022) and 3.2 mm (Perumal et al., 2020). However, there is no proper study to vary the lip thickness in this range from 0.7 to 3.2 mm to understand the flow behavior of a co-flowing jet.