This study seeks to characterise the breakdown of the steady 2D solution in the flow around a 180-degree sharp bend to infinitesimal 3D disturbances using a linear stability analysis. The stability analysis predicts that 3D transition is via a synchronous instability of the steady flows. A highly accurate global linear stability analysis of the flow was conducted with Reynolds number $Re<1150$ and bend opening ratio (ratio of bend width to inlet height) $0.2\leq\beta\leq5$. This range of $Re$ and $\beta$ captures both steady-state 2D flow solutions as well as the inception of unsteady 2D flow. For $0.2\leq\beta\leq1$, the 2D base flow transitions from steady to unsteady at higher Reynolds number as $\beta$ increases. The stability analysis shows that at the onset of instability, the base flow becomes three-dimensionally unstable in two different modes, namely spanwise oscillating mode for $\beta=0.2$, and spanwise synchronous mode for $\beta \geq 0.3$. The critical Reynolds number and the spanwise wavelength of perturbations increase as $\beta$ increases. For $1<\beta\leq2$ both the critical Reynolds for onset of unsteadiness and the spanwise wavelength decrease as $\beta$ increases. Finally, for $2<\beta\leq5$, the critical Reynolds number and spanwise wavelength remain almost constant. The linear stability analysis also shows that the base flow becomes unstable to different 3D modes depending on the opening ratio. The modes are found to be localised near the reattachment point of the first recirculation bubble

Systems with multiple jets are encountered in many engineering applications, for example, propulsion units in aircraft and rockets. When more than one jet is placed close to each other, the resultant aerodynamics is complicated due to the mutual interaction of the jets. In the present work, mean flowfield and the mixing characteristics of free supersonic jets from twin and triple converging-diverging nozzles placed in close proximity are studied experimentally. The nozzles are designed for Mach numbers 1.5 and 2.0, with an inter-nozzle spacing of twice the nozzle exit diameter. The typical interaction process and the evolution of the triple jet are discussed using crosssectional contour plots. The influence of introducing additional similar jets on the near flowfield characteristics such as jet-spread, supersonic core, and the shock wave structure is studied using pressure measurements along the jet centerline. As the number of jets increases, the spreading rate decreases due to a decrease in the entrainment. This causes the jets to decay at a slow rate, and the core length increases in the order of an increased number of jets. Schlieren images of single, twin and triple jets reveal that the supersonic jet core is different in twin and triple when compared with a single jet .