The ‘step stall’ phenomenon of a supersonic Coanda jet

This study presents an experimental investigation into the influence of a step on the detachment characteristics of a supersonic Coanda jet. A parametric analysis was conducted by systematically varying the Coanda surface radius, jet outlet height and step height. Flow morphology was captured using high-speed schlieren and shadowgraph visualisation, while the critical nozzle pressure ratio ( italic NPR NPR $\\textit{NPR}$ ) for jet detachment was precisely measured with pressure transducers. For the baseline configuration without a step, the detachment italic NPR NPR $\\textit{NPR}$ (denoted italic NPR NPR $\\textit{NPR}$ d ) was predicted by a macroscopic mechanical model balancing centrifugal force and the radial pressure gradient, confirming a power-law relationship with the geometric ratio. The introduction of a step revealed a novel ‘step stall’ phenomenon: while increasing the step height initially enhances attachment and raises italic NPR NPR $\\textit{NPR}$ d by simplifying wave structures and suppressing separation bubbles, a further increase beyond a critical optimal value causes a sharp drop in performance. This optimal step height, which maximises the detachment italic NPR NPR $\\textit{NPR}$ , is shown to have a linear relationship with the normalised Coanda radius. The phenomenon is explained by the ineffective wall–jet interaction when an excessive step height prevents the jet’s core from being effectively ‘grabbed’ by the curved surface. These findings provide critical insights and practical design guidelines for optimising the Coanda effect in advanced flow control applications.