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Unsteady Flow Conditions During Dual-Bell Sneak Transition

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A cold-gas test campaign was conducted on a subscale dual-bell nozzle operating under sea-level conditions to study the unsteady flow conditions encountered during sneak transition, which is a phenomenon prevalent just before final transition occurs. The study reveals that the flow during sneak transition is highly unsteady and is the major source of side-load generation in the dual-bell nozzle preceding the final transition. Statistical analysis suggests that, as the separation front moves into the region of wall inflection, the separated shear layer gradually comes in close proximity to the nozzle extension wall that alters the flow development process in the recirculation/backflow region considerably. This sets the entire backflow region into pressure fluctuations, making the flow conditions highly unsteady. It is further observed that the flow during sneak transition is associated with low frequencies in the vicinity of the separation location (0.8 kHz), which decreases as the sneak transition nozzle pressure ratio is approached (0.2 kHz).


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