Council of Scientific and Industrial Research–National Aerospace Laboratories,

Acold-gas test campaignwas conducted on a subscale dual-bell nozzle operatingunder 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 flowduring sneak transition is highly unsteady and is themajor source of side-load generation in the dual-bell nozzle preceding the final transition. Statistical analysis suggests that, as the separation frontmoves 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). Nomenclature αe = wall angle at nozzle exit, deg _m = mass flow rate of test gas, kg∕s Pa = ambient pressure, bar Pw = local wall pressure, bar P0 = stagnation pressure of the test nozzle, bar rt = radius of nozzle throat, mm X = coordinate along the nozzle axis, mm αi = wall angle at the inflection point, deg αP = skewness coefficient;