Unsteady Flow Conditions During Dual-Bell Sneak Transition
Résumé
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). Nomenclature α e = wall angle at nozzle exit, deg _ m = mass flow rate of test gas, kg∕s P a = ambient pressure, bar P w = local wall pressure, bar P 0 = stagnation pressure of the test nozzle, bar r t = radius of nozzle throat, mm X = coordinate along the nozzle axis, mm α i = wall angle at the inflection point, deg α P = skewness coefficient; 1 n P n i0 P w i − P w 3 ∕σ 3 w β P = kurtosis coefficient; 1 n P n i0 P w i − P w 4 ∕σ 4 w ∈ = area ratio of the dual-bell nozzle ∈ b = area ratio of the base nozzle ∈ s = area ratio of the location of flow separation σ w = standard deviation of wall pressure fluctuations; P n i0 P w i −P w 2 n−1 r σ w ∕P w max = nondimensionalized peak value of standard deviation in the region of separation φ = angle in circumferential direction, dege