Unsteady Flow Conditions During Dual-Bell Sneak Transition

Abstract

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

Numerical Analysis of Side-loads Reduction in a Sub-scale Dual-bell Rocket Nozzle

“…The spatial evolution of the mean wall pressure ( p w ∕p a ), averaged in time and in the azimuthal direction, is presented in Fig. 5(a), compared with the reference experimental data (Verma et al 2015). The mean wall pressure decreases until the separation point, which is located at the inflection point, then the oblique shock causes a sudden increase up to a plateau value close to the ambient pressure level.…”

“…The present investigation is carried out on the dual-bell geometry experimentally tested at DLR (Verma et al 2015), characterized by a throat radius r t = 0.01 m, a wall inflection angle (5)…”

“…Génin and Stark (2011) confirmed experimentally the existence of the inflection region and that during the transition of the separation point in this zone the level of the side loads is similar to that suffered by a conventional nozzle. Verma et al (2015) studied the flow unsteadiness when the separation front was located in the inflection region by analyzing the spectral content of the wall-pressure signature. They related the onset of side loads during the transition in the inflection region to the high level of unsteadiness suffered by the flow in this regime.…”

“…In this work, a calibrated version of the delayed detached eddy simulation (DDES) of a sub-scale cold-gas dual-bell nozzle in the first operating mode is carried out. The geometry is inspired by the experimental work of Verma et al (2015) and it is characterized by a high nozzle-exit Mach number, closer to a real application than the dual bell used by Proschanka et al (2012). The simulated NPR is experimentally characterized by a low level of side loads, and the separation front is located at the inflection point, at the very beginning of the inflection region.…”

Large-eddy simulation of a spatially-evolving supersonic turbulent boundary layer atM∞=2

“…The study of supersonic turbulent boundary layers (STBL) is crucial for understanding basic flow physics in turbulent wall-bounded flows. The study has also a great importance in many industrial applications, such as high speed external and internal aerodynamics [1,2,3,4], combustion and detonation [5,6]. For adiabatic STBL, and due to viscous heating, compressibility effects arise mainly from the large change in the fluid properties (variable-density flow).…”