Dual-bell nozzles permit two different operating modes, which provide higher performance than conventional bell nozzles when applied to rocket engines operating from sea level. During the low-altitude mode, the flow is separated and the separation line is located near the inflection of the nozzle wall, in a region characterized by a negative value of the wall pressure gradient, as in conventional nozzles, and in which side loads may occur. The characteristics of this region in hot full-scale applications are addressed by means of numerical simulations, and scaling laws are sought for cold subscale side-load experiments. Moreover, the flow behavior during the transition between the two operating modes is analyzed by time-accurate simulations.
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Martelli et al. (2007) studied this question.
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