Abstract
Abstract Previous research on Dual-Bell nozzle flow always neglected the influence of the outer flow on the nozzle flow and its transition from sea level to altitude mode. Therefore, experimental measurements on a Dual-Bell nozzle with trans- and supersonic external flows about a launcher-like forebody were carried out in the Trisonic Wind Tunnel Munich with particle image velocimetry, static pressure measurements and the schlieren technique. A strongly correlated interaction exists between a transonic external flow with the nozzle flow in its sea level mode. At supersonic external flow conditions, a Prandtl–Meyer expansion about the nozzle’s lip decreases the pressure in the vicinity of the nozzle exit by about 55%. Therefore a new definition for the important design criterion of the nozzle pressure ratio was suggested, which considers this drastic pressure drop. Experiments during transitioning of the nozzle from sea level to altitude mode show that an interaction about the nozzle’s lip causes an inherently unstable nozzle state at supersonic free-stream conditions. This instability causes the nozzle to transition and retransition, or flip-flop, between its two modes. This instability can be eliminated by designing a Dual-Bell nozzle to transition during sub-/transonic external flow conditions.
Highlights
The Ariane 5 space launcher has a geometric discontinuity, similar to a backwardfacing step (BFS), at the end of its main stage ahead of the cryogenic engine
The static pressure reduces by another 2%, resulting in an overall pressure decrease of about 9% from the free-stream to the pressure in close vicinity of the nozzle
About the nozzle lip, the pressure decreases by nearly 55%, resulting in an overall pressure reduction of around 58% in close vicinity of the nozzle exit! This pressure reduction is bound to have an effect on the transitioning of the nozzle, an effective nozzle pressure ratio (N P Reff ), which accounts for the large pressure drop about the nozzle lip, has been defined [4]
Summary
The Ariane 5 space launcher has a geometric discontinuity, similar to a backwardfacing step (BFS), at the end of its main stage ahead of the cryogenic engine. With the application of passive flow control, the root mean square (RMS) of the pressure fluctuations was reduced by 35% [3] This load reduction is achieved through the imprinting of strong streamwise vorticity aft of the BFS with so-called ‘lobes’ on the step. Even at takeoff the sea level mode’s overexpansion is not as extreme as it is the case for a conventional rocket nozzle This increases the thrust integral within the troposphere while avoiding the risk of high side loads due to unsteady flow separation during the start-up of the engine. The interaction of the external flow with the nozzle flow has always been neglected This may have a drastic effect on the transition behavior of a Dual-Bell, since the afterbody flow of a launcher may cause large unsteady deviations in the pressure in close vicinity of the nozzle exit. It has been the aim of the underlying research to characterize severity of these effects
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