Enhanced Thrust-Efficiency of Overexpanded Nozzles by Passive Venting

Abstract

The design of thrust nozzles for airbreathing, hypersonic vehicles requires an efficient propulsion airframe integration. Single expansion ramp nozzles (SERN) have a unique installation advantage because the vehicles aft end can be used as an external expansion ramp to achieve high area ratios [1]. Low thrust efficiencies have been measured at transonic flight regime due to the overexpansion of the nozzle. One approach to improve the off-design performance penalities is the passive cavity concept. The basic idea behind this concept originates from the design of supercritical airfoils [2]. The drag reduction at transonic speeds is achieved by replacing a part of the airfoils upper surface by a porous plate with a vent chamber beneath it. Applied to the overexpanded nozzle the passive ventilation along the ramp enables the self-adjusting capabilty of the jet to low pressure ratios by encouraging flow separation. The objective of the present study is to investigate the potential benefit in thrust efficiency by this method. A parametric study varying the percentage of porosity and cavity geometry is conducted to find an optimal configuration.