The effect of a cylindrical shroud on the performance of a stationary convergent nozzle

Abstract

One possible method of reducing jet noise at take-off is the use of an ejector in which the propelling nozzle discharges into a shroud and thereby induces a secondary flow. The present report describes an experiment on the aerodynamics of such systems. The primary nozzle was axisymmetric and convergent and the shrouds were cylindrical with faired convergent entries. The stagnation temperatures of the primary and secondary streams were equal and the secondary flow was entrained from the ambient static atmosphere. Ratios of jet total pressure to ambient static pressure ranged from 1.0 to 2.0. A systematic study was made of the effect of ejector length, ejector area ratio and jet pressure ratio on the ejector performance. This was specified in terms of the ratio of secondary to primary mass flow, the velocity profile at the shroud outlet and the thrust. The variation in static pressure along the shroud wall and the distribution of total and static pressure within a shroud were also determined. No acoustic measurements were made. The results are in fair agreement with the predictions of a simplified ejector theory which assumes uniform one-dimensional flow at the entry and outlet of the shroud and negligible skin-friction losses. The optimum shroud tested gave 15% more thrust than the unshrouded primary nozzle at a jet pressure ratio of 2.0.