Influence of aspect-ratio on the mixing and acoustic characteristics of plain and modified elliptic slot jets

This paper presents the results of an experimental program that was conducted to examine the effect of aspect-ratio on the flow development and the related acoustic properties of jets issuing from elliptic-slots. The aspect-ratios investigated are 2:1, 3:1 and 4:1. Fully expanded cases show a decrease in shear layer growth with increasing aspect-ratio resulting in a downstream shift in axis-switching location. Further the small aspect-ratio jet shows a dominance of large-scale structures close to slot exit relative to 3:1 and 4:1 jets. In the underexpanded condition the shock-structure development shows appreciable changes with aspect-ratio. At the end of first cell, oblique shock cross-over point in 2:1 jet is gradually replaced by a normal shock in 3:1 and 4:1 jets. The frequency content of jet noise also shows a corresponding increase in amplitude of fundamental screech frequency with increase in aspect-ratio. Introduction of notches along the minor-axis sides further modifies the shock development process which significantly alters the screech amplitudes for notched 3:1 and 4:1 jets indicating the weakening of the feedback process.