On the vortical structures and behaviour of inclined elliptic jets

Abstract

A study has been carried out to clarify vortical structures and behaviour resultant from imposing inclined exits along either the major or minor plane of an elliptic nozzle. Laser-induced fluorescence (LIF) flow visualizations show production of inclined vortex roll-ups along the inclined planes, with corresponding narrowing of jet columns along the non-inclined planes. Minor-plane inclined nozzles result in significant growths in the jet spread along the inclined plane, while major-plane inclined nozzles produce little variations. Formation of rib structures is observed to be suppressed in minor-plane inclined nozzles and linked to braid vortices inducing the formation of streamwise vortices along the minor plane. Particle-image velocimetry measurements show that increasing the incline angle in major-plane inclined nozzles reduce the strengths of the discrete vortex roll-ups, while the opposite occurs in minor-plane inclined nozzles. Although Reynolds shear stress variations correspond well with changes in incline angle and vortex roll-up strength in major-plane inclined nozzles, they demonstrate a non-monotonic relationship in minor-plane inclined nozzles. LIF visualizations further clarify how strong asymmetric interactions between the inclined vortex roll-ups and braid vortices lead to suppression of axis-switching in major-plane inclined nozzles but not in minor-plane inclined nozzles. The more complex flow behaviour in the latter is responsible for the non-linear relationship in Reynolds shear stress levels observed earlier. Comparisons of the half-jet width profiles confirm the suppression of axis-switching in major-plane inclined nozzles only, while momentum thickness profiles show significant variations in the mixing layer characteristics between major- and minor-plane inclined nozzles.