Vortex dynamics in jets from inclined nozzles

Experimental tests were performed on round jets exiting inclined nozzles at a Reynolds number of 9000. Both natural jets and jets forced with single frequencies corresponding to St_D = 0.25, 0.5, 0.75, and 1.0 were examined. In the natural case, the nozzle incline caused a mild increase in the radial spreading in the plane of azimuthal symmetry. The forcing amplified the asymmetric radial spreading by altering the vortex structure. In general, the inclined vortex rings rolled up at an angle slightly smaller than the nozzle incline angle. As the rings moved downstream, they migrated away from the jet centerline and their incline angle increased. Vortex rings generated at St_D = 0.5 did not pair because that Strouhal number was near the "preferred" mode. For nozzles with slight inclines, forcing at larger Strouhal numbers led to pairing near x/D = 2 in order to achieve the "preferred" mode. For nozzles with larger inclines, the vortex cores broke down before pairing could occur. Forcing at a lower Strouhal number (St_D = 0.25) yielded ring formation at St_D = 0.5 and subsequent pairing. Increasing the incline angle moved the pairing location closer to the nozzle lip. Also, the pairing process was found to depend on the nozzle incline angle.