No abstract

No abstract

The efficacy of introducing mixing promoting small-scale vortices by two rectangular tabs, of aspect ratio 1.0, 1.5 and 2.0, placed at diametrically opposite locations at the exit of a Mach 1.73 convergent–divergent circular nozzle has been experimentally investigated, for NPRs from 4 to 8, covering overexpanded, correctly expanded and underexpanded states of the jet. The area blockage due to the each tab was 2.5% of the nozzle exit area. Keeping the blockage constant, the aspect ratio (defined as the ratio of length to width of the tab) was varied. A maximum core length reduction of 84.6% was caused by the tabs of aspect ratio 1.0, at underexpanded conditions corresponding to NPR (nozzle pressure ratio) 6. At this NPR, tabs of aspect ratio 1.5 and 2.0 caused core length reduction of 76.9% and 61.5%, respectively. The mixing promoting efficiency of aspect ratio 1.0 is found to be better than 1.5 and 2.0, at all NPRs of the present study, except NPR 5. The shadowgraph pictures of the uncontrolled and controlled jets clearly demonstrate the effectiveness of the tabs in weakening the waves in the jet core.

The mixing enhancement and core length reduction of a jet without significant loss of thrust are essential for reducing infrared radiation, mitigating aeroacoustic noise, improving combustion characteristics, and thrust vectoring. The jet mixing can be improved by manipulating the flow behavior. In subsonic and sonic jets, the flow manipulation may be achieved by utilizing nozzles with non-circular geometries that shed vortices of varying size due to their non-uniform azimuth curvatures. Non-uniform vortices generate differential spreading along the nozzle’s perimeter, causing axis switching and improving entrainment characteristics. Therefore, the present study examines the effects of two non-circular nozzle exit shapes (elliptic and square) on the mixing augmenting efficacy at subsonic and sonic flow conditions. The circular nozzle is tested for comparison. Both quantitative and qualitative analyses evaluate the efficacy of nozzles with non-circular exit geometries. Among the configurations investigated, the elliptic nozzle is superior in shortening the potential core length and enhancing the jet spread. A maximum reduction of 18.75% in core length with rapid jet decay was accomplished with the elliptic nozzle. The measurement of pressure profiles at different streamwise locations reveals that the spread rate is greater for elliptic and square jets than their circular counterpart. The elliptic jet exhibits the highest spread along the minor-axis direction compared to the major-axis direction. The differential jet spread rate in the elliptical jet causes an early axis-switching––direct evidence of mixing augmentation. Shadowgraph images show the asymmetric pattern of shock cell structures and differential spreading in elliptic and square jets.

No abstract