The control of co-flowing jets by varying lip thickness has been studied experimentally. Lip thickness is defined as the thickness of primary nozzle wall separating primary jet and secondary jet at the co-flowing nozzle exit. Co-flowing jets from a primary nozzle of diameter 10 mm (1.0 Dp) and a secondary duct with lip thickness (LT) 0.2 Dp, 1.0 Dp and 1.5 Dp at Mach numbers 0.6, 0.8 and 1.0 have been studied. Jet centreline total pressure decay, static pressure variation and jet mixing behaviour were analysed. The results show that the mixing of the co-flowing jet with substantial values of lip thickness is superior to the co-flowing jets with comparatively lower values of lip thickness. Co-flowing jets with lip thickness 1.0 Dp and 1.5 Dp experience a significantly higher mixing than the lip thickness 0.2 Dp jet, for all Mach numbers analyzed in the present study. Moreover, in the case of correctly expanded jets, the local static pressure is assumed to be equal to atmospheric pressure. This assumption becomes invalid for co-flowing jets with substantial lip thickness. The centerline static pressure varies sinusoidally above and below atmospheric pressure by a maximum of 11 %, which is due to wake dominance.

The effects of bypass ratio on co-flowing subsonic and correctly expanded sonic jet decay have been studied experimentally. Co-flowing jets with lip thickness 1.0Dp (where Dp is the diameter of primary nozzle and is equal to 10 mm) with bypass ratios of around 0.7, 1.4, and 6.4 at primary jet exit Mach numbers 0.6, 0.8, and 1.0 have been analyzed. A single free jet equivalent to primary nozzle of the co-flowing nozzle was considered for comparison. Primary jet centerline total pressure decay, spread, and static pressure variation were investigated. The results show that the mixing of the high bypass ratio co-flowing jet with lip thickness 1.0Dp is superior to low bypass ratio co-flowing jet. Both lip thickness and bypass ratio have a strong influence on the co-flowing jet mixing. Bypass ratio 6.3 experiences a significantly higher mixing than bypass ratio 0.7 and 1.4. Selected jets were also investigated computationally. The computations capture the salient flow physics and reproduce well with the experiments.

The effect of tab geometry and its orientation placed at the nozzle exit on the evolution of an axi-symmetric sonic under-expanded and fully expanded jet was investigated experimentally. The tab used was a hollow semi-circular tube. The near jet flow field was studied for two configurations of the tab, namely, the concave surface facing the flow exiting nozzle (arc-tab facing-in) and convex surface facing the flow (arc-tab facing-out) for three different protrusions of the tab corresponding to blockages of 3.82, 7.64, and 11.46 per cent (z 1 , z 2 , and z 3 , respectively). The results were compared with that obtained for a plain rectangular tab of the same blockage and a plain circular nozzle. The jet was found to decay at a faster rate in the case of arc-tab facing-in configuration at all blockage levels as compared with the other two configurations. With arc-tab facing-in, the core length was reduced by 80 per cent and the corresponding reduction was 40 per cent for the jet with arc-tab facing-out and rectangular tabs. For the under-expanded sonic jet at the nozzle pressure ratio (NPR) 2, 3, and 5, the arc-tab facing-in configuration was found to reduce the potential core length and weaken the shock structure significantly as compared with the other two configurations as indicated by the lesser amplitude in pitot pressure variations. The shadowgraph pictures showed that the shock-cell structure was significantly perturbed and length of the cells was reduced considerably. This effect was found to be more pronounced at higher blockages. The iso-baric contours indicate that the jet spread is faster and wider in the direction normal to the tab revealing that the streamwise vortices shed by the tabs cause an inward indentation of the ambient air into the jet core and outward ejection of the jet core to the ambient. The geometry and orientation of the tab along with the level of blockage seem to have a profound influence on the development of the jet in the near field.

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.

This paper presents the numerical simulation of the subsonic jets controlled by slanted perforated tabs and its performance of mixing efficiency is compared with the jet controlled by solid tab and free jet. The objective of this paper is to study the performance of slanted per foration geometry tabs in controlling high speed jets to enhance the mixing of jet with the ambient air, to sup press the noise level and to minimize the thrust loss. In this paper the simulations have been carried out using the commercial meshing and analysis software. Due to the effect of tabs the potential core decay occurs and velocity reduces drastically because of enhanced mixing produced by the tabs. From the results it is found that in slanted per forated tab the main jet interacts with the slanted perfo rated jet which causes in effective mixing, instability in jets and lower thrust loss when compared with the free jet. The decay of the potential core and velocity reduction is computed by simulation for 0.4 Mach number. Velocity plots are obtained at both near field and far field down stream locations to study the jet distortion with slanted perforated tabs and solid tabs. The results obtained for perforated tabs for 0.4 Mach number are also compared with various other Mach numbers. They have also been validated with experimental results which show good agreement with the computational results.

Effect of nozzle geometries on the propagation of twin jet issuing from nozzles with circle-circle, circle-ellipse, circle-triangle, circle-square, circle-hexagon and circle-star geometrical combinations was investigated for Mach numbers 0.2, 0.4, 0.6 and 0.8. In all the cases, both jets in the twin jet had the same Mach number. All the twin jets of this study are free jets, discharged into stagnant ambient atmosphere. The result of the twin jets issuing from circle-circle nozzle is kept as the reference in this study. For all the twin jet nozzles, the inter nozzle spacing; the distance between the nozzle axes (S) was 20 mm and all the nozzles had an equivalent area of 78.5 mm2. Thus for all the cases of the present study, S/D ratio is 2. The results show that the mixing of the combined jet, after the merging point is strongly influenced by the combined effect of the nozzle geometry and jet Mach number. Among the six different twin jet nozzle configuration studied, circle-square combination is found to be the most superior mixing promoter.

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