Srinivasan K

An experimental study of the acoustic far field and flow field of co-axial circular and non-circular jets with the vane angles of 0° and 30° are presented in this paper. Flat vanes are fixed in the annular passage of the coaxial jets, which cause trailing edge vortices and/or swirl flow. The co-axial jets with vane angles of 0° and 30° almost eliminate the transonic tones and screech tones entirely, in contrast to non-swirl jet. The broadband shock associated noise is also mitigated by the co-axial jets, irrespective of the vane angles. The co-axial jets are efficient in overall noise suppression at both subsonic and supersonic conditions. The co-axial jets diminish the low frequency noise and enhance the high frequency noise. Reduction in core length is highest for the co-axial jets compared to the non-swirl jet. The flow visualization study also confirms that the co-axial jets weaken or destroy the shock cells and reduce the number of the shock cells compared to non-swirl jet.

Experimental studies are carried out to reduce the jet noise using co-axial swirlers in the form of curved vanes fixed in an annular passage. The swirl numbers considered for the present work ranged from 0 to 1.31, and the corresponding swirl vane angles ranged from 0 to 60°. The nozzle pressure ratios studied ranged from 1.8 to 6. The acoustic far field study at subsonic conditions revealed the presence of transonic tones for the non-swirl jet. However, swirl eliminates the transonic tones and a weak swirl is most efficient for noise reduction at subsonic conditions. The centerline total pressure measurements indicate the reduced core length for the swirl jets compared to the non-swirl jets. At supersonic conditions, the non-swirl jet emits the highest noise at all the emission angles compared to the swirl jets. The swirl jets are free from screech tones, and have lower amounts of shock associated noise, even at high nozzle pressure ratios. The centerline total pressure measurements and schlieren visualization studies show that shock cell spacing and the number of shock cells are reduced in the swirl jets compared to the non-swirl jet.

Supersonic impinging jet noise reduction is an important problem, especially in STOL aircraft, VTOL aircraft and rockets. These types of impinging jets generate a larger amount of noise and highly unsteady flows, which lead to noisy environment, posing a hazard to humans and materials in the proximity. The present study is carried out on supersonic impinging cold jets for nozzle pressure ratio (NPR) values of 2 and 5 with nozzle to plate distance (x / D) of 5, at different swirl numbers. The swirl flows are generated using co-axial curved blades and the results were compared with non-swirl or free jets. The vane angles considered here are 20°, 40°, 60° and the swirl numbers ranged from 0 to 1.31. At NPR 2, the weak swirl number of 0.27 reduced the OASPL level by around 7 dB compared to non-swirl jets. The non-swirl jet emitted impinging tones at NPR 5 and the swirl jet eliminated the impinging tones. At a high swirl number of 1.31, at NPR 5, OASPL is lowered by around 12 dB compared to non-swirl jets. The flow visualization study shows that the swirl disintegrates the repeated shocks and reduces the length of the shock cell system.

Experimental studies in the acoustic far field and flow visualization are carried out on circular and non-circular impinging jets at a nozzle to plate distance ratio of 5, for the nozzle pressure ratios of 1.8, 4, and 6. Spectra and directivity of the overall sound pressure level of circular jets are compared with those of non-circular jets for the same nozzle pressure ratios, at various emission angles. Non-circular jets mitigate the transonic impinging tones at around the critical pressure ratio, compared to the circular jet. Square jet is quieter at low nozzle pressure ratios by 5–8 dB compared to the circular jet. At higher under expansion conditions, the triangular and square jets almost completely eliminate the supersonic impinging tones. Elliptical jet is noisy at a nozzle pressure ratio of 6, even compared to the circular jet. Elliptical minor axis plane is quieter at lower nozzle pressure ratios compared to the major axis plane and the trend is reversed for the higher nozzle pressure ratios. Schlieren flow visualization studies reveal that the shock cell structures are weakened and symmetry of the shock cells are lost for the non-circular jets compared to the circular jet.

Experimental investigations are carried out with an objective of reducing impingement jet noise using coaxial swirl jets. Two nozzle-to-plate distances have been considered and the Mach number values ranged from 0.95 to 1.83. The vane angles of the coaxial swirlers ranged from 0 deg to 60 deg, corresponding to swirl numbers that ranged from weak to high. Flow visualization has also been conducted to understand the shock-cell structure for each case. The results indicate that swirl jets are highly efficient in the control of impingement noise. Transonic, screech, and impinging tones are completely eliminated by the swirl jets. A weak swirl seems efficient for lower Mach numbers while higher amounts of swirl are efficient at higher Mach numbers.

In this work, jet noise reduction using a swirling flow surrounding a circular free jet has been demonstrated. The passive control scheme induces swirl from six flat vanes fixed in an annular passage with vane angles from 0 to 50°, with the corresponding swirl numbers ranging from 0 to 0.91. Noise measurements in terms of overall sound pressure levels, directivity patterns, acoustic spectra; and flow measurements in terms of centerline pitot survey and flow visualization have been carried out to evaluate the efficacy of the passive control scheme. The co-axial swirl jets always reduce the low frequency noise, irrespective of the nozzle pressure ratio. The screech tone is entirely eliminated and broadband shock associated noise mitigated by the co-axial swirl jets. The results of highly under expanded supersonic cases, show that, the weak swirl generates higher noise and high swirl generates lower noise for the same nozzle pressure ratio. The centerline pitot pressure measurements reveal that the co-axial swirl jets decrease the core lengths and the number of shock cells compared to the free jet. The flow visualization study shows that Mach disks are generated at lower pressure ratios for the co-axial swirl jets compared to free jet. The present work proposes swirl as an excellent passive tool for jet noise suppression.

The present experimental work highlights the acoustic far field and flow field characteristics of confined co-axial swirling pipe jets. Co-axial confinements with six vanes at angles of 0°, 20° and 40° are considered here. Two pipe lengths of L/D=0.5 and 2 are studied. The Mach numbers studied range from 0.85 to 1.83. An increase in the pipe length causes suppression of the transonic tones in non-swirl pipe jets. Swirl reduces the low frequency noise components and increases the high-frequency components compared to non-swirl jet. The broadband shock associated noise is mitigated by the swirl pipe jets. However, the screech tone is completely eliminated by the swirl pipe jets. Further, swirl pipe jets radiate low levels of noise at all the emission angles compared to non-swirl pipe jets, for both the pipe length cases at supersonic Mach numbers. Increase in the pipe length enhances the shock associated noise and OASPL for the non-swirl pipe jet. Centreline pitot survey and schlieren visualisation show a reduction in core length, reduction in the number of shock cells, weakening/destruction of the shock cells by the swirl pipe jets compared to the non-swirl pipe jets.