Effect of plate inclination on the noise of impinging jets
This article explains the flow and acoustic variation due to the jet impinging on inclined plates. There is a misconception that the acoustic radiation reduces by changing the angle of impingement. However, in the present study it is found that the reduction or enhancement of the acoustic radiation depends on the location of measurement and nozzle pressure ratio. A detailed directivity study of jet impinging on the inclined plate is carried out. Further, high speed flow visualization is carried out to compare shock oscillation and tonal frequencies.
Noise characteristics of twin-square slot jets
Acoustic measurements have been undertaken on two different twin square jet topologies. OASPL, spectra, directivity measurement and shock-cell visualization have been conducted to evaluate twin square jets in different edge-vertex orientations. The results indicate that twin square jets effectively suppress noise up to 4 dB in terms of OASPL and 6 dB in terms of screech tonal SPL compared to an equivalent single circular jet. The study promises that twin-jet topology could serve as an effective tool for noise suppression.
Passive control of pipe-jet noise using trailing-edge castellations
In the current study, geometric modifications in the form of trailing edge castellations are explored for the reduction of pipe-jet noise. The number of castellations determines the interaction between the streamwise vortices, which alters the sound produced. The noise levels in pipe-jets with and without castellations are compared by measuring the polar and azimuthal directivities. The parameters, namely the number of castellations and the nozzle pressure ratio, are varied in this study. The castellations work best in the underexpanded flow regime of nozzle pressure ratio 3–5. The spectral analysis of the sound revealed that the screech tones are eliminated for all castellated pipes except the one with two castellations. The broadband shock associated noise is found to increase with the number of castellations, and screech tones reappear at a higher nozzle pressure ratio. The maximum noise mitigating and amplifying cases are identified by acoustic power analysis for all castellated pipes with equidistant teeth and gap. As the number of castellations increases, the modified pipes attain the noise characteristics of the base model, which can be considered as an infinitely castellated pipe. These results are supported by the flow visualization using schlieren imaging, which showed the differential expansion in teeth and gap plane, leading to asymmetry in flow. Proper Orthogonal Decomposition of the schlieren images indicated the presence of symmetric and anti-symmetric mode of oscillation in gap and teeth plane, respectively, of the model with two castellations.
Relationship between shock-cell length and noise of jets from rectangular and elliptic disk nozzles
Jets issuing from rectangular and elliptic disk nozzles of various aspect ratios have been studied for their noise characteristics. The overall sound pressure level and directivity were measured for these jets. The effects of aspect ratio, measurement plane (major- vs. minor-) have been analyzed. The results show that the sound pressure levels are entirely different for higher and lower aspect ratio elliptic and rectangular jets. At lower aspect ratios (AR=2:1), the noise emissions from major axis plane dominate while for higher aspect ratios jets (AR>2), the noise from minor axis plane dominate. Further, the relationship between sound pressure level and average shock-cell length is observed to be quadratic at emission angles of 30° and 90° for both rectangular and elliptic jets. © Freund Publishing House Ltd.
Jet noise reduction using co-axial swirl flow with curved vanes
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.
Influence of swirl number on jet noise reduction using flat vane swirlers
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.