Acoustics and Instabilites
Jet noise control using passive grids
Reducing supersonic jet noise by active and passive control techniques has been of great interest to researchers and scientists. Its application in commercial and military aviation is vital for both safety and human comfort. In the present study, the passive method is used to control the under-expanded free jet noise by using passive grids. The main advantage of passive control method over the active control method is that the method does not require any external power source for noise control. The presence of grids successfully eliminate the dominant tones (screech) by disrupting the feedback loop and attenuate the Broadband Shock Associated Noise by converting strong shock cells into weaker multiple shock-lets.
Baskaran, Kabilan, et al. Journal of Sound and Vibration 435 (2018)
Atomization in the acoustic field of a Hartmann whistle
The current work investigates the effect of Hartmann cavity acoustics on the atomization of droplet sprays. Initially, the experiments are conducted on a single droplet to understand its behavior in the sound field of a Hartmann whistle. The atomization studies on single droplet reveal that the existence of sound field causes the droplet to undergo large deformation and become irregular in shape. The degree of droplet deformation is quantified based on smaller circularity and larger Feret's diameter. The increase in cone angle of spray to a higher value in the presence of acoustics in comparison to its absence shows that the acoustics enhances the atomization. The stroboscopic visualization of sprays in the presence of acoustics further reveals the breakup of ligaments, large scatter as well as the formation of more number of droplets, indicating atomization enhancement.
S. Natayanan et al., International Journal of Spray and Combustion Dynamics, 5 (2013)
Spectra and directivity of a Hartmann whistle
Cavities placed in high speed flows can serve as high intensity, narrow band acoustic sources with selective directivity. Such sources can be gainfully used to control the flow, mixing and combustion processes in various propulsion applications.The flow diversion around the cavity explains the observed shift in directivity towards higher angles for the whistle, as compared to the free jet flow. The acoustic power and efficiency are high for small values of stand-off distances and larger cavity lengths.
S. Natayanan et al., Journal of Sound and Vibration, 321 (2009)
A study on the effect of initial conditions on noise from underexpanded pipe jets
An experimental investigation of acoustic radiation from underexpanded air jets of different shear layer thicknesses has been performed. The initial shear layer thickness variation is achieved by allowing the jet to exit through pipes of various lengths. Acoustic radiation is characterized in terms of overall sound pressure level, directivity, tonal, and broadband shock associated noise. Increase in initial shear layer thickness in pipe jets results in the decrease of screech tone amplitude and increase in broadband shock associated noise level. Turbulent mixing noise levels are higher for shorter pipe jets compared to longer ones. Longer pipe jets exhibit more number of screech modes while the shorter pipe jets show only one or two screech modes. The screech frequency and the peak frequency of broadband shock associated noise do not show much variation with increase in initial shear layer thickness.
Studies on swirl jets
Influence of swirl number on jet noise reduction has been studied. In this work, jet noise reduction using a swirling flow surrounding a circular free jet has been demonstrated and the flow visualization also done to confirm the noise reduction. 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 This work proposes swirl as an excellent passive tool for jet noise suppression.
P. Balakrishnan, K. Srinivasan, Aerospace Science and Technology (2018) 256-268
P. Balakrishnan, K. Srinivasan, Appl. Acoust.126 (2017) 149--161