Srinivasan K

The screeching characteristics of an underexpanding cold jet emerging from circular pipe nozzles are discussed in detail. Two screech modes were observed at higher pressure ratios, namely the Mode D and Mode C respectively. The focus is emphasized during the modal change of these screech tones. It is fascinating to understand that, there is a permanent phase shift between the Modes D and C, subsequent to the modes change. This aspect has been analyzed and reported by making phase plots between the acoustic signals obtained for these two screech modes.

This paper investigates temperature measurement using acoustic pyrometry in hot zones and flames. For experiments in flameless hot zones, a Hartmann whistle is used as the acoustic source. Time of flight of sound waves in the heated path is calculated by cross-correlating the signals received by two microphones placed at pipe ends. Acoustic pyrometry is also performed for estimation of flame temperature in kerosene wick and candle flames, using a speaker as the acoustic source. Since length scales of single flames are small, leading to short times-of-flight, appropriate temperature profiling strategies for flame temperature estimation are identified. © 2012 Elsevier Ltd. All rights reserved.

Experiments are conducted to study the influence of surface roughness on the noise emitted from underexpanded pipe jets. Two sets of circular pipes of different orders of surface roughness are considered, and the pipe lengths are varied between 1 and 6 diameters. The blowdown studies are performed in the range of pressure ratios varying from 2 to 7. Acoustic measurements at various underexpansion levels indicate that although screech frequencies remain unaltered with surface roughness, the tonal sound pressure levels differ by almost 10 dB. © 2012 Elsevier Ltd. All rights reserved.

Spectral features of conical and cylindrical Hartmann resonators are compared in this work through a systematic parametric study. Experiments have been conducted by varying the following parameters: stand-off distance, nozzle pressure ratio and cone angle. Resonance frequencies of conical cavities are found to be higher than those of cylindrical cavities of the same length. Low (∼kHz) and high frequency (∼10 kHz) modes are observed in the spectra. Low frequency modes show an oscillatory trend with stand-off distance. The high frequency tones are found to be independent of cavity geometry and cavity length, and are similar to jet impingement tones. © 2007 Elsevier Ltd. All rights reserved.

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. The lengths of the pipes considered for the present study are in the range 1≤L/D≤6. The pressure ratio of the pipe jets is varied from 3 to 7 corresponding to fully expanded jet Mach numbers of 1.35≤Mj≤1.92. 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 (by up to 20 dB), and increase in broadband shock associated noise level. Turbulent mixing noise levels are higher for shorter (L/D) 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. The screech frequency of a jet with negligible shear layer thickness (orifice jet) is found to be higher compared to those of finite shear layer thickness from pipe jets. © 2009 American Institute of Physics.

This paper experimentally investigates the far-field turbulent mixing noise from underexpanded triangular and square jets. The investigations are focused on the noise from vertices and edges of these non-circular jets, and in some cases the results are compared with those of circular jets. The jet pressure ratio is varied in the range of 2 to 6, and the acoustic measurements are carried out along the jet axial plane from 37° to 135° (measured from jet exit axis). The experimental results indicate that turbulent mixing noise levels vary linearly with emission angle, notably with two different gradient values along upstream and downstream. These results are ascribed to the two different turbulent scales, viz.; large scales and fine scales, in the jet flow. The turbulent mixing noise from screeching circular jets are found to be higher compared to triangular and square jets. However at higher pressure ratios, as the screech ceases, the turbulent mixing noise from all the jets (circular and other non-circular jets) becomes similar. © S. Hirzel Verlag.

This paper demonstrates the screech and transonic tonal characteristics from circular jets emerging from orifice. It is well understood that the former is due to the feedback mechanism between the hydrodynamic convection of fluid structures, and the upstream propagating acoustic waves leading to a narrow band, large amplitude tonal noise. Other interesting tones are formed during the transonic regime of Mach numbers. Such tones are not much intense, however initiates with a large number of non-linear tones. This paper describes the characteristics of screech and transonic tones generated from the jet flows.

This paper studies the effect of spacing on the acoustic spectra and directivity of a tapered resonance cavity kept axi-symmetrically in the flow field of a supersonic jet. A parametric study is conducted for various spacings and nozzle pressure ratios. Fundamental frequencies of the tapered cavities for the range of parameters considered are about 500Hz above the fundamental frequency of corresponding case of the cylindrical cavity. The preliminary results indicate that the spacing influences the spectra, and for certain spacings, the fundamental frequency (f1) of the tapered cavity is higher by around 3 times than that of the cylindrical cavity, as manifested in the spectra. The formation of intermediate frequency component between the fundamental frequency and the first harmonic and the disappearance of the intermediate frequency with a slight change of spacing is clearly depicted in the spectra. The periodic variation of fundamental frequency and its occurrence at common positions are discussed.

Time-of-flight (TOF) measurements are valuable in the estimation of distances, displacements and velocities of moving objects, phase differences of wave pulses, temperature of the atmosphere, and so on. The effects of sound source on time-of-flight measurements have been investigated in this paper. The sound sources considered are: electric horn, impact noise source, aerodynamic noise from a free jet, and the Hartmann whistle. The focus of the present study is to highlight the advantage of using Hartmann whistle for TOF measurements as this device is simple and attractive, without any moving parts. Time-of-flight of sound waves is calculated by cross-correlating the signals received by two microphones. Further, the effect of signal filtering on TOF measurements is demonstrated. The results indicate that the sound source has considerable effect on TOF measurements, and the accuracy can be significantly enhanced by appropriate signal conditioning. Hartmann whistle proves to be a good candidate as an acoustic source for TOF measurement. © 2009 Elsevier Ltd. All rights reserved.

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.