The present study investigates the effect of passive grids on the acoustic characteristics of pipe and orifice jets for different Nozzle Pressure Ratios (NPRs). A detailed blow-down and directivity studies have been carried out for various emission angles. To understand the distribution of acoustic radiations with frequency, the spectral analysis is performed. Additionally, the acoustic power is estimated in order to signify the effects of grids. The presence of grids influences the noise level depending on the operating ranges of NPRs. At low NPRs, the grids enhance the overall sound pressure level; mainly due to the turbulent mixing noise. Whereas, at high NPRs, a considerable noise reduction is observed. However, grids assist to shift the noise level from low frequency to high frequency for all ranges of NPR. The eddy fragmentation by the grids at low NPR results in shifting the low frequency noise to a high frequency. In general, at high NPRs the shock associated noise [screech and broadband shock associated noise (BSAN)] is dominant over the turbulent mixing noise. The presence of grids successfully eliminate the dominant tones (screech) by disrupting the feedback loop and attenuate the BSAN by converting strong shock cells into weaker multiple shock-lets. © 2018 Elsevier Ltd

K. Srinivasan

Department of Mechanical Engineering

Kabilan Baskaran

Senthil Kumar Parimalanathan

Abhijit Dhamanekar

ACOUSTIC WAVE TRANSMISSION

Mixing

Noise abatement

Orifices

Shock tubes

Spectrum Analysis

Turbulent flow

Acoustic characteristic

Low-Frequency Noise

Nozzle pressure ratio

Overall sound pressure level

PASSIVE GRID

SCREECH

SHOCK ASSOCIATED NOISE

TURBULENT MIXING NOISE

Acoustic noise

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Journal of Sound and Vibration

The near field evolution of a low Reynolds number free jet and associated turbulent fluctuations are investigated experimentally, in the presence of a grid placed at nozzle exit. The jet spreading characteristics without grid are validated first with available data in the literature and the well known features of vortex roll-up and pairing phenomena in the shear layer region are confirmed. The inclusion of a grid at the nozzle exit appears to reduce velocity fluctuations in the near field, but it accelerates the transition to a full-grown turbulent flow field with respect to axial distance, by the introduction of finer scales. Grids also cause mean velocity overshoot in the radial profile by distorting the approach flow in the boundary layer region ahead of the grid. Distinct vortex roll-up and pairing phenomena in the shear layer region could not be observed with grids. However, weak shear mode instabilities originating in the shear layer for 0 ≤ z/D ≤ 3 appear to interact with the potential core, leading to the formation of a "unified shear/preferred mode" instability. These features are dominantly seen for grid-mounted jet flows having large displacement thickness to grid opening ratio. Power spectra indicate a progressive evolution of turbulent flow features with respect to axial distance, starting with the development of inertial sub-range into a -5/3 slope exponent; followed by the establishment of full range of turbulent scales, and the eventual attainment of isotropy. The establishment of inertial sub-range is attributable to vortex-pairing and turbulent fragmentation in free jets and to the introduction of fine scales at the jet inlet in the cases with grids. © 2015 AIP Publishing LLC.

Fulltext

Physics of Fluids

The effects of grids on the near-field evolution of turbulence in a low-Reynolds number jet

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.

Surface roughness effects on noise from pipe jets

Experimental investigations have been carried out on chevron nozzles to assess the importance of chevron parameters such as the number of chevrons (chevron count) and chevron penetration. Acoustic measurements such as overall sound pressure level, spectra, directivity, acoustic power, and broadband shock noise have been made over a range of nozzle pressure ratio from sub-critical to underexpansion levels. Shadowgraph images of the shock-cell structure of jets from various chevron nozzles have also been captured for different nozzle pressure ratios. The results indicate that a higher chevron count with a lower level of penetration yields the maximum noise suppression for low and medium nozzle pressure ratios. Of all the geometries studied, chevron nozzle with eight lobes and 0° penetration angle gives the maximum noise reduction. Chevron nozzles are found to be free from screech unlike regular nozzles. Acoustic power index has been calculated to quantitatively evaluate the performance of the various chevron nozzles. Chevron count is the pertinent parameter for noise reduction at low nozzle pressure ratios, whereas at high nozzle pressure ratios, chevron penetration is crucial. The results illustrate that by careful selection of chevron parameters substantial noise reduction can be achieved. © 2009 Elsevier Ltd. All rights reserved.

Applied Acoustics

Effect of chevron count and penetration on the acoustic characteristics of chevron nozzles

Experimental and theoretical studies of the spectral and directivity characteristics of a Hartmann whistle are presented for pressure ratios in the range of 4.92-6.88. Cavity resonance is observed for the range of pressure ratios studied and the sound pressure amplitudes are larger than those of free jets. Distinct spectra with high sound pressure levels are observed at the fundamental frequency as well as the higher harmonics. Based on dimensional analysis, an expression for the fundamental frequency is derived in terms of the stagnation sound speed, cavity length, and stand-off distance. The analogy between Hartmann whistle and a classical Helmholtz resonator is discussed. Numerical simulations of the flow field, carried out to supplement the experimental findings, show that the jet regurgitance is significant at smaller values of cavity-stand-off distances. The correspondence between flow pattern seen from numerical simulations and the maximum directivity observed from acoustical measurements is highlighted. 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. © 2008 Elsevier Ltd. All rights reserved.

Spectra and directivity of a Hartmann whistle

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.

Role of initial conditions on noise from underexpanded pipe jets

Effects of passive grids on pipe and orifice jet noise

Journal	Journal of Sound and Vibration
Publisher	Academic Press
ISSN	0022460X
Open Access	No