Far-field and nearfield acoustic measurements have been conducted on equal and unequal twin circular slot jet for different spacing ratios. The twin jets were operated at high, moderate and low levels of underexpanded flow conditions. Measurement of acoustic power index, OASPL, spectra and shock-cell visualization have been carried out to evaluate equal and unequal twin jets. The results indicate that unequal twin jets are efficient screech tone suppressors at high and moderate levels of underexpanded flow conditions but are ineffective at lower levels of underexpansion. The cross correlation studies reveal an antisymmetric coupling for moderately underexpanded equal twin jet and symmetric coupling for low levels of underexpanded unequal twin jet. © 2015 Elsevier Ltd.

K. Srinivasan

Department of Mechanical Engineering

T Sundararajan

V. Boopathi Sabareesh

Acoustic characteristic

ACOUSTIC POWER

Anti-symmetric

Cross correlations

FLOW CONDITION

Near-field acoustics

SCREECH TONES

SYMMETRIC COUPLING

Vibrations (mechanical)

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

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.

International Journal of Aeroacoustics

Noise characteristics of twin-square slot jets

The purpose of this paper is to examine nonlinear processes occurring during the coupling of complex twin jet plumes using higher-order spectral methods. Inadequacies of linear spectral analysis when closely spaced multiple screech sources exist in complex configurations are demonstrated. Cross-bicoherence is used for identifying the nonlinear interactions between jets emanating from rectangular nozzles with spanwise oblique exit geometries. Two basic topologies in twin jets are compared in the paper; one that promotes coupling (referred to as 'co-directed') and another that inhibits coupling (referred to as 'contra-directed'). The primary findings are: (i) some configurations that appeared to be uncoupled by linear spectral analysis metrics were actually found to be nonlinearly coupled, and (ii) two types of nonlinear coupling were observed-one dominated by the fundamental and its interaction with higher modes, and another that displayed clusters of interactions between a frequency component and its sidebands. Further, three new metrics have been proposed: (i) the first metric, 'quadratic phase coupling index', was developed and used to indicate the propensity of jets to couple, (ii) the second metric, namely, 'interaction density' was developed to quantify nonlinear coupling, and (iii) the third metric known as the 'average interaction density' was shown to increase sharply during coupling mode transition. © 2009 Elsevier Ltd. All rights reserved.

Study of coupled supersonic twin jets of complex geometry using higher-order spectral analysis

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.

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