Experimental studies are conducted to investigate the effect of internal chamfer at the mouth of Hartmann resonators. Studies involve a range of nozzle pressure ratios from 4 to 6, and chamfer angles 15°, 30°, and 45°. Further, the effects of cavity length and stand-off distance are also considered. The spectra, directivity, and acoustic power characteristics are studied in detail. Detailed numerical simulations are carried out to capture the flow oscillations inside as well as at the outside of the mouth of the chamfered cavities. Computations show flow diversion in chamfered cavities and explain the shift in the directivity observed experimentally. The fundamental frequency of cavities with 15° and 30° chamfers is observed to be higher than that of regular cavities. Resonance is intensified by the presence of chamfer resulting in higher overall sound pressure levels of chamfered whistles. Thus, chamfered Hartmann whistles are found to emit more than twice the acoustic power of a regular cylindrical whistle. The tonal quality of sound is analyzed using a new metric termed as "resonance index". © 2010 Elsevier Ltd.

K. Srinivasan

Department of Mechanical Engineering

T Sundararajan

K. Ramamurthi

Mathematical models

Vibrations (mechanical)

Acoustic characteristic

FLOW DIVERSION

Flow oscillations

Fundamental frequencies

HARTMANN WHISTLE

Nozzle pressure ratio

Overall sound pressure level

STAND-OFF DISTANCE (SOD)

Signaling

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 efficient way of chamfering at the mouth of Hartmann whistles in generating higher acoustic emission levels are experimentally demonstrated in this paper. The relevant parameters of the present work comprise internal and external-chamfer angles (15, 30), cavity-length, nozzle-to-cavity-distance and jet pressure ratios. The frequency and amplitude characteristics of internal and external, chamfered-Hartmann whistles are compared in detail to ascertain the role of chamfering in enhancing acoustic radiations. The high frequencies possessed by the internal chamfered whistles as compared to the external ones indicate that it amplifies the resonance. It is observed that the internal chamfered whistles exhibit higher directivity than the external chamfered ones. Further, it is noticed that the acoustic-power and efficiency are also higher for the internal chamfered whistles. The shadowgraph sequences reveal the variance in flow-shock oscillations as well as the spill-over features at the mouth of internal and external, chamfered cavities. The presence of large mass flow as well as its subsequent increase of spill-over as a result of enlarged mouth in internal chamfered whistles, leads to the generation of high intensity acoustic radiation than the external chamfered ones. Thus, the internal chamfer proves to be the best passive control device for augmented sound pressure levels and acoustic efficiencies in resonance cavities. © 2013 Elsevier Ltd.

Aero-acoustic features of internal and external chamfered Hartmann whistles: A comparative study

The primary objective of this work is to investigate lip-thickness effect on the various acoustic emission characteristics of Hartmann whistle. Nozzle-to-cavity distance (stand-off distance), cavity length, nozzle pressure ratio and lip thickness form the pertinent parameters of the present study. Two lip thicknesses considered in the present study are a thick-lipped cavity (5 mm) and a thin-lipped cavity (1 mm). Although lip thickness has negligible effect on the resonance frequency, it has significant influence over the sound pressure levels generated. The results showed that thin-lipped-Hartmann whistles could emit up to 2.4 times the acoustic power as compared with thick-lipped whistles. © IMechE 2013.

Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering

Effect of lip thickness on the aero-acoustic features of Hartmann whistle

Experimental investigations are carried out on Hartmann whistles to explore the effect of external chamfer at the cavity mouth. The acoustic performance depends upon the cavity length, jet-to-cavity-spacing and external chamfer angle (15, 30, and 45). The modifications in spectral and directional characteristics of external chamfered Hartmann whistles are studied in detail and are compared with a regular cavity. The frequencies are observed to attain a minimum value at a chamfer angle of 30 along with modification in the acoustic spectra. In general, it is noticed that the external chamfered whistles are directive at an emission angle of around 45 where it is around 37 for a regular whistle. Numerical simulations portray the flow/shock oscillation features in external chamfered cavities. Simulations depict intense flow diversion at the mouth of chamfered cavities and elucidate the directivity shift as well as the enhancement of acoustic power observed experimentally. Thus, it is observed that Hartmann whistles with external chamfering could radiate acoustic power up to 2.3 times that of a non-chamfered whistle. © 2013 Elsevier Ltd. All rights reserved.

Applied Acoustics

Acoustic characteristics of external chamfered Hartmann whistles

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

Hartmann discovered the resonance tube phenomenon in 1918. Although researchers have conducted extensive studies on this topic during the intervening 90 years, no single resource lists, analyzes, synthesizes and interprets the vast body of findings. This review offers a consolidated resource tracing development of the Hartmann tube from discovery to recent advances in understanding, prediction and application of the resonance tube. This review (a) serves as a literature resource for researchers from diverse areas, (b) provides a critical assessment of the state of the art, and (c) provides examples of the vast possibilities for applying this device. Controlled flow-induced resonance can produce high-amplitude dynamic pressures and acoustic emission over a range of frequencies. Studies on such acoustic generators interested researchers during the last half of the 20th century. Hartmann demonstrated the possibility of obtaining high acoustic efficiencies when a jet is aimed at the open end of a tube closed at the other end. His work led to numerous other studies-some that examined the physics and others that developed geometric variants and explored industrial applications. In the last decade there has been renewed interest in powered resonance tubes (PRT) because of their potential as active flow control actuators. This article also evaluates the success of flow-control strategies using PRTs, and attempts to identify promising PRT applications. © 2009 Elsevier Ltd. All rights reserved.

Progress in Aerospace Sciences

The powered resonance tube: From Hartmann's discovery to current active flow control applications

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.

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