Srinivasan K

This paper investigates the effect of chamfer angles on the acoustic spectra and directivity of resonance tubes, kept axi-symmetrically in the flow field of a supersonic jet. Such tubes can be used for effective flow control, mixing, ignition etc. The jet impinges at the open end of the tube which is closed at the other end. The parameters being considered are the chamfer angle of the tube, nozzle pressure ratio and spacing (S) between nozzle exit and the tube inlet. The jet diameter and the tube inlet diameter are kept constant. Nozzle pressure ratio is varied from 4 to 6 in steps of 0.5. The chamfer angles considered are 15°, 30°, 45°. Acoustic pressure is measured in the far field region at emission angles varying from 37° to 135°, from the jet flow direction. The spectra clearly illustrates that the resonance tubes with chamfer has higher fundamental frequency than that of its absence. The fundamental frequency is observed to decrease with L/Dj for all chamfer angles. The frequencies obtained from experiments are compared with standard quarter wavelength theory. It is clear that the frequencies of the chamfered tubes are almost closer to the theory. At large tube lengths all the frequencies match well with theory but at small tube lengths only 30o chamfer is almost close to the theory. The fundamental frequency of 45o chamfer is found to be almost near to that of 0o chamfer. The minimum location of fundamental frequency as marked in Fig. 4 with S/Dj is found to be same for all L/Dj studied. The shadowgraph sequence (Fig. 5) shows that the low frequency components ∼2 kHz in the waterfall spectra (Fig. 6) are due to jet regurgitance. It is observed from Fig.7 that the directivity is seen to be higher for a tube with α = 30°. Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc.

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.

It is well known that initial conditions significantly affect the flow field and the evolution characteristics of the jets. In the present study, experiments on twin pipe jets were performed to understand the effect of developing length on the twin-jet characteristics. The developing length was varied in the range of 4 < L/D < 9, and hot-wire data were acquired up to a downstream distance of 40D. Twin jets combine at farther downstream axial distance with an increase in the developing length due to delayed mixing brought about by relatively less intense vortex action. The range of the energy-containing eddies decreases, resulting in a decrease in the spread rates of the twin jet. © IMechE 2008.

Chevron nozzles are known to be excellent attenuators of jet noise. Conventional chevron nozzles use triangular serrations at the trailing edge of the nozzle. This article proposes two novel chevron concepts and evaluates their noise reduction performance. The new chevron concepts proposed are protrusions with a sinusoidal profile and chevrons with asymmetry. These nozzles are compared against the symmetric chevron nozzle with triangular profile and a baseline circular nozzle without chevrons. The results indicate that the sinusoidal profile chevron nozzle shows better noise reduction at higher pressure ratios for all emission angles. At lower pressure ratios, the acoustic benefit is marginal due to reduced turbulent mixing of jet by low-strength vortices. Shadowgraph images of the asymmetric chevron nozzle shows bevelling due to the relieving effect in the direction of the longer chevron lobes, which reduces noise in the opposite side. This asymmetry of the noise sources and the acoustic field could be advantageously exploited in the implementations of chevron nozzles for aircraft. © 2009 IMechE.

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.

This paper investigates the effect of outer chamfer on the frequency and amplitude characteristics of resonance cavities, placed axi-symmetrically in the flow field of a supersonic jet. Such cavities can be gainfully used for flow control, atomization, mixing, ignition etc. The parameters being considered are the nozzle pressure ratio, chamfer angle at the outer surface of the cavity, cavity length and stand-off distance between nozzle exit and the cavity inlet. The cavity inlet diameter and jet diameter are kept constant. The chamfer angles adopted for the present study includes 15°, 30°, 45°. The acoustic pressure is measured in the far field region at a fixed radius of around 64Dj in order to avoid flow effects. The emission angles considered for the present study varied from 37° to 135° in steps of 2°, measured from the jet flow direction. The frequencies of non-chamfered cavity and 15° chamfered cavity are almost same thereafter it follows decreasing and increasing trend with chamfer angles. It is also noticed that all the frequencies approach a minimum value at a chamfer angle of 30°. The fundamental frequencies of all outlet chamfered cavities at a nozzle pressure ratio (NPR) of 5 are observed to decrease with increase of the non-dimensional cavity length (L/Dj). The frequencies obtained from experiments are compared with those obtained using Quarter wavelength formula (QWL) for an open - closed cavity. The decreasing trend of the frequency with L/Dj is found same for both experiments and theory, but the theoretical values are slightly higher at small L/Dj's. The spectral variation of frequency components at NPR = 4 show that there is no modification of the frequency components at L/Dj = 4.28 but the increase of L/Dj to 5.71 causes the increase of broad part between two successive peaks at 15o and 45o chamfer. It is seen that the minimum location of frequency index (frequency with chamfer/frequency without chamfer) with S/Dj at NPR = 4 is same for all chamfer angles. Shadowgraph sequence clearly illustrates the flow oscillation in front of the cavity mouth for all stand-off distances normalized with jet diameter. It is observed that the overall sound pressure level attains maximum at small L/Dj almost for all chamfer angles. It is seen that for all chamfer angles the overall sound pressure level follows a decreasing trend with S/Dj. Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc.

In this paper we focus on understanding the behavior of twin nozzles of complex geometry in various yaw orientations. To the best of our knowledge there are no published studies addressing the effects of nozzle orientation on the coupling of twin jets of complex exit geometry. We study the behavior of 1) uniform-exit rectangular nozzles, 2) single-beveled nozzles in a codirected configuration, and 3) single-beveled nozzles in a contradirected configuration. Experiments were carried out at fully expanded Mach numbers ranging from 1.28 to 1.72. Bevel angles of 10 and 30 deg were considered, and microphones located at the nozzle exit plane quantified the coupling using both linear and nonlinear spectral-analysis methods. Nonlinear characteristics were quantified using the nonlinear interaction density metric with a cross-bicoherence cut-off threshold of 0.4. The following interesting results emerged from this study: 1) When nozzles having uniform rectangular exits are yawed, the sound-pressure levels in the internozzle region reduce as the yaw angles are increased, and, at a very high yaw angle, the symmetric coupling regime that existed at the high fully expanded Mach number range (without yaw) is replaced by an antisymmetric coupling regime in the same range. 2) Geometrically similar exits from uniform-exit rectangular nozzles and beveled nozzles in the contradirected configuration showed similar characteristics when studied using linear techniques. However, they revealed information that was hitherto unknown when studied using nonlinear spectral-analysis techniques. It is believed that the results presented in this paper will provide benchmark data to those simulating/designing complex-geometry nozzle systems.

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 addresses erosive burning of a cylindrical composite propellant grain. Equations governing the steady axisymmetric, chemically reacting boundary layer are solved numerically. The turbulence is described by the two equation (k-ε) model and Spalding's eddy break up model is employed for the gas phase reaction rate. The governing equations are transformed and solved in the normalized stream function coordinate system. The results indicate that the dominant reaction zone lies within 20% of the boundary layer thickness close to the wall. The sharp gradient of the temperature profile near the wall is found responsible for bringing the maximum heat release zone near the surface and hence enhancement in the burning rate. The model reproduces the experimental observation that erosive burning commences only above a threshold value of axial velocity. © 2007 Springer-Verlag.

The Hatmann whistle, a high efficiency acoustic device with various sounding advantages, differs from other tone generators due to its high acoustic output capability and working principles. The advantages of the whistle include purely aeroacoustic sound generation, high sound quality due to intense tonal content and high amplitude, and tunable frequency of sound using the geometrical parameters. The tone frequency of the whistle corresponds to the quarter-wavelength mode applicable for tubes closed at one end and open at the other. Non-isentropic flow in the tube causes the heating mechanism and the heating effect is more pronounced when the resonance generates high amplitude pressure oscillations. The geometrical resonance of the tubes in Hartmann whistle include factors such as enhancement of acoustic efficiency, thermal effects, and adaptation to practical situations.