The present work deals with an experimental investigation of flow of air through a square-edged circular orifice at the downstream end of a circular duct. Self-excited acoustic oscillations at the natural duct modes are observed for certain flow velocities when the orifice is sufficiently thick. For a specific Reynolds number based on the orifice diameter and the mean jet velocity (9150<Re <9850), the jet forks into two trains, with the alternating vortices falling into the same branch of the forked train. Whereas this phenomenon has been reported earlier to have occurred when the density ratio of the jet is less than 0.72, the present results show that it is possible for a jet having the same density as the ambient atmosphere. The jet forking is coincident with jump in the acoustic frequency from one natural acoustic mode to another with comparable amplitudes of both the modes. © 2003 Acoustical Society of America.

Satyanarayanan R. Chakravarthy

Department of Aerospace Engineering

R. I. Sujith

B. Balanaga Karthik

Ducts

Jets

Orifices

Oscillations

Reynolds number

JET FORKING

Acoustic waves

acoustics

Airflow

article

flow rate

fourier transformation

oscillation

Piezoelectricity

Pressure

priority journal

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

Jet forking driven by pipe tone

Journal of the Acoustical Society of America

The acoustic oscillations excited by flow past an axisymmetric cavity in a circular duct are experimentally investigated. The focus of the work is on the effects of different dimensions and shapes of the cavity, namely, rectangular, V, and U shapes. A large number of transducers are used to simultaneously record the variation of acoustic pressure axially and azimuthally in the duct. The trapped acoustic modes excited in the duct are confirmed by the observation of axial decay of the acoustic pressure amplitude away from the cavity. Shifts are observed between tangential natural acoustic modes of the duct, corresponding to which the amplitudes rise and fall. The subdominant mode amplitude increases simultaneously as the dominant mode decreases during a mode shift. The experimentally observed azimuthal variation of amplitude and phase indicate the prevalence of mixed standing and spinning wave modes, which are separated so as to evaluate the spin ratio, defined as the ratio of the amplitude of the spinning part to the total amplitude comprising both the spinning and standing modes. It is found that the spin ratio reaches low values corresponding to when the mode shifts occur. Accordingly, the ratio of the dominant to subdominant amplitudes registers a spike. The results suggest that, when a dominant mode prevails over a velocity range, the acoustic energy is substantially contributed towards the spinning mode, whereas during conditions of mode shifts, the amplitudes of the competing modes decrease, leaving only their standing wave patterns to prevail mostly. The depth of the cavity affects the natural frequency excited, whereas the length influences the amplitude. The U- and V-shaped cavities offer lesser effective geometric depths to the acoustic modes. The shape of the cavity influences the amplitude excited, with the U-shaped cavity registering the lowest levels. It is expected that the dynamics in the shear layers between the central large-scale vortex and the corner vortices accommodated by the cavity shape contributes to the acoustic oscillations excited. © 2017 Elsevier Masson SAS

Aerospace Science and Technology

Shape effects of single axisymmetric cavity in a circular duct on flow induced acoustic oscillations

In this paper, we show how the phenomenon of intermittency observed in systems with turbulent flow-sound interaction is related to the formation of homoclinic orbits in the phase space. Such orbits that emerge via the intersection of the stable and unstable manifold of an equilibrium configuration result from interactions that happen at multiple spatial/temporal scales associated with turbulent convection and wave propagation. Through a quantification of the time spent by the dynamics in the aperiodic states using recurrence plots, we show how the presence of homoclinic orbits in the dynamics may be convincingly demonstrated, which is often not possible through a visual inspection of the phase space of the attractor. © 2013 AIP Publishing LLC.

Fulltext

Chaos

Identifying homoclinic orbits in the dynamics of intermittent signals through recurrence quantification

Experimental data on acoustic pressure measurements obtained over a wide range of conditions is reported for two simple geometries that are commonly studied for their combustion dynamics behaviour. These geometries are the confined bluff-body and the confined backward-facing steps. The data indicate regimes of flow-acoustic lock-on that signifies the onset of combustion instability, marked by the excitation of high-amplitude discrete tones of sound in the combustor. The high-speed chemiluminescence imaging of the combustion zone indicates heat-release-rate fluctuations occurring at the same frequencies as observed in the acoustic spectra. Attention is then devoted to the data obtained under cold-flow conditions to illustrate distinctly different behaviour than when combustion instability occurs, contrary to the commonly held view that the combustion process does not alter the underlying fluid mechanical processes under low-Mach number conditions. © Printed in India.

Sadhana - Academy Proceedings in Engineering Sciences

Vortex-acoustic lock-on in bluff-body and backward-facing step combustors

A two-dimensional laminar premixed flame is stabilized over a burner in a confined duct and is subjected to external acoustic forcing from the downstream end. The equivalence ratio of the flame is 0.7. The flame is stabilized in the central slot of a three-slotted burner. The strength of the shear layer of the cold reactive mixture through the central slot is controlled by the flow rate of cold nitrogen gas through the side slots. The frequency range of acoustic excitation is 400-1200 Hz, and the amplitude levels are such that the acoustic velocity is less than the mean flow velocity of the reactants. Time-averaged chemiluminescence images of the perturbed flame front display time-mean changes as compared to the unperturbed flame shape at certain excitation frequencies. Prominent changes to the flame front are in the form of stretching or shrinkage, asymmetric development of its shape, increased/preferential lift-off of one or both of the stabilization points of the flame, and nearly random three-dimensional fluctuations over large time scales under some conditions. The oscillations of the shear layer and the response of the confined jet of the hot products to the acoustic forcing, such as asymmetric flow development and jet spreading, are found to be responsible for the observed mean changes in the flame shape. A distinct low-frequency component (∼60-90 Hz) relative to the excitation frequency is observed in the fluctuations of the chemiluminescent intensity in the flame under most conditions. It is observed that fluctuations in the flame area predominantly contribute to the origin of the low-frequency component. This is primarily due to the rollup of vortices and the generation of enthalpy waves at the burner lip. Both of these processes are excited at the externally imposed acoustic time scale, but convect/propagate downstream at the flow time scale, which is much larger. © 2006 The Combustion Institute.

Journal	Journal of the Acoustical Society of America
ISSN	00014966
Open Access	No