Srinivasan K

Understanding the occurrence of various feedback mechanisms of an under-expanded impinging supersonic jet is a crucial task in research. The presence of several jet modes is examined in this study for the flat and corrugated impinging plate geometries. The behavior of impinging plate configurations during mode switching is investigated by varying the flow state, such as the jet Mach number. The staging behavior at various jet Mach numbers is observed using acoustic spectral plots and schlieren flow visualization. To explore the presence of various types of modes during the jet impingement due to the modification of jet Mach number, ensemble averaging and Proper Orthogonal Decomposition of schlieren images are carried out. In the majority of situations, the corrugated design shows a reduction in tonal noise and overall sound pressure level. In exceptional cases, for the corrugated plates, the enhanced overall sound pressure level is caused by the existence of axisymmetric instability (A1, A2).

The screeching characteristics of an underexpanding cold jet emerging from circular pipe nozzles are discussed in detail. Two screech modes were observed at higher pressure ratios, namely the Mode D and Mode C respectively. The focus is emphasized during the modal change of these screech tones. It is fascinating to understand that, there is a permanent phase shift between the Modes D and C, subsequent to the modes change. This aspect has been analyzed and reported by making phase plots between the acoustic signals obtained for these two screech modes.

This paper investigates temperature measurement using acoustic pyrometry in hot zones and flames. For experiments in flameless hot zones, a Hartmann whistle is used as the acoustic source. Time of flight of sound waves in the heated path is calculated by cross-correlating the signals received by two microphones placed at pipe ends. Acoustic pyrometry is also performed for estimation of flame temperature in kerosene wick and candle flames, using a speaker as the acoustic source. Since length scales of single flames are small, leading to short times-of-flight, appropriate temperature profiling strategies for flame temperature estimation are identified. © 2012 Elsevier Ltd. All rights reserved.

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.

In this work, effects of burner configurations on the natural oscillations of methane laminar diffusion flames under atmospheric pressure and normal gravity conditions have been studied experimentally. Three regimes of laminar diffusion flames, namely, steady, intermittent flickering and continuous flickering have been investigated. Burner configurations such as straight pipe, contoured nozzle and that having an orifice plate at the exit have been considered. All burners have the same area of cross section at the exit and same burner lip thickness. Flame height data has been extracted from direct flame video using MATLAB. Shadowgraph videos have been captured to analyze the plume width characteristics. Results show that, the oscillation characteristics of the orifice burner is significantly different from the other two burners; orifice burner produces a shorter flame and wider thermal plume width in the steady flame regime and the onset of the oscillation/flickering regimes for the orifice burner occurs at a higher fuel flow rate. In the natural flickering regime, the dominating frequency of flame flickering remains within a small range, 12.5 Hz to 15 Hz, for all the burners and for all fuel flow rates. The timeaveraged flame length-scale parameters, such as the maximum and the minimum flame heights, increase with respect to the fuel flow rate, however, the difference in the maximum and the minimum flame heights remains almost constant.

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.

The investigation of the aeroacoustics of an underexpanded pipe-cavity jet is carried out experimentally. Two different aspect ratios of the cavity are tested for a wide range of nozzle pressure ratios. Both internal and externally radiated pipe-cavity acoustics are studied. Linear and higher-order spectral analyses are implemented on the unsteady cavity pressure to comprehend the nature of the cavity acoustics and nonlinear interactions of different acoustic modes of the pipe-cavity system. Results show that an increase in depth leads to an enhancement in the nonlinear interactions. Furthermore, the power spectral and overall sound pressure level analyses of pipe and pipe-cavity jet noise radiation are carried out. High-speed schlieren imaging techniques are used to understand jet dynamics. Highly unsteady motion of the jet initial shear layer is observed due to an upstream disturbance of the cavity. In addition, proper orthogonal and dynamic mode decomposition methods are used to extract the spatial and dynamic modes of the jet structure. These methods are used to segregate the cavity associated jet dynamics and screech dynamics.

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.

The current work experimentally investigates the effect of Hartmann cavity acoustics on the atomization of droplet sprays. Initially, the experiments are conducted on a single droplet to understand its behavior in the sound field of a Hartmann whistle. The atomization studies on single droplet reveal that the existence of sound field causes the droplet to undergo large deformation and become irregular in shape. The degree of droplet deformation is quantified based on smaller circularity and larger Feret's diameter. The increase in cone angle of spray to a higher value in the presence of acoustics in comparison to its absence shows that the acoustics enhances the atomization. The stroboscopic visualization of sprays in the presence of acoustics further reveals the breakup of ligaments, large scatter as well as the formation of more number of droplets, indicating atomization enhancement.

The burning characteristics of unsteady partially premixed flames have been investigated in this paper. Two reaction zones namely rich inner premixed zone and outer diffusion wing were observed for all the equivalence ratios considered. This kind of structure is called double flame structure. Flickering of flames were observed due to the formation of toroidal vortices around the flame surface. These toroidal vortices are convected up as a result of buoyant acceleration. The flames are observed to oscillate in two different modes namely, bulk flickering and tip flickering. In the bulk flickering mode, the flame tip is chopped off from the flame surface and in the tip flickering mode, the flame surface is observed to oscillate up and down without any breakage of flame tip. Flames with higher equivalence ratio (greater than 6) exhibit bulk flickering and flames with lower equivalence ratio (less than 6) experience tip flickering. As the equivalence ratio is lowered by partial premixing of the fuel with more air, the flame height is reduced and the strength of inner premixed flame is increased. The amplitude of oscillation of the flame height also decreases with reduction in the equivalence ratio due to the formation of smaller toroidal vortices but the frequency of oscillation is found to increase slightly. A generalised Strouhal number - Froude number correlation has been obtained in the form St ∝Fr-β, by varying the flow rates of air or fuel systematically, keeping the other parameters constant. The effect of increasing air flow rate is to increase the flickering frequency and increasing fuel flow rate is to decrease the flickering frequency.