Srinivasan K

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 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.

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.

This paper explores jet noise control using castellations at the exit of pipe-jets. Far-field acoustic measurements and schlieren visualizations are performed for two configurations of castellations; namely, two and four counts. The results are compared with that of pipe-jet without castellations. The nozzle pressure ratio range of the study is 1.5 to 4.5. With each configuration, the position and strength of vortices vary causing it to interact in different manners. For pipe with two castellations, the screech is amplified and the overall sound pressure level is higher than the reference jet in most of the nozzle pressure ratios under study. For pipe with four castellations, there is no noise variation in the azimuthal direction, and screech is eliminated at all nozzle pressure ratios.

The present article compares the impingement noise generated due to the impact of jet issued from non-circular orifices such as triangular and elliptical with the circular orifice of the equivalent area. The experiments are performed for at various Nozzle pressure ratios ranging from subsonic to supersonic. The NPR variation is performed for nozzle plate spacing of h/d=5.0. Apart from this the effect of impingement plate porosity also studied. The acoustic measurement is carried out in the acoustic far-field region. The noise is compared in terms of overall sound pressure level, frequency spectra, and acoustic power.

Noise produced from impinging jets can lead to environmental and health hazards due to its complex flow physics. Many noise-reduction techniques have been applied in past, however these techniques would have been more effective if sources in jet plume were properly characterized. Thus reconstruction of the acoustic sources in fluid flow plays an important role in understanding noise sources and devising effective methods for controlling them. Among many techniques used for noise source localization, Near-field Acoustic Holography (NAH) has proved to be an effective tool which captures rapidly decaying evanescent waves in the near-field thus producing high resolution mapping of sources. The objective of this study is to visualize the aero-acoustic sound sources in impinging wall jet on various frequencies at different Mach numbers at particular nozzle-plate spacing. For this NAH study, patch and scan based measurements were taken parallel to wall jet using planar microphone array and reference microphones. Then, Partial Field Decomposition procedure was followed and incoherent scans having fixed phase relationship were stitched. Further, Tikhonov Regularization method was used to suppress the effects of high frequency noise components in measurements before backward holographic projection towards source plane to reconstruct the sound field.

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 αFr-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.

This paper explores jet noise control using castellations at the exit of pipe-jets. Far-field acoustic measurements and schlieren visualizations are performed for two configurations of castellations; namely, two and four counts. The results are compared with that of pipe-jet without castellations. The nozzle pressure ratio range of the study is 1.5 to 4.5. With each configuration, the position and strength of vortices vary causing it to interact in different manners. For pipe with two castellations, the screech is amplified and the overall sound pressure level is higher than the reference jet in most of the nozzle pressure ratios under study. For pipe with four castellations, there is no noise variation in the azimuthal direction, and screech is eliminated at all nozzle pressure ratios.

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.

Spectral features of cylindrical and conical Hartmann resonators are compared in this work. Experiments have been conducted at different nozzle pressure ratios at a fixed stand-off distance. Resonance frequencies of conical cavities are found to be higher than those of cylindrical cavities of the same length. Spectra, also shows higher resonant modes excited along with the fundamental. Some qualitative differences were observed between spectra of cylindrical and conical resonator. The directivity is always maximum in the downstream direction for both cavities. These devices are capable of producing high amplitude tones, over a wide range of frequencies.