Krishna Viswanathan

Bio: Krishna Viswanathan is an academic researcher. The author has contributed to research in topics: Jet noise & Jet (fluid). The author has an hindex of 21, co-authored 52 publications receiving 2030 citations.

The sources of jet noise: experimental evidence

Abstract: The primary objective of this investigation is to determine experimentally the sources of jet mixing noise. In the present study, four different approaches are used. It is reasonable to assume that the characteristics of the noise sources are imprinted on their radiation fields. Under this assumption, it becomes possible to analyse the characteristics of the far-field sound and then infer back to the characteristics of the sources. The first approach is to make use of the spectral and directional information measured by a single microphone in the far field. A detailed analysis of a large collection of far-field noise data has been carried out. The purpose is to identify special characteristics that can be linked directly to those of the sources. The second approach is to measure the coherence of the sound field using two microphones. The autocorrelations and cross-correlations of these measurements offer not only valuable information on the spatial structure of the noise field in the radial and polar angle directions, but also on the sources inside the jet. The third approach involves measuring the correlation between turbulence fluctuations inside a jet and the radiated noise in the far field. This is the most direct and unambiguous way of identifying the sources of jet noise. In the fourth approach, a mirror microphone is used to measure the noise source distribution along the lengths of high-speed jets. Features and trends observed in noise source strength distributions are expected to shed light on the source mechanisms. It will be shown that all four types of data indicate clearly the existence of two distinct noise sources in jets. One source of noise is the fine-scale turbulence and the other source is the large turbulence structures of the jet flow. Some of the salient features of the sound field associated with the two noise sources are reported in this paper.

Aeroacoustics of hot jets

Abstract: A systematic study has been undertaken to quantify the effect of jet temperature on the noise radiated by subsonic jets. Nozzles of different diameters were tested to uncover the effects of Reynolds number. All the tests were carried out at Boeing's Low Speed Aeroacoustic Facility, with simultaneous measurement of thrust and noise. It is concluded that the change in spectral shape at high jet temperatures, normally attributed to the contribution from dipoles, is due to Reynolds number effects and not dipoles. This effect has not been identified before. A critical value of the Reynolds number that would need to be maintained to avoid the effects associated with low Reynolds number has been estimated to be 400 000. It is well-known that large-scale structures are the dominant generators of noise in the peak radiation direction for high-speed jets. Experimental evidence is presented that shows the spectral shape at angles close to the jet axis from unheated low subsonic jets to be the same as from heated supersonic jets. A possible mechanism for the observed trend is proposed. When a subsonic jet is heated with the Mach number held constant, there is a broadening of the angular sector in which peak radiation occurs. Furthermore, there is a broadening of the spectral peak. Similar trends have been observed at supersonic Mach numbers. The spectral shapes in the forward quadrant and in the near-normal angles from unheated and heated subsonic jets also conform to the universal shape obtained from supersonic jet data. Just as for unheated jets, the peak frequency at angles close to the jet axis is independent of jet velocity as long as the acoustic Mach number is less than unity. The extensive database generated in the current test programme is intended to provide test cases with high-quality data that could be used for the evaluation of theoretical/semi-theoretical jet noise prediction methodologies.

Jet Aeroacoustic Testing: Issues and Implications

Abstract: Issues that are important for jet aeroacoustic tests and the critical role of good data in the development of jet noise technology are reviewed and discussed. A major effort for improving the quality of aeroacoustic data acquired at the Boeing Low Speed Aeroacoustic Facility has been carried out. This extensive undertaking targeted all aspects of model-scale testing and acquisition of good quality data and covered issues of flow quality, nozzle performance, and acoustics. Significant improvements have been made in all of the named categories. Simultaneous measurement of nozzle aerodynamic performance and noise is important, especially for the development of noise suppression devices. The capabilities of a jet rig incorporated with a six-component force balance are described. It is clearly demonstrated that the measured thrust with the current rig is in excellent agreement with that obtained using a dedicated force balance over a wide range of nozzle pressure ratios. Results of the efforts at rig refurbishment, carried out over the last few years, are presented. The high quality of noise measurements is established through good spectral agreement with data obtained with a blowdown jet, for a wide range of nozzle conditions. An extensive study of available jet noise data from various jet noise facilities has been completed. Implications of contaminated data from most tests and the obligations of the experimental community for the advancement of jet noise technology are discussed.

The Sources of Jet Noise: Experimental Evidence

Abstract: *† ‡ § The primary object of this investigation is to determine experimentally the sources of jet noise. In the present study, four different approaches are used. It is reasonable to assume that the characteristics of the noise sources are imprinted on their radiation fields. Under this assumption, it becomes possible to analyze the characteristics of the far field sound and then infer back on the characteristics of the sources. The first approach is to make use of the spectral and directional information measured by a single microphone in the far field. A detailed analysis of a large collection of far field noise data has been carried out. The purpose is to identify special characteristics that can be linked directly to those of the sources. The second approach is to measure the coherence of the sound field using two microphones. The autocorrelations and cross-correlations of these measurements offer not only valuable information on the spatial structure of the noise field in the radial and polar angle directions, but also on the sources inside the jet. The third approach involves measuring the correlation between turbulence fluctuations inside a jet and the radiated noise in the far field. This is the most direct and unambiguous way of identifying the sources of jet noise. In the fourth approach, a mirror microphone is used to measure the noise source distribution along the lengths of high-speed jets. Features and trends observed in noise source strength distributions are expected to shed light on the source mechanisms. It will be shown that all four types of data indicate clearly the existence of two distinct noise sources in jets. One source of noise is the fine scale turbulence and the other source is the large turbulence structures of the jet flow. Some of the salient features of the sound field associated with the two noise sources are reported in this paper.

Analysis of the two similarity components of turbulent mixing noise

Abstract: It is now widely believed that the turbulence in free shear layers is not completely random, but more coherent and orderly, and that turbulent flows contain both fine-scale and large-scale structures. Both fine-scale turbulence and large-scale turbulence generate noise. Experimental measurements have shown conclusively that the mean flow as well as the turbulence statistics exhibit self-similarity. Based on these observations, Tam et al. (Tam, C. K. W., Golebiowski, M., and Seiner, J. M., Two Components of Turbulent Mixing Noise from Supersonic Jets, AIAA Paper 96-1716, 1996) proposed that because noise is generated by the turbulence of the jet, the noise spectra generated by fine-scale and large-scale turbulence should also exhibit self-similarity. By the examination of a large set of supersonic jet noise data acquired at NASA Langley Research Center, Tam et al. offered evidence that the turbulent mixing noise of high-speed jets does consist of two, independent self-similar components. We first provide additional independent confirmation of the universal shapes of the two components of mixing noise for a single jet. The significance of an important effect, due to atmospheric absorption, is illustrated with detailed analysis of The Boeing Company jet noise data. The Tam et al. analysis is based on the examination of data from single-stream nozzles. We provide evidence from the analysis of noise from dual-stream nozzles that the measured spectra in the forward quadrant and near-normal angles conform to the shape of the fine-scale spectrum, regardless of nozzle geometry and operating conditions. We clearly demonstrate that the spectral shape associated with the large-scale structures of single jets does not characterize the noise of coaxial jets at large aft angles. Finally, we show that the noise of hot subsonic jets at low angles also conform to the shape of the fine-scale spectrum.

Wave Packets and Turbulent Jet Noise

Abstract: Turbulent jet noise is a controversial fluid mechanical puzzle that has amused and bewildered researchers for more than half a century. Whereas numerical simulations are now capable of simultaneously predicting turbulence and its radiated sound, the theoretical framework that would guide noise-control efforts is incomplete. Wave packets are intermittent, advecting disturbances that are correlated over distances far exceeding the integral scales of turbulence. Their signatures are readily distinguished in the vortical, turbulent region; the irrotational, evanescent near field; and the propagating far field. We review evidence of the existence, energetics, dynamics, and acoustic efficiency of wave packets. We highlight how extensive data available from simulations and modern measurement techniques can be used to distill acoustically relevant turbulent motions. The evidence supports theories that seek to represent wave packets as instability waves, or more general modal solutions of the governing equations, and confirms the acoustic importance of these structures in the aft-angle radiation of high subsonic and supersonic jets. The resulting unified view of wave packets provides insights that can help guide control strategies.

The sources of jet noise: experimental evidence

Abstract: The primary objective of this investigation is to determine experimentally the sources of jet mixing noise. In the present study, four different approaches are used. It is reasonable to assume that the characteristics of the noise sources are imprinted on their radiation fields. Under this assumption, it becomes possible to analyse the characteristics of the far-field sound and then infer back to the characteristics of the sources. The first approach is to make use of the spectral and directional information measured by a single microphone in the far field. A detailed analysis of a large collection of far-field noise data has been carried out. The purpose is to identify special characteristics that can be linked directly to those of the sources. The second approach is to measure the coherence of the sound field using two microphones. The autocorrelations and cross-correlations of these measurements offer not only valuable information on the spatial structure of the noise field in the radial and polar angle directions, but also on the sources inside the jet. The third approach involves measuring the correlation between turbulence fluctuations inside a jet and the radiated noise in the far field. This is the most direct and unambiguous way of identifying the sources of jet noise. In the fourth approach, a mirror microphone is used to measure the noise source distribution along the lengths of high-speed jets. Features and trends observed in noise source strength distributions are expected to shed light on the source mechanisms. It will be shown that all four types of data indicate clearly the existence of two distinct noise sources in jets. One source of noise is the fine-scale turbulence and the other source is the large turbulence structures of the jet flow. Some of the salient features of the sound field associated with the two noise sources are reported in this paper.

Aeroacoustics of hot jets

Abstract: A systematic study has been undertaken to quantify the effect of jet temperature on the noise radiated by subsonic jets. Nozzles of different diameters were tested to uncover the effects of Reynolds number. All the tests were carried out at Boeing's Low Speed Aeroacoustic Facility, with simultaneous measurement of thrust and noise. It is concluded that the change in spectral shape at high jet temperatures, normally attributed to the contribution from dipoles, is due to Reynolds number effects and not dipoles. This effect has not been identified before. A critical value of the Reynolds number that would need to be maintained to avoid the effects associated with low Reynolds number has been estimated to be 400 000. It is well-known that large-scale structures are the dominant generators of noise in the peak radiation direction for high-speed jets. Experimental evidence is presented that shows the spectral shape at angles close to the jet axis from unheated low subsonic jets to be the same as from heated supersonic jets. A possible mechanism for the observed trend is proposed. When a subsonic jet is heated with the Mach number held constant, there is a broadening of the angular sector in which peak radiation occurs. Furthermore, there is a broadening of the spectral peak. Similar trends have been observed at supersonic Mach numbers. The spectral shapes in the forward quadrant and in the near-normal angles from unheated and heated subsonic jets also conform to the universal shape obtained from supersonic jet data. Just as for unheated jets, the peak frequency at angles close to the jet axis is independent of jet velocity as long as the acoustic Mach number is less than unity. The extensive database generated in the current test programme is intended to provide test cases with high-quality data that could be used for the evaluation of theoretical/semi-theoretical jet noise prediction methodologies.

Current Status of Jet Noise Predictions Using Large-Eddy Simulation

Abstract: A survey of the current applications of large-eddy simulation for the prediction of noise from single stream turbulent jets is given. After summarizing the numerical techniques used, the data predicted by the simulations are given at conditions from subsonic, heated jets to supersonic, unheated jets. Mach numbers between 0.3 and 2.0 are considered. Following the data presentation, an analysis of the trends exhibited by the data is given, with special attention paid to relationship between numerical and/or modeling choices and the prediction accuracy. The data support the conclusion that the most limiting factor in current large-eddy simulations is the thickness of the initial shear layer, which is commonly one order of magnitude thicker than what is found experimentally. There is also a large amount of uncertainty regarding the influence of the subgrid scale model on the predictions. The influence of inflow conditions is discussed in depth. Uncertainties in the inflow conditions currently prohibit the simulations from reliably predicting the potential core length. The centerline evolution of the mean and fluctuating axial velocity is strongly coupled to the resolution of the initial shear layers, but can be made to agree within experimental uncertainty when sufficiently thin initial shear layers are used. The maximum achieved Strouhal number of the sound in the acoustic far field is 1.5-3.0, depending on flow condition; this limit is due to numerical resources. A listing of some of the open questions and future directions concerning jet noise predictions using large-eddy simulation concludes the survey.

On using large-eddy simulation for the prediction of noise from cold and heated turbulent jets

Abstract: The results of a series of large-eddy simulations of heated and unheated jets using approximately 106 grid points are presented. The computations were performed on jets at operating conditions originally investigated by Tanna in the late 1970s [H. K. Tanna, “An experimental study of jet noise Part I: Turbulent mixing noise,” J. Sound Vib., 50, 405 (1977)]. Three acoustic Mach numbers are investigated (Uj∕a∞=0.5, 0.9, and 1.5) at cold (constant stagnation temperature) and heated conditions (Tj∕T∞=1.8, 2.7, and 2.3, respectively). The jets’ initial annular shear layers are thick relative to experimental jets and are quasi-laminar with superimposed disturbances from linear instability theory. It is observed that qualitative changes in the jets’ mean- and turbulent field structure with Uj and Tj are consistent with previous experimental data. However, the jets exhibit a faster centerline mean velocity decay rate relative to the existing data, with a corresponding 3–4 % over-prediction of the peak root-mean-square level. The acoustic pressure fluctuations in the far field are analyzed in detail. The accuracy of the overall sound pressure level predictions is found to be a strong function of the jet Mach number, with the lowest speed jets being the least accurate. At all conditions the peak acoustic frequency occurs at approximately St=fDj∕Uj=0.25. The limited resolution of the computations is shown to impact the radiated sound by yielding effectively low-pass filtered versions of the experimental spectra, with a maximum frequency of St≈1.2.