Brief discussion on Crackle
Crackle can be defined as a distinguishable sound that can be subjectively investigated in the noises coming out of a high speed jet. However, a Crackle cannot be categorized simply with standard description regarding the spectrum of a noise. Nevertheless, the sound produced is considered to be particularly annoying and it has a dominant component on the overall noise production of a high speed jet. It was first noticed and further brought to the public attention after the microphone measurements of the noise emanation for Rolls-Royce/SNECMA Olympus 593 were studied, in which the engine was performing at a high-specific thrust (Russavage et al., 2018). Following would be a report on the different researches that have been conducted on the Crackle noises. The report would consist of the brief discussion about the phenomenon of Crackle, the researches that have been conducted so far and the inferences of those researches. This would present an overview about the research about Crackle that has been conducted till date.
When closely listened to the jet aircraft of high specific thrust at the time of take off, anybody would be recognizable to the sound of a ‘crackle’. The outsized jet engines used for transporting in the first generation used for military purposes needed high level of performance working at full power. These aircrafts working at full power tend to produce a sudden spasmodic burst emerging from a rasping fricative sound which somewhat resembles the sound of tearing of a piece of dry paper. It has also been reported that some of the observers have reported the sound to resemble an electric arc welder or a poorly-connected loud speaker (Daniel et al., 2018). Otherwise, there have been incidences that the observes have reported the sounds to resemble that of a reaction of spitting happening between extremely hot fat and water being added to it. The word ‘Crackle’ itself clarifies the effect of the sound. It is an extremely and subjectively distinct feature amongst the sounds produced by a jet aircraft in high performance level. There has been constant testing of engines involving technicians that have produced views upon the different causes of crackle. It has been found that the tendency to produce crackle increases with increased performance of the aircraft at highest power settings (Murray and Lyons 2016). This is because at this point of time, the jet becomes super critical and is sometimes imposed more with the presence of an afterburner. This is where the process of combustion is superbly rough compared to other stages of the aircraft combustions. Many researchers are of the opinion that a reheated jet compels the production of Crackle and some are of the opinion that the crackle sound originates from the jet aircraft as soon as it exhausts at superficial conditions from a nozzle that has an imperfectly expanded flow (Reichman et al., 2018). Although, there is no systematic base for the views that the researchers have been providing, yet, there have been impressions that form as a result of practical experience.
Researches conducted on Crackle
Crackle cannot be anyways quantified with respect to any common scales of sound and also it cannot be distinguished with any common scales of sound. Another case, it cannot even be evaluated whether a jet has been cracking in order to present the sound or not. However, the researchers have concluded the fact that measurement of a crackle is dependent on the skewness of the time series considering it to be one of the primary parameter (Baars and Tinney 2014). This helps in checking whether a jet is cracking or not. Nevertheless it has been found that the previous researches did not provide much interest on the subject of Crackle, its origin or the cause by which it has been produced. Very little attention was paid to the phenomenon of Crackle (Tam et al., 2018). This has been found to be of the fact that crackle cannot be usually detected or quantified while microphone signal analysis also fails to acknowledge its existence. It has also been found as per reports that the presence of crackle in supersonic helium jets issuing into air that is of ambient nature has been found. The next discussion would thus be ensued upon the researches that have been conducted so far about the jet noises or crackle noises witnessed so far.
The first paper that has conducted research on the presence of Crackle in jet aircrafts has been a subjective investigation resulting to the proper distinguishing of the sound in high speed jets. The ‘crackle’ has although not been written off by the standard description regarding the spectrum of noise, however, it has proven to be the intense spasmodic short-duration compressive elements occurring in a waveform. These noises have seen to have lower energy speed that spreads over a wide range of frequency. The groups happen to appear randomly and stay there for an amount of about 10 – 18, each group happens to hold an average of 10 compressions, which generally has a strength of 5 x 10W atoms at a distance of 50 m (Williams, Simson and Virchis 1975). It has also been found from the above reports that the skewness reports produced from the aircrafts that resulted in producing a skewness of less than 0.3 are crackle-free. The paper concludes thus that the crackle noise is produced out of local shock formation that occurs on account of a non linear wave steepening at the source and does not appear on account of non linear propagation in the long term.
In the next paper about crackles in high amplitude jet aircrafts also refers that crackle forming can be quantifiable with the skewness of the time waveform. The investigation that has been done in this paper refers to a simulated waveform having a almost indistinguishable probability density function and power spectrum to a near about amount of F/A-18E afterburner recording that has been created through the nonlinearly that helps to transform a Gaussian waveform in a statistical way (Gee et al., 2007). The two waveforms thus producing have further been examined revealing that although their statistics might be identical virtually, but there happens to be considerable differences with the rate of change in time in the intense compressive portions in the waveforms.
Results of the researches
The next paper also focuses on the research that has been done to identify the generation of the ‘Crackle’ which was first observed by Ffowcs Williams in the year 1975 within a full scale jet aircraft. The data generated due to the research focused on the origin of crackle off the axisymmetric supersonic jets that were used for the experimentation and thus further the characteristics of the crackle were justified. It is believed by the experiment that the small scale laboratory experiment also could validate the experimental data for full scale jet engines. The experimental data expanded over a value of velocity range expanding from 600m/s to 1.05 km/sec (Krothapalli, Venkatakrishnan and Lourenco 2000). The research suggests that micro explosions of cold ambient fluid in the hot jet happen to produce these concentrated waves associated with the origination of crackle. It has also been observed that a cold lump of fluid when exposed to a hot jet fluid suddenly, it expand by giving provision to the monopole source of sound, which is quantified as the sound produced by a crackle.
The last paper in question focuses on the experiment that the organization General Electric has done to quantify the generation of Crackle in collaboration with the United States Navy. They have inferred that the consequence of chevrons that appear on the crackle noise component in full scale on the F404 engine, and in small scale on the F414 engine nozzle in the twin configuration (Martens, Spyropoulos and Nagel 2011).
The results that the researches have imposed upon gives rise to various inferences about the production of the jet noise called crackle. The first paper concludes that the crackle noise is produced out of local shock formation that occurs on account of a non linear wave steepening at the source and does not appear on account of non linear propagation in the long term (Mora et al., 2015). In the next paper it can be seen that the two waveforms thus producing have further been examined revealing that although their statistics might be identical virtually, but there happens to be considerable differences with the rate of change in time in the intense compressive portions in the waveforms (Swift, Gee and Neilsen 2017). The next research suggests that micro explosions of cold entrained ambient fluid in the hot jet happen to generate these intense waves associated with the origination of crackle.
Conclusion
Thus, it can be concluded from the above report about the ‘crackle’ sound derived from several journal articles that ‘crackle’ is a noise witnessed in high-speed jet aircrafts working at extreme levels. It was first noticed and further brought to the public attention after the microphone measurements of the noise emanation for Rolls-Royce/SNECMA Olympus 593 were studied, in which the engine was performing at a high-specific thrust. The report has been started off with a report on the different researches that have been conducted on the Crackle noises. The report consisted of the brief discussion about the phenomenon of Crackle, the researches that have been conducted so far and the inferences of those researches. This would present an overview about the research about Crackle that has been conducted till date. The various findings of the papers have resulted in the production of the initial idea of the Crackle and so far have achieved the levels where it can be said that Crackle is not characterized by any sound wave form but can only be felt as a chemical reaction happening between different elements of a jet aircraft needed for the combustion process when it works at high speed. The researches have proven the fact by taking the amplitude diagram of various produced Crackles and plotting it on the graph. Mostly these have been done on the aircraft Rolls-Royce/SNECMA Olympus 593. The paper thus concludes the brief but extensive idea on the concept of ‘Crackle’.
References
Baars, W.J. and Tinney, C.E., 2014. Shock-structures in the acoustic field of a Mach 3 jet with crackle. Journal of Sound and Vibration, 333(12), pp.2539-2553.
Daniel, K., Mayo, D.E., Lowe, T. and Ng, W., 2018. Experimental Investigation of the Very Near Pressure Field of a Heated Supersonic Jet with a Total Temperature Non-Uniformity. In 2018 AIAA/CEAS Aeroacoustics Conference (p. 3145).
Gee, K.L., Sparrow, V.W., Atchley, A.A. and Gabrielson, T.B., 2007. On the perception of crackle in high-amplitude jet noise. AIAA journal, 45(3), pp.593-598.
Krothapalli, A., Venkatakrishnan, L. and Lourenco, L., 2000. Crackle-A dominant component of supersonic jet mixing noise. In 6th Aeroacoustics Conference and Exhibit (p. 2024).
Martens, S., Spyropoulos, J.T. and Nagel, Z., 2011, January. The effect of chevrons on crackle: Engine and Scale model results. In ASME 2011 Turbo Expo: Turbine Technical Conference and Exposition (pp. 315-326). American Society of Mechanical Engineers.
Mora, P.A., Kastner, J.F., Gutmark, E.J. and Kailasanath, K., 2015. Investigation of a heated supersonic jet chevrons nozzle. In 53rd AIAA Aerospace Sciences Meeting (p. 0233).
Murray, N.E. and Lyons, G.W., 2016. On the convection velocity of source events related to supersonic jet crackle. Journal of Fluid Mechanics, 793, pp.477-503.
Reichman, B.O., Gee, K.L., Neilsen, T.B., Downing, J.M., James, M.M., Wall, A.T. and McInerny, S.A., 2018. Characterizing acoustic shocks in high-performance jet aircraft flyover noise. The Journal of the Acoustical Society of America, 143(3), pp.1355-1365.
Russavage, P.B., Neilsen, T.B., Gee, K.L. and Swift, S.H., 2018, May. Rating the perception of jet noise crackle. In Proceedings of Meetings on Acoustics 175ASA (Vol. 33, No. 1, p. 040001). ASA.
Swift, S.H., Gee, K.L. and Neilsen, T.B., 2017. Testing two crackle criteria using modified jet noise waveforms. The Journal of the Acoustical Society of America, 141(6), pp.EL549-EL554.
Tam, C.K., Spyropoulos, J.T., Aubert, A. and Powers, R., 2018. Crackle in the noise of high-performance aircraft. In 2018 AIAA/CEAS Aeroacoustics Conference (p. 3306).
Williams, J.F., Simson, J. and Virchis, V.J., 1975. ‘Crackle’: An annoying component of jet noise. Journal of Fluid Mechanics, 71(2), pp.251-271.
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