Classical Theories of Jet Noise
Noise generated form the jets with high-velocity is generally termed as jet noise. This is also produced by eddies of turbulence that is emitted as a result of shearing flow. Jet noise is often been described as the source of loudest noise that is produced by the humans. This paper illustrates the classical theories of jet noise theory, along with the noise pollution for which it is responsible for. Additionally the paper discusses the health effects of this noise pollution that it has on the human body.
Most of the works that has been conducted on jet noise are built on the fundamentals of the acoustic analogy. The jet velocity relies on this acoustic analogy which is the eighth-power dependence of the overall power level (OAPWL) (Clark, Head and Stansfeld 2013). The quadrupole, acts an important part for the unheated jets and is responsible for causing the V8 dependence on jet velocity. This can be showed by the equation –
T ij = ρ υi υj + (p – ρa2) δij
Here the variables p, ρ, υ, δ and T refers to the pressure, the density, and the velocity components vi and vj respectively, in order to evaluate the flow. a refers to an assumed speed of sound in an unbounded medium, taken here to be the ambient speed of sound. The jet theory also suggests that the there is an impact of the temperature intensity, that results in fluctuations that is responsible for the turbulent velocity fluctuations intensity. Researchers estimated the intensity ratios of the dipole, related to the quadrupole noise in context of a heated jet. This is expressed as:
(Tj /Ta) refers to the jet static temperature ratio. The switch from the quadrupole to dipole dominance for the heated jet was related to the enthalpy ratio (hj /ha) (Kopiev and Chernyshev 2014). Other theories given by Morfey (1973), Tester and Morfey (1976), and Morfey et al (1978), provided support to the impact of the dipole characteristics as a source of second term for the jet noise (Bres et al., 2015). After reviewing all the classical formulations, assumption was made that there was independence and correlation between the terms of stress and entropy, provided by Reynolds. This general form can be expressed as:
Suggestions have also been made of “shear noise” and “self-noise” as the source terms, with respect to jet noise. According to several researchers, a different version of acoustic analogy was described which used linearized Euler equations that helped in the easy identification of the source terms in the continuity and the momentum equations and there interpretation (Jordan and Colonius 2013).
Health Effects of Jet Noise Pollution
Noise is produced in various ways by a jet engine however the main source is the nozzle at the rear of the engine, from which emission of the high-speed exhaust stream takes place. A tremendous shear is produced on contact of the air with the exhaust steam. This leads to the formation of a turbulence that subsides on being unstable, forming the roar of the engine. This phenomenon takes place in the length of the nozzle, where there is initial mixing of the air in the annular shear layer. In case of Supersonic, or “choked” jets, there is a flow in the cells, along with continuous expansion and contraction. This phenomenon is responsible for the formation of screech tones” and associated broadband “shock associated noises” (Papamoschou 2018).
Any sort of noise pollution can lead to impairment of mental performances. Apparently the noise pollution produced by the jet engines lead to occurrence of hypertensive diseases. The stress cause due to noise can prevail for a long time that can exhaust the compensatory mechanisms and leads to decrease in the regulatory capacity of the body. Often these health problems become chronic. Such disorders as the chronic arterial hypertension can affect a major population segment, and emerge as a leading factor for the cause of myocardial infarction and stroke. Reports suggested that when 2959 adults were exposed to jet noise, it led to the increase in high blood pressure. Additionally it was perceived that a continuous 24 hours exposure to aircraft noise level (FBN) above 55 dB (A) and at maximum levels above 72 dB (A), resulted in increasing effects of hypertension (Fosso Pouangué et al., 2014). Other studies have showed that jet noise exposure during the nocturnal hours have an effect on the hypertension. Jet noise also has an effect on the cardiac health of individuals. Exposure to higher levels of jet noise have been established as the cause of cardiovascular diseases. Additional health impacts of general noise pollution and jet noise includes sleep interference, loss of hearing, and several other non-auditory effects. Disturbance in sleep is the major cause of annoyance in human. The jet noise is of intermittent nature which is responsible for more disturbance compared to general noise. An analysis conducted by the U.S. Air Force, suggested the main concerns related to the effect of noise on sleep (Basner et al. 2014).
An important impact is the loss of hearing. The fact that continuous exposure to high noise levels will lead to damage human hearing was established by studies of the U.S. Environmental Protection Agency, 1978. The normal hearing range is upto 120 dB. There is shift in the higher sound level of the sensitivity of the ear and the accuracy with which sound is perceived. The change can either be temporary which is called a temporary threshold shift (TTS), or of permanent nature, called a permanent threshold shift (PTS) (Kearney-Fischer, Sinha and Samimy 2013). Determination whether correlations exist between noise exposures is conducted and is related to the cardiovascular problems, birth weight, and mortality rates. Exposure to noise for prolonged periods is the major cause of non-auditory effects of noise pollution. There are also incidents of performance disruption due to the general effect of noise pollution. Effect is seen on the quantity and the quality of work as a result of noise pollution. Performance impairment occurs due to noise pollution as it places high demands on the workers. Cognitive and learning abilities of the individuals are also disrupted due to the ill effects of noise. Background noise in excessive amount can interfere with the communication abilities of individuals, creating barriers of acoustic learning. There is also an interference in the writing, reading and other such activities as a result of the negative effects of noise pollution (Halperin 2014). Sometimes incidences may occur like difficulty in memory recalling, comprehending or puzzle solving abilities of individuals due to prolonged expose to high range of noise which can be general noise pollution or jet noise.
Conclusion
From the above discussions it can be concluded that the theories of jet noise are formulated on the fundamentals of acoustic analogy, which can be interpreted in different forms. Nosie pollution formed as a result of this jet noise is probably the source of the loudest noise that is produced by mankind. The deleterious effects that it has on the human health can be many which includes auditory as well as non-auditory disorders.
References
Basner, M., Babisch, W., Davis, A., Brink, M., Clark, C., Janssen, S. and Stansfeld, S., 2014. Auditory and non-auditory effects of noise on health. The lancet, 383(9925), pp.1325-1332.
Bres, G.A., Jaunet, V., Le Rallic, M., Jordan, P., Colonius, T. and Lele, S.K., 2015. Large eddy simulation for jet noise: the importance of getting the boundary layer right. In 21st AIAA/CEAS aeroacoustics conference (p. 2535).
Clark, C., Head, J. and Stansfeld, S.A., 2013. Longitudinal effects of aircraft noise exposure on children’s health and cognition: a six-year follow-up of the UK RANCH cohort. Journal of Environmental Psychology, 35, pp.1-9.
Correia, A.W., Peters, J.L., Levy, J.I., Melly, S. and Dominici, F., 2013. Residential exposure to aircraft noise and hospital admissions for cardiovascular diseases: multi-airport retrospective study. Bmj, 347, p.f5561.
Fosso Pouangué, A., Sanjosé, M., Moreau, S., Daviller, G. and Deniau, H., 2014. Subsonic jet noise simulations using both structured and unstructured grids. AIAA Journal, 53(1), pp.55-69.
Halperin, D., 2014. Environmental noise and sleep disturbances: A threat to health?. Sleep science, 7(4), pp.209-212.
Jordan, P. and Colonius, T., 2013. Wave packets and turbulent jet noise. Annual Review of Fluid Mechanics, 45, pp.173-195.
Kearney-Fischer, M., Sinha, A. and Samimy, M., 2013. Intermittent nature of subsonic jet noise. AIAA journal, 51(5), pp.1142-1155.
Kopiev, V.F. and Chernyshev, S.A., 2014. Simulation of azimuthal characteristics of turbulent jet noise by correlation model of quadrupole noise sources. International Journal of Aeroacoustics, 13(1-2), pp.39-60.
Papamoschou, D., 2018. Wavepacket modeling of the jet noise source. International Journal of Aeroacoustics, 17(1-2), pp.52-69.
Powers, R.W., Kuo, C.W., McLaughlin, D.K. and Morris, P.J., 2014. Supersonic jet noise reduction by nozzle fluidic inserts with simulated forward flight. In 20th AIAA/CEAS aeroacoustics conference (p. 2474).
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