Dans ce travail de thèse, les mécanismes à l'origine de la formation des chocs associés à la perception de crackle proche de jets supersoniques axisymétriques sont étudiés à l'aide de simulations numériques. Dans ces simulations, les équations de Navier-Stokes instationnaires et compressibles sont résolues en coordonnées cylindriques à l'aide de différences finies d'ordre élevé peu dissipatives et peu dispersives. Quatre jets temporels à des nombres de Mach de 2 et~3 et à des nombres de Reynolds compris entre 3125 et 50000 sont simulés dans un premier temps. Des ondes acoustiques de forte amplitude présentant d'importants gradients de pression sont mises en évidence à proximité des jets. Elles se forment par un mécanisme de raidissement à la source qui est étudié par le calcul de moyennes conditionnelles synchronisées autour des pics de pression en champ proche. Ces moyennes montrent un lien direct entre ces ondes non linéaires et la convection de structures cohérentes à desvitesses supersoniques dans les couches de~mélange. L'influence de la température sur la formation de ces ondes est examinée dans un second temps par le calcul de cinq jets temporels à des rapports de température de 1, 2 et 4, et à des nombres de Mach acoustique compris entre 2 et 4. À vitesse d'éjection constante, les niveaux de bruit produits par les jets chauds sont moins élevés que ceux du jet isotherme, mais les ondes non linéaires qu'ils rayonnent sont peu affectées par une hausse de température. À nombre de Mach constant, les niveaux augmentent avec la température, de même que l'asymétrie des fluctuations de pression, traduisant un renforcement du caractère non linéaire des ondes rayonnées. Ces variations pourraient être dues à celles de la vitesse de convection des structures cohérentes, qui augmente de façon significative avec la température lorsque le nombre de Mach est constant, mais diminue légèrement à vitesse~constante. Finalement, trois simulations de jets spatiaux isothermes et chauds à un nombre de Mach acoustique de 2 et à des nombres de Reynolds de 12500 et 50000 sont mises en \oe uvre. Des ondes de Mach présentant d'importants gradients de pression sont visibles au voisinage direct des jets. La formation de ces ondes est liée, comme dans le cas des jets temporels, à la convection supersonique de structures cohérentes dans les couches de mélange. Le champ lointain acoustique est enfin déterminé par des méthodes d'extrapolation linéaire et non linéaire. Lorsque la propagation est non linéaire, un raidissement additionnel des fronts d'onde est constaté en champ lointain. / Numerical simulations are carried out with the aim of investigating the formation of nonlinear steepened waves at the origin of crackle in the near acoustic field of supersonic jets. In these simulations, the compressible Navier-Stokes equations are solved in cylindrical coordinates using high-order low-dissipative and low-dispersive finite difference schemes.Four temporally-developing isothermal round jets are first simulated at Mach numbers of~2 and~3 and at Reynolds numbers ranging from 3,125 to 50,000. Strong acoustic waves containing sharp pressure variations are observed in the vicinity of the jets. Their formation process is described by the computation of conditional averages which are triggered by the detection of strong pressure peaks in the near field. Such steepened waves are then shown to be produced by the supersonic motion of coherent structures inside the jet shear layers.Temperature effects are then investigated by considering five temporal round jets at temperature ratios of 1, 2 and~4 and at acoustic Mach numbers of 2, 2.8 and 4. For a given jet speed, the sound levels produced by the hot jets are lower than those of the isothermal one. However, the properties of the steepened waves they generate are not significantly affected by a rise of temperature. On the contrary, when the Mach number is held constant, pressure levels are higher at high temperature. The skewness and kurtosis factors of pressure fluctuations are also increased, which indicates a strengthening of the asymmetry and the intermittency of the pressure fluctuations. It is likely that the influence of temperature on these waves results from the variations of the convection speed, which is found to significantly increase with temperature at constant Mach number, but to slightly decrease at constant jet speed.Finally, three simulations of spatially-developing axisymmetric, isothermal and hot jets at a Mach number of~2 and at Reynolds numbers of 12500 and 50000 are performed. Strong Mach waves possessing the distinctive features of crackle are visible in the near vicinity of the jets. As observed for temporal simulations, their formations are associated with the supersonic motion of large-scale coherent structures inside the jet shear layers. The far acoustic field is determined using linear as well as nonlinear extrapolation methods. When nonlinear propagation effects are taken into account, a further steepening of the wavefronts is observed with increasing propagation distance.

Uncertainties still exist about the safety of cell phone use and the level of cell phone-driven radiation. The purpose of the current inquiry was to determine the long-term health impacts of cell phone-driven radiation via the use of cell phones. In this cross-sectional study, which was based on socio-ecological theory, secondary data from the 2012 National Health Interview Survey were analyzed to assess the difference in the prevalence of thyroid cancer, mouth/tongue/lip cancer, and heart disease between exposed and non-exposed/less exposed cell phone-driven radiation groups in the United States. Logistic regression was used to address three research questions. Findings initially showed that cell phone use was associated with cancer outcome. However, there was no statistically significant relationship between individuals who were heavy users or sometimes users of cell phones and thyroid or mouth/tongue/lip cancer when compared to individuals who rarely or do not use cell phones. There was a relationship between heavy/sometimes users and heart disease when compared to individuals who rarely/do not use cell phones. Yet, when all the confounders/covariates were included in the model, there was no statistically significant difference between the groups compared. For assessment of thyroid cancer cases among individuals who received 'all/almost all calls' via the cell phones and those who received calls 'sometimes' on cell phones, age and sex were added in the model. Based on the study findings, policy-makers could further explore the implementation of comprehensive regulatory measures to address cell phone safety.

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