CENTRALE LYON - Ph.D position Propagation of infrasonic waves in a turbulent atmosphere

ECL and Laboratory presentation

Founded in 1857, École Centrale de Lyon is one of the top 10 engineering schools in France. It trains more than 3,000 students of 50 different nationalities on its campuses in Écully and Saint-Étienne (ENISE, in-house school): general engineers, specialized engineers, masters and doctoral students. With the Groupe des Écoles Centrale, it has three international locations. The training provided benefits from the excellence of the research carried out in the 6 CNRS-accredited laboratories on its campuses, the 2 international laboratories, the 6 international research networks and the 10 joint laboratories with companies. Its excellent research and high-level teaching have enabled it to establish double degree agreements with prestigious universities and advanced partnerships with numerous companies. With its focus on sobriety, energy, the environment and decarbonization, Centrale Lyon intends to respond to the problems faced by socio-economic players in the major transitions.

Research field presentation

Propagation of infrasonic waves in a turbulent atmosphere

Infrasonic waves are acoustic waves with frequencies lower than the human hearing threshold, around 20 Hz, and wavelengths between 20 m and 8 km. They can be generated by natural hazards, like earthquakes and volcanic eruptions, and human-made sources, such as chemical and nuclear explosions or the sonic booms produced by supersonic aircraft (see Figure 1a). Infrasound can travel through the atmosphere over horizontal distances ranging from a few hundred to several thousand kilometers and vertically up to the thermosphere above a hundred kilometers altitude. At great distances from their source, infrasonic pressure signals generally exhibit distinct wave packets known as arrivals (as seen in Figure 1b). These arrivals provide important information about the excitation mechanisms and the interaction between the infrasonic waves and the propagation medium.

The atmosphere is a complex, unsteady, and intrinsically turbulent flow. In addition to large-scale variations in temperature and winds, primarily controlled by solar activity and planetary waves, turbulent fluctuations with spatio-temporal scales close to acoustic wavelengths and frequencies are continuously observed. These fluctuations are notably generated by the breaking of gravity waves (mechanical waves due to the buoyancy of the air and with frequencies lower than a few millihertz) and can considerably affect the infrasonic arrivals. Although several studies have been carried out on acoustic propagation in turbulent flows, the interaction between infrasound and atmospheric turbulence remains a topic of ongoing research. Understanding this interaction is essential to improve our ability to interpret infrasonic recordings.

Description of the activities

The atmospheric propagation of infrasonic waves has conventionally been modeled using approximate methods, such as ray tracing or normal modes, which keep computational costs low. However, the efficiency of these techniques comes at the expense of a detailed description of the physics of infrasound propagation. Thus, recent research conducted at the Center for Acoustic Research has investigated infrasonic waves by direct numerical simulations of the Navier-Stokes equations [1--6]. These equations describe acoustic propagation without approximations, providing unparalleled insight into the physical phenomena affecting infrasound, such as refraction caused by large-scale temperature and wind variations, non-linear effects, absorption due to viscous and thermal effects, caustics, and diffraction.

The proposed doctoral research aims to build on the aforementioned investigations and study the propagation of infrasonic waves generated by an impulsive source in a turbulent atmosphere induced by the breaking of gravity waves. To this end, three-dimensional numerical simulations of the Navier-Stokes equations will be carried out using an algorithm based on high-order finite difference schemes [7]. This approach will enable the simultaneous description of the spatio-temporal evolution of the turbulent atmosphere (including the generation of gravity waves, their breaking, and the turbulent cascade) and the acoustic propagation through the turbulent atmospheric field. The computations will be executed on clusters of GPUs (Graphics Processing Units) using a code written in C/C++/CUDA. A preliminary two-dimensional investigation was conducted in 2019 [5] (see Figure 2).

As part of the doctoral research, a method for exciting a realistic spectrum of gravity waves will first be developed. The interaction between the turbulent inhomogeneities generated by the breaking of these waves and the pressure signals recorded a few hundred kilometers from the acoustic source will then be analyzed. Finally, the effects of turbulence on arrival waveforms and frequency content will be examined for various source characteristics (energy, spectrum).

References

[1] G. Hanique-Cockenpot, “Étude numérique de la propagation non linéaire des infrasons dans l’atmosphère,” Thèse de doctorat, ECL - No. 2011-32, 2011.

[2] R. Sabatini, O. Marsden, C. Bailly and C. Bogey, “A numerical study of nonlinear infrasound propagation in a windy atmosphere,” The Journal of the Acoustic Society of America, 140(1), 641-656, 2016.

[3] R. Sabatini, C. Bailly, O. Marsden and O. Gainville, “Characterization of absorption and nonlinear effects in infrasound propagation using an augmented Burgers’ equation,” Geophysical Journal International, 207, 1432-1445, 2016.

[4] R. Sabatini, O. Marsden, C. Bailly and O. Gainville, “Three-dimensional direct numerical simulation of infrasound propagation in the Earth’s atmosphere,” Journal of Fluid Mechanics, 859, 754-789, 2019.

[5] R. Sabatini, J. B. Snively, C. Bailly, M. P. Hickey and J. L. Garrison, “Numerical modeling of the propagation of infrasonic acoustic waves through the turbulent field generated by the breaking of mountain gravity waves,” Geophysical Research Letters, 46, 5526-5534, 2019.

[6] R. Sabatini, J. B. Snively, M. P. Hickey and J. L. Garrison, “An analysis of the atmospheric propagation of underground-explosion-generated infrasonic waves based on the equations of fluid dynamics: ground recordings,” The Journal of the Acoustic Society of America, 146, 4576-4591, 2019.

[7] C. Bogey and C. Bailly, “A family of low dispersive and low dissipative explicit schemes for flow and noise computations,” J. Comput. Phys., 194(1), 194-214, 2004.

Required skills / qualifications________________________________

Diplomas : a master’s degree in aerospace or mechanical engineering, physics, mathematics, or a related field.

Experience : none required.

Knowledge required: a strong background in fluid mechanics, acoustics, and computational fluid mechanics.

Operational skills : hands-on experience with computing in C/C++, Fortran, Python, or similar programming languages; excellent written and verbal communication skills in English.

Date de début : 06/10/2025