Date of Award

Spring 2025

Access Type

Dissertation - Open Access

Degree Name

Doctor of Philosophy in Engineering Physics

Department

Physical Sciences

Committee Chair

Jonathan B. Snively

Committee Co-Chair

Matthew Zettergren

Committee Advisor

Jonathan B. Snively

First Committee Member

Roberto Sabatini

Second Committee Member

Matthew Zettergren

Third Committee Member

Frederique Drullion

College Dean

Peter Hoffman

Abstract

Nonlinear atmospheric models have provided important insight into acoustic waves generated by natural and man-made hazards, which may steepen into shocks or N-waves while also dissipating when propagating in the thermosphere. Although models have yielded results that agree with observations of ionospheric perturbations, dynamical models for the diffusive and stratified lower thermosphere often use single gas approximations with height-dependent physical properties that omit the dynamics of the major and minor constituents. Thus, the inter-species diffusion associated with these flows (e.g. variations of mean molecular weight, and specific heat) are not accounted for. This approximation is simpler and less computationally expensive than a true multi-fluid model, yet captures the important physical transitions between molecular and atomic gases in the lower thermosphere. However, models with time-dependent composition have been shown to outperform commonly used models with fixed composition; these time-dependent effects have been included in a one-gas model by adding an advection equation for the molecular weight, finding closer agreement to a true binary-gas model (e.g. Walterscheid [2012]). Here, we solve the hydrodynamic conservation equations for a multi-component flow, based on the method described by [Ern and Giovangigli 1994], with an emphasis on the effects of mass-diffusion on a vertically propagating atmospheric waves that reach the lower thermosphere. The application of the transport equations to the neutral, non-reactive atmosphere resembles an extension of the Navier-Stokes equations applied to a multi-component fluid (also known as the Multi-component Navier-Stokes equations), allowing for the modeling of species interactions [Arnault, 2022]. This description includes classical dissipative terms, such as viscosity and thermal conduction, as done in Pineyro [2018], and molecular diffusion. To investigate the impact of mass diffusion, the equations were analyzed under conditions where linear acoustic and gravity wave solutions are permitted. From this analysis, a dispersion relation was obtained that measured the impact of mass-diffusion on any atmosphere, given certain neutral atmospheric parameters. Nonlinear solutions were also analyzed to investigate the interplay between nonlinear effects and dissipative processes. The model developed, a modified version of Pineyro [2018], is applied to various studies in 1D and 2D to analyze the nonlinear behavior of gravity waves under conditions where species diffusion can impact the wave. Mass diffusion will additionally affect the fluctuations of the composition of the upper atmosphere as it is modulated by waves. For the well-mixed lower atmosphere, the impact is insufficient to necessitate the full multi-component flow equations. However, when the wave reaches thermospheric altitudes, where the diffusive separation is more apparent, the impact of mass-diffusion may become relevant. The relative contributions of diffusion processes to determining the dynamics of short-period waves in the lower thermosphere are investigated and quantified in this thesis. The results indicate that barodiffusion is the dominant diffusion mechanism driving wave dissipation, with acoustic attenuation most pronounced when atmospheric constituents have markedly different molecular masses, a condition typically found in the upper layers of planetary atmospheres. On Earth, species diffusion plays an increasingly significant role in acoustic attenuation at altitudes above approximately 150km, contributing up to 16% of the total absorption. This effect is primarily due to the high concentrations of atomic oxygen and helium. On Venus, species diffusion can account for as much as 45% of the total dissipation above about 200km, where the atmosphere is characterized by a ternary mixture of helium, hydrogen, and atomic oxygen. Mars exhibits a pattern similar to Earth, with species diffusion contributing around 17% of the total absorption due to its quaternary atmospheric composition of carbon dioxide, nitrogen, atomic oxygen, and carbon monoxide. In contrast, on Titan, Uranus, and Neptune, species diffusion plays a comparatively minor role, with bulk and shear viscosity effects dominating wave attenuation in these atmospheres.

Comments

Roberto Sabatini served as Co-Advisor.

Additional Files
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