Date of Award

8-2018

Document Type

Thesis - Open Access

Degree Name

Master of Science in Engineering Physics

Department

Physical Sciences

Committee Chair

Dr. Jonathan B. Snively

First Committee Member

Dr. Michael Hickey

Second Committee Member

Dr. Roberto Sabatini

Abstract

Recent nonlinear atmospheric models have provided important insight into acoustic waves generated by seismic events, which may steepen into shocks or saw-tooth trains while also dissipating strongly in the thermosphere. Although they 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 (e.g. mean molecular weight, specific heats) that do not vary with time (fixed composition). This approximation is simpler and less computationally expensive than a true multi-fluid model, yet captures the important physical transition between molecular and atomic gases in the lower thermosphere. Models with time-dependent composition and properties have been shown to outperform commonly used models with fixed properties; 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 and Hickey [2012]).

Here, a one-dimensional nonlinear mass fraction approach to multi-constituent gas modeling, motivated by the results of Walterscheid and Hickey, is presented. A flux-differencing finite volume method of is implemented in Clawpack with a Riemann Solver to solve the Euler Equations including multiple species, defined by their mass fractions, as they undergo advection. Viscous dissipation and thermal conduction are applied via a fractional step method. The model is validated with shock tube problems for two species, and then applied to investigate propagating nonlinear acoustic waves from ground to thermosphere, such as following the 2011 Tohoku Earthquake and rocket launches. The limits of applicability are investigated for vertically propagating acoustic waves near the cut-off frequency, and for simulations of steepening waves at finite spatial resolution. The addition of a mass fraction density introduces noticeable fluctuations to the state of the atmosphere that can account for modulation of acoustic waves. The model developed also has potential uses in parametric studies, complementing more costly 2D and 3D models.

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