Foundations for Development of Three-Dimensional Energy Flux Acoustic Propagation Models
Open Access
- Author:
- Langhirt, Mark
- Graduate Program:
- Acoustics
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- October 14, 2024
- Committee Members:
- Daniel C. Brown, Thesis Advisor/Co-Advisor
Sheri Lynn Martinelli, Committee Member
Julianna Simon, Program Head/Chair
Victor Ward Sparrow, Committee Member
Charles Holland, Committee Member - Keywords:
- underwater
acoustic
propagation
model
energy
flux
three
dimensional - Abstract:
- Within the field of ocean acoustics there are a variety of acoustic propagation models used to solve the linear acoustic partial differential equations; the three most common being ray tracing, normal modes, and the parabolic equation. Each model is typically built upon some set of approximations or assumptions that make complex and realistic solutions tractable by constructing them from simpler ideal solutions. The classic energy flux method is another acoustic propagation model that utilizes several of the assumptions used in normal mode and ray tracing models to efficiently calculate the averaged acoustic intensity within a waveguide. This approach has not received as much focus as other models over the years, but the method was extended a decade ago to reincorporate some of the modal interference from neighboring modes which allowed the model to resolve caustic-like structures within the acoustic field. This semi-coherent energy flux model was still computationally efficient and allowed for limited horizontal environmental range dependence, but also captured the most significant structure of the acoustic field within the waveguide. The most often used ocean acoustic propagation models assume azimuthal symmetry when constructing a solution along a particular radial bearing from the source location and stitch the solutions together for three-dimensional environments. With the advancement of computational resources and new applications for acoustics in more complicated underwater environments, truly three-dimensional ocean acoustic propagation models have been developed using normal modes, ray tracing, and parabolic equation methods that can capture the horizontal refraction within the underwater waveguide, but these are significantly more computationally intensive. Even though the energy flux method is computationally efficient and analytically extensible, no sources for deriving and implementing a practical three-dimensional energy flux model were found in the academic literature. Within this thesis, foundational models and analytical tools are developed and tested for the purpose of deriving a truly three-dimensional energy flux ocean acoustic propagation model that clearly captures horizontal refraction in three-dimensional underwater environments using the semi-coherent interference of neighboring modes. The development of these tools is explored through two test models that investigate the theoretical foundations for the adiabatic approximation, the free-field Green's function solution for linear acoustics, a hybrid normal mode and energy flux model that demonstrates the use of energy flux methods to solve the horizontal component of a vertical normal mode problem. Comparisons for idealized test environments with exact solutions and other ocean acoustic propagation models are used to validate the energy flux models and identify inherent differences that result from underlying assumptions made in the model derivations.