Date of Award
2024-08-01
Degree Name
Doctor of Philosophy
Department
Mechanical Engineering
Advisor(s)
Vinod Kumar
Second Advisor
Arturo Bronson
Abstract
Turbulence medium poses significant challenges to the propagation of laser beams, impacting applications such as free-space optical communication, remote sensing, and laser weapons systems. Therefore, comprehending and mitigating turbulence effects are vital for enhancing the performance of these systems. Analysis of a strong turbulence impact on electromagnetic wave propagation requires extensive knowledge of the Earth's atmospheric and Oceanic phenomena and Maxwell's electromagnetic theory of light as a propagating wave of electric and magnetic fields. Optical systems' propagation often encounters Strong Turbulence, causing interference and diffraction. Nonetheless, light propagation through randomly fluctuating media has been an exciting and active area of extensive research over many decades. This research holistically examined Maxwell's electromagnetic theory of light propagation through unstable medium and its effects (diffraction/interference) on optical systems. Generally, Turbulence causes scintillations, beam spreading, phase fluctuations, beam-wander, and jitter that can degrade and limit the performance of imaging and laser systems. However, the need for further investigation into the adequate characterization of strong turbulent waves and their effects on electromagnetic wave propagation is urgent to improve the existing mitigation measures. The Directed Energy (DE) Directorate of the Air Force Research Laboratory (AFRL), the U.S. Air Force, and the U.S. Space Force need to consider the impact of strong and substantial Turbulence on optical systems. Our holistic approach examined the problem and evaluated existing mitigation measures for gaps. We focused on refractive index fluctuation (scintillation), a primary cause of Turbulence in optical systems and light propagation. Our investigation into Refractive Index Structure Function, Dn(r), Power Spectral Density (PSD), and associated atmospheric and oceanic parameters was thorough and meticulous. We explored multiple concepts leading to the Refractive Index Structure Function, Dn(r), and different Power Spectral Density (PSD) mathematical formulations. Additionally, we formulated an analytical approach to solving the wave equation using the multiple integrals technique, which involves complex integrands. We investigated and implemented various mathematical and computational algorithms governing strong Turbulence and spatial-temporal imaging. We proposed developing a mathematical framework to simulate optical propagation through Turbulence. Our research continued with a meticulous design and setup of an apparatus for controlled simulated experiments using a 76.2cm (about 30inches) by 31cm (about 12.21inches) acrylic tank. We carefully varied temperature and salinity to observe the behavior of light under different turbulence levels. The results of these significant experiments, conducted with utmost precision, revealed how turbulence affects light propagation. They demonstrated the intricate relationship between turbulence levels and the resulting patterns of light intensity in laser beam propagation. These results offer valuable visual insights into how turbulence impacts how laser beams travel through a medium and provide valuable information about the physics of turbulent flow dynamics. In turbulent conditions, the movement of fluid particles becomes irregular and chaotic, making light propagation through the medium complex. As the laser beam moves through the turbulent medium, it encounters fluctuations in refractive index due to temperature, density, and velocity variations in the flow field. These fluctuations cause spatial and temporal changes in the laser beam's intensity, resulting in the intricate patterns of intensity observed in the distribution. Higher salt concentrations result in reduced turbulence intensity. This reduction is evident in the decreased amplitude of fluctuations, indicating that saline water, which is denser, creates a stabilizing effect in the convection cell. As the salt concentration increases, the fluid becomes more stratified, suppressing vertical motions and leading to less pronounced turbulence. The use of SVD in analyzing signal amplitude versus frame number and amplitude versus frequency plots offers valuable insights into the temporal and spectral characteristics of turbulence. The amplitude versus frequency plots reveal that higher salt concentrations are associated with lower amplitudes across various frequency bands. This indicates a decrease in turbulent energy and a reduction in the strength of turbulent fluctuations. The normalized amplitude versus mode number plots show that higher salt concentrations lead to fewer distinct oscillatory patterns, indicating simpler and less varied convective motions. This analysis reveals that increasing salinity reduces both the intensity and complexity of turbulent structures within the convection cell. These findings have practical implications for understanding fluid dynamics under varying salinity conditions, potentially informing engineering and environmental applications where turbulence control is critical. This significant level is a defining feature of seawater. At a salinity of 3.5%, seawater presents a set of physical properties that differ from freshwater. These include higher density, a lower freezing point, and unique buoyancy characteristics. Understanding these practical implications is crucial for our research, as they directly apply to various fields. Controlled laboratory experiments provide valuable knowledge and insights.
Language
en
Provenance
Received from ProQuest
Copyright Date
2024-08-01
File Size
115 p.
File Format
application/pdf
Rights Holder
Richard Owusu Adansi
Recommended Citation
Adansi, Richard Owusu, "Experimental And Computational Investigation Of Green Laser Propagation Through Turbulent Medium In A Water Tank" (2024). Open Access Theses & Dissertations. 4167.
https://scholarworks.utep.edu/open_etd/4167