Expansive soils are treated with anhydrous or hydrated lime. The use of calcium-based stabilizers such as calcium oxide (lime) in sulfate-bearing clay soils has historically led to distress due to the formation of an expansive mineral called ettringite and possibly another such mineral, thaumasite. Predicting the precipitation of these minerals is a complex problem related not only to soil composition but also construction methods, availability of water, ion migration, and whether the expansive mineral growth can be accommodated by the void structure in the surrounding soil. In trying to control the damage associated with such occurrences, engineers have attempted to determine a threshold value of soluble sulfates, a quantity that is relatively easy and quick to measure, at which significant ettringite growth and, therefore, structural distress occurs. Unfortunately, experience alone and “rules-of-thumb” based on experience are not sufficient to deal with this complex issue. This thesis describes how thermodynamic geochemical models of lime-treated soil can be used as a first step toward establishing problematic threshold levels of soluble sulfates for a specific soil. A foundation for the model development is presented, and two different soils are compared to illustrate their very different sensitivities to ettringite growth upon the addition of hydrated lime. Various soil series along the route of SH 130 between Austin and San Antonio have been identified to contain soluble sulfate that may pose a problem for soil stabilization using lime and cement. Since the model predicts ettringite growth based upon site-specific properties, this thesis also shows how the model can be used to assess the potential amelioration effects of soluble silica. Research was conducted at the Texas Transportation Institute to develop a methodology for identifying areas which are susceptible for ettringite formation. The proposed methodology uses a magnetometer to quickly screen large areas for high sulfate. Application of GIS to identify ettringite formation using soils, topographical, and geological maps is also illustrated in this thesis.

This dissertation examines the assessment and mitigation of airborne noise from power generation equipment. The first half of the dissertation investigates the diagnosis and treatment of combustion oscillations in boilers. Sound is produced by the flame and is reflected downstream from the combustion chamber. The reflected sound waves perturb the mixture flow or equivalence ratio increasing the heat release pulsations and the accompanying sound produced by the flame. A feedback loop model for determining the likelihood of and diagnosing combustion oscillations was reviewed, enhanced, and then validated. The current work applies the feedback loop stability model to two boilers, which exhibited combustion oscillations. Additionally, a feedback loop model was developed for equivalence ratio fluctuations and validated. For the first boiler, the combustion oscillation problem is primarily related to the geometry of the burner and the intake system. For the second boiler, the model indicated that the combustion oscillations were due to equivalence ratio fluctuations. Principles for both measuring and simulating the acoustic impedance are summarized. An approach for including the effect of structural-acoustic coupling was developed. Additionally, a method for determining the impedance above the plane wave cut-off frequency, using the acoustic FEM, of the boiler was proposed. The second half of the dissertation examines the modeling of bar silencers. Bar silencers are used to mitigate the airborne noise from large power generation equipment (especially gas turbines). Due to the large dimensions of the full cross section, a small representative cell is isolated from the entire array for analysis purposes. To predict the acoustical performance of the isolated cell for different geometric configurations, a numerical method based on the direct mixed-body boundary element method (BEM) was used. An analytical solution for a simplified circular geometry was also derived to serve as a comparison tool for the BEM. Additionally, a parametric study focusing on the effects of flow resistivity, perforate porosity, length of bars, and cross-sectional area ratio was performed. A new approach was proposed to evaluate the transmission loss based on a reciprocal work identity. Moreover, extension of the transmission loss computation above the plane wave cut-off frequency was demonstrated.

We develop a physics-based reduced-order model of the aero-acoustic sound sources in reacting mixing layers as a method for fast and accurate predictions of the radiated sound. Instabilities in low-speed mixing layers are known to be dominated by the traditional Kelvin–Helmholtz (K–H)-type “central” mode, which is expected to be superseded by the “outer” modes as the chemical-reaction-based heat-release modifies the mean density, yielding new peaks in the density-weighted vorticity profiles. Although, these outer modes are known to be of lesser importance in the near-field mixing, how these radiate to the far-field is uncertain, on which we focus primarily, when the mixing layer is supersonic, but also report subsonic cases. On keeping the flow compressibility fixed, the outer modes are realized via biasing the respective mean density of the fast (oxidizer) or slow (fuel) side. In the linearized model that we use, the mean flow are laminar solutions of two-dimensional compressible boundary layers with an imposed composite turbulent spread rate, which we show to correctly predict the growth of instability waves by saturating them earlier, similar to in non-linear calculations, but obtained here via solving the linear parabolized stability equations (PSE). The chemical reaction is modeled via a single-step, single-product overall process which introduces a heat release term in the mean temperature equation. As the flow parameters are varied, modes that are unstable on the slow side are shown to be more sensitive to heat release, potentially exceeding equivalent central modes, as these modes yield relatively compact sound sources with lesser spreading of the mixing layer, when compared to the corresponding fast modes. In contrast, the radiated sound, obtained directly from the PSE solutions, seems to be relatively unaffected by a variation of mixture equivalence ratio, except for a lean mixture which is shown to yield a pronounced effect on the slow mode radiation by reducing its modal growth. For subsonic mixing layers, the sensitivity of central mode is explored, which in addition requires an acoustic analogy based method (e.g. the Lilley–Goldstein equations) to predict the sound from the linearized PSE sources, as used here, unlike in supersonic cases.