The research objective of this dissertation is to advance the understanding of rotorcraft broadband noise and reduce the noise impacts. Rotorcraft broadband noise has recently become a critical topic for vertical take-off and landing (VTOL) aircraft, due to the rapid progresses in Advanced Air Mobility (AAM) technologies. However, the current noise assessment tools of VTOL broadband noise rely heavily on semi-empirical models, which are short of both prediction accuracy and flow physics. To close this gap, a state-of-the-art prediction tool, namely UCD-QuietFly, is developed to assess rotor broadband noise using physics-based approaches. Extensive validations of UCD-QuietFly are performed against experiments. The effects of rotor design parameters on broadband noise are studied. Broadband noise impacts are investigated on Urban Air Mobility (UAM) conceptual designs, quiet helicopter designs, and small-scale drones. Finally, the noise reduction technique using trailing-edge serrations is analyzed.

The first part of this dissertation investigates the effects of rotorcraft design and operating parameters on trailing-edge noise. A rotor trailing-edge noise prediction method is first developed. It is found that helicopter broadband noise scales with the 4.5th to 5.0th power of the tip Mach number in which the range is determined by the typical helicopter collective pitch angle in operation. Detailed trend analyses of noise levels as a function of frequency are presented in terms of the collective pitch angle, twist angle, rotor solidity, rotor radius, disk loading, and number of blades.

Second, broadband noise of multi-rotor UAM vertical take-off and landing (VTOL) vehicles is studied. The multi-rotor broadband noise prediction capability is developed. It is found that UAM VTOL vehicles' broadband noise is important in the high-frequency range. For the same mission specifications, broadband noise is found to be higher for VTOL designs with more rotors. Multi-rotor vehicles at the same rotational speeds have weaker amplitude modulations than a single rotor, which demonstrates the benefits of using multiple rotors in terms of noise annoyance.

Third, tonal and broadband noise are studied for rotor designs used on UAM vehicles. The rotor aerodynamics in edge-wise forward flight is calculated by an in-house code. With the forward flight capability developed in UCD-QuietFly, it is found that broadband noise is the dominant noise source for the rotor designs with low tip speeds and fewer blades, while tonal noise is dominant for the high-tip-speed designs. A low tip speed and more blades are found to be the preferable design features in terms of psychoacoustic metrics.

Fourth, a physics-based broadband noise prediction approach is applied for small-scale drone rotors. LBL-VS noise is found to be an important noise source for untripped blades. The effect of leading-edge back-scattering in broadband noise is found to be important for small-scale rotors at low frequencies. Finally, at the same thrust, the ideally twisted rotor generates slightly higher broadband noise than the linearly twisted rotor, while the tapered blade tip is shown to reduce the broadband noise levels significantly.

Fifth, machine learning-based fast-predicting models of rotorcraft trailing-edge broadband noise are developed, using artificial neural network (ANN) and linear regression. It is found that the ANN model accurately captures the variations of the noise levels, and the linear regression models are also capable of predicting the general trends of noise levels.

This thesis examines the feasibility of Delayed Detached Eddy Simulations (DDES) in terms of predicting aerodynamic loads and capturing complex flow physics relating to the dynamic stall process at a transitional Reynolds number of 200,000 and Mach number of 0.10, using NASA’s OVERFLOW 2.3 code. This investigation tests the performance and capabilities of state-of-the-art transition models in terms of their ability to capture the underlying viscous mechanisms from which dynamic stall is onset. This work focuses on the events leading up to stall, rather than the analysis of the stalled flow itself, hence the focus will be on the upstroke region. This is due to the modern-day modeling tools being more so calibrated for pre-stall. Furthermore, an accurate prediction of the pre-stall regime could have a significant impact in employing flow control technologies in the future. All grids generated are based on current best practices of Chimera Grid Tools (CGT). While current Large Eddy Simulations (LES) standards recommend a low y + value in the boundary layer, a conservative value of 0.4 is used. Deeper investigations into the flow physics are performed on the 2nd finest grid out of 3 configurations. While by nature it would be expected that the finest mesh would yield the most accurate results, another motivation for this study is to test capabilities performed on a scale parallel with current industry standards, using coarser meshes, and lower fidelity models, resulting in quicker turnaround times. It is found that the overall flow physics is captured in detail even with a mesh composed of roughly 16 million grid points. A leading-edge separation region and formation of a dynamic stall vortex is examined in detail, and results suggest that the role of a laminar separation bubble is significant in the dynamic stall process, which has otherwise been a controversial point in the past regarding this flow phenomenon. Furthermore, it is found that DDES matches up accurately with respect to benchmark LES results for the same case, but with coarser spatial and temporal resolutions. James Coder’s SA Amplification Factor Transport (AFT) transition model yielded the most accurate results when compared with LES results and shows promising results for future viii use in more high-fidelity case studies. Langtry and Menter’s SST correlation-based transition model and its SA counterpart developed by Medida and Baeder are also investigated in detail.

Dynamic stall is a prevalent phenomenon affecting the aerodynamic performance of a wide array of fluid-dynamic machinery, including maneuvering aircraft, fighter jets, helicopters, and wind turbines. The dynamic stall process induces sudden and undesirable variations in aerodynamic forces and pitching moments, critically affecting the controllability and structural integrity. The characterization of dynamic stall is still an arduous and complicated challenge.

This research investigates the effect of airfoil design parameters, effect of airfoil trailing-edge morphing, and the effect of particle induced surface roughness on dynamic stall through the use of high-fidelity computational fluid dynamics (CFD). OVERFLOW Reynolds-averaged Navier-Stokes (RANS) CFD and delayed detached eddy simulations (DDES) CFD are used to simulate the pitching and ramp-up motion of the airfoil undergoing dynamic stall. An airfoil morphing method investigation involving six parametric airfoil design parameters, including the camber, thickness, thickness crest position, leading-edge radius, trailing-edge camber, and boat-tail angle, is undertaken. The CFD numerical simulations are validated against experimental data for analyzing the aerodynamic loading, which includes the coefficient of lift, drag, and pitching moment. Then, the peaks of these coefficients are compared to identify the trends. Morris' sensitivity analysis is also used to analyze the effects of the different design parameters and further rank and assess their relative influences on the dynamic stall characteristics. Overall, it is found that the thickness crest position has a sizable influence on all the dynamic stall characteristics. Additionally, it is found that the design parameter that modifies the trailing-edge (boat-tail angle) is able to to markedly diminish the pitching moment, while concurrently sustaining a comparable lift coefficient. The complex features of the dynamic stall development stages and flow physics are further analyzed. The study reveals that morphed trailing-edge airfoil with the largest positive selected boat-tail angle design parameter causes the laminar separation bubble (LSB) to burst at an earlier stage, resulting in the dynamic stall vortex (DSV) occurring at an earlier stage. Mainly, the results indicate that morphed trailing-edge airfoils exhibit stronger secondary shear layer separation in the middle and aft airfoil sections at higher angles of attack (AoAs), compared to the baseline airfoil. Finally, the effects of particle impact induced surface roughness are examined. Lawrence Livermore National Laboratory's (LLNL) code called ParticleTSim is used to obtain the particle strike maps to record the range of particle strikes during the dynamic stall motion. A comparison study between different roughness height values and impacted surface areas is performed. The results illustrate that, as the surface roughness height and impacted surface roughness area increase, the dynamic stall progression stages develop much quicker, the stall behavior occurs at an earlier AoA, and the accumulation of kinetic energy above the airfoil diminishes. This research dissertation ventures into the comprehension and mitigation methods for the adverse ramifications of dynamic stall.

This study aims to advance the fundamental flow physics of acoustic source generation, propagation, and mitigation using large-eddy simulations with specific emphasis on flows around an airfoil.First, a wavelet-based recursive denoising algorithm is applied to airfoil flow field. The pressure field around the airfoil is decomposed into coherent contributions corresponding to denoised pressure and incoherent pressure corresponding to background noise. It is found that the denoised pressure represents physical phenomena associated with near-wall hydrodynamic wavy structures and sound propagation generated near the boundary-layer tripping region and trailing edge. On the other hand, the incoherent pressure or background noise exhibits a small and constant amplitude closely adhering to the Gaussian distribution. Second, wavenumber-frequency decomposition and Amiet's theory are used to separate total pressure into two distinct components: hydrodynamic (incident) pressure and acoustic (scattered) pressure. The findings reveal that hydrodynamic pressure consists of turbulent coherent structures characterized by high-energy spectra but acts as non-propagating sources due to destructive interference within the streamwise correlation length. In contrast, acoustic pressure exhibits an in-phase nature that facilitates efficient sound propagation. Amiet's theory shows similarities in magnitude and directivity patterns, but the disparity between numerical and analytical studies exists, which is investigated in detail. Third, this study delves into the detailed sound generation and propagation mechanisms associated with trailing-edge scattering and flow perturbations of boundary-layer tripping. Two distinct boundary-layer tripping techniques, namely a geometrically resolved stair strip and an artificially modeled trip using suction and blowing, are investigated. It is found that boundary-layer tripping notably leads to intricate, scenario-specific noise generation: there is an interaction between the laminar separation bubble and tripping for the stair strip case, whereas laminar boundary layer instability is evident for the suction and blowing scenario. Aerodynamic flow fields involving acoustic noise sources, their propagating natures near the wall, and far-field acoustics are cross-examined in detail. Fourth, a cross-spectrum method is proposed to identify the origin of noise sources and understand sound production mechanisms. This novel method shows its potential by effectively detecting the acoustic source and propagation that are on par with or surpass those of dynamic mode decomposition and spectral proper orthogonal decomposition modes while simultaneously providing the spectral magnitude and phase topologies when applied to different flow transition mechanisms of an airfoil. Fifth, the effect of misaligned flow on trailing-edge noise is studied to examine fundamental mechanisms governing noise mitigation. Flow misalignment observed in the swept airfoil is found to generate destructive interference, playing a crucial role in the noise reduction mechanism. Critical flow physics responsible for the noise source attenuation is discovered in the numerical simulations, which are cross-examined with Amiet's swept trailing-edge noise theory. Sixth, the effects of trailing-edge morphing on aerodynamic and aeroacoustic performances are studied. Concave (M1) and convex (M2) shapes are imposed near the trailing edge of the airfoil. The M1 airfoil demonstrates enhanced performance in both aerodynamics and aeroacoustics, while the M2 airfoil shows deteriorated performances compared to the baseline airfoil. Flow features impacting the airfoil performances are examined in detail.

Aerodynamic interactions between spinning rotor blades, rotor wake systems, and the fuselage present formidable challenges to rotorcraft designers. These interactions underpin much of the noise and aerodynamic challenges faced by such vehicles. With the emergence of urban air mobility (UAM) aircraft, especially vehicles with multiple rotors, the complexities and challenges of these interactions have increased dramatically when compared to simpler single-rotor helicopter designs. The need for quieter urban air vehicles requires an in-depth understanding of these interactions. This dissertation seeks to bridge existing knowledge gaps through the application of high-fidelity computational fluid dynamics (CFD) models combined with a high-fidelity model to predict resultant rotorcraft noise. By accurately simulating the aerodynamics and the noise generated by multi-rotor UAM aircraft, this research aims to pave the way for innovative low-noise designs that not only meet technical benchmarks but also fit seamlessly into urban settings by securing the acceptance of city dwellers.A foundational step in this investigation is to ensure the robustness and accuracy of the employed CFD models. To achieve this, the models are meticulously validated against experimental data. A keen emphasis is placed on examining rotor-vortex interactions, especially parallel blade-vortex interactions (BVI). These BVIs are of particular interest because of their propensity to induce rapid aerodynamic changes and significant noise, which can be detrimental to the efficacy and acceptance of UAM vehicles. Progressing from this foundational step, the research delves deeper into a case study involving NASA’s proposed side-by-side rotor vehicle. This study comprehensively evaluates the hovering rotor performance, aerodynamics, and the associated noise generation under varied design scenarios, specifically considering rotor overlaps of 0%, 5%, 15%, and 25%. Furthermore, simulations are performed for scenarios in both free-air and in proximity to the ground, reflecting the varied conditions that urban aerial vehicles might encounter. In out-of-ground situations, higher rotor overlap leads to more intense noise, primarily because of the rotor-to-rotor BVIs. However, for in-ground scenarios, lower or no overlap results in increased noise levels. This rise in noise is attributed to the interaction between the rotor and the upwash flow created by the ground, especially when the rotor is in closer proximity to the ground. This dissertation also undertakes an all-encompassing analysis of the UAM side-by-side rotor aircraft design. This involves not just the rotors, but also the fuselage and wing and its interplay with the aerodynamics and noise. Similar to the ground effect, the upwash flow from the fuselage significantly influences noise generation in scenarios with low or no overlap. The outcomes of this research shed light on the physical mechanisms behind noise generation, the directional patterns of noise propagation, and the intricacies of aerodynamic interactions. In essence, this dissertation offers more than just a deep dive into the technical aspects. It provides invaluable insights, robust models, and guidance for engineers and designers striving to create the next generation of low-noise UAM rotorcraft, ensuring they are both effective, quiet, and amenable to urban integration.