<p> Huanglongbing (HLB) is an incurable bacterial disease that kills citrus trees and threatens to decimate California's $2.2 billion citrus industry. A solution for limiting the spread of HLB is to rapidly detect infected trees with a chemical sensor equipped Unmanned Aerial System (UAS), which lands within tree proximity and deploys an extendable boom for air-sample collection. The Agricultural UAS project is a multidisciplinary engineering effort to conduct chemical sample collection and analysis in remote locations, to study a tilt-frame UAS concept performance, and to test a novel Propeller Thrust Governing System (PTGS). Simulated flight metrics show that a tilt-frame UAS concept significantly increases endurance, range, cruising performance, and service envelope over a conventional multi-rotor UAS design. A UAS prototype has been built by integrating the following subsystems: tilt-frame aircraft design, PTGS, and an attitude control system. The PTGS is a novel subsystem designed for regulating thrust of a constant velocity, non-variable pitch propeller through the use of actuated aerodynamic surfaces (flaps) for vehicle attitude control. Experiments conducted on a custom-built force measuring platform show that a standard/inverted flap combination produces a high force-to-flap deflection angle ratio, preserves a linear response, and minimizes coupling between downwards/sideways forces. An attitude controller was designed using a cascade PID scheme with a Mahony filter for rapid attitude estimation. By modeling system dynamics and using airfoil theory, predicted dynamic response and simulated flight metrics are generated and then experimentally validated with a functional prototype vehicle. Collected flight data deviates from predicted performance by less than 5%.</p><p>

<p> Road vehicle aerodynamics are primarily focused on developing and modeling performance at steady-state conditions, although this does not fully encompass the entire operating envelope. Considerable vehicle acceleration and deceleration occurs during operation, either because of driver input or from transient weather phenomenon such as wind gusting. With this considered, high performance road vehicles experience body acceleration rates well beyond &plusmn;1G to navigate courses during efficient transition in and out of corners, accelerating from maximum straight-line speed to manageable cornering speeds, and then back to maximum straight-line speed. This dissertation aims to answer if longitudinal acceleration is important for road vehicle aerodynamics with the use of transient Computational Fluid Dynamics (CFD) to develop a method for obtaining ensemble averages of forces and flow field variables. This method was developed on a simplified bluff body, a channel mounted square cylinder, achieving acceleration through periodic forcing of far field velocity conditions. Then, the method was applied to an open-source road vehicle geometry, the DrivAer model, and a high performance model which was created for this dissertation, the DrivAer-GrandTouringRacing (GTR) variant, as a test model that generates considerable downforce with low ground proximity. Each test body experienced drag force variations greater than &plusmn;10% at the tested velocities and acceleration rates with considerable variations to flow field distributions. Finally, an empirical formulation was used to obtain non-dimensional coefficients for each body from their simulated force data, allowing for force comparison between geometries and modeling of aerodynamic force response to accelerating vehicle conditions.</p><p>

<p> Aluminum particles are widely used as a metal fuel in solid propellants. However, poor combustion efficiencies and two-phase flow losses result due in part to particle agglomeration. Recently, engineered composite particles of aluminum (Al) with inclusions of polytetrafluoroethylene (PTFE) or low-density polyethylene (LDPE) have been shown to improve ignition and yield smaller agglomerates in solid propellants. Reductions in agglomeration were attributed to internal pressurization and fragmentation (microexplosions) of the composite particles at the propellant surface. </p><p> Here, we explore the mechanisms responsible for microexplosions in order to better understand the combustion characteristics of composite fuel particles. Single composite particles of Al/PTFE and Al/LDPE with diameters between 100&ndash;1200 &mu;m are ignited on a substrate to mimic a burning propellant surface in a controlled environment using a CO₂ laser in the irradiance range of 78&ndash;7700 W/cm². The effects of particle size, milling time, and inclusion content on the resulting ignition delay, product particle size distributions, and microexplosion tendencies are reported. For example, particles with higher PTFE content (30 wt.%) had laser flux ignition thresholds as low as 77 W/cm², exhibiting more burning particle dispersion due to microexplosions compared to the other materials considered. Composite Al/LDPE particles exhibit relatively high ignition thresholds compared to Al/PTFE particles, and microexplosions were observed only with laser fluxes above 5500 W/cm² due to low LDPE reactivity with Al resulting in negligible particle self-heating. However, results show that microexplosions can occur for Al containing both low and high reactivity inclusions (LDPE and PTFE, respectively) and that polymer inclusions can be used to tailor the ignition threshold. This class of modified metal particles shows significant promise for application in many different energetic materials that use metal fuel.</p><p>

Offshore floating wind turbines represent the future of wind energy. However, significant challenges must be overcome before these systems can be widely used. Because of the dynamics of offshore floating wind turbines—surge, sway, heave, roll, pitch, and yaw—and the resulting interactions between the rotor and generated wake, the aerodynamic analysis methods and design codes that have found wide use throughout the wind energy industry may be inadequate. Application of these techniques to offshore floating wind turbine aerodynamics may result in off-optimal designs, effectively handicapping these next-generation systems, thereby minimizing their full potential. This dissertation will demonstrate that the aerodynamics of offshore floating wind turbines are sufficiently different from conventional offshore and onshore wind turbines, warranting the use of higher fidelity analysis approaches. It will outline the development and validation of a free vortex wake code, the Wake Induced Dynamics Simulator, or WInDS, which uses a more physically realistic Lagrangian approach to modeling complex rotor-wake interactions. Finally, results from WInDS simulations of various offshore floating wind turbines under different load conditions will be presented. The simulation results indicate that offshore floating wind turbine aerodynamics are more complex than conventional offshore or onshore wind turbines and require higher fidelity analysis approaches to model adequately. Additionally, platform pitching modes appear to drive the most aerodynamically-significant motions, followed by yawing modes. Momentum balance approaches are shown to be unable to accurately model these dynamic systems, and the associated dynamic inflow methods respond to velocity changes at the rotor incorrectly. Future offshore floating wind turbine designs should strive to either minimize platform motions or be complementarily optimized, via higher fidelity aerodynamic analysis techniques, to account for them. It is believed that this dissertation is the first in-depth study of offshore floating wind turbine aerodynamics and the applicability of various analysis methods.^

The sound generated by high speed trains can be exacerbated by the presence of trackside structures. Tunnels are the principal structures that have a strong influence on the noise produced by trains. A train entering a tunnel causes air to flow in and out of the tunnel portal, forming a monopole source of low frequency sound ["infrasound"] whose wavelength is large compared to the tunnel diameter. For the compact case, when the tunnel diameter is small, incompressible flow theory can be used to compute the Green's function that determines the monopole sound. However, when the infrasound is "shielded" from the far field by a large "flange" at the tunnel portal, the problem of calculating the sound produced in the far field is more complex. In this case, the monopole contribution can be calculated in a first approximation in terms of a modified Compact Green's function, whose properties are determined by the value at the center of a. disk (modelling the flange) of a diffracted potential produced by a thin circular disk. In this thesis this potential is calculated numerically. The scattering of sound by a thin circular disk is investigated using the Finite Difference Method applied to the three dimensional Helmholtz equation subject to appropriate boundary conditions on the disk. The solution is also used to examine the unsteady force acting on the disk.

Thesis (Ph.D.)--Boston University / This thesis presents methods for interpolating transfer functions of damped vibrating systems. Primary applications lie in the design and control of damped structures. The interpolations reduce the number of frequencies at which the transfer function must be computed or measured. The transfer functions are assumed to have impulse responses that are real-valued and causal, so a method is developed for constructing interpolations that implicitly satisfy these conditions. The method is applied to a particular choice of basis function that corresponds to a Fourier series in the time domain. Numerical results indicate that satisfaction of the causality condition increases the accuracy of the interpolation. A detailed investigation is made into interpolations for viscously damped systems, whose transfer functions are linear combinations of basis functions derived from the complex-valued eigenpairs of the system. Since the estimation of all eigenpairs is computationally expensive, a method is developed to estimate only those eigenpairs that significantly contribute to the transfer function in the specified frequency band. The method uses eigenvalues of the corresponding undamped system, which are much easier to compute, as starting guesses in an iterative algorithm. One advantage of the method is the assurance that it finds all eigenvalues in a specified region of the complex plane.