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Introduction. Statement of problem: The primary purpose of this study is to indicate the manner in which a Boundary Value problem in Physics leads to the solution in generalized Fourier Series. The conditions to be met in problems of this sort are generally the Partial Differential Equation and several unique physical conditions which are imposed on the distribution sought after. The problem is solved when a mathematical solution of the Differential Equation is found which satisfies all of the restrictions levied by the physical considerations. The secondary purpose of this study is to obtain a view of the generalized problem which leads ultimately to the Sturm-Liouville theory.

This thesis continues the work started with my previous supervisor, Dr. Shaohua Chen. In [15], the authors developed some tools that showed the boundedness or blowup of solutions to a semilinear parabolic system with homogeneous Neumann boundary conditions. This system, the so called ’Activator-Inhibitor Model’, is of interest as it is used to model biological processes and pattern formation. Similar tools were later adapted to deal with the same parabolic system in [3], in which the authors prove global boundedness of solutions under homogeneous Dirichlet conditions. This new problem is of mathematical interest as the solutions may grow singular near the boundary. Shortly after, a different system was considered in [4], where the authors proved global boundedness of solutions to a system featuring similar singular reaction terms. The goal of this thesis is twofold: first, the tools developed that allow us to tackle these sorts of problems will be demonstrated in detail to showcase its utility; the second is to then use these tools to generalize some of these previous results to a larger class of singular parabolic systems. In doing so, we expand the classical literature found in [14] and other notable works, where nonsingular equations are extensively treated. The motivation for the first should be clear. While there are numerous bodies of text treating nonsingular problems, there are no collections available dealing with these types of singularities exclusively. This is of practical use to other mathematicians studying partial differential equations. The motivation for the second is, perhaps, more practical. There are a growing number of models found in physics, chemistry and biology that may be generalized to a singular type system. Through allowing those individuals to treat these problems, we may gain valuable insights into the real world and how these processes behave. / Thesis / Master of Science (MSc)

In practical multibody systems, various factors such as friction, joint clearances, and external events play a significant role and can greatly influence the optimal design of the system and its controller. This research focuses on the use of gradient-based optimization methods for multibody dynamic systems with the incorporation of joint friction. The dynamic formulation has been derived in using two distinct techniques: Index-1 DAE and the tangent-space formulation in minimal coordinates. It employs a two different approaches for gradient computation: direct sensitivity approach and the adjoint sensitivity approach. After a comprehensive review of different friction models developed over time, the Brown McPhee model is selected as the most suitable due to its accuracy in dynamic simulations and its compatibility with sensitivity analysis. The proposed methodology supports the simultaneous optimization of both the system and its controller. Moreover, the sensitivities obtained using these formulations have been thoroughly validated for numerical accuracy and benchmarked against other friction models that are based on dynamic events for stiction to friction transition. The approach presented is particularly valuable in applications like robotics and servo-mechanical systems where the design and actuation are closely interconnected. To obtain numerical results, a new implementation of the MBSVT (Multi-Body Systems at Virginia Tech) software package, known as MBSVT 2.0, is reprogrammed in Julia and MATLAB to ensure ease of implementation while maintaining high computational efficiency. The research includes multiple case studies that illustrate the advantages of the concurrent optimization of design and control for specific applications. Efficient techniques for control signal parameterization are presented using linear basis functions. A special focus has been made on the computational efficiency of the formulation and various techniques like sparse-matrix algebra and Jacobian-free products have been employed in the implementation. The dissertation concludes with a summary of key results and contributions and the future scope for this research. / Doctor of Philosophy / In simpler terms, this research focuses on improving the design and control of complex mechanical systems, like robots and automotive systems, by considering factors such as friction in the joints. Friction in a system can greatly affect how it performs for the desired task. The research uses a method called gradient-based optimization, which essentially means finding the most optimal parameters of the system and its controller such that they achieve a desired goal in the most optimal way. Before a model for such a system can be developed, various techniques need to be researched for incorporation of friction mathematically. A model known as Brown McPhee friction is one such model suitable for such an analysis. When optimizing any system on a computer, an iterative process needs to be performed which may prove to be very expensive in terms of computational resources required and the time taken to achieve a solution. Hence, proper mathematical and computational techniques need to be employed to ensure that the resources of a computer are utilized in the most efficient way to get the solution is the quickest way possible. Among the various novelties of this research, it is worth noting that this method that allows for simultaneous design and control optimization, which is particularly useful for applications such as robotics and servo-mechanical systems. Considering the design and control together, leads to more efficient and effective systems. The approach is tested using a software package called MBSVT 2.0, which was specifically developed as part of this research. The software is available in 3 languages: Julia, MATLAB and Fortran for universal access to people from various communities. The results from various case studies are presented that demonstrate this simultaneous design and control approach and highlights its effectiveness making the systems more robust and better performing.

No / This title provides detailed description of how Partial Differential Equations are used in the field of geometric design, and supplies clear and concise explanations of how to implement the techniques described. It also offers extensive discussions (with examples) or practical applications of Partial Differential Equations in geometric design.

Novel analytical nonperturbative techniques are developed in the area of nonlinear and linear stochastic differential equations and applications are considered to a variety of physical problems. First, a method is introduced for deriving first- and second-order moment equations for a general class of stochastic nonlinear equations by performing a renormalization at the level of the second moment. These general results, when specialized to the weak-coupling limit, lead to a complete set of closed equations for the first two moments within the framework of an approximation corresponding to the direct-interaction approximation. Additional restrictions result in a self-consistent set of equations for the first two moments in the stochastic quasi-linear approximation. The technique is illustrated by considering two specific nonlinear physical random problems: model hydrodynamic and Vlasov-plasma turbulence. The equations for the phenomenon of hydrodynamic turbulence are examined in more detail at the level of the quasi-linear approximation, which is valid for small turbulence Reynolds numbers. Closed form solutions are found for the equations governing the random fluctuations of the velocity field under the assumption of special time-dependent, uniform or sheared, mean flow profiles. Constant, transient and oscillatory flows are considered. The smoothing approximation for solving linear stochastic differential equations is applied to several specific physical problems. The problem of a randomly perturbed quantum mechanical harmonic oscillator is investigated first using the wave kinetic technique. The equations for the ensemble average of the Wigner distribution function are defined within the framework of the smoothing approximation. Special attention is paid to the so-called long-time Markovian approximation, where the discrete nature of the quantum mechanical oscillator is explicitly visible. For special statistics of the random perturbative potential, the dependence of physical observables on time is examined in detail. As a last example of the application of the stochastic techniques, the diffusion of a scalar quantity in the presence of a turbulent fluid is investigated. An equation corresponding to the smoothing approximation is obtained, and its asymptotic long-time version is examined for the cases of zero-mean flow and linearly sheared mean flow. / Ph. D.