Browsing by Author "Nguyen, Lam"

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    Multiscale imaging-through-analysis methods in computational mechanics
    (2018-11) Nguyen, Lam
    The goal of the thesis is to develop a methodology which enables an autonomous computational framework for the high-fidelity and efficient analysis of imaging-based geometries with complex microstructures. The thesis presents three new techniques: the diffuse boundary condition approach, the multiscale predictor/corrector scheme, and the residual-driven iterative corrector scheme. These methods are developed to target major challenges in the analysis of imaging-based geometries. The diffuse boundary approach overcomes the obstacle in the imposition of boundary conditions in the context of the finite cell method, which requires the explicit parametrization of boundary surfaces. Its essential component is a diffuse geometry model generated from metastable phase-field solutions of the Allen-Cahn problem. Phase-field approximations of the boundary and its gradient are then employed to transfer all boundary terms in the variational formulation into volumetric terms. The multiscale predictor/corrector scheme is developed to tackle the high computational cost in nonlinear voxel finite elements. The core components of our method are top-down displacement and bottom-up stress projectors for the exchange of information between coarse and fine scales. These projectors enable the solution in terms of a series of small uncoupled systems at a fraction of the computing power and memory required by the fully coupled fine-scale system. Although the multiscale predictor/corrector approach yields the fine-mesh accuracy, it requires the balance of the approximation power of coarse-scale and fine-scale meshes. The residual-driven corrector scheme is developed to completely resolve this issue. The method is based on the concept of the multiscale finite element method. The definition of a local corrector problem for each multiscale basis function that mitigates the error introduced by local boundary conditions is the core idea of our approach. Each corrector problem results in a local corrector solution that improves the accuracy of the corresponding multiscale basis function at element interfaces. We employ several carefully chosen numerical examples in two and three dimensions, covering both linear and nonlinear problems, to demonstrate the accuracy, robustness, and versatility of these methods.
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    Understanding and Mitigating the Dynamic Behavior of RICWS and DMS Under Wind Loading
    (Minnesota Department of Transportation, 2020-06) Linderman, Lauren; Guala, Michele; French, Catherine; Schillinger, Dominik; Finley, Nicole; Heisel, Michael; Nguyen, Lam; Stoter, Stein; Vievering, Josh; Zhu, Qiming
    Dynamic Messaging Signs (DMS) and Rural Intersection Conflict Warning Signs (RICWS) are roadside signs that feature much larger and heavier signs than are typically placed on their respective support systems. The excess weight and size of these signs, in conjunction with their breakaway support systems, introduces vibration problems not seen in the past. The AASHTO 2015 LRFD Specification for Structural Supports for Highway Signs, Luminaires, and Traffic Signals (SLTS) does not yet address vibration design for these nontraditional roadside signs. DMS and RICWS were instrumented in the field and numerically modeled to explore their wind-induced behavior. A dynamic numerical model was validated with experimental field data and used to evaluate the fatigue life of the DMS support system instrumented in the field. The resulting fatigue life differed significantly from the equivalent static pressure analysis prescribed in the AASHTO specification. The fatigue life of the DMS instrumented in the field was conservatively estimated to be 23.8 years. Based on data collected from a RICWS instrumented in the field and experiments done on a scaled model of the RICWS at the St. Anthony Falls Laboratory, vortex shedding was identified as the predominant wind phenomena acting on the RICWS structure. Three modifications were proposed to reduce the impacts of vortex shedding. The investigation of these newer sign types highlights the importance of considering the impact of dynamic behavior and vortex shedding on the structural design.