Browsing by Subject "multiphysics"
Now showing 1 - 2 of 2
Results Per Page
Sort Options
Item Contact Detection and Constraints Enforcement for the Simulation of Pellet/Clad Thermo-Mechanical Contact in Nuclear Fuel Rods(2014-03-05) Lebrun-Grandi?, Damien ThomasAs fission process heats up the fuel rods, UO2 pellets stacked on top of each other swell both radially and axially, while the surrounding Zircaloy cladding creeps down, so that the pellets eventually come into contact with the clad. This exacerbates chemical degradation of the protective cladding and high stress values may enable the formation and propagation of cracks, thus threatening the integrity of the clad. Along these lines, pellet-cladding interaction establishes itself as a major concern for fuel rod design and core operation in light water reactors. Accurately modeling fuel behavior is challenging because the mechanical contact problem strongly depends on temperature distribution and the pellet-clad coupled heat transfer problem is, in turn, affected by changes in geometry induced by body deformations and stresses generated at the contact interface. Our work focuses on active set strategies to determine the actual contact area in high-fidelity coupled physics fuel performance codes. The approach consists of two steps: in the first one, we determine the boundary region on standard finite element meshes where the contact conditions shall be enforced to prevent objects from occupying the same space. For this purpose, we developed and implemented an efficient parallel search algorithm for detecting mesh inter-penetration and vertex/mesh overlap. The second step deals with solving the mechanical equilibrium taking into account the contact conditions computed in the first step. To do so, we developed a modified version of the multi-point constraint strategy. While the original algorithm was restricted to the Jacobi preconditioned conjugate gradient method, our approach works with any Krylov solver and does not put any restriction on the type of preconditioner used. The multibody thermo-mechanical contact problem is tackled using modern numerics, with continuous finite elements and a Newton-based monolithic strategy to handle nonlinearities (the one stemming from the contact condition itself as well as the one due to the temperature-dependence of the fuel thermal conductivity, for instance) and coupling between the various physics components (gap conductance sensitive to the clad-pellet distance, thermal expansion coefficient or Young?s modulus affected by temperature changes, etc.). We will provide different numerical examples for contact problems using one and multiple bodies in order to demonstrate the performance of the method.Item Dynamic Adaptive Multimesh Refinement for Coupled Physics Equations Applicable to Nuclear Engineering(2013-06-21) Dugan, KevinThe processes studied by nuclear engineers generally include coupled physics phenomena (Thermal-Hydraulics, Neutronics, Material Mechanics, etc.) and modeling such multiphysics processes numerically can be computationally intensive. A way to reduce the computational burden is to use spatial meshes that are optimally suited for a specific solution; such meshes are obtained through a process known as Adaptive Mesh Refinement (AMR). AMR can be especially useful for modeling multiphysics phenomena by allowing each solution component to be computed on an independent mesh (Multimesh AMR). Using AMR on time dependent problems requires the spatial mesh to change in time as the solution changes in time. Current algorithms presented in the literature address this concern by adapting the spatial mesh at every time step, which can be inefficient. This Thesis proposes an algorithm for saving computational resources by using a spatially adapted mesh for multiple time steps, and only adapting the spatial mesh when the solution has changed significantly. This Thesis explores the mechanisms used to determine when and where to spatially adapt for time dependent, coupled physics problems. The algorithm is implemented using the Deal.ii fiinite element library [1, 2], in 2D and 3D, and is tested on a coupled neutronics and heat conduction problem in 2D. The algorithm is shown to perform better than a uniformly refined static mesh and, in some cases, a mesh that is spatially adapted at every time step.