Sequential Refinement Solver using Space-Time Domain Decomposition for Non-linear Multiphase Flow Problems

Convergence failure and slow convergence rate are among the biggest challenges with solving the system of non-linear equations numerically. While using strictly small time steps sizes and unconditionally stable fully implicit scheme mitigate the problem, the computational load becomes enormous. We introduce a sequential local refinement scheme in space-time domain that improves convergence rate and prevents convergence failure while not restricting to small time step, thus boosting computational efficiency. We rely on the non-linear two-phase flow model. The algorithm starts by solving the coarsest mesh. Then regions with certain features such as saturation front is refined to the finest resolution sequentially. Such process prevents convergence failure. After each refinement, the solution from the previous mesh is used to estimate initial guess of the current mesh for faster convergence. Numerical results are presented to confirm accuracy of our algorithm as compared to the traditional fine time step approach. We also observe 5 times speedup in the runtime by using our algorithm

Micromechanistic study of hydrogen embrittlement in pipeline steels

Hydrogen embrittlement, which causes the premature failure of steel pipelines, poses a long-standing challenge to hydrogen energy utilization. Ferrite-pearlite steels dominate the in-service hydrogen pipelines market. Yet hydrogen embrittlement mechanisms for the highly susceptible pearlite phase have remained inconclusive since the complicated microstructures in the bulk ferrite-pearlite steels interfere with categorizing the contribution of pearlite to hydrogen-induced failure. Here we provide a protocol combining in-situ micromechanical testing and ex-situ electrochemical hydrogen charging to successfully examine the effects of hydrogen on the mechanical behavior of pearlite and ferrite micropillars. In this project atom probe tomography with cryogenic-transfer technique was conducted on hydrogen-charged pearlite samples and observed hydrogen is trapped in the cementite lamellae rather than at the ferrite-cementite interfaces. The introduction of hydrogen reduces the yield strength of pearlite micropillars to a narrow range, which means that hydrogen weakens the anisotropic yielding of pearlite. Slip occurs at the ferrite-cementite interface for uncharged micropillars with inclined lamellae but after hydrogen charging it takes place in the ferrite matrix. Shear deformation dominates in micropillars with vertical and horizontal lamellae, where fracture occurs in the presence of hydrogen. Unlike pearlite, hydrogen only slightly reduces the yield strength of ferrite but has a greater impact on plasticity. Hydrogen softens ferrite micropillars and weakens intermittency during plastic deformation. These phenomena are attributed mainly to the hydrogen-enhanced local plasticity mechanism. This thesis also provides a new scanning electron microscope-based protocol to test the effect of hydrogen on the mechanical behavior of ferrite-pearlite steels that can facilitate fundamental studies on the interactions between hydrogen, microstructure, and deformation behavior

Design, characterization, and sensitivity of the supernova trigger system at Daya Bay

Providing an early warning of galactic supernova explosions from neutrino signals is important in studying supernova dynamics and neutrino physics. A dedicated supernova trigger system has been designed and installed in the data acquisition system at Daya Bay and integrated into the worldwide Supernova Early Warning System (SNEWS). Daya Bay's unique feature of eight identically-designed detectors deployed in three separate experimental halls makes the trigger system naturally robust against cosmogenic backgrounds, enabling a prompt analysis of online triggers and a tight control of the false-alert rate. The trigger system is estimated to be fully sensitive to 1987A-type supernova bursts throughout most of the Milky Way. The significant gain in sensitivity of the eight-detector configuration over a mass-equivalent single detector is also estimated. The experience of this online trigger system is applicable to future projects with spatially distributed detectors.Comment: 8 pages, 6 figures, to be submitted to Astroparticle Physic