Finite Element Modeling of Thermal Effect on Airport Slabs

A sub-ground tunnel connecting two concourses of a terminal at Chicago O’Hare International Airport suffered from water leakage. The airport engineers suspected that the problem was caused by high stresses impinging on the tunnel structure by the pavement sections above the tunnel, acting to damage joints and waterproofing membranes that should have prevented such leakage. To provide a fundamental solution to this problem while minimizing the interference with airport operation, a repair strategy needs to be determined based on a clear understanding of the root cause. A preliminary field inspection confirmed that the concrete pavement slabs moved significantly when ambient temperature changes caused concrete contraction or expansion. This study used finite element modeling to infer the slab displacements and stresses driven by the daily and seasonal thermal changes based on actual temperature statistics. We proposed a reasonable 3D model configuration for the entire pavement section. The simulated slab displacements agree with the results of actual manual measurements. Subsequent analysis predicts the stresses applied to the tunnel structure, whereby insightful data are obtained for preparing the repair plan

Applications of finite element method in studying mechanical behavior of concrete materials on different scales

Computer simulation is a powerful tool for understanding material behavior at a wide range of scales. This research uses finite element modeling for two investigations at two very different scales. The first study considers the microstructure of foam concrete material. The objective is providing insights for understanding the foam concrete crushing behavior, where the real foam concrete microstructure is studied. With a micro-CT scan providing the foam geometry information, tetrahedral meshes are generated for rendering the microstructure, which is further used for simulating a uniaxial compression test in Abaqus/CAE. The simulation result is qualitatively reasonable, suggesting a potential of using this approach for further exploring the foam crushing behavior. The second study focuses on investigating the thermal induced movement of the full-scale airport slabs at O’Hare International Airport. The simulation is implemented to evaluate four model configurations for estimating the opening at concrete expansion joints. After further considering the actual boundary condition, the modeling effort demonstrates acceptable accuracy of the simulation result. The good agreement between the FEM simulation and the field measurement confirms a realistic prediction overall. By studying the stress concentration in the concrete slabs in subsequent, it is found that expansion joint width plays the most critical role in alleviating the stress buildup issue as concerned by the field engineers

Envisioning a Next Generation Extended Reality Conferencing System with Efficient Photorealistic Human Rendering

Meeting online is becoming the new normal. Creating an immersive experience for online meetings is a necessity towards more diverse and seamless environments. Efficient photorealistic rendering of human 3D dynamics is the core of immersive meetings. Current popular applications achieve real-time conferencing but fall short in delivering photorealistic human dynamics, either due to limited 2D space or the use of avatars that lack realistic interactions between participants. Recent advances in neural rendering, such as the Neural Radiance Field (NeRF), offer the potential for greater realism in metaverse meetings. However, the slow rendering speed of NeRF poses challenges for real-time conferencing. We envision a pipeline for a future extended reality metaverse conferencing system that leverages monocular video acquisition and free-viewpoint synthesis to enhance data and hardware efficiency. Towards an immersive conferencing experience, we explore an accelerated NeRF-based free-viewpoint synthesis algorithm for rendering photorealistic human dynamics more efficiently. We show that our algorithm achieves comparable rendering quality while performing training and inference 44.5% and 213% faster than state-of-the-art methods, respectively. Our exploration provides a design basis for constructing metaverse conferencing systems that can handle complex application scenarios, including dynamic scene relighting with customized themes and multi-user conferencing that harmonizes real-world people into an extended world.Comment: Accepted to CVPR 2023 ECV Worksho

Superconducting fault current limiter (SFCL): Experiment and the simulation from finite-element method (FEM) to power/energy system software

The superconducting fault current limiter (SFCL) has been regarded as one of most popular superconducting applications. This article reviews the modern energy system with two major issues (the power stability and fault-current), and introduces corresponding approaches to mitigate these issues, including the importance of using SFCL. Then the article presents the experiment of a resistive-type SFCL used for a power electronic circuit. The experiment well matched the advanced finite-element method (FEM) SFCL model, from which the reliability of FEM SFCL model was confirmed. Afterwards, the FEM model and the power system software PSCAD were used to model a large-scale resistive-type SFCL. Under the same simulation conditions the FEM model well matched the PSCAD model. The FEM method has the advantages of offering specific electromagnetic modeling on superconducting part. The PSCAD SFCL model has much faster simulation speed and can directly cope with all ranges of power networks. This article presents a new vision and an all-in-one study to link the experiment, the numerical model, and the power/energy system software model, and their agreement can be extremely helpful for researchers and engineers to find useful evidences and reliable methods to confidently carry out successful SFCL designs for the electrical energy system

Losses in the saturated iron-core superconducting fault current limiter for VSC-HVDC system

This paper presents the loss analysis on the saturated iron-core superconducting fault current limiter (SISFCL) for a VSC-HVDC transmission system. The numerical model of SISFCL as well as its loss calculation on superconducting parts were carried out by the finite-element method (FEM) using the H -formulation merged into the commercial package COMSOL. The SISFCL model was established for a practical ±10 kV VSC-HVDC system, and the fault current situation was simulated using the PSCAD with a SISFCL. The capability of fault current limiting was verified using the analysis of electromagnetic characteristics, and the corresponding patterns of magnetic field in the iron-core were studied. During the process of fault current limiting, the instantaneous power losses in the superconducting components were studied with the increasing DC bias current. Even in a DC grid system, results proved there were considerable amounts of losses occurred in the superconducting parts, when the SISFCL encountered the fault currents

Superconducting Conductor on Round Core (CORC) cables: 2D or 3D modeling?

The HTSConductor on Round Core (CORC) Cables have been widely recognized as the proper candidate for superconducting applications such as the high-field magnets and power transmission cables. This article presents the modeling of HTS CORC cables using the FEM H-formulation model merged into the package COMSOL Multi-physics. The 2D and 3D H-formulation models of HTS CORC cables were compared and analyzed. There were two important parameters in our CORC design: (1) the gap angle between the HTS tapes in a single layer, and, (2) the degree of twist for HTS tapes (normally identified as the pitch). The 2D CORC model has much faster solving speed over the 3D model and sometimes is able to model the AC loss in a reasonable range, while, the 3D CORC model has the advantages over 2D CORC model with respect to more information in each sector with the different conditions of two parameters above, and more realistic loss calculation

Altered resting-state dynamic functional brain networks in major depressive disorder: Findings from the REST-meta-MDD consortium

Background: Major depressive disorder (MDD) is known to be characterized by altered brain functional connectivity (FC) patterns. However, whether and how the features of dynamic FC would change in patients with MDD are unclear. In this study, we aimed to characterize dynamic FC in MDD using a large multi-site sample and a novel dynamic network-based approach. Methods: Resting-state functional magnetic resonance imaging (fMRI) data were acquired from a total of 460 MDD patients and 473 healthy controls, as a part of the REST-meta-MDD consortium. Resting-state dynamic functional brain networks were constructed for each subject by a sliding-window approach. Multiple spatio-temporal features of dynamic brain networks, including temporal variability, temporal clustering and temporal efficiency, were then compared between patients and healthy subjects at both global and local levels. Results: The group of MDD patients showed significantly higher temporal variability, lower temporal correlation coefficient (indicating decreased temporal clustering) and shorter characteristic temporal path length (indicating increased temporal efficiency) compared with healthy controls (corrected p < 3.14 x 10(-3)). Corresponding local changes in MDD were mainly found in the default-mode, sensorimotor and subcortical areas. Measures of temporal variability and characteristic temporal path length were significantly correlated with depression severity in patients (corrected p < 0.05). Moreover, the observed between-group differences were robustly present in both first-episode, drug-naive (FEDN) and non-FEDN patients. Conclusions: Our findings suggest that excessive temporal variations of brain FC, reflecting abnormal communications between large-scale bran networks over time, may underlie the neuropathology of MDD