The high penetration of Inverter-Based Resources (IBRs) necessitates computationally intensive Electro-Magnetic Transient (EMT) simulations to analyze controller interactions. A practical challenge is that these studies are considerably time-consuming for large systems, and need to be done for different operating conditions and contingencies. Reducing the order of the model to form a reduced-order equivalent is a viable approach to speed up the simulation. In such models, it is important to ensure that (a) the reduced order model is accurate over a wide range of frequencies, and (b) the equivalent system is passive to reduce the chance of instability of the simulation. To address this issue, this paper presents a novel, passivity-preserving model order reduction framework that achieves wide-band accuracy. The proposed approach extends the classical moment-matching approach to match the system’s response at multiple frequency points to construct a passivity-preserving reduced-order model. The efficacy of the proposed technique is validated on the IEEE 39-bus system. Results demonstrate that the reduced models provide high fidelity over a wide frequency range, enabling a significant reduction in EMT simulation time without compromising the stability of the simulation.
Effective thermal management is essential to maintain the reliability and service life of high-power LED devices. Radial heat sinks offer a compact cooling solution, and their effectiveness largely depends on their geometric parameters. This study investigates a radial heat sink configuration with interrupted branching fins of non-uniform heights. Three fin height distributions, including radially decreasing (Type 1), radially increasing (Type 2), and uniform fin height, are evaluated through numerical simulations. The Type 2 heat sink array exhibits 15% lower thermal resistance and a 1.7°C reduction in the peak wall temperature compared to the Type 1 design. The improvement in performance is due to reduced flow resistance, delayed boundary-layer merging, and buoyant-assistive flow from inner branching fins. A numerical design exploration is conducted to analyze the effects of height difference, fin number, and branching angle on heat sink performance. A parametric dataset is generated and used to train a Gaussian Process Regression model (GPR) to predict thermal resistance across the design space. Subsequently, multi-objective optimization is performed using the trained GPR model as a surrogate model, and a set of Pareto-optimal solutions is obtained for different height differences and heat fluxes. Among the investigated configurations, the Pareto-optimal design with a 10 mm height difference shows 31.5% lower thermal resistance than the constant-height design, despite a 27.6% increase in mass. Under a constant mass constraint, the design with varying fin heights shows an 11-18% reduction in thermal resistance compared to the uniform height design. Furthermore, the studied designs remain invariant across the applied heat fluxes considered in this study.
Fracture initiation and propagation in porous geomaterials arise from the complex coupling between solid deformation, damage evolution, and fluid flow in the fractures. Capturing these processes numerically poses significant challenges due to the strong hydro-mechanical interaction and the emergence of evolving discontinuities. This study presents a novel coupled meshfree–grid computational framework that combines Smoothed Particle Hydrodynamics (SPH) for solid deformation and fracture modeling with a Finite Difference Method (FDM) formulation for pore fluid flow. Unlike conventional approaches, the proposed method does not require the knowledge of predefined cracks. Fracture initiation and propagation emerge naturally from the evolving stress and pore pressure fields. A robust coupling algorithm is developed to dynamically exchange stress, pressure, and damage information between the SPH and FDM domains, enabling consistent simulation of fluid-driven fracture evolution and solid deformation through Biot’s theory. A damage-driven fracture detection scheme is used which enables automatic transition between Darcy flow in the porous medium and cubic-law flow within open fractures, thereby capturing the evolving hydro-mechanical response. The framework is validated against analytical benchmark solutions including Mandel’s poroelastic consolidation problem and a confined five-spot pattern flow problem, as well as elasticity solutions for stress distribution around a pressurized circular opening. Further validation is performed through comparison with laboratory hydraulic-fracturing experiments conducted on basalt and marble samples under different hydrostatic confining stresses. The numerical results show excellent agreement with analytical solutions and experimental observations, and the method successfully captures key hydro-mechanical phenomena, including stress redistribution, pressure relief, and the transition from fracture initiation to propagation.
The integrative regeneration of articular cartilage and subchondral bone remains a major clinical challenge due to the difficulties of mimicking the spatial and compositional complexity of native osteochondral tissues in artificial implants. To overcome these limitations, 3D printing has enabled personalized solutions, going beyond traditional plug-shaped scaffolds that are limited to repairing small, cylindrical focal defects. In this study, we report solvent-free 4D printing of olive oil-derived fatty amide-based acrylate photoresin for osteochondral tissue engineering applications. The physicochemical properties of the printed polymers were fine-tuned using acrylic acid as a comonomer. With optimized printing conditions, the resin can be printed into high-resolution objects with each layer of ∼50-100 μm. Depending on the resin composition, the mechanical properties of the printed polymers vary between ∼200-1400 kPa with ∼210-420% strain under tensile setting, and ∼26-98 MPa at ∼80% strain under compressive setting. The mechanical properties of the printed polymers are comparable to those of various interfacial soft tissues, including ligaments, articular cartilage, and soft collagenous bone, indicating their potential for interfacial tissue engineering applications. Furthermore, the printed polymers showed near-body temperature-responsive shape memory (4D) properties with a recovery ratio of ∼99% for optimized resin composition. In addition, the printed polymers exhibited excellent bactericidal antimicrobial activity against both Gram-negative and Gram-positive bacteria, highlighting their multifunctional nature. More importantly, the printed polymers exhibited good cytocompatibility (cell viability ∼≥90% across all samples compared to control) toward adipose-derived human mesenchymal stem cells, facilitating growth factor-free osteogenic and chondrogenic differentiation, which confirms their potential for osteochondral tissue regeneration. Overall, the newly developed biobased resin offers a low-cost, sustainable alternative for 4D printing of personalized implants for osteochondral tissue regeneration.
This study investigates the performance of the Constant Radial Stiffness Triaxial (CRST) test for simulating pavement loading conditions compared to the traditional repeated load triaxial test. Through the integration of an advanced constitutive model capable of accurately simulating cyclic hardening in unsaturated soils, a fully coupled Finite Element (FE) model for unsaturated soils is developed. This FE model enables the exploration of the effects of varying degrees of saturation on soil behaviour under a large number of load cycles, accounting for the complex interactions between the soil skeleton, pore water pressure, and suction. FE examples show how initial soil conditions like the degree of saturation and void ratio, and loading conditions, influence permanent deformations in flexible pavements. These findings can provide insights into pavement permanent deformation mechanisms and demonstrate the effectiveness of the CRST test to provide a reasonable representation of in-situ pavement loading conditions. Furthermore, the study illustrates the FE model’s utility in refining the CRST test design, particularly in determining appropriate radial stiffness values that effectively mimic the influence of surrounding soils. This aspect is vital for improving the relevance of the laboratory testing method to in-situ pavement loading scenarios.
