Non-metallic crystalline materials conduct heat by the transport of quantized atomic lattice vibrations called phonons. Thermal conductivity depends on how far phonons travel between scattering ...events-their mean free paths. Due to the breadth of the phonon mean free path spectrum, nanostructuring materials can reduce thermal conductivity from bulk by scattering long mean free path phonons, whereas short mean free path phonons are unaffected. Here we use a breakdown in diffusive phonon transport generated by high-frequency surface temperature modulation to identify the mean free path-dependent contributions of phonons to thermal conductivity in crystalline and amorphous silicon. Our measurements probe a broad range of mean free paths in crystalline silicon spanning 0.3-8.0 μm at a temperature of 311 K and show that 40±5% of its thermal conductivity comes from phonons with mean free path >1 μm. In a 500 nm thick amorphous silicon film, despite atomic disorder, we identify propagating phonon-like modes that contribute >35±7% to thermal conductivity at a temperature of 306 K.
Objective In situ fenestration of endovascular stent grafts has been used as a method for branch vessel revascularization in urgent and emergent settings. The objective of this manuscript was to ...review the clinical and experimental evidence related to this technique. Methods PubMed, MEDLINE, and Embase databases were searched for papers published until December 2015 describing in situ fenestration of aortic stent grafts. Benchtop, animal, and human studies were included. Results The literature review identified 118 articles, of which 28 studies were selected for inclusion. These included 16 clinical papers (2 case series and 14 case reports) reporting in situ fenestration of 46 aortic branch vessels in 44 patients. There were 42 retrograde and 4 antegrade instances of in situ fenestration. The most frequent target vessel for in situ fenestration was the left subclavian artery (72%), and the most frequent indication for stent graft implantation was a degenerative aortic aneurysm (43%). Technical success was reported in 44 of 46 attempted fenestrations (96%). The combined rate of perioperative mortality, stroke, and paralysis was 7%. In situ fenestration was predominantly performed with the Talent (Medtronic, Santa Rosa, Calif) stent graft (54%), followed by the Zenith (Cook Medical, Bloomington, Ind) stent graft (37%) and the TAG (W. L. Gore & Associates, Newark, Del) stent graft (9%). In vitro benchtop evaluations of in situ fenestration showed minimal change in fenestration size after 1 year of pulsatile fatigue testing. The use of energy-based fenestration techniques (radiofrequency or laser) has been associated with less fabric fraying than in needle-based techniques. The larger caliber initial fenestration created by these devices also avoids the need for cutting balloons, which have also been linked with increased fabric tears and fraying of the fibers surrounding the fenestration. In addition, the Zenith stent graft was shown in benchtop testing to be the strongest in postfenestration mechanical testing, but it was also the most resistant to balloon dilation. Conclusions In the short to moderate term, in situ fenestration appears to be a reasonable and effective method to extend the proximal landing zone for revascularization of the left subclavian artery. However, longer follow-up is needed to fully assess the long-term durability of this procedure. Based on studies of material properties, an energy-based fenestration technique (radiofrequency or laser) is recommended, along with the avoidance of cutting balloons for dilation of the fenestration.
•Methodology to determine heat capacity and direction thermal conductivities of LIBs.•The methodology combines numerical simulations with experimental measurements.•The proposed methodology is ...applied to cylindrical LIBs of various chemistries.•Methodology uses simple and inexpensive experimental setup.•Results verified with calorimetry and weighted sum.
This paper proposes a methodology to determine the specific heat capacity and the directional components of the thermal conductivity of cylindrical lithium-ion batteries (LIBs) by combining numerical simulations with experimental measurements. These thermophysical properties are crucial for the modeling and simulation of LIBs, and for assessing the performance of battery thermal management systems. The heat capacity of LIBs can be obtained directly with an isothermal battery calorimeter or can be estimated indirectly as a weighted sum of the heat capacities of its constituent materials. However, calorimeters are expensive, and materials compositions are typically not disclosed by LIB manufacturers. In this paper, we propose a cost-effective methodology that uses conventional experimental measurements of battery operating voltage and transient surface temperatures at multiple battery locations to predict the heat generation with a single-particle electrochemical model and the thermophysical properties with an inverse thermal simulation. To assess the accuracy of the proposed methodology, these thermophysical properties are then used to predict the battery surface temperatures at different charging/discharging rates and thermal boundary conditions. The proposed methodology is applied to cylindrical LIBs of various chemistries.
In this paper, we study the effect of porosity heterogeneity on the bulk hydrodynamic properties (permeability and tortuosity) of simulated gas diffusion layers (GDLs). The porosity distributions of ...the heterogeneous reconstructed samples are similar to those previously reported in the literature for Toray TGP-H 120TM GDLs. We use the lattice Boltzmann method to perform pore-level flow simulations in the reconstructed GDL samples. Using the results of pore-level simulations, the effect of porosity distribution is characterized on the predicted in- and cross-plane permeability and tortuosity. It was found that porosity heterogeneity causes a higher in-plane permeability and lower in-plane tortuosity, while the effect is opposite in the cross-plane direction, that is a lower cross-plane permeability and a higher cross-plane tortuosity. We further investigate the effect of adding poly-tetra-fluoro-ethylene (PTFE) & binder material to the reconstructed GDL samples. Three fiber volume percentages of 50, 75, and 100% are considered. Overall, increasing the fiber volume percentage reduces the predicted in- and cross-plane permeability and tortuosity values. A previously reported relationship for permeability of fibrous materials is fitted to the predicted permeability values, and the magnitude of the fitting parameter is reported as a function of fiber volume percentage.
•A mathematical programming approach is proposed to solve the WFLO problem.•Differentiable mathematical models are developed to handle land-use constraints.•Test cases with significant land-use ...constraints are solved efficiently.•The proposed approach outperforms genetic algorithm.
The Wind Farm Layout Optimization (WFLO) problem has attracted a lot of attention from researchers and industry practitioners, as it has been proven that better placement of wind turbines can increase the overall efficiency and the total revenue of a wind farm. Although the engineering wake models are commonly used for layout optimization, the literature seems to have settled on using metaheuristics and stochastic optimization methods. In the present study, we show the effectiveness of non-linear mathematical programming in solving continuous-variable WFLO problems by utilizing exact gradient information of the problem’s objective and constraints. Moreover, mathematical models for handling land-use constraints are developed to solve highly constrained practical problems. For demonstration purposes, the results were compared with those obtained by a genetic algorithm, using a set of test cases that have been frequently used in the WFLO literature. Additional test cases with higher dimensionality, significant land-availability constraints and higher wind farm turbine densities (i.e., turbines per square kilometer) are devised and solved to show the merits of the proposed approach. Our results show the superiority of mathematical programing in solving this problem, as evidenced by the resulting wind farm efficiency and the computational cost required to obtain the solutions.
Strain engineering is an effective way to tune the thermal and electrical properties of novel two-dimensional (2D) materials. In this work, first-principles density functional theory (DFT) is used to ...systematically investigate the strain-dependent lattice thermal conductivity and phonon properties of buckled arsenene and phosphorene, which are the 2D materials with the highest thermal conductivities among monolayers in group-VA. We implemented an iterative self-consistent solution to the Peierls-Boltzmann transport equation (PBTE). Our results showed that the thermal conductivity in both monolayers exhibits an up-and-down behavior when biaxial tensile strain is applied in the range from 0% to 9%. The peak values in the thermal conductivities occur at 5% of strain in arsenene and 3% in phosphorene, with the maximum conductivities of strained arsenene and phosphorene being 1.4 and 1.2 times higher than those of unstrained samples, respectively. We provide a rigorous description of the underlying phonon physics responsible for these thermal responses to strain, addressing the interplay between phonon group velocities, heat capacities, and relaxation times. The acoustic-optical phonon band gaps in arsenene and phosphorene were found to reduce with strain, being the reduction more significant in phosphorene. Our results also showed that the use of the single mode relaxation time approximation (SMRTA) predict substantially lower thermal conductivities for arsenene and phosphorene than those predicted by the iterative solution of the PBTE.
Abstract The capability to spatially control stem cell orientation and differentiation simultaneously using a combination of geometric cues that mimic structural aspects of native extracellular ...matrix (ECM) and biochemical cues such as ECM-bound growth factors (GFs) is important for understanding the organization and function of musculoskeletal tissues. Herein, oriented sub-micron fibers, which are morphologically similar to musculoskeletal ECM, were spatially patterned with GFs using an inkjet-based bioprinter to create geometric and biochemical cues that direct musculoskeletal cell alignment and differentiation in vitro in registration with fiber orientation and printed patterns, respectively. Sub-micron polystyrene fibers (diameter ∼ 655 nm) were fabricated using a Spinneret-based Tunable Engineered Parameters (STEP) technique and coated with serum or fibrin. The fibers were subsequently patterned with tendon-promoting fibroblast growth factor-2 (FGF-2) or bone-promoting bone morphogenetic protein-2 (BMP-2) prior to seeding with mouse C2C12 myoblasts or C3H10T1/2 mesenchymal fibroblasts. Unprinted regions of STEP fibers showed myocyte differentiation while printed FGF-2 and BMP-2 patterns promoted tenocyte and osteoblast fates, respectively, and inhibited myocyte differentiation. Additionally, cells aligned along the fiber length. Functionalizing oriented sub-micron fibers with printed GFs provides instructive cues to spatially control cell fate and alignment to mimic native tissue organization and may have applications in regenerative medicine.
The lattice Boltzmann method is a discrete representation of the Boltzmann transport equation that has been employed for modeling transport of particles of different nature. In the present work, we ...describe the lattice Boltzmann methodology and implementation techniques for the phonon transport modeling in crystalline materials. We show that some phonon physical properties, e.g., mean free path and group velocity, should be corrected to their effective values for one- and two-dimensional simulations, if one uses the isotropic approximation. We find that use of the
D2Q9 lattice for phonon transport leads to erroneous results in transient ballistic simulations, and the
D2Q7 lattice should be employed for two-dimensional simulations. Furthermore, we show that at the ballistic regime, the effect of direction discretization becomes apparent in two dimensions, regardless of the lattice used. Numerical methodology, lattice structure, and implementation of initial and different boundary conditions for the
D2Q7 lattice are discussed in detail.
Tissue engineering predominantly relies on trial and error in vitro and ex vivo experiments to develop protocols and bioreactors to generate functional tissues. As an alternative, in silico methods ...have the potential to significantly reduce the timelines and costs of experimental programs for tissue engineering. In this paper, we propose a methodology to formulate, select, calibrate, and test mathematical models to predict cell population growth as a function of the biochemical environment and to design optimal experimental protocols for model inference of in silico model parameters. We systematically combine methods from the experimental design, mathematical statistics, and optimization literature to develop unique and explainable mathematical models for cell population dynamics. The proposed methodology is applied to the development of this first published model for a population of the airway-relevant bronchio-alveolar epithelial (BEAS-2B) cell line as a function of the concentration of metabolic-related biochemical substrates. The resulting model is a system of ordinary differential equations that predict the temporal dynamics of BEAS-2B cell populations as a function of the initial seeded cell population and the glucose, oxygen, and lactate concentrations in the growth media, using seven parameters rigorously inferred from optimally designed in vitro experiments.
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Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
•A gradient multi-objective optimization algorithm is proposed for WFLO problems.•The problem formulation includes energy, land, cabling and environmental impact.•Exact gradients are derived for the ...optimization objectives and constraints.•NSGA-II is unable to cover the solution space of high-density wind farms.•The proposed approach outperforms NSGA-II in coverage, spread and efficiency.
In addressing the multi-criteria Wind Farm Layout Optimization (WFLO) problem, the literature has been focused on the simple weighted sum approach using single-objective stochastic and evolutionary algorithms, in addition to Pareto formulations using evolutionary algorithms. There is no single solution to a multi-criteria problem with conflicting objectives; therefore, the Pareto approach is useful to provide the developer with a non-dominated set of solutions. However, the evolutionary optimization algorithms tend to be computationally prohibitive, especially when optimizing large-scale wind farms. Additionally, most of WFLO problems are highly constrained, where many unfeasible zones can exist inside the proposed wind farm boundaries, which in turn complicates the optimization process. To remedy these drawbacks, we propose a gradient-based approach to Pareto optimization of the multi-criteria WFLO problems considering land footprint, energy output, electrical infrastructure and environmental impact. Mathematical functions and their derivatives are developed to represent the four objectives, land-based constraints, and their gradients. The developed models were validated by devised numerical experiments; and the optimized layouts using the proposed algorithm were compared to those by the Non-Dominated Sorting Genetic Algorithm (NSGA-II). Our results provide some evidence regarding the inability of the NSGA-II to cover the objective space when optimizing wind farms with large power-densities. In contrast, our proposed approach succeeds in obtaining high-density layouts efficiently. Furthermore, we demonstrated the superiority of the developed algorithm, in the aspects of coverage, spread, and computational cost.
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