477 research outputs found
sj-docx-1-wso-10.1177_17474930241228956 – Supplemental material for Type of anesthesia for endovascular therapy in acute ischemic stroke: A literature review and meta-analysis
No full textSupplemental material, sj-docx-1-wso-10.1177_17474930241228956 for Type of anesthesia for endovascular therapy in acute ischemic stroke: A literature review and meta-analysis by Yitong Jia, Yao Feng, Yanhui Ma, Guang Feng, Na Xu, Meng Li, Miao Liu, Zhen Fan and Tianlong Wang in International Journal of Stroke</p
Influence of axial spacing on stall development of FBCDZ-10-No20 mode contra-rotating fan
No full textThe rotating fan is prone to instability such as stall and even surge when operating at low flow rates. In order to study the influence of axial spacings on the stall inception and its development and propagation process of a contra-rotating fan , a Shear Stress Transport(SST) k-ω turbulence model was used to numerically simulate the unsteady flow in the whole passage of a FBCDZ-10-No20 contra-rotating fan. The stall evolution of fan under two axial spacings are investigated. The results show that the axial spacing has significant influence on the inception and development of fan stall.In the stall inception stage, when the axial spacing between the two-stage impellers is 170 mm, the stall inception first occurs within the rear impeller, which locates in the trailing edge area on the suction surface of the blade root. Then stall inception develops from the blade root to the tip area along the radial direction, and accumulates towards a blade passage along the circumferential direction and falls off on the way to form a blocking area, which eventually leads to stall. When the axial spacing is 70 mm, the stall inception successively occurs the tip area of the two-stage impellers. Then stall inception keeps increasing, which eventually leads to stall. During the full stall stage, the axial spacing has a significant effect on the propagation of the stall vortex in the circumferential, axial and radial directions. When the axial spacing is 170 mm, the type of stall vortex shows the single vortex full-blade high stall. The propagation range of the stall vortex in the axial direction is limited to the region of rear impeller, and rotates at 33.3% of the rear impeller speed in the circumferential direction; When the axial spacing is 70 mm, the type of stall vortex shows the multi-vortex partial blade high stall. The stall vortex are successively generated in the tip area of the two-stage impellers, propagating upward and downstream in the axial direction, and are dispersed in the area above 70% of the blade height of each blade channel in the radial direction. Due to the change of the axial distance between the two-stage impellers, the type of stall inception of the fan is changed from “partial surge type” to “Spike-type”
Effect of axial spacing on rotating stall performance of FBCDZ-10-No20 contra-rotating fan
No full textMining contra-rotating fan are prone to rotational stall when it operates at low flow rates, which seriously affects the operational stability. The unsteady flow in the full flow passage of a FBCDZ-10-No20 contra-rotating fan at five axial spacings was numerically simulated by using the SST k-ω turbulence model. The effect of axial spacing on the stall process of contra-rotating fan was studied, and the mechanism of stall inception and development at different axial spacings was revealed. The results shown that the axial spacing had a significant influence on the initial position, type and development of stall inception. For the axial spacings of 70 mm and 100 mm, the stall inception first occurred at the tip of the front stage, and subsequently appeared at the tip of the rear stage due to the rotor-rotor interaction between the two stages. However, compared with the axial spacing of 70 mm, the rotor-rotor interaction at the axial spacing of 100 mm was relatively weak, making it take longer for a stall inception to occur in the rear stage. For all three axial spacings of 140 mm, 170 mm and 225 mm, the stall inception occurred first in the root of rear stage. The difference was that for the axial spacing of 140 mm, the leakage flow at the tip of the front stage cannot completely flow out of the channel with the main flow, and a localized tip blockage area was formed. However, for both spacings of 170 mm and 225 mm, there was almost no blockage area at the tip of the front stage, and eventually only mature stall vortices formed in the rear stage. With the increase of the axial spacing, the blockage area formed by both the leading edge overflow and the trailing edge reverse flow those originated from the leakage flow at the tip of the front stage gradually reduced. In contrast, the blockage area formed by the radial vortex on the suction surface near the root of the rear stage gradually increased. When the leakage flow at the tip of the front stage failed to form blockage area, the stall type changed from the “spike type” induced by the tip leakage flow at the front stage to the “localized surge type” induced by the radial vortex flow at the root of the rear stage
Low-frequency vibrational modes of benzoic acid investigated by terahertz time-domain spectroscopy and theoretical simulations
No full textIn this paper, the low-frequency vibrational modes of crystalline benzoic acid (BA) have been investigated by terahertz time-domain spectroscopy (THz-TDS) and theoretical simulations based on the linearity combination of atomic orbital within the Density Functional Theory (DFT) as well as ab initio molecular orbital method at second-order Møller-Plesset Perturbation Theory (MP2) level for single molecule and dimer. Experimentally, a series of prominent absorption features of pure benzoic acid relevant to intra- and inter-molecular vibrational modes have been obtained below 4 THz at room temperature. For the theoretical simulations, geometry-optimization results of bond lengths and dihedral angles in both BA monomer and dimer are very close to experimental neutron diffraction measurements. Furthermore, the simulation results demonstrate absorption profile centered at 1.89 THz contains low-frequency modes of Ph-COOH twisting due to intramolecular motion and cogwheel owing to intermolecular motion. All the intra- and inter-molecular vibrational modes measured have also been assigned
Pore-scale Study of Flow and Transport in Energy Georeservoirs
No full textOptimizing proppant pack conductivity and proppant-transport and -deposition patterns in a hydraulic fracture is of critical importance to sustain effective and economical production of petroleum hydrocarbons. In this research, a numerical modeling approach, combining the discrete element method (DEM) with the lattice Boltzmann (LB) simulation, was developed to provide fundamental insights into the factors regulating the interactions between reservoir depletion, proppant-particle compaction and movement, single-/multiphase flows and non-Darcy flows in a hydraulic fracture, and fracture conductivity evolution from a partial-monolayer proppant concentration to a multilayer proppant concentration. The potential effects of mixed proppants of different sizes and types on the fracture conductivity were also investigated.
The simulation results demonstrate that a proppant pack with a smaller diameter coefficient of variation (COV), defined as the ratio of standard deviation of diameter to mean diameter, provides better support to the fracture; the relative permeability of oil was more sensitive to changes in geometry and stress; when effective stress increased continuously, oil relative permeability increased nonmonotonically; the combination of high diameter COV and high effective stress leads to a larger pressure drop and consequently a stronger non-Darcy flow effect. The study of proppant mixtures shows that mixing of similar proppant sizes (mesh-size-20/40) has less influence on the overall fracture conductivity than mixing a very fine mesh size (mesh-size-100); selection of proppant type is more important than proppant size selection when a proppant mixture is used. Increasing larger-size proppant composition in the proppant mixture helps maintain fracture conductivity when the mixture contains lower-strength proppants. These findings have important implications to the optimization of proppant placement, completion design, and well production.
In the hydraulic-mechanical rock-proppant system, a fundamental understanding of multiphase flow in the formation rock is critical in achieving sustainable long-term productivity within a reservoir. Specifically, the interactions between the critical dimensionless numbers associated with multiphase flow, including contact angle, viscosity ratio, and capillary number (Ca), were investigated using X-ray micro computed tomography (micro-CT) scanning and LB modeling. The primary novel finding of this study is that the viscosity ratio affects the rate of change of the relative permeability curves for both phases when the contact angle changes continuously. Simulation results also indicate that the change in non-wetting fluid relative permeability was larger when the flow direction was switched from vertical to horizontal, which indicated that there was stronger anisotropy in larger pore networks that were primarily occupied by the non-wetting fluid. This study advances the fundamental understanding of the multiphysics processes associated with multiphase flow in geologic materials and provides insight into upscaling methodologies that account for the influence of pore-scale processes in core- and larger-scale modeling frameworks.
During reservoir depletion processes, reservoir formation damage is an issue that will affect the reservoir productivity and various phases in fluid recovery. Invasion of formation fine particles into the proppant pack can affect the proppant pack permeability, leading to potential conductivity loss. The combined DEM-LB numerical framework was used to evaluate the role of proppant particle size heterogeneity (variation in proppant particle diameter) and effective stress on the migration of detached fine particles in a proppant supported fracture. Simulation results demonstrate that a critical fine particle size exists: when a particle diameter is larger or smaller than this size, the deposition rate increases; the transport of smaller fines is dominated by Brownian motion, whereas the migration of larger fines is dominated by interception and gravitational settling; this study also indicates that proppant packs with a more heterogeneous particle-diameter distribution provide better fines control. The findings of this study shed lights on the relationship between changing pore geometries, fluid flow, and fine particle migration through a propped hydraulic fracture during the reservoir depletion process.Doctor of PhilosophyHydraulic fracturing stimulation design is required for unconventional hydrocarbon energy (e.g., shale oil and gas) extraction due to the low permeability and complex petrophysical properties of unconventional reservoirs. During hydrocarbon production, fractures close after pumping due to the reduced fluid pressure and increased effective stress in rock formations. In the oil and gas industry, proppant particles, which are granular materials, typically sand, treated sand, or man-made ceramic materials, are pumped along with fracturing fluids to prevent hydraulic fractures from closing during hydrocarbon extraction. In order to relate the geomechanical (effective stress), geometric (pore structure and connectivity), and transport (absolute permeability, relative permeability, and conductivity) properties of a proppant assembly sandwiched in a rock fracture, a geomechanics-fluid mechanics framework using both experiment and simulation methods, was developed to study the interaction and coupling between them. The outcome of this research will advance the fundamental understanding of the coupled, multiphysics processes with respect to hydraulic fracturing and benefit the optimization of proppant placement, completion design, and well production
中国“五育并举”研究 ——以石家庄市第十五中学为例
No full text对于“教育应该培育什么样的人”、“教育是在为谁培育人”的问题已经明确，那么，新时代教育就需要回答“教育应该如何育人？”、“怎样提升教育质量？”等问题。2019 年6 月，国务院办公厅印发《关于深化教育教学改革全面提高义务教育质量的意见》（以下简称《意见》），提出：“坚持德智体美劳‘五育’并举，全面发展素质教育”[1]。为了进一步推动教育现代化，《中国教育现代化2035》（以下简称《2035》）强调：更多地关注道德，更多地关注发展，更多地关注每个人，更多地关注终生的学习，更多地注重知识和行为的一致性。[2]这说明“五育并举，融合育人”已成为基础教育人才培养的大势所趋，也标志着共建共享共生的中国教育新时代已经到来。
本论文总结了国内文献中对“五育并举”、“五育融合”、“五育”关系的相关论述，对中国“五育并举”的历史进行了梳理，这些构成了本论文的理论基础。紧接着，论文列举了当前中小学“五育并举”实践中存在的问题，经过对问题的深度思考，本论文提出：融理念、融课程、融机制、融评价的改进思路，并对比新加坡全面育人理念，从中获取了不少有用的借鉴和启示，以期从顶层设计的角度，来研究探讨有关课题，以便能更深刻的改进和落实“五育并举，融合育人”的相关举措
Direct numerical simulation of supersonic turbulent boundary layer subjected to a curved compression ramp
No full textNumerical investigations on a supersonic turbulent boundary layer over a longitudinal curved compression ramp are conducted using direct numerical simulation for a free stream Mach number M-infinity = 2.9 and Reynolds number Re-theta = 2300. The total turning angle is 24 degrees, and the concave curvature radius is 15 times the thickness of the incoming turbulent boundary layer. Under the selected conditions, the shock foot is transferred to a fan of the compression wave because of the weaker adverse pressure gradient. The time-averaged flow-field in the curved ramp is statistically attached where the instantaneous flow-field is close to the intermittent transitory detachment state. Studies on coherent vortex structures have shown that large-scale vortex packets are enhanced significantly when the concave curvature is aligned in the spanwise direction. Consistent with findings of previous experiments, the effect of the concave curvature on the logarithmic region of the mean velocity profiles is found to be small. The intensity of the turbulent fluctuations is amplified across the curved ramp. Based on the analysis of the Reynolds stress anisotropy tensor, the evolutions of the turbulence state in the inner and outer layers of the boundary layer are considerably different. The curvature effect on the transport mechanism of the turbulent kinetic energy is studied using the balance analysis of the contributing terms in the transport equation. Furthermore, the Gortler instability in the curved ramp is quantitatively analyzed using a stability criterion. The instantaneous streamwise vorticity confirms the existence of the Gortler-like structures. These structures are characterized by an unsteady motion. In addition, the dynamic mode decomposition analysis of the instantaneous flow field at the spanwise/wall-normal plane reveals that four dynamical relevant modes with performance loss of 16% provide an optimal low-order representation of the essential characteristics of the numerical data. The spatial structures of the dominated low-frequency dynamic modes are found to be similar to that of the Gortler-like vortices. Published by AIP Publishing
Effect of coordination number of particle contact force on rutting resistance of asphalt mixture
No full textOptimizing asphalt mix design at the indoor stage is of significant importance for enhancing the rutting resistance of asphalt mixture, which is affected by its structural characteristics. In this work, the coordination number of particle contact force (CNpcf) was proposed as an indicator to represent contact characteristics of skeleton structure aggregates in asphalt mixture. Nine asphalt mixtures with different gradations were designed, and the relationship of CNpcf with the number of aggregate contact zones (CZ) was established by combining rutting tests and digital image processing technique (DIP). The Mann-Whitney U test was implemented to analyze the distribution properties of inter-particle contacts before and after the rutting test. In addition, the resistance to the further expansion of rutting was analyzed. The results revealed a significant positive correlation (PCCs = 0.843, R2 = 0.711) between CNpcf and CZ. The content of coarse aggregates in the dominant structure did not exhibit monotonic related to anti-rutting performance of the asphalt mixture. Therefore, an optimum aggregate content of 57% was utilized. The Mann-Whitney U test revealed that the mesoscale skeleton structure of the asphalt mixes before and after rutting exhibited excellent stability. This study further indicated the applicability of combining CNpcf to adjust the mix design to enhance the rutting resistance of asphalt mixture and to prevent rutting expansion in flexible pavement.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Pavement Engineerin
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