Microscale Effects Research Articles

Evaluation of curing residual stresses in three-phase thin composite laminates considering micro-scale effects

Generally, in thin laminates, residual stresses are studied in two scales; micro mechanics and macro-mechanics. So far, no study has included both the micro- and macro-scale analysis for predicting the thermal residual stresses of composite laminates. In other words, no literature model is proper for both the micro and macro scale at the same time. This paper presents a procedure to integrate micro and macro thermal residual stresses in composite laminates. The combination of micromechanics-based analytical results and hole-drilling calibration factors leads to predict the macro thermal residual stresses of a unidirectional laminate. Classical lamination theory (CLT) is then employed to predict macroscopic thermal residual stress of each layer in laminated composites (with different orientations). Comparison of longitudinal, transverse and in-plane shear stresses between the proposed approach and published experimental results is made for glass/epoxy laminates in cross-ply and quasi-isotropic configurations. Available experimental data from incremental hole-drilling method show that the present combination procedure yields more precise residual stresses predictions in comparison with the state that the CLT is used only. In other words, the presented procedure which adds the micro residual scale results to the CLT, gives rise to a better prediction of residual stress fields. Consequently, ignoring the micro-mechanical effects may result in some uncertainties in thermal residual stress evaluation of thin composite laminates. Furthermore, while the CLT is not able to take into account the in-plane shear residual stresses for the cross-ply composites, the present procedure can provide an estimation of shear residual stress field.

Analysis of slip flow heat transfer between two unsymmetrically heated parallel plates with viscous dissipation

This paper presents an analytical investigation to study the heat transfer and fluid flow characteristics in the slip flow region for hydrodynamically and thermally fully developed flow between parallel plates. Both upper and lower plates are subjected to asymmetric heat flux boundary conditions. The effect of first order velocity slip, temperature jump, asymmetric heat flux ratio and viscous dissipation on the heat transfer performance is analyzed. Closed form expressions are obtained for the temperature distribution and Nusselt number. Present predictions are verified for the cases that neglect the viscous heating and microscale effects. The effect of asymmetric heat flux ratio with and without viscous dissipation on Nusselt number for both macroscale and microscale is highlighted. The heat transfer characteristics are found to depend on various modeling parameters, namely, modified Brinkman number, Knudsen number and heat flux ratio.

Coupled on-site measurement/CFD based approach for high-resolution wind resource assessment over complex terrains

Wind resource assessment is essential for development of wind energy, particularly in the regions with complex terrains. This study proposes a coupled on-site measurement/CFD (Computational Fluid Dynamics) based approach to reproduce the spatial variability of wind speed for a region with complex terrain conditions. A complete framework is presented for wind resource assessment, which involves on-site measurement, CFD simulations and statistical analysis. Next, a case study on wind resource assessment for an offshore island with complex terrain features where is equipped with anemometers for long-term wind measurement is performed using the developed approach. The microscale effects in the assessment region are justified from CFD simulations with modified RNG k–ε model. A cross-validation of the numerical simulations against wind tunnel experimental results and on-site measurements indicates a good agreement. Consequently, a detailed wind resource map of the offshore island is attained through the wind data from a single measurement site combined with the CFD simulations, which is of great use for future wind farm siting and turbine micro-siting. The coupled on-site measurement/CFD based approach is expected to enable the efficient and reliable wind resource assessment and facilitate the wind energy development in the areas with complex terrain conditions.

The micro-scale analysis of the micro direct methanol fuel cell

In this paper, the behavior of micro-scale effect in a micro direct methanol fuel cell (μDMFC) is investigated through both simulations and experiments. A model is built to describe the methanol distribution in the diffusion layer with different feature sizes. In addition, the dynamic movement of a single CO2 bubble is also simulated to study the two-phase characteristics in the micro channels with various aspect ratios. Furthermore, a metal-based transparent μDMFC with the active area of 0.64 cm2 is designed and fabricated to evaluate the two-phase flow characteristic as well as the corresponding cell performance. The experimental results reveal that when the feature size scales down to 0.6 mm and 0.4 mm, the peak power density of 27.1 mW cm−2 and 26.3 mW cm−2 are achieved at room temperature. Finally, the effect of adjusting channel aspect ratio is experimentally investigated to improve the inner convection transport and the cell output, and the results are well in agreement with the simulation.

Collembola at three alpine subarctic sites resistant to twenty years of experimental warming.

This study examined the effects of micro-scale, site and 19 and 21 years of experimental warming on Collembola in three contrasting alpine subarctic plant communities (poor heath, rich meadow, wet meadow). Unexpectedly, experimental long-term warming had no significant effect on species richness, effective number of species, total abundance or abundance of any Collembola species. There were micro-scale effects on species richness, total abundance, and abundance of 10 of 35 species identified. Site had significant effect on effective number of species, and abundance of six species, with abundance patterns differing between sites. Site and long-term warming gave non-significant trends in species richness. The highest species richness was observed in poor heath, but mean species richness tended to be highest in rich meadow and lowest in wet meadow. Warming showed a tendency for a negative impact on species richness. This long-term warming experiment across three contrasting sites revealed that Collembola is capable of high resistance to climate change. We demonstrated that micro-scale and site effects are the main controlling factors for Collembola abundance in high alpine subarctic environments. Thus local heterogeneity is likely important for soil fauna composition and may play a crucial role in buffering Collembola against future climate change.

Numerical investigation of dynamic microorgan devices as drug screening platforms. Part II: Microscale modeling approach and validation.

The authors have previously reported a rigorous macroscale modeling approach for an in vitro 3D dynamic microorgan device (DMD). This paper represents the second of a two-part model-based investigation where the effect of microscale (single liver cell-level) shear-mediated mechanotransduction on drug biotransformation is deconstructed. Herein, each cell is explicitly incorporated into the geometric model as single compartmentalized metabolic structures. Each cell's metabolic activity is coupled with the microscale hydrodynamic Wall Shear Stress (WSS) simulated around the cell boundary through a semi-empirical polynomial function as an additional reaction term in the mass transfer equations. Guided by the macroscale model-based hydrodynamics, only 9 cells in 3 representative DMD domains are explicitly modeled. Dynamic and reaction similarity rules based on non-dimensionalization are invoked to correlate the numerical and empirical models, accounting for the substrate time scales. The proposed modeling approach addresses the key challenge of computational cost towards modeling complex large-scale DMD-type system with prohibitively high cell densities. Transient simulations are implemented to extract the drug metabolite profile with the microscale modeling approach validated with an experimental drug flow study. The results from the author's study demonstrate the preferred implementation of the microscale modeling approach over that of its macroscale counterpart.

Spatial scale effects on Chironomidae diversity and distribution in a Mediterranean River Basin

Chironomidae are a very common insect group in running waters of Greece; however, they have been rarely studied. The aim of this work was to investigate Chironomidae distribution and abundance in a Mediterranean intermittent river basin, identify the environmental factors that are linked to variation in their assemblages and to partition the influence of environmental and spatial components, alone and in combination, on Chironomidae community composition. Multivariate ordination techniques were used to assess the association between environmental variables and species abundance, while variation partitioning was performed using partial canonical correspondence analysis to understand the importance of different explanatory variables in Chironomidae variation. Chironomidae variation was divided into independent and joint effects of micro-scale (physical and chemical variables, microhabitat composition, etc.), meso-scale (land use/cover) and macro-scale (altitude and distance to source, etc.) variables. Stream width and depth, water discharge, land uses, pH, and ecological quality were the most important factors structuring Chironomidae assemblages. Micro-scale variables accounted for 55.6% of the total explained variation followed by meso-scale (10.7%) and macro-scale (10.4%) variables. The results of partial constraint analyses suggest that micro-scale variables play a major role in Chironomidae assemblages.

Multi‐Scale Effects in the Strength of Ceramics

Multiple length-scale effects are demonstrated in indentation-strength measurements of a range of ceramic materials under inert and reactive conditions. Meso-scale effects associated with flaw disruption by lateral cracking at large indentation loads are shown to increase strengths above the ideal indentation response. Micro-scale effects associated with toughening by microstructural restraints at small indentation loads are shown to decrease strengths below the ideal response. A combined meso-micro-scale analysis is developed that describes ceramic inert strength behaviors over the complete indentation flaw size range. Nano-scale effects associated with chemical equilibria and crack velocity thresholds are shown to lead to invariant minimum strengths at slow applied stressing rates under reactive conditions. A combined meso-micro-nano-scale analysis is developed that describes the full range of reactive and inert strength behaviors as a function of indentation load and applied stressing rate. Applications of the multi-scale analysis are demonstrated for materials design, materials selection, toughness determination, crack velocity determination, bond-rupture parameter determination, and prediction of reactive strengths. The measurements and analysis provide strong support for the existence of sharp crack tips in ceramics such that the nano-scale mechanisms of discrete bond rupture are separate from the larger scale crack driving force mechanics characterized by continuum-based stress-intensity factors.

Microscale effects of melt infiltration into the lithospheric mantle: Peridotite xenoliths from Xilong, South China

Melting and reactions between minerals and melts are important processes in the evolution of the lithospheric mantle, and are usually inferred from their geochemical fingerprints in mantle samples. However, a suite of mantle-derived peridotite xenoliths from the Xilong area, South China, records the reaction of successive silicate melts of different compositions with mineral assemblages in the mantle, preserved by quenching during entrainment. These xenoliths form two groups and record a compositionally layered mantle. Group 1 has olivine Mg# ~91, (and is thus relatively refractory), is derived from depths of ~50–65km, and shows the trace-element geochemical signature of “old” carbonatitic metasomatism. Group 2 is more fertile with olivine Mg# mainly ~89–90, is derived from ~40 to 55km and has ubiquitous modal spinel. Xenoliths of both groups then show sequential infiltration by two compositionally distinct melts (Na-rich and K-rich) not long before eruption.The Na-rich melts are enclosed in spongy clinopyroxene and spinel rims and are inferred to have triggered the reactions that formed the spongy rims, which have lower Al2O3, Na2O and Mg#, but higher FeO, TiO2 and Cr# than the primary phases. The undersaturated Na-rich mafic melts were probably formed in the asthenosphere by low-degree melting.The K-rich melts occur mainly in reaction zones around orthopyroxene and in reaction patches containing fine-grained secondary olivine, clinopyroxene and minor spinel. The melts have high contents of SiO2, K2O (mean 14.3wt.%), Rb, Ba, and LREE but very low Na2O/K2O (0.01–0.29), positive anomalies in Eu and Sr, and variable HFSE anomalies. These compositional characteristics are consistent with an origin as low-degree partial melts of pre-existing phlogopite-bearing rocks.The K-rich melts also react with primary olivine, and the spongy-textured secondary clinopyroxene and spinel inferred to have formed by reaction with the Na-rich melts, yielding secondary olivine, vermicular clinopyroxene and spinel compositionally similar to that in the reaction patches. This observation suggests that the infiltration of K-rich melt occurred after the percolation of Na-rich melts.This study demonstrates the complexity of metasomatic processes in the lithospheric mantle, and emphasizes the caution required when interpreting the geochemical signatures of mantle-derived xenoliths in which multiple overprinting by compositionally diverse fluid and melt interactions with mantle minerals cannot be so clearly observed and characterized as in these samples.

A non-classical Timoshenko beam element for the postbuckling analysis of microbeams based on Mindlin’s strain gradient theory

Based on Mindlin’s strain gradient elasticity theory capturing microscale effects, a new extended Timoshenko beam element is proposed to study the postbuckling behavior of microbeams. So as to develop the size-dependent finite element formulation, the higher-order tensors of energy pairs in the energy functional are vectorized and represented in the quadratic form. In comparison with the standard Timoshenko beam element, the present one needs two further nodal degrees of freedom including derivatives of lateral translation and rotation. The Hermite polynomials are also implemented as shape functions. The developed model is general so that its formulation can be used for modified couple stress, modified strain gradient and classical elasticity theories. In the numerical results, the influences of the small-scale factor, geometrical parameters and boundary conditions on the bifurcation diagrams of microbeams are examined.

Effective boundary conditions for compressible flows over rough boundaries

Simulations of a flow over a roughness are prohibitively expensive for small-scale structures. If the interest is only on some macroscale quantity it will be sufficient to model the influence of the unresolved microscale effects. Such multiscale models rely on an appropriate upscaling strategy. Here the strategy originally developed by Achdou et al. [Effective boundary conditions for laminar flows over periodic rough boundaries, J. Comput. Phys. 147 (1998) 187–218] for incompressible flows is extended to compressible high Reynolds number flow. For proof of concept a laminar flow over a flat plate with partially embedded roughness is simulated. The results are compared with computations on a rough domain.

Age-related vascular stiffening: causes and consequences.

Arterial stiffening occurs with age and is closely associated with the progression of cardiovascular disease. Stiffening is most often studied at the level of the whole vessel because increased stiffness of the large arteries can impose increased strain on the heart leading to heart failure. Interestingly, however, recent evidence suggests that the impact of increased vessel stiffening extends beyond the tissue scale and can also have deleterious microscale effects on cellular function. Altered extracellular matrix (ECM) architecture has been recognized as a key component of the pre-atherogenic state. Here, the underlying causes of age-related vessel stiffening are discussed, focusing on age-related crosslinking of the ECM proteins as well as through increased matrix deposition. Methods to measure vessel stiffening at both the macro- and microscale are described, spanning from the pulse wave velocity measurements performed clinically to microscale measurements performed largely in research laboratories. Additionally, recent work investigating how arterial stiffness and the changes in the ECM associated with stiffening contributed to endothelial dysfunction will be reviewed. We will highlight how changes in ECM protein composition contribute to atherosclerosis in the vessel wall. Lastly, we will discuss very recent work that demonstrates endothelial cells (ECs) are mechano-sensitive to arterial stiffening, where changes in stiffness can directly impact EC health. Overall, recent studies suggest that stiffening is an important clinical target not only because of potential deleterious effects on the heart but also because it promotes cellular level dysfunction in the vessel wall, contributing to a pathological atherosclerotic state.

Study on the steady and transient pressure characteristics of shale gas reservoirs

The flow law was obtained by combining the mechanics of continuous media and molecular kinematics method for shale gas reservoirs with nano/micropores. The new non-linear seepage model considering the diffusion, slippage and desorption effects was established and simplified. The steady and transient governing equations were constructed on the basis of the non-linear model. The pressure characteristic analytical solution was then obtained. The permeability adjustment factor changes largely when the pore throat radius is less than 100 nm. Therefore, the flow law in nano/micropore departs significantly from Darcy's law. Numerical simulation results show that the flux of gas changes significantly with the permeability of the pore throat radius, and the flow in nano/micropore reservoir shows the microscale effect. Compared with the pressure distribution calculated by Darcy's law, the pressure distribution calculated by the new model decreases slowly because of the diffusion and desorption effect. The results provide a theoretical foundation for developing shale gas reservoirs.

Numerical simulation of a backward-facing step flow in a microchannel with external electric field

A backward-facing step flow in the microchannel with external electric field was investigated numerically by a high-order accuracy upwind compact difference scheme in this work. The Poisson–Boltzmann and Navier–Stokes equations were computed by the high-order scheme, and the results confirmed the ability of the new solver in simulation of micro-scale electric double layer effects. The flow fields were displayed for different Reynolds numbers; the positions of the vortex saddle point of model with external electric field and model without external electric field were compared. The average velocity increases linearly with the electric field intensity; however, the Joule heating effects cannot be neglected when the electric field intensity increases to a certain level.

Progress on Flow Mechanism in Low Permeability Formation

Some progress concerning low permeability flow in pore structure and interaction with formation fluid, micro-scale effect, threshold pressure gradient, relative permeability curve is reviewed. Pore structure and distribution patterns have no distinct difference in different permeability rocks while pore-throats have. In low permeability formation conventional Newtonian fluid showed non-Newtonian characteristics which performed as no-Darcy flow. The existence of threshold pressure gradient was questioned and denied because of the drawbacks of experiment procedure and apparatus, as well as great success of tight oil reservoirs development without considering threshold pressure gradient. Smaller diameter of microtubule leads to higher degree of nonlinearity. Effects of solid-fluid interaction to percolation should be noticed in low permeability formation. One well-accepted model indicated that solid liquid interface potential has a positive relationship with reciprocal of permeability or pore radius, thus interface force becoming too large to neglect when permeability or pore radius decreased to a certain value. Boundary effects fluid started to flow with overcoming surface molecule force only when displacement pressure drop increased to a certain degree, which accounts for the threshold pressure gradient. Interfacial tension was the key factor effecting fluid interface shape and thus having a significant influence on percolation mechanism.

Measurement of the Thermal Environment in Temporary Settlements with High Building Density after 2008 Wenchuan Earthquake in China

After 2008 Wenchuan earthquake in China, a large amount of temporary settlements with high building density had appeared in the earthquake-stricken area. These temporary settlements all consisted of low-energy temporary buildings: prefab houses. In order to make clear the indoor and outdoor physical environments in the settlements, we measured the relevantly important parameters such as wind velocity, air temperature in Happy Homeland, a typical post-disaster temporary settlement. The field measurements’ results showed as followed: (1) the indoor thermal environment of the prefab houses in summer was always worse than that in winter, (2) the wind velocity inside settlement was much lower than that outside, and indoor ventilation was very poor, (3) a micro-scale heat island effect further deteriorated both the summer outdoor and indoor thermal environment.

微流体技术制备多级结构材料的研究进展

Hierarchically structured materials, which exhibit tailorable properties and multi-functionalization at the micro-scale, have recently attracted extensive attention. Microfluidic technique offers significant advantages such as unique micro-scale effects and versatile operation in the preparation of these materials, leading to successful fabrication of various materials with complex structure or unusual morphology. In China, many progresses have also been made in this area, some of which are extrodinory in terms of skillful design and configuration of the materials. Thus, it is quite necessary to summarize these progresses. This paper reviews the latest progresses and development in the preparation of hierarchically structured materials by microfluidic technique, with the emphasis put on those developed in China. Various synthesis methods such as interfacial polymerization, interfacial extraction, liquid-phase separation, multi-emulsions, etc., and the corresponding scientific principles behind these methods are introduced. The design of microfluidic devices and the structures of these as-synthesized hierarchical materials are also elucidated. The perspectives for future research and developments are also described.

Electro-osmotic slip flow of Eyring fluid in a slit microchannel

The electro-osmotic flow of a non-Newtonian fluid in a slit micro-channel under the Navier's slip boundary condition is investigated. The Eyring constitutive relationship model is adopted to describe the non-Newtonian characteristics of the flow driven by the applied electric field force and pressure. In consideration of the micro-scale effects, electric field, non-Newtonian behavior and slip boundary condition, a mechanical model is built and the effects of these factors on the flow are studied. Analytical expressions are derived for the electric potential and velocity profile by solving the linearized Poisson-Boltzmann equation and the modified Cauchy equation. Approximate expressions of the velocity distribution are also given and discussed. Furthermore, by comparing the effects of electric force with that of pressure on the velocity distribution, some meaningful conclusions are drawn from the obtained graphics.

Static and eigenvalue problems of Sigmoid Functionally Graded Materials (S-FGM) micro-scale plates using the modified couple stress theory

Bending, vibration and buckling analysis of Sigmoid Functionally Graded Materials (S-FGM) micro-scale plates using the modified couple stress theory is presented in this paper. The micro-scale effects of S-FGM micro-scale plates are captured using the single material length scale parameter of a modified couple stress theory. The material properties are assumed to vary only through the thickness of the S-FGM micro-scale plate, by mixing two different materials. The shear deformation effects are included, using the third-order shear deformation theory. The present theory does not require a shear correction factor, because of the parabolic distribution of the transverse shear strains through the thickness. The third-order theory that accounts for the functionally graded material, and the micro-scale effects have been derived using the principle of virtual displacements. In the present theory, the variation of two materials through thickness is embedded into the plate constitutive relation. The static and eigenvalue problems are solved by the Navier method. Numerical examples of bending, vibration and buckling problems are presented, to illustrate the effects of the sigmoid power-law distribution of the two materials, and the dimensionless material length scale parameter. In particular, in the case of biaxial compression and tension, the magnitude and direction of the buckling load should be considered simultaneously.

Effects of Viscous Dissipation and Rarefaction on Parallel Plates with Constant Heat Flux Boundary Conditions

AbstractThe effect of viscous dissipation and rarefaction on heat transfer characteristics of hydrodynamically and thermally fully developed flow between parallel plates with constant heat flux conditions is analyzed. Three different types of heat flux boundary conditions, i.e., both plates kept at different constant heat fluxes, both plates kept at equal constant heat fluxes, and one plate insulated, are applied. In all cases, closed form expressions are obtained for the temperature distribution and Nusselt number. Viscous dissipation, rarefaction, and heat flux ratio are found to influence the heat transfer substantially. The present predictions are verified for the cases which neglect the microscale and viscous heating effect. The obtained results are in good agreement with other analytical results.