A micromechanics model for molecular diffusion in materials with complex pore structure          下载免费PDF全文

Jithender J. Timothy  Günther Meschke 《国际地质力学数值与分析法杂志》2016,40(5):686-712

Molecular diffusion in fully saturated porous materials is strongly influenced by the pore space, which, in general, is characterized by a complex topological structure. Hence, information on macroscopic diffusion properties requires up‐scaling of transport processes within nano‐pores and micro‐pores over several spatial scales. A new model in the framework of continuum micromechanics is proposed for predicting the effective molecular diffusivity in porous materials. Considering a representative volume element, characterizing a porous material without any information about the pore space microstructure complexity, the uniform flux is perturbed by recursively embedding shape information hierarchically in the form of the ESHELBY matrix‐inclusion morphology to obtain the effective diffusivity as a function of the recurrence level and the porosity. The model predicts a threshold value for the porosity, below which no molecular diffusion can occur because of the presence of isolated pore clusters that are not connected and unavailable for transport. The maximum porosity, below which no molecular transport is possible, is predicted as one‐third for spherical inclusions. The model allows for extensions to more complex morphologies of the inclusions. We also identify, that the effects of the micro‐structure on molecular transport are characterized by porosity dependent long‐range and short‐range interactions. The developed framework is extended to incorporate realistic pore size distributions across several spatial scales by means of a distribution function within the hierarchical homogenization scheme. Available experimental results assert the model predictions. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

Risk management tools for sustainable fisheries management under changing climate: a sea cucumber example     

Éva E. Plagányi  Timothy D. Skewes  Natalie A. Dowling  Malcolm Haddon 《Climatic change》2013,119(1):181-197

Sustainable fisheries management into the future will require both understanding of and adaptation to climate change. A risk management approach is appropriate due to uncertainty in climate projections and the responses of target species. Management strategy evaluation (MSE) can underpin and support effective risk management. Climate change impacts are likely to differ by species and spatially. We use a spatial MSE applied to a multi-species data-poor sea cucumber/béche-de-mer fishery to demonstrate the utility of MSE to test the performance of alternative harvest strategies in meeting fishery objectives; this includes the ability to manage through climate variability and change, and meeting management objectives pertaining to resource status and fishery economic performance. The impacts of fishing relative to the impacts of climate change are distinguished by comparing future projection distributions relative to equivalent no-fishing no-climate-change trials. The 8 modelled species exhibit different responses to environmental variability and have different economic value. Status quo management would result in half the species falling below target levels, moderate risks of overall and local depletion, and significant changes in species composition. The three simple strategies with no monitoring (spatial rotation, closed areas, multi-species composition) were all successful in reducing these risks, but with fairly substantial decreases in the average profit. Higher profits (for the same risk levels) could only be achieved with strategies that included monitoring and hence adaptive management. Spatial management approaches based on adaptive feedback performed best overall.  相似文献   

Laterally‐propagating Rossby waves in the easterly acceleration phase of the quasi‐biennial oscillation     

Timothy J. Dunkerton 《大气与海洋》2013,51(1):55-68

Abstract

It is shown that oscillating mean flow solutions exist in the one‐dimensional Holton‐Lindzen (1972) model in the presence of a single Kelvin wave, mean flow diffusion, and an easterly zonal force per unit mass that is constant in height and time except at those points in the time‐height cross‐section where the latitudinally‐integrated mean flow is less than some prescribed easterly value. The latter forcing is intended to crudely represent the absorption of quasi‐stationary planetary Rossby waves at the tropical zero‐wind line. Our results suggest an alternative, and somewhat simpler, possible interpretation of the quasi‐biennial mean zonal wind oscillation in the equatorial lower stratosphere.  相似文献   

Abrupt CO2 experiments as tools for predicting and understanding CMIP5 representative concentration pathway projections     

Peter Good  Jonathan M. Gregory  Jason A. Lowe  Timothy Andrews 《Climate Dynamics》2013,40(3-4):1041-1053

A fast simple climate modelling approach is developed for predicting and helping to understand general circulation model (GCM) simulations. We show that the simple model reproduces the GCM results accurately, for global mean surface air temperature change and global-mean heat uptake projections from 9 GCMs in the fifth coupled model inter-comparison project (CMIP5). This implies that understanding gained from idealised CO₂ step experiments is applicable to policy-relevant scenario projections. Our approach is conceptually simple. It works by using the climate response to a CO₂ step change taken directly from a GCM experiment. With radiative forcing from non-CO₂ constituents obtained by adapting the Forster and Taylor method, we use our method to estimate results for CMIP5 representative concentration pathway (RCP) experiments for cases not run by the GCMs. We estimate differences between pairs of RCPs rather than RCP anomalies relative to the pre-industrial state. This gives better results because it makes greater use of available GCM projections. The GCMs exhibit differences in radiative forcing, which we incorporate in the simple model. We analyse the thus-completed ensemble of RCP projections. The ensemble mean changes between 1986–2005 and 2080–2099 for global temperature (heat uptake) are, for RCP8.5: 3.8 K (2.3 × 10²⁴ J); for RCP6.0: 2.3 K (1.6 × 10²⁴ J); for RCP4.5: 2.0 K (1.6 × 10²⁴ J); for RCP2.6: 1.1 K (1.3 × 10²⁴ J). The relative spread (standard deviation/ensemble mean) for these scenarios is around 0.2 and 0.15 for temperature and heat uptake respectively. We quantify the relative effect of mitigation action, through reduced emissions, via the time-dependent ratios (change in RCPx)/(change in RCP8.5), using changes with respect to pre-industrial conditions. We find that the effects of mitigation on global-mean temperature change and heat uptake are very similar across these different GCMs.  相似文献   

A sledge microtome for high resolution subsampling of freeze cores     

Andrew L. Macumber  R. Timothy Patterson  Lisa A. Neville  Hendrik Falck 《Journal of Paleolimnology》2011,45(2):307-310

We describe a sledge microtome designed for the high-resolution subsampling of freeze cores. This inexpensive freeze-core microtome is capable of producing precise subsamples at mm to sub-mm resolution without sediment loss and cross-contamination. Such a subsampling resolution permits recognition of sub-decadal to annual events even in systems with low sedimentation rates. The freeze-core microtome is particularly useful for obtaining high-resolution subsamples at the environmentally important sediment-water interface due to freeze corers being capable of capturing this boundary with minimal disturbance as compared to other coring methods.  相似文献   

Radiocarbon and 230Th data reveal rapid redistribution and temporal changes in sediment focussing at a North Atlantic drift     

Gesine Mollenhauer  Jerry F. McManus  Thomas Wagner  I. Nick McCave  Timothy I. Eglinton 《Earth and Planetary Science Letters》2011,301(1-2):373-381

In locations of rapid sediment accumulation receiving substantial amounts of laterally transported material the timescales of transport and accurate quantification of the transported material are at the focus of intense research. Here we present radiocarbon data obtained on co-occurring planktic foraminifera, marine haptophyte biomarkers (alkenones) and total organic carbon (TOC) coupled with excess Thorium-230 (²³⁰Th_xs) measurements on four sediment cores retrieved in 1649–2879 m water depth from two such high accumulation drift deposits in the Northeast Atlantic, Björn and Gardar Drifts. While ²³⁰Th_xs inventories imply strong sediment focussing, no age offsets are observed between planktic foraminifera and alkenones, suggesting that redistribution of sediments is rapid and occurs soon after formation of marine organic matter, or that transported material contains negligible amounts of alkenones. An isotopic mass balance calculation based on radiocarbon concentrations of co-occurring sediment components leads us to estimate that transported sediment components contain up to 12% of fossil organic matter that is free of or very poor in alkenones, but nevertheless appears to consist of a mixture of fresh and eroded fossil material. Considering all available constraints to characterize transported material, our results show that although focussing factors calculated from bulk sediment ²³⁰Th_xs inventories may allow useful approximations of bulk redeposition, they do not provide a unique estimate of the amount of each laterally transported sediment component. Furthermore, our findings provide evidence that the occurrence of lateral sediment redistribution alone does not always hinder the use of multiple proxies but that individual sediment fractions are affected to variable extents by sediment focussing.  相似文献   

Are there basic physical constraints on future anthropogenic emissions of carbon dioxide?     

Timothy J. Garrett 《Climatic change》2011,104(3-4):437-455

Global Circulation Models (GCMs) provide projections for future climate warming using a wide variety of highly sophisticated anthropogenic CO₂ emissions scenarios as input, each based on the evolution of four emissions ??drivers??: population p, standard of living g, energy productivity (or efficiency) f and energy carbonization c (IPCC WG III 2007). The range of scenarios considered is extremely broad, however, and this is a primary source of forecast uncertainty (Stott and Kettleborough, Nature 416:723?C725, 2002). Here, it is shown both theoretically and observationally how the evolution of the human system can be considered from a surprisingly simple thermodynamic perspective in which it is unnecessary to explicitly model two of the emissions drivers: population and standard of living. Specifically, the human system grows through a self-perpetuating feedback loop in which the consumption rate of primary energy resources stays tied to the historical accumulation of global economic production??or p×g??through a time-independent factor of 9.7±0.3 mW per inflation-adjusted 1990 US dollar. This important constraint, and the fact that f and c have historically varied rather slowly, points towards substantially narrowed visions of future emissions scenarios for implementation in GCMs.  相似文献   

The influence of synoptic airflow on UK daily precipitation extremes. Part I: Observed spatio-temporal relationships   总被引：1，自引：1，他引：0  

Douglas Maraun  Timothy J. Osborn  Henning W. Rust 《Climate Dynamics》2011,36(1-2):261-275

We study the influence of synoptic scale atmospheric circulation on extreme daily precipitation across the United Kingdom, using observed time series from 689 rain gauges. To this end we employ a statistical model, that uses airflow strength, direction and vorticity as predictors for the generalised extreme value distribution of monthly precipitation maxima. The inferred relationships are connected with the dominant westerly flow, the orography, and the moisture supply from surrounding seas. We aggregated the results for individual rain gauges to regional scales to investigate the temporal variability of extreme precipitation. Airflow explains a significant fraction of the variability on subannual to decadal time scales. A large fraction of the especially heavy winter precipitation during the 1980s and 1990s in north Scotland can be attributed to a prevailing positive phase of the North Atlantic Oscillation. Our statistical model can be used for statistical downscaling and to validate regional climate model output.  相似文献   

Erratum to: Segregating gas from melt: an experimental study of the Ostwald ripening of vapor bubbles in magmas     

Nicole C. Lautze  Thomas W. Sisson  Margaret T. Mangan  Timothy L. Grove 《Contributions to Mineralogy and Petrology》2011,161(2):349

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Geometric and structural river channel complexity and the prediction of urban inundation   总被引：1，自引：0，他引：1  

Timothy J. Fewtrell  Jeffrey C. Neal  Paul D. Bates  Peter J. Harrison 《水文研究》2011,25(20):3173-3186

Recent research modelling floodplain inundation processes has concentrated on issues surrounding the level of physical, topographical, and numerical solver complexity needed to represent floodplain flows adequately. However, during flooding episodes the channel typically still conveys the bulk of the flow. Despite this, the effect of channel physical processes and topographic complexity on model results has been largely unexplored. To address this, the impact of channel cross‐section geometry, channel long‐profile variability and the representation of hydraulic structures on floodplain inundation are explored using a coupled dynamic 1D‐2D hydraulic model (ESTRY‐TUFLOW) of the Carlisle floods of January 2005. These simulations are compared with those from a simplified 1D‐2D model, LISFLOOD‐FP. In this case, the simpler model is sufficient to simulate the far‐field peak flood elevations. However, comparison of channel dynamics suggests that the full shallow water approximation used by ESTRY‐TUFLOW gives a more robust performance when models calibrated on maximum floodplain water elevations are used to predict channel water levels. Examination of the response of ESTRY‐TUFLOW to variations in channel geometric complexity shows that downstream variations in the channel long profile are more important than cross‐section variability for obtaining a dataset‐independent calibration. The results show, in general, that as model physical complexity is increased, calibrated parameters become less ‘effective’, and as a consequence, the values of performance measures reduce less rapidly away from the optimum value. This means that often more physically complex models are less likely to yield different optimum parameter values when calibrated on different datasets resulting in a more robust numerical model. Lastly, the inclusion of bridge structures can simulate substantial local backwatering effects, but the variability in observed water and wrack marks is such that it is not possible to discern the effect of the bridges at this site in the post‐event observational dataset. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献