Controls that expedite first-passage times in disordered systems.

First-passage time statistics in disordered systems exhibiting scale invariance are studied widely. In particular, long trapping times in energy or entropic traps are fat-tailed distributed, which slow the overall transport process. We study the statistical properties of the first-passage time of biased processes in different models, and we employ the big-jump principle that shows the dominance of the maximum trapping time on the first-passage time. We demonstrate that the removal of this maximum significantly expedites transport. As the disorder increases, the system enters a phase where the removal shows a dramatic effect. Our results show how we may speed up transport in strongly disordered systems exploiting scale invariance. In contrast to the disordered systems studied here, the removal principle has essentially no effect in homogeneous systems; this indicates that improving the conductance of a poorly conducting system is, theoretically, relatively easy as compared to a homogeneous system.

Global warming accelerates soil heterotrophic respiration.

Carbon efflux from soils is the largest terrestrial carbon source to the atmosphere, yet it is still one of the most uncertain fluxes in the Earth's carbon budget. A dominant component of this flux is heterotrophic respiration, influenced by several environmental factors, most notably soil temperature and moisture. Here, we develop a mechanistic model from micro to global scale to explore how changes in soil water content and temperature affect soil heterotrophic respiration. Simulations, laboratory measurements, and field observations validate the new approach. Estimates from the model show that heterotrophic respiration has been increasing since the 1980s at a rate of about 2% per decade globally. Using future projections of surface temperature and soil moisture, the model predicts a global increase of about 40% in heterotrophic respiration by the end of the century under the worst-case emission scenario, where the Arctic region is expected to experience a more than two-fold increase, driven primarily by declining soil moisture rather than temperature increase.

Failure of ureteral stents subject to extrinsic ureteral obstruction and stent occlusions.

PURPOSE: To quantify the occurrence of stent failure and the dynamic behavior of urine flow in ureter-stent systems, including the relative flow in the ureter and stent lumina, subject to various degrees of ureter and stent blockage. METHODS: Numerical simulations based on computational fluid dynamics (CFD) were used to quantify urine flow behavior in stented ureters, in the presence of extrinsic ureteral obstruction (EUO) and stent occlusions. Two stented ureter configurations were considered, one with circumferential occlusion of the ureter and the second with pressure on one side of the ureter wall. The pressure within the renal unit for different degrees of ureter closure and stent lumen occlusion was determined systematically. Onset of stent failure and the distribution of urine flow between stent and ureter lumina were determined. RESULTS: In the case of EUO completely encircling the ureter, causing 100% obstruction of the ureter lumen, pressure in the renal unit is essentially unaffected until the stent lumen reaches ~ 90% occlusion, and fails only with > 95% occlusion. Occlusions of 50% in stent side holes in the vicinity of the EUO only alter local flow patterns but have no significant influence on renal unit pressure. For EUO deforming and compressing the ureter from one side, with ~ 50% reduction in ureter lumen, urine drainage proceeds with negligible increase in renal pressure even with 100% occlusion in the stent lumen. CONCLUSION: CFD simulations show that stent failure under EUO tends to occur suddenly, only when both ureter and stent lumina become almost fully blocked.

Anomalous transport dependence on Péclet number, porous medium heterogeneity, and a temporally varying velocity field.

We investigate the effects of the Péclet number (Pe) on transport of an inert chemical tracer in heterogeneous porous media. We simulate fluid flow and transport through two-dimensional pore-scale matrices with varying structural complexity. With increasing Pe, the anomalous nature of the transport becomes enhanced as the host domain becomes more heterogeneous, due to the increasingly dominant effects of the complex velocity field. The sensitivity of (anomalous) transport to Pe is shown to be controlled by the medium structure. We quantify the effects of Pe by interpreting the numerical simulations within the continuous time random walk method (CTRW) framework, and incorporating Pe within the underlying tracer transition time distribution. We then investigate transport behavior subjected to temporal variation in the velocity field magnitude, accounting for tracer propagation controlled by Pe. Because of the nonlinear influence of Pe on the transport behavior, we show that temporal variations in the velocity field can lead to an increase in the anomalous nature of the transport.

Inertial Effects on Flow and Transport in Heterogeneous Porous Media.

We investigate the effects of high fluid velocities on flow and tracer transport in heterogeneous porous media. We simulate fluid flow and advective transport through two-dimensional pore-scale matrices with varying structural complexity. As the Reynolds number increases, the flow regime transitions from linear to nonlinear; this behavior is controlled by the medium structure, where higher complexity amplifies inertial effects. The result is, nonintuitively, increased homogenization of the flow field, which leads in the context of conservative chemical transport to less anomalous behavior. We quantify the transport patterns via a continuous time random walk, using the spatial distribution of the kinetic energy within the fluid as a characteristic measure.