On the capillary stress tensor in wet granular materials

This paper presents a micromechanical study of unsaturated granular media in the pendular regime, based upon numerical experiments using the discrete element method, compared to a microstructural elastoplastic model. Water effects are taken into account by adding capillary menisci at contacts and their consequences in terms of force and water volume are studied. Simulations of triaxial compression tests are used to investigate both macro and micro-effects of a partial saturation. The results provided by the two methods appear to be in good agreement, reproducing the major trends of a partially saturated granular assembly, such as the increase in the shear strength and the hardening with suction. Moreover, a capillary stress tensor is exhibited from capillary forces by using homogenisation techniques. Both macroscopic and microscopic considerations emphasize an induced anisotropy of the capillary stress tensor in relation with the pore fluid distribution inside the material. In so far as the tensorial nature of this fluid fabric implies shear effects on the solid phase associated with suction, a comparison has been made with the standard equivalent pore pressure assumption. It is shown that water effects induce microstrural phenomena that cannot be considered at the macro level, particularly when dealing with material history. Thus, the study points out that unsaturated soil stress definitions should include, besides the macroscopic stresses such as the total stress, the microscopic interparticle stresses such as the ones resulting from capillary forces, in order to interpret more precisely the implications of the pore fluid on the mechanical behaviour of granular materials.Comment: 39 page

Flow-injection of branched polymers inside nanopores

Flexible chains (linear or branched) can be forced to enter into a narrow capillary by using a hydrodynamic flow. Here, we correct our earlier description of this problem by considering the progressive nature of the suction process. We find that the critical current for penetration, $J\_c$, is controlled by the entry of a single blob of the capillary size, and that its scaling structure is the same for branched and linear chains.Comment: Submitted to Europhysics Letter

Capillary barrier effect on the response of residual slope to rainfall infiltration

Rainfall-induced slope failure is a common problem in areas covered by residual soil in tropical countries. The soil exists in unsaturated condition as ground water table is located well below the ground surface. Rainfall infiltration results in a reduction of matric suction of soil which in turn reduces the soil shear strength, and subsequently triggers the slope failure. Natural formation of the residual soil has lead to the variation of hydraulic conductivity in which soil closer to the ground surface usually has lower permeability as compared to the deeper layer. This condition causes the development of capillary barrier effect at the interface. The water accumulates at the interface and flow for some distance down-slope before it manages to infiltrate into the deeper layer. The distance that the water has to travel before breakthrough is referred as the water diversion length. Numerical simulation using SEEP/W was performed in this study to determine the water diversion length for two cases representing natural slopes i.e. Silty SAND over SAND and Silty SAND over Highly Weathered Granite. Parametric study was performed to study the effect of several variables including hydraulic conductivity of soil, thickness of layers, slope dip angle and the rate of infiltration. Results show that the diversion length is linearly correlated with the difference in the permeability of two soil layers and slope dip angle. The effect of rainfall infiltration depends on the saturated permeability of the upper layer (MRL). The optimum thickness of MRL obtained in this study is 1.5m. Results of numerical analysis are compared with analytical method by Ross model, however good agreement between the two methods was not reached because the difference in saturated hydraulic conductivity of the soils used is not very big. Moreover, under an infiltration rate, the maximum suction existing in the CBL should be as low as possible while the maximum suction attained in the MRL should be as high as possible, which was not the case especially for Silty SAND over Highly Weathered Granite

Development, Validation, and Application of a Refrigerator Simulation Model

This report describes the further development and validation of the Refrigerator/Freezer Simulation (RFSIM) model. The reports also describes the first major application of the model as an analysis tool for new refrigerator designs; several aspects of multi-speed compressor operation were examined with the model. Several improvements were made to the model that facilitated the validation process and the examination of multi-speed compressors: the model was made more general so that it could operate in numerous configurations in addition to the original design and simulation modes; many improvements were made in the modeling logic and robustness of the capillary tube-suction line heat exchanger model; and the equation-of-statebased property routines that calculated the thermodynamic properties were replaced with interpolation routines that were much faster. The RFSIM model, in design and simulation mode, was validated with data from two refrigerators. In both modes, the average model errors were less than ??5% for several important variables such as evaporator capacity and coefficient of performance. The errors of the simulation mode were reduced from the previous model validation primarily by using a different void fraction correlation in the refrigerant charge equations. The results from the validated RFSIM model indicate that a two-speed compressor could yield energy savings of 4% to 14% due to the increased steady-state efficiency at the low speed and an additional 0.5 to 4% savings due to the decreased cycling frequency. The results also showed that the capillary tube-suction line heat exchanger, when designed for the low speed, did not adversely affect the pull-down capacity when the compressor operated at the high speed. Lastly, it was found that a refrigerator operating at low ambient temperatures could actually benefit from a decrease in the condenser fan speed. This change in fan speed increased the evaporator capacity by reallocating charge to the evaporator and subsequently reducing the superheat at the evaporator exit.Air Conditioning and Refrigeration Project 6

A Bio-Wicking System to Mitigate Capillary Water in Base Course

Water within pavement layers is the major cause of pavement deteriorations. High water content results in significant reduction in soil’s resilient behavior and increase in permanent deformation. Conventional drainage systems can only drain gravity water but not capillary water. Both preliminary lab and field tests have proven the drainage efficiency of a newly developed H2Ri geotextile with wicking fabrics. This bio-wicking system aims at resolving the potential issues that the original design may encounter: (1) H2Ri ultraviolet degradation, (2) H2Ri mechanical failure, (3) loss of drainage function under high suction, and (4) clogging and salt concentration. Both elemental level and full-scale test results indicated that the bio-wicking system is more effective in draining capillary water within the base courses compared with original design, in which the geotextile is directly exposed to the open air. However, a good drainage condition is required for the bio-wicking system to maintain its drainage efficiency. Accumulation of excess water will result in water re-entering the road embankment. Moreover, grass root and geotextile share the same working mechanism in transporting water. In the proposed bio-wicking system, the relatively smaller channels in the grass roots further ensures water moving from H2Ri geotextile, transporting through the stems of grass, and eventually evapo-transpiring into the air at the leaf-air interfaces. In sum, the bio-wicking system seemed to successfully address the concerns in the preliminary design and is a more efficient system to dehydrate the road embankment under unsaturated conditions.TenCate Geosynthetic