Hydrodynamic modeling of ephemeral flow in the Iishana channel systems of the Cuvelai Basin—Northern Namibia

The transboundary region of the Iishana system in the western Cuvelai Basin, between southern Angola and northern Namibia, is frequently affected by floods at irregular intervals. As a result, the predominantly rural, subsistence farming population has experienced crop failures, human, and economic losses. To date, very little is known about the generation of floods, flood concentration, and stormwater drainage dynamics in this region. In this study, 2D-hydrodynamic modeling was applied to reconstruct one of the latest major flood events during the rainy season from November 2008 to March 2009 in order to study the runoff behavior and interconnectivity of the Iishana system. The model focused on the eastern part of the Iishana system, which was most affected by floods and flood damage due to the high population density in and around Oshakati, the regional capital. Two main streams were identified noteworthy because they merge and subsequently affect Oshakati. Regarding the simulated flood event water depths vary from 0.1 m to 14 m, with an average of 0.2 m, while water depths above 5 m were attributed to borrow pits. The inundation area ranged up to 1860 km² and the amount of water left after the rainy season on March 25th, 2009, was determined between 0.116 and 0.547 km³, depending on the amount of evapotranspiration considered in the model. Thus, in the Angolan part of the Iishana system, significantly larger quantities of water are available for longer periods of time during the subsequent dry season, whereas the system in Namibia stores less water, resulting in a shorter water retention period.     

Asymmetric Three-Phase Surface Tension Forces in Agglomerating Particulate Systems

In accurate simulation of surface tension dominating systems, such as particulate systems in the nano- and mesoscale, the computation of interface curvature is essential. In a previous work a formulation using discrete differential geometry was demonstrated that could be used to find exact equilibrium solutions of surfaces. However, it was also shown that this formulation, as well all other tested formulations from those found in literature, fail to find accurate point-wise estimates for curvatures for particle-particle capillary liquid bridges. In this communication we elaborate on how total curvatures can be used for computing forces of parameterized models as well as in direct numerical simulations and demonstrate the infeasibility of using point-wise estimates on any asymmetric discretization. Near-exact numerical solutions are demonstrated for asymmetrically triangulated Catenoids and a minimal surface suitable for modeling liquid bridges between particles of different diameters.     

Laser-based sample preparation for high-resolution X-ray-computed tomography of glasses and glass ceramics

X-ray-computed tomography with sub-micron resolution (nano-CT) is one of the most useful techniques to examine the 3D microstructure of materials down to voxel sizes 10 nm. However, since size and shape of samples have considerable influence on acquisition time and data quality, adapted and universally applicable workflows are needed. Three novel workflows for sample preparation using ultra-short pulsed lasers are presented which allow for reproducible fabrication, safe extraction and mounting of samples. Their application potential is illustrated via nano-CT measurements of glass ceramics as well as a laser-modified glass. Since the according sample geometries take also the requirements of other analytical techniques such as transmission electron microscopy into account, samples prepared according to the new workflows can be furthermore seen as a starting point for correlative microstructural analyses involving multiple techniques.     

Micromechanics of ITZ–Aggregate Interaction in Concrete Part I: Stress Concentration

At the macroscopic scale, concrete appears as a composite made of a cement paste matrix with embedded aggregates. The latter are covered by interfacial transition zones (ITZs) of reduced stiffness and strength. Cracking in the ITZs is probably the key to the nonlinear stress–strain behavior in the prepeak regime. For a deeper understanding of this effect triggered by tensile microstress peaks, we here employ and extend the framework of continuum micromechanics, as to develop analytical solutions relating the macroscopic stresses acting on a piece of concrete, to microtractions at the aggregates' surfaces and to three‐dimensional stress states within the ITZs. In the latter context, a new aggregate‐to‐ITZ stress concentration tensor is derived based on the separation‐of‐scale principle, which implies that ITZs may be modeled as two‐dimensional interfaces at the concrete scale, but as three‐dimensional bulk phases at the scale of a few micrometers. Microtensile peaks occur both under uniaxial macroscopic tension and compression. To describe the respective microtraction and microstress fields, it is suitable to define aggregate's “poles” and “equator” by an “axis” through the aggregate center, directed in the uniaxial macroscopic loading direction. Accordingly, tensile microtraction peaks, induced by macro‐tension and macro‐compression, respectively, occur at the “poles” and at the “equator”, respectively. The largest tensile ITZ‐microstresses occur at an offset of about π/8 from the “poles” and the “equator”, respectively. These fields of microtractions and ITZ microstresses are prerequisites for upscaling ITZ‐related strength to the macroscopic concrete level, as presented in the companion paper (Part II).