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  • 1
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    Non-invasive detection and localization of microplastic particles in a sandy sediment by complementary neutron and X-ray tomography (2021)
    Springer Berlin Heidelberg
    Details
    Publication Date: 2024-01-12
    Description: Purpose: Microplastics have become a ubiquitous pollutant in marine, terrestrial and freshwater systems that seriously affects aquatic and terrestrial ecosystems. Common methods for analysing microplastic abundance in soil or sediments are based on destructive sampling or involve destructive sample processing. Thus, substantial information about local distribution of microplastics is inevitably lost. Methods: Tomographic methods have been explored in our study as they can help to overcome this limitation because they allow the analysis of the sample structure while maintaining its integrity. However, this capability has not yet been exploited for detection of environmental microplastics. We present a bimodal 3D imaging approach capable to detect microplastics in soil or sediment cores non-destructively. Results: In a first pilot study, we demonstrate the unique potential of neutrons to sense and localize microplastic particles in sandy sediment. The complementary application of X-rays allows mineral grains to be discriminated from microplastic particles. Additionally, it yields detailed information on the 3D surroundings of each microplastic particle, which supports its size and shape determination. Conclusion: The procedure we developed is able to identify microplastic particles with diameters of approximately 1 mm in a sandy soil. It also allows characterisation of the shape of the microplastic particles as well as the microstructure of the soil and sediment sample as depositional background information. Transferring this approach to environmental samples presents the opportunity to gain insights of the exact distribution of microplastics as well as their past deposition, deterioration and translocation processes.
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Description: Deutsche Forschungsgemeinschaft (DE)
    Keywords: ddc:550.724 ; Neutron imaging ; Sediment core ; Non-destructive analysis ; Microplastic detection ; Shape and size
    Language: English
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  • 2
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    AVISA: anisotropic visco-ISA model and its performance at cyclic loading (2020)
    Springer Berlin Heidelberg
    Details
    Publication Date: 2023-06-23
    Description: In this work, a constitutive model able to capture the strain rate dependency, small strain effects and the inherent anisotropy is proposed considering the influence of the overconsolidation ratio (OCR). Small strain effects are captured by using an extended ISA plasticity formulation (Fuentes and Triantafyllidis in Int J Numer Anal Methods Geomech 39(11):1235–1254, 2015). The strain rate dependency is reproduced by incorporating a third strain rate mechanism (in addition to the elastic and hypoplastic strain rate). A loading surface has been incorporated to define a three-dimensional (3D) overconsolidation ratio and to account for its effects on the simulations. Experimental investigations using Kaolin Clay and Lower Rhine Clay with horizontal bedding plane have shown that under undrained cycles of small strain amplitudes (\10-4 ), the effective stress path in the p–q space is significantly inclined towards the left upper corner of the p - q plane. Consequently, a transversely (hypo)elastic stiffness has been successfully formulated to capture this behaviour. The performance of the model has been inspected by simulating the database of approximately 50 cyclic undrained triaxial (CUT) tests on low-plasticity Kaolin Clay (Wichtmann and Triantafyllidis) considering different deviatoric stress amplitudes, initial stress ratios, displacement rate, overconsolidation ratio and cutting direction. Furthermore, 4 CUT tests conducted on high-plasticity Lower Rhine Clay were simulated, whereby the influence of the displacement rate, as well as the deviatoric stress amplitude, has been analysed. The simulations showed a good congruence with the experimental observations.
    Keywords: ddc:550.724 ; Anisotropy ; Clay ; Cyclic loading ; Excessive pore water pressure ; Rate dependency ; Soft soils ; Silt ; Time dependency ; Viscosity
    Language: English
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  • 3
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    Investigations into the opening of fractures during hydraulic testing using a hybrid-dimensional flow formulation (2021)
    Springer Berlin Heidelberg
    Details
    Publication Date: 2023-12-19
    Description: We applied a hybrid-dimensional flow model to pressure transients recorded during pumping experiments conducted at the Reiche Zeche underground research laboratory to study the opening behavior of fractures due to fluid injection. Two distinct types of pressure responses to flow-rate steps were identified that represent radial-symmetric and plane-axisymmetric flow regimes from a conventional pressure-diffusion perspective. We numerically modeled both using a radial-symmetric flow formulation for a fracture that comprises a non-linear constitutive relation for the contact mechanics governing reversible fracture surface interaction. The two types of pressure response can be modeled equally well. A sensitivity study revealed a positive correlation between fracture length and normal fracture stiffness that yield a match between field observations and numerical results. Decomposition of the acting normal stresses into stresses associated with the deformation state of the global fracture geometry and with the local contacts indicates that geometrically induced stresses contribute the more the lower the total effective normal stress and the shorter the fracture. Separating the contributions of the local contact mechanics and the overall fracture geometry to fracture normal stiffness indicates that the geometrical stiffness constitutes a lower bound for total stiffness; its relevance increases with decreasing fracture length. Our study demonstrates that non-linear hydro-mechanical coupling can lead to vastly different hydraulic responses and thus provides an alternative to conventional pressure-diffusion analysis that requires changes in flow regime to cover the full range of observations.
    Description: Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347
    Description: Bundesministerium für Bildung und Forschung (DE)
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Description: Universität Stuttgart (1023)
    Keywords: ddc:550.724 ; Hydro-mechanics of fractures ; Hybrid-dimensional modeling ; Fracture contact mechanics ; Fracture stiffness ; Hydraulic testing of fractures ; Reiche Zeche underground research laboratory
    Language: English
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  • 4
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    Implications of the principle of effective stress (2020)
    Springer Berlin Heidelberg
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    Publication Date: 2023-06-23
    Description: While Terzaghi justified his principle of effective stress for water-saturated soil empirically, it can be derived by means of the neutrality of the mineral with respect to changes of the pore water pressure p w. This principle works also with dilating shear bands arising beyond critical points of saturated grain fabrics, and with patterns of shear bands as relics of critical phenomena. The shear strength of over-consolidated clay is explained without effective cohesion, which results also from swelling up to decay, while rapid shearing of water-saturated clay can lead to a cavitation of pore water. The p w-neutrality is also confirmed by triaxial tests with sandstone samples, while Biot’s relation with a reduction factor for p w is contestable. An effective stress tensor is heuristically legitimate also for soil and rock with relics of critical phenomena, particularly for critical points with a Mohr–Coulomb condition. Therein, the p w-neutrality of the solid mineral determines the interaction of solid fabric and pore water, but numerical models are questionable due to fractal features.
    Description: Karlsruher Institut für Technologie (KIT) (4220)
    Keywords: ddc:550.724 ; Effective stress ; Interaction of solid fabric and pore water ; Pore pressure neutrality of mineral ; Shear bands and cracks
    Language: English
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  • 5
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    Accumulation of germanium (Ge) in plant tissues of grasses is not solely driven by its incorporation in phytoliths (2020)
    Springer International Publishing
    Details
    Publication Date: 2023-06-19
    Description: Ge/Si ratios of plant phytoliths have been widely used to trace biogeochemical cycling of Si. However, until recently, information on how much of the Ge and Si transferred from soil to plants is actually stored in phytoliths was lacking. The aim of the present study is to (i) compare the uptake of Si and Ge in three grass species, (ii) localize Ge and Si stored in above-ground plant parts and (iii) evaluate the amounts of Ge and Si sequestrated in phytoliths and plant tissues. Mays (Zea mays), oat (Avena sativa) and reed canary grass (Phalaris arundinacea) were cultivated in the greenhouse on soil and sand to control element supply. Leaf phytoliths were extracted by dry ashing. Total elemental composition of leaves, phytoliths, stems and roots were measured by ICP-MS. For the localization of phytoliths and the determination of Ge and Si within leaf tissues and phytoliths scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX) and laser ablation inductively coupled mass spectrometry (LA-ICP-MS) was used. The amounts of Si and Ge taken up by the species corresponded with biomass formation and decreased in the order Z. mays 〉 P. arundinacea, A. sativa. Results from LA-ICP-MS revealed that Si was mostly localized in phytoliths, while Ge was disorderly distributed within the leaf tissue. In fact, from the total amounts of Ge accumulated in leaves only 10% was present in phytoliths highlighting the role of organic matter on biogeochemical cycling of Ge and the necessity for using bulk Ge/Si instead of Ge/Si in phytoliths to trace biogeochemical cycling of Si.
    Keywords: ddc:550.724 ; Germanium ; Ge/Si ratio ; Phytoliths ; Poaceae ; LA-ICP-MS
    Language: English
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  • 6
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    Evolution and Features of Dust Devil‐Like Vortices in Turbulent Rayleigh‐Bénard Convection—An Experimental Study (2023)
    Details
    Publication Date: 2024-02-05
    Description: We present an experimental study simulating atmospheric dust devils in a controlled laboratory experiment. The experimental facility, called the “Barrel of Ilmenau” (www.ilmenauer-fass.de) represents a classical Rayleigh‐Bénard set‐up and is believed to model the phenomena in a convective atmospheric boundary layer fairly well. Our work complements and extends the numerical work of Giersch and Raasch (2021)https//doi.org/10.1029/2020jd034334 by experiments. Dust devils are thermal convective vortices with a vertical axis of rotation visualized by entrained soil particles. They evolve in the convective atmospheric boundary layer and are believed to substantially contribute to the aerosol transport into the atmosphere. Thus, their evolution, size, lifetime, and frequency of occurrence are of particular research interest. Extensive experimental studies have been conducted by field measurements and laboratory experiments so far. Beyond that, our study is the first attempt of Rayleigh‐Bénard convection (RBC) in air to investigate dust devil‐like vortices in a laboratory experiment. Up to now, this set‐up mimics the natural process of dust devil evolution as closest to reality. The flow measurement was carried out by particle tracking velocimetry using neutrally buoyant soap bubbles. We initially identified dust devil‐like vortices by eye from the Lagrangian velocity field, and in a later, more sophisticated analysis by a specific algorithm from the corresponding Eulerian velocity field. We analyzed their frequency of occurrence, observation time, and size. With our work, we could demonstrate that turbulent RBC is an appropriate model to mimic the natural process of the evolution of dust devils in the convective atmospheric boundary layer without artificial stimulation.
    Description: Plain Language Summary: We could experimentally demonstrate that dust devil‐like vortices spontaneously arise in turbulent Rayleigh‐Bénard convection. This first‐time experimental survey simulates the evolution of dust devil‐like vortices in a laboratory experiment which mimics the convective atmospheric boundary layer quite closely and gets by without any artificial input of rotation. Dust devil‐like vortices are measured and identified using the particle tracking velocimetry technique. Within an observation period of 2 hr, 56 dust devil‐like vortices could be detected in total. Their properties coincide quite well with those structures identified in very recent direct numerical simulations (DNS) by Giersch and Raasch (2021, https//doi.org/10.1029/2020jd034334). As well, they show similarity to atmospheric dust devils. The size of our experimentally generated dust devil‐like vortices starts at about 1 dm and ranges up to about 1 m. This is larger than in DNS, but still smaller than in the atmosphere or in large eddy simulation.
    Description: Key Points: Dust devil‐like vortices spontaneously evolve in turbulent Rayleigh‐Bénard convection at sufficiently high Rayleigh numbers Ra 〉 1010. We studied their properties in a large‐scale Rayleigh‐Bénard experiment using Lagrangian particle tracking velocimetry. The vortical structures in the laboratory experiment are weaker than atmospheric dust devils, but they exhibit similar features.
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Description: https://ftp.tu-ilmenau.de/hpc-private/mb/Dust_Devil_Scripts.rar
    Description: https://ftp.tu-ilmenau.de/hpc-private/mb/Kaestner_et_al_2022_dust_devil-like_vortex_in_turbulent_Rayleigh_Benard_convection.avi
    Keywords: ddc:550.724 ; dust devils ; particle tracking velocimetry ; Rayleigh‐Bénard convection ; turbulence ; atmospheric boundary layer
    Language: English
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  • 7
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    Fine‐Sediment Erosion and Sediment‐Ribbon Morphodynamics in Coarse‐Grained Immobile Beds (2022)
    Details
    Publication Date: 2023-10-26
    Description: In rivers, fine sediments are often transported over immobile coarse grains. With low sediment supply, they tend to aggregate in longitudinal ribbons. Yet, the long‐term evolution of such ribbons and the influence of immobile grains on the erosion of fine sediments are still not well understood. Flume experiments without sediment supply were therefore performed to investigate the erosion of an initially uniform fine‐sediment bed covering an immobile bed of staggered spheres through topographic and flow measurements. The topographic measurements yielded the spheres' protrusion above the fine sediment (P) and revealed long‐lived ribbons with ridges and troughs. The ridges are the main long‐term sediment source as the troughs are quickly eroded to a stable bed level resulting from the spheres' sheltering. The ridges stabilize with a spacing of 1.3 effective water depths, their number resulting from the integer number of wavelengths fitting into the effective channel width which excludes side‐wall accumulations. The ridges' erosion is damped by the local upflow of secondary current cells, which displaces the strongest sweep events above the bed. The upflow intensity is controlled by the ridges' height for low P, while for high P by the lateral roughness heterogeneity. The trends in erosion rates over ridges and troughs are similar and characterized by the following sequence of four regimes with increasing P: a drag sheltering, a turbulence‐enhancement, a wake‐interference sheltering, and a skimming‐flow sheltering regime. The critical P levels at the transitions are independent of the flow above the canopy, depending only on the geometrical configuration of the immobile bed.
    Description: Key Points: Four erosive regimes are identified: drag sheltering, turbulence‐enhancement, wake‐interference sheltering and skimming‐flow sheltering. Secondary currents influence the momentum redistribution but the erosive behavior is controlled by the protrusion of the immobile grains. The sediment‐ribbon spacing is about 1.3 effective water depths.
    Description: Landesgraduiertenförderung of the Land Baden‐Württemberg
    Description: Graduate School for Climate and Environment, KIT
    Description: https://doi.org/10.5281/zenodo.5787371
    Keywords: ddc:550.724 ; sediment erosion ; rough beds ; sediment ribbons ; secondary currents ; open‐channel ; ridge morphology
    Language: English
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  • 8
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    Quantification of Impact‐Induced Melt Production in Numerical Modeling Revisited (2022)
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    Publication Date: 2023-12-16
    Description: Melting and vaporization of rocks in impact cratering is mostly attributed to be a consequence of shock compression. However, other mechanism such as plastic work and decompression by structural uplift also contribute to melt production. In this study we expand the commonly used method to determine shock‐induced melting in numerical models from the peak shock pressure by a new approach to account for additional heating due plastic work and internal friction. We compare our new approach with the straight‐forward method to simply quantify melting from the temperature relative to the solidus temperature at any arbitrary point in time in the course of crater formation. This much simpler method does account for plastic work but suffers from reduced accuracy due to numerical diffusion inherent to ongoing advection in impact crater formation models. We demonstrate that our new approach is more accurate than previous methods in particular for quantitative determination of impact melt distribution in final crater structures. In addition, we assess the contribution of plastic work to the overall melt volume and find, that melting is dominated by plastic work for impacts at velocities smaller than 7.5–12.5 km/s in rocks, depending on the material strength. At higher impact velocities shock compression is the dominating mechanism for melting. Here, the conventional peak shock pressure method provides similar results compared with our new model. Our method serves as a powerful tool to accurately determine impact‐induced heating in particular at relatively low‐velocity impacts.
    Description: Plain Language Summary: During the collision of cosmic bodies such as planets and asteroids on various scales, the involved material is heated such that melting or vaporization can occur. The vast amount of heat is considered to be generated during shock compression, however recent studies found that plastic deformation during decompression also contribute to the heating process. In this study, we introduce a new approach to quantify impact‐induced melting more accurately under consideration of the latter heating mechanisms. We demonstrate that our approach is more accurate than previous attempts and quantify the contribution from plastic work on impact‐induced melting. We systematically study the effect of impact velocity and material strength on melt production and find, that it is dominated by plastic work for impact velocities up to 7.5–12.5 km/s in rocks, depending on the material strength.
    Description: Key Points: We propose an improved method to quantify impact‐induced melt production for rocks. We quantify impact‐induced melt production and separate between heating due to shock compression and plastic work. Melting due to frictional heating (plastic work) dominates over shock melting for impact velocities below 7–13 km/s depending on strength.
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Description: Japan Society for the Promotion of Science London http://dx.doi.org/10.13039/501100000646
    Description: http://www.isale-code.de/redmine/projects/isale/wiki/Terms_of_use
    Description: https://doi.org/10.35003/HVTJQD
    Keywords: ddc:550.724 ; impact heating ; numerical modeling ; impact melt ; melt quantification
    Language: English
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  • 9
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    A robust potential-based contact force solution approach for discontinuous deformation analysis of irregular convex polygonal block/particle systems (2020)
    Springer Berlin Heidelberg
    Details
    Publication Date: 2023-06-08
    Description: Contact interaction of two bodies can be modeled using the penalty function approach while its accuracy and robustness are directly associated with the geometry of contact bodies. Particularly, in the research fields of rock mechanics, we need to treat polygonal shapes such as mineral grains/particles at a mesoscale and rock blocks at a macroscale. The irregular shapes (e.g., polygons with small angles or small edges) pose challenges to traditional contact solution approach in terms of algorithmic robustness and complexity. This paper proposed a robust potential-based penalty function approach to solve contact of polygonal particles/block. An improved potential function is proposed considering irregular polygonal shapes. A contact detection procedure based on the entrance block concept is presented, followed by a numerical integral algorithm to compute the contact force. The proposed contact detection approach is implemented into discontinuous deformation analysis with an explicit formulation. The accuracy and robustness of the proposed contact detection approach are verified by benchmarking examples. The potential of the proposed approach in analysis of kinetic behavior of complex polygonal block systems is shown by two application examples. It can be applied in any discontinuous computation models using stepwise contact force-based solution procedures.
    Description: Alexander von Humboldt Foundation
    Description: National Natural Science Foundation of China http://dx.doi.org/10.13039/501100001809
    Keywords: ddc:550.724 ; Block system ; Explicit discontinuous deformation analysis ; Irregular polygon ; Penalty function method ; Potential contact force
    Language: English
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  • 10
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    An automated testing device for continuous measurement of the hysteretic water retention curve of granular media (2020)
    Springer Berlin Heidelberg
    Details
    Publication Date: 2023-06-08
    Description: The water retention curve (WRC), representing an important key for the modelling of hydro-mechanical behaviour of unsaturated soils, is still not fully understood, because it originates from microscopic hydraulic and capillary phenomena. Furthermore, its experimental measurement, especially for cyclic drainage and imbibition paths, is challenging and time-consuming. In this contribution, a recently developed low-cost easy-to-use miniature testing device for the investigation of the WRC of unsaturated granular soils, such as coarse-grained sand and a packing of glass beads, is presented. With the help of the new device, that can be controlled by a Raspberry Pi single-board computer, the hysteretic WRC can be investigated in a conventional macroscopic approach by plotting the macroscopic specimen degree of saturation versus measured matric suction. The test set-up allows an automatic measurement of the WRC which is measured continuously following a programmed test procedure. In addition to the technical realisation of the new device, this contribution focuses on macroscopic results of water retention tests. Moreover, the testing device has been designed in a miniaturised size, in order to obtain microscopic insights into the phase distribution during cyclic drainage and imbibition paths with the help of computed tomography in future applications.
    Description: German Research Foundation (Deutsche Forschungsgemeinschaft, DFG)
    Keywords: ddc:550.724 ; Single-board computers ; Suction measurement ; Unsaturated granular soils ; Water retention behaviour ; X-ray computed tomography
    Language: English
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