02 Fakultät Bau- und Umweltingenieurwissenschaften
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/3
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Item Open Access Diagnosis of model errors with a sliding time‐window Bayesian analysis(2022) Hsueh, Han‐Fang; Guthke, Anneli; Wöhling, Thomas; Nowak, WolfgangDeterministic hydrological models with uncertain, but inferred‐to‐be‐time‐invariant parameters typically show time‐dependent model errors. Such errors can occur if a hydrological process is active in certain time periods in nature, but is not resolved by the model or by its input. Such missing processes could become visible during calibration as time‐dependent best‐fit values of model parameters. We propose a formal time‐windowed Bayesian analysis to diagnose this type of model error, formalizing the question “In which period of the calibration time‐series does the model statistically disqualify itself as quasi‐true?” Using Bayesian model evidence (BME) as model performance metric, we determine how much the data in time windows of the calibration time‐series support or refute the model. Then, we track BME over sliding time windows to obtain a dynamic, time‐windowed BME (tBME) and search for sudden decreases that indicate an onset of model error. tBME also allows us to perform a formal, sliding likelihood‐ratio test of the model against the data. Our proposed approach is designed to detect error occurrence on various temporal scales, which is especially useful in hydrological modeling. We illustrate this by applying our proposed method to soil moisture modeling. We test tBME as model error indicator on several synthetic and real‐world test cases that we designed to vary in error sources (structure and input) and error time scales. Results prove the successful detection errors in dynamic models. Moreover, the time sequence of posterior parameter distributions helps to investigate the reasons for model error and provide guidance for model improvement.Item Open Access Hydraulically induced fracturing in heterogeneous porous media using a TPM‐phase‐field model and geostatistics(2023) Wagner, Arndt; Sonntag, Alixa; Reuschen, Sebastian; Nowak, Wolfgang; Ehlers, WolfgangHydraulically induced fracturing is widely used in practice for several exploitation techniques. The chosen macroscopic model combines a phase‐field approach to fractures with the Theory of Porous Media (TPM) to describe dynamic hydraulic fracturing processes in fully‐saturated porous materials. In this regard, the solid's state of damage shows a diffuse transition zone between the broken and unbroken domain. Rocks or soils in grown nature are generally inhomogeneous with material imperfections on the microscale, such that modelling homogeneous porous material may oversimplify the behaviour of the solid and fluid phases in the fracturing process. Therefore, material imperfections and inhomogeneities in the porous structure are considered through the definition of location‐dependent material parameters. In this contribution, a deterministic approach to account for predefined imperfection areas as well as statistical fields of geomechanical properties is proposed. Representative numerical simulations show the impact of solid skeleton heterogeneities in porous media on the fracturing characteristics, e. g. the crack path.Item Open Access Experimental methods and imaging for enzymatically induced calcite precipitation in a microfluidic cell(2021) Weinhardt, Felix; Class, Holger; Vahid Dastjerdi, Samaneh; Karadimitriou, Nikolaos; Lee, Dongwon; Steeb, HolgerEnzymatically induced calcite precipitation (EICP) in porous media can be used as an engineering option to achieve precipitation in the pore space, for example, aiming at a targeted sealing of existing flow paths. This is accomplished through a porosity and consequent permeability alteration. A major source of uncertainty in modeling EICP is in the quantitative description of permeability alteration due to precipitation. This report presents methods for investigating experimentally the time‐resolved effects of growing precipitates on porosity and permeability on the pore scale, in a poly‐di‐methyl‐siloxane microfluidic flow cell. These methods include the design and production of the microfluidic cells, the preparation and usage of the chemical solutions, the injection strategy, and the monitoring of pressure drops for given fluxes for the determination of permeability. EICP imaging methods are explained, including optical microscopy and X‐ray microcomputed tomography (XRCT), and the corresponding image processing and analysis. We present and discuss a new experimental procedure using a microfluidic cell, as well as the general perspectives for further experimental and numerical simulation studies on induced calcite precipitation. The results of this study show the enormous benefits and insights achieved by combining both light microscopy and XRCT with hydraulic measurements in microfluidic chips. This allows for a quantitative analysis of the evolution of precipitates with respect to their size and shape, while monitoring their influence on permeability. We consider this to be an improvement of the existing methods in the literature regarding the interpretation of recorded data (pressure, flux, and visualization) during pore morphology alteration.Item Open Access High‐speed fatigue testing of high‐performance concretes and parallel frequency sweep characterization(2023) Madadi, Hamid; Steeb, HolgerCycling loading of brittle materials like ultra‐high‐performance concrete (UHPC), which is often used in marine and civil structures, results in unexpected failures. When a material is subjected to cyclic loading, its mechanical properties change due to the evolution of (micro‐)fractures often denoted as damage. To better understand the effective material's properties under such kind of fatigue load and to relate the material's properties to the specific time‐dependent loading characteristics, the mechanical response of the material shall be characterized at characteristic harmonic excitations. Therefore, cyclic loading experiments are conducted to determine how the evolution of microfractures, that is, fatigue, affects the material's effective mechanical properties and after how many cycles microfractures further evolve towards meso‐ and macrofractures leading finally to a critical number of cycles to material's failure. The problem with such cyclic fatigue tests is that they are potentially “expensive” to conduct as the number of loading cycles at failure can be extremely high. Moreover, it is not possible to observe and characterize further the evolution of (micro‐)fractures within the different damage phases of the cycling experiment. Further, it is challenging to characterize the material's small‐strain stiffness evolution. In this investigation, a combination of a (high‐amplitude) high‐frequency excitation and a high‐speed fatigue testing approach is used for the high cycle fatigue experiment along with a characterization approach of the material properties using a (low‐amplitude) dynamic mechanical analysis (DMA). The test setup applies harmonic excitations for high and low amplitudes using a high‐voltage piezoelectric actuators. Furthermore, the failure modes of the material will be examined. The excitation frequency 𝑓 for the fatigue test is significantly higher than in classical low- and high-cyclic fatigue approaches, that is, 10 < 𝑓 < 200 Hz, allowing to reduce the overall time of the experimental investigation time to failure. Further, the frequency-dependent number of cycles to failure is studied. Similar to standard DMA, effective complex mechanical properties of the material in tangential space are obtained in frequencies between 0.01 and 1000 Hz; while the observed mechanical properties of these materials change with increasing frequency. In the case of materials' behavior, by increasing the frequency, Young's modulus increases and Poisson's ratio decreases. Experimental fatigue results will be presented for UHPC samples. Harmonic experimental data include (direct) strain measurements in axial and circumferential directions as well as forces in axial directions. In addition, the resulting complex Young's modulus and evolving damage‐like “history” of UHPC will be shown.Item Open Access A continuum mechanical porous media model for vertebroplasty : numerical simulations and experimental validation(2023) Trivedi, Zubin; Gehweiler, Dominic; Wychowaniec, Jacek K.; Ricken, Tim; Gueorguiev, Boyko; Wagner, Arndt; Röhrle, OliverThe outcome of vertebroplasty is hard to predict due to its dependence on complex factors like bone cement and marrow rheologies. Cement leakage could occur if the procedure is done incorrectly, potentially causing adverse complications. A reliable simulation could predict the patient-specific outcome preoperatively and avoid the risk of cement leakage. Therefore, the aim of this work was to introduce a computationally feasible and experimentally validated model for simulating vertebroplasty. The developed model is a multiphase continuum-mechanical macro-scale model based on the Theory of Porous Media. The related governing equations were discretized using a combined finite element-finite volume approach by the so-called Box discretization. Three different rheological upscaling methods were used to compare and determine the most suitable approach for this application. For validation, a benchmark experiment was set up and simulated using the model. The influence of bone marrow and parameters like permeability, porosity, etc., was investigated to study the effect of varying conditions on vertebroplasty. The presented model could realistically simulate the injection of bone cement in porous materials when used with the correct rheological upscaling models, of which the semi-analytical averaging of the viscosity gave the best results. The marrow viscosity is identified as the crucial reference to categorize bone cements as ‘high- ’or ‘low-’ viscosity in the context of vertebroplasty. It is confirmed that a cement with higher viscosity than the marrow ensures stable development of the injection and a proper cement interdigitation inside the vertebra.