Publications

Abstract

To maximize the usefulness of groundwater flow models for the protection of aquifers and abstraction wells, it is necessary to identify and decrease the uncertainty associated with the major parameters such as permeability. To do this, there is a need to develop set of estimates representing subsurface heterogeneity or representative soil permeability estimates. Here, we use a coupled Random Field and extended Theory of Porous Media (eTPM) simulation to develop a robust model with a good predictive ability that reduces uncertainty. The coupled model is then validated with a physical sandbox experiment. Uncertainty is reduced by using 500 realisations of the permeability parameter using the eTPM approach. A multi-layer contaminant transport scenario with varying permeabilities, similar to what could be expected with shallow alluvial sediments, is simulated. The results show that the contaminant arrival time could be strongly affected by random field realizations of permeability compared with a modelled homogenous permeability parameter. The breakthrough time for heterogeneous permeabilities is shorter than the homogeneous condition. Using the 75% confidence interval (CI), the average contaminant concentration shows 4.4% variation from the average values of the considered area and 8.9% variation in the case of a 95% confidence interval.

Abstract

Spasticity, following the neurological disorder of cerebral palsy (CP), describes a pathological condition, the central feature of which is involuntary and prolonged muscle contraction. The persistent resistance of spastic muscles to stretching is often followed by structural and mechanical changes in musculature. This leads to functional limitations at the respective joint. Focal injection of botulinum toxin type-A (BTX-A) is effectively used to manage spasticity and improve the quality of life of the patients. By blocking acetylcholine release at the neuromuscular junction and causing temporary muscle paralysis, BTX-A aims to reduce spasticity and hereby improve joint function. However, recent studies have indicated some contradictory effects such as increased muscle stiffness or a narrower range of active force production. The potential of these toxin- and atrophy-related alterations in worsening the condition of spastic muscles that are already subjected to changes should be further investigated and quantified. By focusing on the effects of BTX-A on muscle biomechanics and overall function in children with CP, this review deals with which of these goals have been achieved and to what extent, and what can await us in the future.

Abstract

Abstract The present study applies two different machine learning (ML) algorithms to predict the stress-strain mapping for the non-linear behaviour of thermoplastic materials: a Long Short-Term Memory (LSTM) algorithm and a Feed-Forward Neural Network (FFNN). The approach of this work requires the generation of the stress-strain curve for specific material parameters. The training data are obtained from the von Mises material law and the Ramberg-Osgood equation. The four combinations of ML algorithms with constitutive laws are evaluated and show a good agreement with numerical data.

Abstract

Botulinum toxin type-A (BTX-A) is commonly used for spasticity management aiming at reducing joint stiffness and increasing joint range of motion in CP patients. However, previous animal studies showed acutely increased passive forces and a narrowerlength range of active force exertion (lrange) for muscles exposed. BTX-A can spread affecting mechanics of several muscles in a compartment, but it was shown acutely to diminishepimuscular myofascial force transmission (EMFT). Yet, our understanding of these effects in the long-term is limited and they need to be tested in a bi-articular muscle. The goal was to test the following hypotheses in a long-term rat model: exposure to BTX-A (i) has no effects onlrangeand passive forces of bi-articular extensor digitorum longus (EDL) muscle and (ii) diminishes EMFT. Male Wistar rats were divided into two groups: BTX-A and control (0.1 units of BTX-A or only saline was injected into the tibialis anterior). Isometric proximal and distal EDL forces were measured simultaneously, one-month post-injection. Proximally and distally lengthening the muscle showed that BTX-A causes a significantly narrowerlrange(by 14.7% distally and 32.2% proximally) and significantly increased passive muscle forces (over 2-fold both distally and proximally). Altering muscle position at constant length showed that BTX-A does not change EMFT. The findings reject both hypotheses showing that long-term exposure to BTX-A compromises bi-articular muscle’s contribution to motion for both joints and the muscle’s mechanical interaction with the surroundings remains unaffected. These effects which may compromise long-term spasticity management should be studied in CP patients.

Abstract

Abstract The increasing number of cartilage diseases negatively affects the quality of life for a large part of the population. Understanding the mechanical properties of cartilage is a key component in investigating and mitigating the effects of such diseases. To describe the behavior of articular cartilage, a biphasic fiber-reinforced numerical model based on the Theory of Porous Media (TPM) has been developed. To possibly provide an alternative for the corresponding time-consuming computational simulations, the suitability of Artificial Neural Networks (ANNs) as surrogate models is investigated. For this purpose, the simulation results of a compression-relaxation test are compared with the predictions of the ANN.

Abstract

Abstract Quantum computing is a promising new computing paradigm that may yield both significant speedup over classical algorithms as well as new ways to think about problems and finding novel solution algorithms. While large-scale error-corrected quantum computers are still under development, a selection of noisy intermediate-scale quantum processing units is readily made available by some companies via cloud access. Despite the lack of error correction, these units can be used to test established quantum algorithms on custom problem setups. Here, we solve the finite element problem of a two-dimensional cantilever, completely fixed on one side and loaded on the opposite side, on a 15-qubit QPU from IBM.

Abstract

Botulinum toxin type-A (BTX-A) is widely used for spasticity management and mechanically aims at reducing passive resistance at the joint and widening joint range of movement. However, recent experiments on acute BTX-A effects showed that the injected rat tibialis anterior (TA) muscle's passive forces increased, and the length range of active force exertion (l (range)) did not change. Additionally, BTX-A was shown to spread into non-injected muscles in the compartment and affect their mechanics. Whether those effects persist in the long term is highly important, but unknown. The aim was to test the following hypotheses with experiments conducted in the anterior crural compartment of the rat: In the long term, BTX-A (1) maintains l (range), (2) increases passive forces of the injected TA muscle, and (3) spreads into non-injected extensor digitorum longus (EDL) and the extensor hallucis longus (EHL) muscles, also affecting their active and passive forces. Male Wistar rats were divided into two groups: BTX-A and Control (0.1 units of BTX-A or only saline was injected into the TA). Isometric forces of the muscles were measured simultaneously 1-month post-injection. The targeted TA was lengthened, whereas the non-targeted EDL and EHL were kept at constant length. Hydroxyproline analysis was done to quantify changes in the collagen content of studied muscles. Two-way ANOVA test (for muscle forces, factors: TA length and animal group) and unpaired t or Mann-Whitney U test (for l (range) and collagen content, where appropriate) were used for statistical analyses (P < 0.05). BTX-A caused significant effects. TA: active forces decreased (maximally by 75.2\% at short and minimally by 48.3\%, at long muscle lengths), l (range) decreased (by 22.9\%), passive forces increased (by 12.3\%), and collagen content increased (approximately threefold). EDL and EHL: active forces decreased (up to 66.8\%), passive force increased (minimally by 62.5\%), and collagen content increased (approximately twofold). Therefore, hypothesis 1 was rejected and 2 and 3 were confirmed indicating that previously reported acute BTX-A effects persist and advance in the long term. A narrower l (range) and an elevated passive resistance of the targeted muscle are unintended mechanical effects, whereas spread of BTX-A into other compartmental muscles indicates the presence of uncontrolled mechanical effects.

Abstract

Visualisation of the groundwater flow and contaminant transport can play a significant role for a better understanding of contaminant fate, which helps decision-makers and contaminated site planners to choose and implement the best remediation strategies. In this paper, a microfluidic chip coated with nanoclay was developed to mimic soil behaviour. Scanning electron microscopy (SEM) images and Fourier-transform infrared spectroscopy (FTIR) analysis confirmed that all the features and surfaces are coated with nanoclay. The change of contact angle for the native polydimethylsiloxane (PDMS) from 151° $\pm$ 4° to 73° $\pm$ 6° for modified ones is indicative of a considerable shift to hydrophilic behaviour. Moreover, the transport process in the developed chip was simulated utilising the Theory of Porous Media (TPM) and computational fluid dynamic (CFD) approaches. Although the results of both numerical approaches are in good agreement with experiments, the Root Mean Square Error (RMSE) of the predicted contaminant concentration by TPM at two observation points is less than that of CFD.

Abstract

Abstract The viscous-plastic sea ice model based on 2 describes the motion of sea ice for scales of several thousand kilometers. The numerical model for the simulation of sea ice circulation and evolution over a seasonal cycle includes the consideration of the sea ice thickness and sea ice concentration. Transient advection equations describe the physical behavior of both thickness and concentration with the velocity of the sea ice as the coupling field. Recent research on a finite element implementation of the sea ice model is devoted to formulations based on the (mixed) Galerkin variational approach, compare to 1 and 6 for instance. Here, particular treatments are necessary regarding the stabilization of the complex numerical scheme, especially for the first-order advection equation. It is therefore suggested to utilize the mixed least-squares finite element method (LSFEM), which is well established in the branch of, e.g., fluid mechanics. A significant advantage of the method is its applicability to first-order systems. Thus, this method results in stable and robust formulations also for not self-adjoint operators like the tracer equations. Moreover, in 7, the authors provide a promising higher-order time integration scheme for transient advection equations denoted as Taylor-least-squares scheme, which is investigated in this article. The presented least-squares finite element formulation is based on the unsteady sea ice equations including two tracer equations of transient advection type. The numerical problem of a Box test case is investigated.

Abstract

Rheumatic heart disease (RHD) is identified as a serious health concern in developing countries, specifically amongst young individuals, accounting for between 250 000 and 1.4 million deaths annually. As such, attention is initially placed on the importance of the development of a cardiac analysis toolbox with functionality for pathophysiological analysis of the disease. Subsequently, in order to develop a toolbox to further the understanding of the mechanisms of the disease as linked to changes in the cytoskeletal architecture and hypertrophy of cardiac myocytes, a continuum bi-phasic model applicable to cardiac tissue is formulated based on the theory of porous media (TPM). This makes it possible to account for interactions and contributions of multiple phases of constituent materials as well as concentrations of solved components, which in computational cardiac modelling are the solid phase -- the cardiac tissue -- and the liquid phase -- blood and interstitial fluid. Therefore, subsequent attention is paid to the cardiac model development in order to implement a sound base on which to add strain- and nutrient-driven phase transition, in addition to a nutrient phase contained within the liquid phase. To this end, based on thermodynamical restrictions, constitutive relations are proposed for stress, permeability, seepage velocity and interaction forces. The approach is implemented in the in-house computational cardiac mechanics toolbox SESKA which supports finite element as well as Element-free Galerkin-based approximations. This paper considers the passive and active non-linear elastic material behaviour of the myocardium of the left ventricle coupled with porous media theory, along with an additional coupling to the haemodynamics of the circulatory system, facilitating modelling of the full cardiac cycle. In order to illustrate the potential and efficacy of the approach with qualitative results, a human heart affected by RHD is investigated, making use of cardiovascular magnetic resonance scans taken over a period of two years to generate realistic 3D computer models.

Abstract

Abstract A numerical simulation of a solidification experiment is presented. The experimental setup consists of an empty screwed up mould, which is filled up with molten steel and cools down due to atmospheric conditions. During the cooling process, the temperature at the centre as well as the shrinkage at the narrow side, respectively, have been recorded. The simulation has been run via the software ANSYS modified by specific provided programmer interfaces, the User Programmable Features (UPFs). All required temperature dependent material parameters have been provided by ThyssenKrupp Steel Europe. (? 2017 Wiley?VCH Verlag GmbH & Co. KGaA, Weinheim)

Abstract

Biological methane oxidation may be regarded as a method of aftercare treatment for landfills to reduce climate relevant methane emissions. It is of social and economic interest to estimate the behavior of bacterial methane oxidation in aged landfill covers due to an adequate long-term treatment of the gas emissions. Different approaches assessing methane oxidation in laboratory column studies have been investigated by other authors recently. However, this work represents the first study in which three independent approaches, ((i) mass balance, (ii) stable isotope analysis, and (iii) stoichiometric balance of product (CO2) and reactant (CH4) by CO2/CH4-ratio) have been compared for the estimation of the biodegradation by a robust statistical validation on a rectangular, wide soil column. Additionally, an evaluation by thermal imaging as a potential technique for the localization of the active zone of bacterial methane oxidation has been addressed in connection with stable isotope analysis and CO2/CH4-ratios. Although landfills can be considered as open systems the results for stable isotope analysis based on a closed system correlated better with the mass balance than calculations based on an open system. CO2/CH4-ratios were also in good agreement with mass balance. In general, highest values for biodegradation were determined from mass balance, followed by CO2/CH4-ratio, and stable isotope analysis. The investigated topsoil proved to be very suitable as a potential cover layer by removing up to 99\% of methane for CH4 loads of 35--65gm--2d--1 that are typical in the aftercare phase of landfills. Finally, data from stable isotope analysis and the CO2/CH4-ratios were used to trace microbial activity within the reactor system. It was shown that methane consumption and temperature increase, as a cause of high microbial activity, correlated very well.

Abstract

This paper addresses a continuum-mechanical, bi-phasic, two-scale numerical model for casting and processing of metallic alloys. The solid and liquid physical states, which represents the solid and molten alloy, are formulated in the framework of the theory of porous media (TPM) including thermal coupling, finite plasticity superimposed by a secondary power creep law and visco-elasticity associated by Darcy's permeability for the solid and the liquid phase, respectively. In view of phase transition during solidification, a two-scale approach considering the phase-field on the micro-scale is proposed, where a double-well potential with two local minima for completely solid and liquid physical states is utilized. The finite element method based on the standard Gallerkin element formulation and the finite difference method was employed for the macro-scale and the micro-scale, respectively. Finally, the performance of the discussed model is demonstrated by the recalculation and validation of a solidification experiment.

Abstract

It is of high interest to describe alloy solidification processes with numerical simulations. In order to predict the material behavior as precisely as possible, a ternary phase, bi-scale numerical model will be presented. This paper is based on a coupled thermo-mechanical, two-phase, two-scale finite element model developed by Moj et~al. 2, where the theory of porous media (TPM) 1 has been used. Finite plasticity extended by secondary power-law creep is utilized to describe the solid phase and linear visco-elasticity with Darcy's law of permeability for the liquid phase, respectively. Here, the microscopic, temperature-driven phase transition approach is replaced by the diffusion-driven 0D model according to Wang and Beckermann 3. The decisive material properties during solidification are captured by phenomenological formulations for dendritic growth and solute diffusion processes. A columnar as well as an equiaxial solidification example will be shown to demonstrate the principal performance of the presented model. (\copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)

Abstract

The human liver is important for hepatic metabolism regulations which depend on complex and time depending biochemical reactions coupled to the hemodynamic blood perfusion. Growth effects can impair the perfusion and therefore affect the metabolism. Such influences can be observed during the accumulation of fat in the liver tissue. The fat accumulation results from the lipid metabolism which comprises synthesis and degradation of lipids in liver cells. Individual addictions to alcohol and high fat diets are possible causes which result in an excessive synthesis of lipids. The liver sensitively reacts to such changes which could affect the viability of the organ. To observe these coupled influences of perfusion-metabolism-growth effects we use a computational model with a multi-component/tri-phasic/bi-scale approach to simulate the functionality of the human liver with respect to fat deposition. (\copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)

Abstract

In civil engineering, the frost durability of partly liquid saturated porous media under freezing and thawing conditions is a point of great discussion. Ice formation in porous media results from coupled heat and mass transport and is accompanied by ice expansion. The volume increase in space and time corresponds to the moving freezing front inside the porous solid. In this paper, a macroscopic model based on the Theory of Porous Media (TPM) is presented which describes energetic effects of freezing and thawing processes. For simplification a ternary model consisting of the phases solid, ice and liquid is used. Attention is paid to the description of the temperature development, the determination of energy, enthalpy and mass supply as well as volume deformations due to ice formation during a freezing and thawing cycle. For the detection of energetic effects regarding the characterization and control of phase transition of water and ice, a physically motivated evolution equation for the mass exchange between ice and liquid is presented. Comparing experimental data with numerical examples shows that the simplified model is indeed capable of simulating the temperature development and energetic effects during phase change.

Abstract

Fatty liver is a common disease in the western society. Different stimuli, such as obesity or abuse of alcohol, cause the liver to store fat within liver cells, the so called hepatocytes. This growth induced by storage of fat influences the perfusion of the overlying structures, i. e. sinusoids, liver lobules, liver lobes, and consequently the liver itself. The scales of these structures reach from few µm (size of a hepatocyte) to several cm (size of liver). In view of finite element (FE) simulations, the necessity of introducing different scales becomes obvious. For the description of growth effects in the liver the relevant quantities are: perfusion of the liver and amount of substances that are needed for production of fat (glycogen and fatty acids). In this work we present a multiphasic continuum mechanical model for the description of micro-perfusion in the liver lobule using a homogenized multiphasic approach based on the theory of porous media (TPM). Into this model we inserted a zero-dimensional (0D) kinetic model calculating rates of all relevant substances. So far, the model focuses on the time dependent and spatial zonation of glycogen, since production of fat heavily depends on the stored glycogen. Additionally, a possible method for incorporating fatty acids and therefor also growth will be presented. (\copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)

Abstract

This study focuses on a formulation within the theory of porous media (TPM) for continuum multicomponent modeling of osmotic driven deformation in articular cartilage. The cartilage consists of porous solid matrix (extracellular matrix(ECM)),which is reinforced by collagen fibers and saturated by a fluid phase. The solid and fluid phases are considered as immiscible constituents occupying spatially their individual volume fraction. However, both phases include electrolytes, which are responsible for the osmotic pressure. The collagen fiber induce both, a transverse-isotropic stress and permeability behavior which is represented in the modal approach by an invariant-based description for the stress and anisotropic permeability tensor for the fluid flow, respectively. The osmotic pressure is captured by considering the electro-chemical potential of the electrolytes. After discussing the main details of the large-strain coupled poro-osmotic-hyper-elastic model approach, a brief numerical example is given. (\copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)