Coupled FE-Analysis Involving Partially Saturated Soils

A short introduction to the governing equations and the corresponding FE-formulation of a three phase model for partially saturated soils is given and a constitutive law of the soil skeleton and its numerical integration is discussed briefly. Finally, the application of the numerical model is presented. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Modeling,simulation and optimization of general solar updraft towers

A model to describe a solar chimney power plant with a generally sloped collector field and for the general situation of humid air is presented. This is a significant development of existing simple models for solar updraft towers with planar collector fields for the situation of purely dry air. The model describing the gas dynamics in the collector and in the chimney includes a turbine model, friction and heat transfer losses, evaporation and condensation models etc. However, the relevant physics can be modeled in one space dimension. It is the result of a fully compressible gas dynamic model in the small Mach number limit. A numerical algorithm is defined which admits very fast simulations. Therefore optimization procedures can easily be applied. Numerical results on optimization with respect to geometric and physical parameters which may be considered both in the planning and the operational phase are presented. The results are compared qualitatively and – if available – quantitatively to prototype data and to simulations from the literature.     

Frictional contact of a nonlinear spring

We describe and analyze a frictional problem for a system with a compressed spring which behaves as if it has a spring constant that is negative over a part of its extension range. As a result, the problem has three critical points. The friction is modeled by the Coulomb law. We show that there are three separate stick regions for some values of the parameters, centered on the critical points. We model three other versions of the process. Then we describe a numerical scheme for the models and present a number of computer simulations.     

非饱和土本构关系的混合物理论(Ⅰ)——非线性本构方程和场方程

以混合物理论为基础建立了非饱和土非线性本构方程和场方程.把非饱和土作为3种组分构成的饱和混合物来研究.首先根据土力学成果提出了非饱和土混合物的基本假设,推导出适用于非饱和土混合物的熵不等式;然后采用混合物理论处理本构问题的常规方法得出了非饱和土非线性本构方程;最后把非线性本构方程代入混合物组分动量守恒定律,获得了非饱和土各组分运动的非线性场方程;并且给出了非饱和土混合物的能量守恒方程,从而形成了解决非饱和土混合物热力学过程的完备方程组.     

Analytical prediction of tunneling-induced ground movements and liner deformation in saturated soils considering influences of shield air pressure

Complex underground constructions in urban areas require strict predictions for ground movements and liner deformation induced by shield-driven tunneling, in which the complex interaction mechanics between ground and liner play a substantial role. Previous studies, however, provided little information on the ground-liner interaction and less attention to the effects of groundwater and compressed air during the shield operation. This paper presents a closed-form analytical solution for predicting long- and short-term ground deformation and liner internal forces induced by tunneling in saturated soils in which shield excavation effects with and without air pressure are both considered. The oval-shaped convergence deformation pattern is incorporated as the boundary condition of displacements around the tunnel section. This paper also investigates the difference between uniform radial and oval-shaped convergence deformation patterns on the ground and tunnel responses. Generally, the predicted ground movements by the oval-shaped deformation pattern aligns well with measured data of actual tunnels with and without considering the shield air pressure. It is comparatively observed that the shield excavation under air pressure obtains larger ground deformation than the non-pressure condition, and the long-term ground settlements induced by tunneling in saturated soils are confidently larger than the short-term. Moreover, the effects of sensitive parameters, including the shield air pressure, the long- and short-term effects on the tunneling-induced ground movements are assessed based on the oval-shaped deformation pattern. Furthermore, parametric analyses are conducted to measure the influences of concerned tunneling coefficients on the liner displacements and internal forces, namely, soil Young's modulus, soil unit weight, coefficient of lateral soil pressure, tunnel radius, tunnel buried depth and gap parameter. In summary, the analytical approach proposed in this research provides an effective insight into the ground-liner interaction mechanics related with the shield air pressure, which can serve as an alternative approach in the preliminary design for conservatively estimating the excavation influences caused by tunneling in saturated soils.     

A continuum-mechanical analysis of the influence of mechanical stimuli on biological tissue

Mechanical stimuli play a crucial role in the differentiation process of mesenchymal stem cells (MSC). The resulting mechanical signals are important in the regulation of various cell functions and maintenance of many tissues. The underlying molecular and biophysical mechanisms of the differentiation process are poorly understood. Present remodelling and growth models are purely phenomenological without linkage to cell mechanisms. The presented macroscopic model of MSC mechanics is based on a multiphasic-multicomponent formulation within the framework of Theory of Porous Media (TPM), where a single cell is considered as a mixture of interacting constituents. In particular, the constituents are the solid cytoskeleton saturated by a fluid phase (cytoplasm), which itself consists of a liquid solvent and mobile components, e. g., chemical messengers, proteins, etc. To demonstrate the capabilities of the developed model, first qualitative numerical simulations of the impact of external forces on MSC are presented. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Turbulent Entrainment in the Atmospheric Boundary Layer

Turbulent entrainment – the process by which turbulence inside of the atmospheric boundary layer entrains air from the free troposphere above it – has been investigated using direct numerical simulations in two configurations, one without a cloud and one with a cloud. With the first configuration, we have learned that the entrainment zone in a convective boundary layer growing into a linearly stratified troposphere is better described in terms of a two-layer structure, with different characteristic scales associated with each of the two sub-layers. With the second configuration, we have explained how wind shear across the entrainment zone capping a stratocumulus cloud can render evaporative cooling as important as radiative cooling in driving convective motions. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Aerated autoclaved concrete: Stochastic structure model and elastic properties

Aerated autoclaved concrete (AAC) is a modern and important construction material, whose elastic properties are primarily defined by its porosity. The possibility to predict elastic properties of AAC based on the voids distribution is very important. The report describes simulations of the mechanical properties of AAC, based on a stochastic-geometric model of its structure. The model is the well-known “cherry-pit” model, which presents a random system of partially overlapping spheres. In the mechanical analysis the solid phase is approximated by a network model with the help of the so-called radical tessellation with respect to the hard spheres of the “cherry-pit” model. The network edges are modelled in ANSYS as 3D beams. In this approach, the discretized elements (the edges) have in distinction to FE calculations with small polyhedral same dimension as the air voids and so the numerical costs can be drastically reduced. The FE simulations calculate the elastic constants and energy concentrations, which are responsible for the material failures, in large samples. Comparisons with fracture tests showed good matching between simulations and experiments. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Response of Kidney Tissue to Dynamical Loading

In shock-wave lithotripsy – a medical procedure to fragment kidney stones – the patient is subjected to hypersonic waves focused at the kidney stone. Although this procedure is widely applied, the physics behind this medical treatment, in particular the question of how the injuries of the surrounding kidney tissue arise, is still under investigation. Here we contribute to the solution of this problem with large scale numerical simulations of a human kidney under shock-wave loading. For this purpose we developed a complex constitutive model of the bio-mechanical kidney system. Assuming a multiplicative decomposition of the deformation gradient and adopting an internal variable formulation for the inelastic deformation the model is able to handle large deformations, time-effects, rate-sensitivity and material damage. By finite element simulations we study the shock-wave propagation into the kidney tissue and analyze the resulting stress states. Unknown material parameters are adapted and special attention is paid on the bubble expansion within the soft tissue. The numerical simulations predict localized damage in the human kidney within the focal region of the shock waves. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the simulation of soils under rapid cyclic loading conditions

When simulating soils, which are subjected to dynamic loadings conditions, base on a convenient soil model, special attention has to be paid to the approximation of the real problem by a suitable initial-boundary-value problem (IBVP). In this regard, the semi-infinite half-space is split into a near-field, which is, in general, the domain of interest, and a far-field. However, truncating the half-space at the near field, which is often sufficient in quasi-static simulations, introduces artificial boundaries at which, in dynamic analysis, the incident waves are reflected back into the domain of interest. Several methods have been proposed in the literature to overcome this problem. The present contribution proceeds from a coupling approach, where the near and the far field are discretised with finite and infinite elements, respectively, and where additionally an energy-absorbing layer is introduced at the interface. The aim of the present contribution is to perform a parametric study associated with a biphasic soil model based on the Theory of Porous Media (TPM). Therein, numerical simulations are carried out in order to judge the energy-absorbing capabilities of the present far-field treatment under different parametric conditions. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Surrogate Modelling for Real-time Predictions of Mechanised Tunnelling Processes with Interval Data

Finite element simulations are commonly used to investigate the soil-structure interactions of mechanised tunnelling processes, such as to provide predictions on the expected surface settlement field. For real-time predictions during the tunnel construction, the finite element model is substituted by a hybrid surrogate model combining Artificial Neural Network and Proper Orthogonal Decomposition approaches. The surrogate model is employed to predict time-variant interval surface settlement fields for selected scenarios of the tunnelling process steering parameters in real-time considering uncertain geotechnical parameters as intervals. For this purpose, the surrogate model in [1], which is based on a Recurrent Neural Network and Gappy Proper Orthogonal Decomposition technique, is split into surrogate models for midpoint and radius computations of the interval data. The online computation time of the new surrogate modelling approach is only a few seconds, which enables tu apply it for real-time predictions and to support the Tunnel Boring Machine steering. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Deformation induced martensite transformation in a cold-worked forming process of austenitic stainless steel

Within the last years the goal of industrial manufacturing processes – such as tube forming – has shifted towards an optimization of technological as well as mechanical properties of the manufactured structures. For example, during the forming procedure of sheets made of austenitic stainless steel X5CrNi18-10, the content of strain-induced martensite needs to be controlled. In order to achieve optimal structural properties of the manufactured tube with respect to very high-cycle fatigue (VHCF), a martensite ratio of approximately 25% needs to be obtained [1]. On the basis of experimental investigations this contribution deals with the numerical simulation of the tube-forming process with special consideration of the martensite ratio c as a function of temperature and deformation field. For this purpose we extend an existing martensite model on polyaxial states of stress and compare experimental results and numerical simulations for the modified model. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optimization the shape of finite element in shot peening process

This paper contains modelling and numerical simulations of shot peening process. The application in Ansys/LS – Dyna programme were elaborated. The phenomena of shot peening process on a typical incremental step were described using a step-by-step incremental procedure, with an updated Lagrangian formulation.Finite elements methods (FEM) and the dynamic explicit method (DEM) were used to obtain the solution. The main purpose of this article is to determinate optimal model of shot peening process (convergence resulting for minimal number of finite elements and their optimal shapes). Examples of calculations were presented. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical solution of a one-dimensional problem of filtration consolidation of saline soils in a nonisothermal regime

A mathematical model of a problem of filtration consolidation of saline soils in a nonisothermal regime with allowance for the presence of salts in the liquid and solid phases is constructed. A numerical solution of the corresponding one-dimensional boundary-value problem is found by the method of finite differences. As an example, we investigate a problem of filtration consolidation of a massif of clay soil of finite thickness. Results of numerical investigations and their analysis are presented. Translated from Matematychni Metody ta Fizyko-Mekhanichni Polya, Vol. 51, No. 1, pp. 197–204, January–March, 2008.     

Computational Excavation Analysis of a Single Cutting Tool using a Hypoplastic Constitutive Model

We propose a computational model for the numerical analysis of the dynamic interaction between a single excavation tool and the surrounding soil. An incremental non-linear (hypoplastic) constitutive model is employed to capture the complex response of soft soils. Large displacements and deformations are handled by an Updated Lagrangian formulation, the particle finite element method (PFEM). (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)