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This publication presents a novel numerical avenue for the assessment of masonry structures. To circumvent the labor-intensive finite element method modeling and pre-processing step, the isogeometric analysis is enhanced within this contribution to cope with the necessities for masonry structures. This incorporates the study of apparent kinematic shell formulations, including their respective properties. Additionally, the connection towards non-linear material laws is outlined, and an empirically evaluated approach for the element size regularization is proposed. This generic approach would be inherently valid for other size dependent constitutive material models, as e.g. concrete models. The current paper is explored in the context of a number of experimental problems. These validation examples are used to provide structural assessments that extend beyond the scope of the experiment. Finally, a potential example that provides an outlook towards anticipated applications is presented. • Advancing the isogeometric analysis to cope with shell-based masonry structures. • Proposing a damage model in the context of the isogeometric analysis. • Newly proposed element length regularization technique for NURBS. • Enhancement of parametric and CAD-integrated workflow for masonry analysis. • Comprehensive mechanical assessments of masonry structures. [ABSTRACT FROM AUTHOR]

This paper investigates data mapping between non-matching meshes and geometries in fluid-structure interaction. Mapping algorithms for surface meshes including nearest element interpolation, the standard mortar method and the dual mortar method are studied and comparatively assessed. The inconsistency problem of mortar methods at curved edges of fluid-structure-interfaces is solved by a newly developed enforcing consistency approach, which is robust enough to handle even the case that fluid boundary facets are totally not in contact with structure boundary elements due to high fluid refinement. Besides, tests with representative geometries show that the mortar methods are suitable for conservative mapping but it is better to use the nearest element interpolation in a direct way, and moreover, the dual mortar method can give slight oscillations. This work also develops a co-rotating mapping algorithm for 1D beam elements. Its novelty lies in the ability of handling large displacements and rotations. [ABSTRACT FROM AUTHOR]

This contribution presents a computational engineering tool for the purposeful and systematic investigation how sensitive responses of a structural analysis model react due to variations of model parameters. The methodical basis of the approach is the relationship between adjoint sensitivity analysis and the classical and well-known technique of influence functions. The strong connection between the two approaches allows the extension of the traditional method of influence functions as a tool for the evaluation of sensitivities for general responses, parameters and structures. The paper provides a detailed discussion of that methodical relationship and shows how the influence function of important responses in structural engineering can be identified in the adjoint sensitivity equation. Following the ability of the traditional method of influence functions to provide structural comprehension as visual means, also the results of sensitivity analysis are graphically processed as maps. Finally, the application of the presented method and the sensitivity maps is demonstrated by means of expressive engineering structures. The examples provide insights how sensitivity analysis can support the structural design process by fast and purposeful identification and further assessment of the importance of model parameters. • The strong relation between influence functions and sensitivity analysis is shown. • The method of influence functions is generalized for sensitivity analysis. • Interpretation of sensitivity terms for practically important responses is given. • Sensitivity analysis is elaborated as engineering tool to enrich structural design. • Discussion concerning the significance and interpretation of sensitivities. [ABSTRACT FROM AUTHOR]

'Leicht Bauen mit Beton' und 'form follows force' sind die Motivation dieses Artikels. Es wird ein Prozess vorgestellt, wie das Entwerfen, Berechnen und Bemessen von sehr leichten und filigranen Freiformschalen aus Betonfertigbauteilen in der Zukunft aussehen könnte. Dafür werden neueste Methoden aus dem Bereich CAD-basierte Verfahren verwendet, welche eine sehr elegante Vereinigung von Design und Analyse ermöglichen. Der vorgestellte Prozess zeigt den Weg von der Entwurfsidee bis zur Bestimmung der optimalen Geometrie und Anordnung der Betonfertigbauteile. Design and Analysis of Curved Precast Concrete Components Using CAD-based Approaches - to build lightweight structures in the future 'Leicht Bauen mit Beton' and 'form follows force' are the motivation for this article. In this contribution a process is presented, which shows how the design and analysis of very light, free-form shells, made of precast concrete components, could be done in the future. Therefore, new techniques from the area of CAD-based methods are used, which allow for a very elegant way of integrating design and analysis. The presented process shows the path from the idea of the design to the determination of the optimal geometry and assembly of the precast concrete components. [ABSTRACT FROM AUTHOR]

This paper introduces a general fully stabilized mesh based shape optimization strategy, which allows for shape optimization of mechanical problems on FE-based parametrization. The well-known mesh dependent results are avoided by application of filter methods and mesh regularization strategies. Filter methods are successfully applied to SIMP (Solid Isotropic Material with Penalization) based topology optimization for many years. The filter method presented here uses a specific formulation that is based on convolution integrals. It is shown that the filter methods ensure mesh independency of the optimal designs. Furthermore they provide an easy and robust tool to explore the whole design space with respect to optimal designs with similar mechanical properties. A successful application of optimization strategies with FE-based parametrization requires the combination of filter methods with mesh regularization strategies. The latter ones ensure reliable results of the finite element solutions that are crucial for the sensitivity analysis. This presentation introduces a new mesh regularization strategy that is based on the Updated Reference Strategy (URS). It is shown that the methods formulated on this mechanical basis result in fast and robust mesh regularization methods. The resulting grids show a minimum mesh distortion even for large movements of the mesh boundary. The performance of the proposed regularization methods is demonstrated by several illustrative examples. [ABSTRACT FROM AUTHOR]

Abstract: A methodology is presented which enables isogeometric analysis on trimmed NURBS surfaces. The general limitations of NURBS-based isogeometric analysis are roughly delineated to resolve the associated issues. A local reconstruction technique using a geometric basis is developed and applied to evaluate the finite element constituents of the trimmed knot spans in terms of the underlying control variables. This paper covers the algorithmic treatment of single patches as well as of multi-patches. Moreover, the proposed methodology gives rise to a new feature in the case of load application, what we refer to as Isogeometric Load Design. It provides the possibility to apply an arbitrarily shaped loading area onto a patch. Consequently, the restriction of the alignment of the loading with the underlying parameterization is canceled. In several significant examples the capabilities and effectiveness of this new method are demonstrated and confirm that the proposed method produces compelling results. [Copyright &y& Elsevier]

Abstract: A multi-purpose general rotation free element for the analysis of thin Kirchhoff–Love type shells is presented: Isoparametric displacement elements are used as basic geometrical description and to determine the membrane strains. Calculation of curvature to determine bending strains is based on discrete nodal directors, which are assigned to each node and determined by the geometry of the surrounding elements. The kinematic is formulated completely nonlinear. The technology is modular allowing for adding bending stiffness to an initial membrane model using the same geometrical discretization and, thus, supporting the entire design procedure from form finding to ultimate load or dynamic analysis of thin shells. Different examples ranging from static wrinkling analysis of a membrane to nonlinear dynamic analysis of a thin metal shelter show the multitude of possible applications. [Copyright &y& Elsevier]

Abstract: Free form shells optimized for stiffness under given loading and membrane structures act in a pure membrane state of stresses, either because bending is minimized or not even present by definition. Physical experiments as soap films and hanging models have been used since centuries to generate optimal shapes of membranes in tension and shells in compression. The paper presents numerical methods to simulate the physical experiments as well as how they can be merged among each other and with the most general technology of structural optimization. The combined approach represents the combined power of each technique. Several examples illustrate the methods and typical applications. [Copyright &y& Elsevier]

With the ability to manufacture complex functional structures, Additive Manufacturing (AM) enables new advanced applications which could not be realized before. An example of such structures is open porous structures. In the presented approach, the open porous structures are a result of the selected manufacturing parameters, instead of being defined in CAD models. Thereby, various porosities can be achieved flexibly without design adjustments. However, to comprehensively understand the correlation of manufacturing parameters and different properties, various tests are required. As an alternative, properties can be determined by means of simulation. This study presents 3D reconstructions of complex open porous structures, which are based on μ-CT imaging. The influence of the downsampling during voxelization on the 3D reconstructed geometry is studied. Further, different polyhedral mesh settings have been tested in order to find a mesh that minimizes the deviation between the CFD simulation results and the laboratory test results for flow resistance. A guideline for the creation of a calibrated CFD model based on μ-CT imaging is proposed. As it is acknowledged by the authors that the data availability to reconstruct shown cases is crucial, data for a reference case have been made freely available online. [ABSTRACT FROM AUTHOR]

Abstract: Uncertainty quantification for multi‐physics problems like fluid‐structure interaction (FSI) is challenging in terms of the required computational effort and complexity of physics involved. A Multilevel Monte Carlo (MLMC) approach for stochastic analysis of FSI problems is presented in the current study. An overall reduction in computational cost is achieved by variance reduction in MLMC method. The efficacy of MLMC algorithm for expensive problems like FSI is compared with classical Monte Carlo(MC) method and MLMC is found to achieve the required accuracy at a significantly reduced computational cost. An FSI benchmark test case with uncertain inputs is presented here. The possibility of uncertainty quantification for FSI systems with a plausible computational cost using MLMC is demonstrated. [ABSTRACT FROM AUTHOR]

Lightweight shell structures have gained significant popularity in the engineering design as they offer high load carrying capacity for low cost. While classical Finite Element models of such structures have been extensively used in practice, the shell formulations for Isogeometric Analysis (IGA), particularly for Kirchhoff‐Love shell structures [1], have demonstrated benefits arising from the smooth geometry description and the highly accurate solutions that are enabled. Moreover, the direct use of CAD‐models for the structural simulations is possible based e.g. on the Isogeometric B‐Rep Analysis (IBRA) [2]. Herein the application of IBRA to Fluid‐Structure Interaction (FSI) simulations of wind turbines is demonstrated, thus naturally extending IBRA to coupled multiphysics problems. [ABSTRACT FROM AUTHOR]

The Material Point Method (MPM) is an established and powerful numerical method particularly useful for simulating large-scale, rapid soil deformations. Therefore, it is often used for the numerical investigation of mass movement hazards such as landslides, debris flows, or avalanches. It combines the benefits of both mesh-free and mesh-based continuum-based discretization techniques by discretizing the physical domain with Lagrangian moving particles carrying the history-dependent variables while the governing equations are solved at the Eulerian background grid, which brings many similarities with commonly used finite element methods. However, due to this hybrid nature, the material boundaries do not usually coincide with the nodes of the computational grid, which complicates the imposition of boundary conditions. Furthermore, the position of the boundary may change at each time step and, moreover, may be defined at arbitrary locations within the computational grid that do not necessarily coincide with the body contour, leading to different interactions between the material and the boundary. To cope with these challenges, this paper presents a novel element-wise formulation to weakly impose non-conforming Dirichlet conditions using Lagrange multipliers. The proposed formulation introduces a constant Lagrange multiplier approximation within the constrained elements in combination with a methodology to eliminate superfluous constraints. Therefore, in combination with simple element-wise interpolation functions classically utilized in MPM (and FEM) to approximate the unknown field, a suitable Lagrange multiplier discretization is obtained. In this way, we obtain a robust, efficient, and user-friendly boundary imposition method for immersed methods specified herein for implicit MPM. Furthermore, the extension to frictionless slip conditions is derived. The proposed methodologies are assessed by comparing the numerical results with both analytical and experimental data to demonstrate their accuracy and wide range of applications. [ABSTRACT FROM AUTHOR]

Embedded and immersed methods have become essential tools in computational mechanics, as they allow discretizing arbitrarily complex geometries without the need for boundary-fitted meshes. One of their main challenges is the accurate numerical integration of cut elements. Among the various integration schemes developed for this purpose, moment fitting has proven to be a powerful technique that provides highly efficient and accurate integration rules. This publication presents a framework for the robust and efficient numerical integration of embedded solids described by oriented boundary meshes using moment fitting. The developments include an intersection algorithm that aims to drastically accelerate the computation of the necessary moments while achieving high accuracy. A closed surface parameterization of each cut domain is computed to facilitate the direct application of the divergence theorem. The algorithm is subject to a single quality criterion that guarantees the accurate evaluation of boundary integrals. At the same time, it allows to disregard classical mesh criteria, such as high aspect ratios, strongly varying angles, etc., resulting in extremely fast runtimes. In addition, an existing robust flood fill-based element classification scheme is further developed to initiate filling from arbitrary seed elements and to enable parallel execution, increasing its flexibility and efficiency. The successful application of all proposed algorithms to 4948 valid and flawed STLs from the Thingi10K database (Zhou and Jacobson, 2016) demonstrates their extraordinary robustness. In all cases, the wall-clock time scales at most linearly with the number of elements in the background mesh. We show that higher-order quadrature rules on the boundary elements enable efficient computation of the moments via the divergence theorem with near-machine precision. Finally, the presented methodologies are used to perform direct FE analyses on clean and flawed B-Rep models. All proposed algorithms are publicly available in the open-source C++ framework QuESo – Quadrature for Embedded Solids (https://github.com/manuelmessmer/QuESo), where the moment fitting equations are assembled and solved. • Fast assembly of the moment fitting equations for arbitrarily complex faceted B-Reps. • Efficient intersection algorithm to facilitate the application of the divergence theorem. • Generalized and parallel-executable element classification scheme based on flood filling. • Successful application of the framework to 4948 valid and flawed STLs. [ABSTRACT FROM AUTHOR]

The CAD‐integrated parametric design process of membrane structures The design of membrane structures entails three strongly linked and often very interactive disciplines: formfinding, structural analysis and cutting pattern generation. Powerful numerical tools and environments are needed for the design of a viable membrane structure while consistently performing the necessary analyses. CAD‐integrated analysis allows for the performance of these highly non‐linear analyses within the CAD environment. This leads to an optimal geometry description for the analysis model, since it is modeled with smooth NURBS curves and surfaces. Furthermore, the links between analysis steps can be modeled ideally within the CAD framework that contains both pre‐ and postprocessing. A formfound geometry can e. g. directly be used for consecutive analyses. The basis for this is Isogeometric B‐Rep Analysis (IBRA), which will be explained briefly in this paper. In the isogeometric method, NURBS are used for the basis functions of Finite Element Method as well as the geometry description. The parametrization of geometric and mechanical properties enables the fully integrated performance of formfinding, structural analysis and patterning based on a common numerical model. The consideration of nearly infinite configurations of one model is thus provided with a minimum of effort. This article presents an overview of current methods and advantages of the parametric CAD‐integrated design cycle. The plugin Kiwi!3D for Rhino and Grasshopper is used for the demonstration and the mechanical basis of all design steps will be laid out. The textile roofs of the Cologne Tanzbrunnen serve as exemplary structures. [ABSTRACT FROM AUTHOR]

An adjoint-based procedure to determine weaknesses, or, more generally the material properties of structures is developed and tested. Given a series of force and displacement/strain measurements, the material properties are obtained by minimizing the adequately weighted differences between the measured and computed values. The approach is directly based on the finite element model of the structure of interest, which can be arbitrarily complex and be composed of any kind of element formulation. This is especially advantageous in complicated real-world applications. As a consequence, the procedure can provide highly resolved parameter distributions within the structure and allows for the localization of e.g. damage regions or other zones with deviations from the planned configuration. Several examples with truss, plain strain and volume elements show the viability, accuracy and efficiency of the proposed methodology using both displacement and strain measurements. An important finding was that in order to obtain reliable, convergent results the gradient of the cost function has to be smoothed appropriately. [ABSTRACT FROM AUTHOR]

Membranes have been extensively used for the design of architectural and general structural models due to their low cost and high load carrying capacity. Traditionally such models were discretized using the standard low order Finite Element Method (FEM) which typically results in a compromised description of the geometry. However, the accurate geometric description of membrane structures is essential as for instance bifurcation points in geometrically nonlinear analysis may be inaccurately predicted when the geometric description of the model is not accurate enough. Moreover, the design of membrane structures typically requires several cycles of form-finding and subsequent structural analysis under various loads which can benefit from a direct connection to the Computer-Aided Design (CAD) environment using its exact geometric description. In this contribution, the form-finding analysis using the Updated Reference Strategy (URS) and the geometrically nonlinear transient analysis of membranes is extended to Isogeometric Analysis (IGA) on multipatch surfaces with Non-Uniform Rational B-Splines (NURBS). As typical in IGA for real CAD geometries, multiple patches with non-matching parametrizations are considered and therefore the continuity of the solution field along with the application of weak Dirichlet boundary conditions need to be addressed. Thus, four different constraint enforcement methods are elaborated and compared, namely, the Penalty , the Lagrange Multipliers , the augmented Lagrange Multipliers and a Nitsche -type method. For the latter method, a solution dependent stabilization approach is employed in order to render the Nitsche-type method coercive. All methods are elaborated and systematically compared in both form-finding analysis, whenever necessary, and subsequently in geometrically nonlinear transient analysis. It should be noted that the Nitsche-type method is more computationally demanding amongst these methods due to the additional nonlinear terms. However, the results suggest that the Nitsche-type method is advantageous for these kinds of problems as no parameter or discretization other than the isogeometric discretization within each patch needs to be specified prior to the analysis. • NURBS-based isogeometric analysis for non-conforming multipatch membrane structures. • Comparison of various weak constraint enforcement methods in isogeometric analysis. • Extension of the Nitsche's method for 3D nonlinear transient membrane analysis. • Full framework for design, form-finding, modal and transient analysis of membranes. • Numerical investigations ranging from benchmark to real world engineering examples. [ABSTRACT FROM AUTHOR]

Membranes constitute an important category of thin‐walled and in particular tensile structures [1], for which in contrast to shells their structural rigidity is obtained by means of prestress rather than relying on bending stiffness. In particular, the structures considered in this study act in pure membrane state and have no bending stiffness. A big challenge of such structures is finding the shape of static equilibrium. This is not a trivial task since not every shape under any prestress distribution or loading condition renders a static equilibrium configuration. This is even more prominent when prestressed cables are attached to the structure. To overcome this problem for general tensile structures, various types of form‐finding methods were developed [2, 3]. In this study, the Updated Reference Strategy (URS) [4, 5] is presented in combination with multipatch isogeometric analysis [6] as discretization method. The enforcement of the continuity constraints along the common patch interfaces [7] is realized using a Nitsche‐type method [8] while the results are evaluated and compared with the Penalty‐based approach [9]. [ABSTRACT FROM AUTHOR]

This paper investigates the aerodynamic performance of a semiflexible membrane wing concept using twoway coupled fluid-structure interaction simulations. The sailwing concept consists of a rigid leading-edge mast, ribs, tensioned edge cables, and membranes forming the upper- and lower-wing surfaces. For membrane structures like the sailwing, the exact shape of the membrane surface in the absence of external loading depends on the prestress levels of both membranes and supporting edge cables. A form-finding analysis is used for calculating the equilibrium shape of prestressed membrane surfaces with no external loading. The exact shape of the sailwing in operating condition depends on the pressure distribution on the membrane's surface, which itself depends on the wing's shape and the structural parameters. For faster design space exploration, the fluid problem is modeled using an unsteady vortex panel method. Among all the factors influencing the fluid-structure interaction problem, the role of the prestress level is specifically investigated. A nonlinear dynamic analysis of the structural problem is performed using the finite element method. The influence of the wing's flexibility on its aerodynamic performance and structural response is examined by changing the prestress level in the membranes. Aerodynamic characteristics of the membrane wing are compared with an equivalent conventional rigid wing. A higher lift slope for the membrane wing and improvement of the lift coefficient due to trailing-edge flexibility are observed. [ABSTRACT FROM AUTHOR]

This contribution gives a short introduction into the topic of fluid-structure-control interaction (FSCI). This is a multi-field problem, where three fields are coupled, namely a fluid-flow, structural dynamics and closed-loop control of structures. The interactions are identified as surface-coupling of fluid flow and structural dynamics as well as signal-coupling of structural dynamics and closed-loop control. At first a simple model problem is introduced, which allows an efficient investigation of the convergence behavior and stability of such kind of problem. In this context an iterative partitioned approach, using a serial Gauß-Seidel scheme, is presented. Finally, the insights gained by the model problem are used to set up a multi-dimensional numerical experiment. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

• Finite element-based damage localization with coda wave interferometry. • Application of different solving algorithms that improve the damage localization. • Validation of the finite element-based approach with synthetic data. • Application of damage localization with coda wave interferometry at a real experiment. • Localization and distinguishing of multiple cracks in a real experiment. Coda wave interferometry is a structural health monitoring technique that allows the detection of cracks in concrete using diffuse ultrasound. This study develops a finite element-based methodology that computes a substitute model referred to as a sensitivity kernel. This kernel simulates the decorrelation development of two compared ultrasonic signals over the signals' length and allows to locate damage by solving an inverse problem. Results are compared to an established analytical approach. Two different solving techniques namely the LSMR method and one that respects upper and lower bounds on the variables called STIR are successfully applied in this study. The finite element-based methodology is validated with a reference simulation. In addition to the synthetic wave signals, the approach is tested on a real four-point bending test of a reinforced concrete specimen, where multiple cracks were successfully detected. [ABSTRACT FROM AUTHOR]

This publication presents a study aiming a macro characterization of the mechanical properties, suitable for future macro-scale numerical continuum-based analyses of a structured masonry bond, employing modern hollow bricks. The experiments themselves were initially simulated across different configurations, sizes and loading scenarios to determine the minimal specimen size invariant to boundary and size effects that scales consistently. From these simulations, a numerically informed experimental program was executed across compression, tension, shear, triplet shear, and out-of-plane bending conditions to fully quantify orthotropic material parameters of the masonry pattern suitable for aimed macro-scale simulation. The outcome of this publication is a comprehensive quantitative understanding of the apparent masonry type for engineering design purposes. • Structured masonry bonds with modern hollow bricks. • Numerical masonry classification. • Orthotropic macroscopic plane-stress. • Macro scale experimental tests. [ABSTRACT FROM AUTHOR]

In this study, we evaluate lattice Boltzmann and finite element methods for wind load estimation of parabolic trough solar collectors. Mean, root-mean-square (RMS) and peak values of aerodynamic load coefficients of an isolated collector are estimated using large-eddy simulation and compared with experimental results obtained in a boundary layer wind tunnel. Despite their fundamental differences, the two numerical approaches yield similar values for the drag, lift and pitching moment indicating that the results are essentially independent of the numerical schemes. Time-varying inlet boundary conditions are obtained using an efficient synthetic generation technique. The statistics of the numerical boundary layer are investigated by comparing mean and turbulence intensity profiles at varying distances from the inlet as well as the spectra and autocorrelation at the position of the structure. Through appropriate modelling of the boundary layer, the numerical models are shown to reproduce the mean, RMS and peak load behaviour measured in the wind tunnel. [ABSTRACT FROM AUTHOR]

The latest updates on the Vertex Morphing technique for large optimization problems are shown in this work. Discussions about the challenges of node-based shape optimization in academic and industrial applications are included. The adaptive Vertex Morphing technique is demonstrated, which is easy to use in practice and allows the full exploitation of the potential of node-based shape optimization to find new designs in large-scale applications. We also show an efficient optimization method to handle different physical responses with many geometrical constraints. A state-of-the-art example of industrial importance supports the work. [ABSTRACT FROM AUTHOR]

Co-simulation is a popular numerical method to solve complex multiphysics problems. The co-simulation method has an intrinsic advantage: it allows well-established and specialized simulation tools to be reused and combined with minor adaptations, in contrast to the monolithic approach. We employ a novel co-simulation approach to simulate a fully-coupled emergency brake maneuver scenario of the National Renewable Energy Laboratory (NREL) Unsteady Aerodynamics Experiment (UAE) Phase VI wind turbine. Within this simulation the acceleration and deceleration of the turbine are a result of the interaction of the blades, generator/gearbox, control unit and the flow field. The simulation results are validated against measurement data from the NREL UAE Phase VI performed in the NASA AMES wind tunnel. Using the co-simulation approach, in contrast to the investigations of various other research groups, this work provides a methodological framework to perform simulations much closer to real physics. This is realized by taking into account the interaction of blades, generator/gearbox, control unit and fluid at the same time. Thus, the usual approximations and assumptions on the single fields, which become necessary due to the artificial decoupling, are not needed, e.g. prescribing certain behavior. Moreover, the full flexibility of co-simulation is combined with the stability of a monolithic approach applying the novel Interface Jacobian-based Co-Simulation Algorithm (IJCSA). [ABSTRACT FROM AUTHOR]

This paper introduces new initial value problems for the design and analysis of fluid-structure interaction algorithms. The problems are constructed from a finite number of sinusoidal perturbations of thin structures superposed on incompressible fluids. Explicit analytical solutions are provided for rigorous quantitative comparisons including viscous effects. The equation governing a single inviscid mode is split into fluid and structure subproblems and numerical parameters are derived for several partitioned algorithms based on a Dirichlet-Neumann decomposition and a second-order backward difference time discretization. Numerical experiments are performed using a stabilized fractional-step finite element method for the fluid and membrane finite elements for the structure. The convergence behaviour of the finite element solutions to the analytical solutions is demonstrated. Finally, the stability and performance of the numerical parameters derived from a single inviscid mode are presented for more general problems including multiple modes and viscous effects. [ABSTRACT FROM AUTHOR]

This work presents a partitioned solution procedure to compute shape gradients in fluid–structure interaction (FSI) using black-box adjoint solvers. Special attention is paid to project the gradients onto the undeformed configuration due to the mixed Lagrangian–Eulerian formulation of large-deformation FSI in this work. The adjoint FSI problem is partitioned as an assembly of well-known adjoint fluid and structural problems. The sub-adjoint problems are coupled with each other by augmenting the target functions with auxiliary functions, independent of the concrete choice of the underlying adjoint formulations. The auxiliary functions are linear force-based or displacement-based functionals which are readily available in well-established single-disciplinary adjoint solvers. Adjoint structural displacements, adjoint fluid displacements, and domain-based adjoint sensitivities of the fluid are the coupling fields to be exchanged between the adjoint solvers. A reduced formulation is also derived for the case of boundary-based adjoint shape sensitivity analysis for fluids. Numerical studies show that the complete formulation computes accurate shape gradients whereas inaccuracies appear in the reduced gradients. Mapping techniques including nearest element interpolation and the mortar method are studied in computational adjoint FSI. It is numerically shown that the mortar method does not introduce spurious oscillations in primal and sensitivity fields along non-matching interfaces. [ABSTRACT FROM AUTHOR]

This work presents a novel application of the Shifted Boundary Method (SBM) within the Isogeometric Analysis (IGA) framework, applying it to two-dimensional and three-dimensional Poisson problems with Dirichlet and Neumann boundary conditions. The SBM boundary condition imposition is achieved by means of a fully penalty-free formulation, eliminating the need for penalty calibration. The numerical experiments demonstrate how order elevation, coupled with SBM through higher-order Taylor expansions, consistently achieves optimal convergence rates. Additionally, analyzing the condition number of the problem matrix reveals that SBM, when integrated with IGA, effectively circumvents the small cut-cell problem, a common issue in numerical methods with unfitted boundaries. [ABSTRACT FROM AUTHOR]

One of the biggest challenges in optimizing the shape of complex solids is the requirement to maintain a reasonable mesh quality not only at the boundary but also for the bulk discretization of the interior. Thus, additional regularization and, in many cases, re-meshing of the structure during the iterative process is unavoidable with a Lagrangian description. By tracking the shape update using an Eulerian representation, embedded boundary methods are a promising technique for eliminating mesh distortion problems. This work consistently combines the unique features of implicit Vertex-Morphing and embedded boundary methods, facilitating the node-based shape optimization of solids with industrial complexity. One of the crucial elements for solving the primal problem on a fixed background grid is an efficient and robust quadrature scheme. To this end, we incorporate the open-source C++ framework QuESo (https://github.com/manuelmessmer/QuESo) developed for the numerical integration of arbitrarily complex embedded solids defined by oriented boundary meshes, e.g., in STereoLithography (STL) format. Meanwhile, applying the Helmholtz/Sobolev-based (implicit) filter to the vertices of the embedded boundary mesh not only exploits the extensive design space of node-based optimization but also ensures robust control over the feature size. To realize the above methodology, this work introduces a novel sensitivity analysis that yields mesh-independent shape gradients with respect to the immersed boundary. Through a specific sensitivity weighting, we recover a continuous gradient field from discrete values calculated at the nodes of the background grid. In addition, the presented workflow ensures robust enforcement of challenging geometrical constraints, including minimum wall thicknesses and design space limitations. We critically assess our approach with benchmarks and structures of industrial relevance. In all examples, trivariate B-Spline bases span the background grid, providing highly accurate finite element solutions and shape sensitivities at every iteration. Moreover, the elimination of mesh distortion problems enables a successful termination at the local optimum, even for large shape modifications. • Extension of Vertex-Morphing to embedded boundary methods. • Rigorous elimination of mesh distortion problems. • Discretization-independent sensitivity analysis. • Robust enforcement of design space constraints. • Application to solid structures with industrial complexity. [ABSTRACT FROM AUTHOR]

Hybrid structures, in general, combine different structural elements or materials. In the context of the present paper, the combination of tensile structures and elastic members (bending active elements) and its effects on the mechanical description of the design process of such structures are discussed. A von-Mises-truss example is used to elaborate the underlying problem of choosing the "correct" reference configuration and the resulting differences between the presented approaches. For lightweight hybrid structures the implications on modelling aspects as well as the structural behaviour are discussed. The developed methods are applied to a real-scale umbrella structure which is part of an on-going research project. [ABSTRACT FROM AUTHOR]

In many geomechanics applications, material boundaries are subjected to large displacements and deformation. Under these circumstances, the application of boundary conditions using particle methods, such as the material point method (MPM), becomes a challenging task since material boundaries do not coincide with the background mesh. This paper presents a formulation of penalty augmentation to impose nonhomogeneous, nonconforming Dirichlet boundary conditions in implicit MPM. The penalty augmentation is implemented utilizing boundary particles, which can move either according to or independently from the material deformation. Furthermore, releasing contact boundary condition, as well as the capability to accommodate slip boundaries, is introduced in the current work. The accuracy of the proposed method is assessed in both 2D and 3D cases, by convergence analysis reaching the analytical solution and by comparing the results of nonconforming and classical grid-conforming simulations. [ABSTRACT FROM AUTHOR]

Following the growth of computational fluid dynamics in engineering applications, adjoint methods for sensitivity analysis are being applied to an increasing range of industrial problems. In recent years, growth in computational power has led to widespread use of high‐fidelity, time‐resolved techniques, such as large‐eddy simulation, for applications, including bluff‐body aerodynamics, where time‐averaged equations may be less reliable. The solution of time‐dependent adjoint equations, however, becomes unstable in time as the Reynolds number increases and the flow becomes chaotic. The source of this instability has been explained by analysis of the system's Lyapunov exponents and several solutions have been proposed in the literature based on least‐squares shadowing. Currently, such techniques impose a higher computational cost compared to classical adjoint methods and may become infeasible when the number of unstable Lyapunov exponents is large. In this work, we explore a more recently proposed approach, which balances the adjoint energy source term with added artificial dissipation. The stability of this approach is investigated for a discrete adjoint solver based on the time‐dependent incompressible Navier‐Stokes equations. The accuracy of the resulting shape sensitivities is analyzed using a linear regression technique for the flow around a circular cylinder. [ABSTRACT FROM AUTHOR]

Monitoring and Modeling of a Wind Turbine Support Structure to Create a Digital Twin The basis for creating a digital twin is a suitable model with a close connection to reality in the form of measurement data. In this paper it is explained how model and measurement data of the support structure of a wind turbine can be obtained. The procedure is implemented on a real wind turbine with a concrete/steel hybrid tower. The monitoring includes the acquisition of vibrations, strains and temperature as well as the determination of the dynamic Young's modulus of the concrete structure by sound velocity measurements. For modeling, a finite element model with shell elements was chosen, which takes into account stiffening effects from static loads. A comparison between model and reality is the most important premise for the use of a digital twin: Therefore, a multi‐stage model validation based on modal parameters and local material stresses is performed. The Digital Twin offers different usage scenarios; here one is carried out: the fatigue calculation and remaining useful life (RUL) estimation. For this purpose, material stresses at highly stressed positions are determined using measurement data and model. The results are currently still unrealistically high lifetimes for the concrete part. The results are based on relatively short (1 h, 24 h) stress time series. This could be one reason for the high values. It could also be an indication that fatigue is a non‐critical load case for the concrete part. [ABSTRACT FROM AUTHOR]

Motivated by the applications of the node-based shape optimization problems, where various response evaluations are often considered in constrained optimization, we propose a modified search direction method for optimization design updates. There exist numerous methods for solving constrained optimization problems. The methods of the class active-set strategy try to travel along the active constraints to solve constrained optimization problems. Contrary to the active-set methods, the algorithms of the class interior-point method reach an optimal solution by traversing the interior of the feasible domain. This characteristic is considered to be beneficial for shape optimization because the usual zig-zagging behavior when traveling along the active constraints is avoided. However, the interior-point methods require the solution of a Newton problem in each iteration step, which is generally considered to be difficult for the shape optimization problem. In the present work, we propose a modified search direction method that applies the singular-value decomposition method for the normalized objective and constraint gradients. The modified search direction is a descent direction of the objective function. Using this search direction for the design update in each iteration, we observe that the centrality conditions for the interior-point method are approached iteratively. We are able to achieve a local minimum by traversing the interior of the feasible domain by only using the gradient information of the objective and constraint functions. The results are shown first with analytical 2D problems, and then the results of shape optimization problems with a large number of design variables are discussed. To robustly deal with complex geometries, the Vertex Morphing method is used. [ABSTRACT FROM AUTHOR]

CAD integrated analysis for the design process Lightweight structures are characterized by their delicate supporting structures, featuring a variety of graceful shapes and organically appealing free‐form surfaces. Modern CAD systems use NURBS (Non‐uniform rational B‐Splines) to access the geometric language of lightweight structures with the computer. Isogeometric B‐Rep analysis (IBRA) uses NURBS to formulate structural problems. Using the original geometric and topological information is beneficial for the data exchange between CAD and structural analysis. A uniform model description facilitates parametric design. Whenever the solution of an analysis must be forwarded to CAD, advantages appear. Form‐finding and construction staging are examples where the generated geometry must be considered for further design steps. Regarding bending‐active structures, the smoothness of a model's geometry and the representation of its deformation is of great importance. NURBS enable this smoothness. This article introduces the method and shows its advantages with two selected examples. [ABSTRACT FROM AUTHOR]

Within this study the influence of the interface description for partitioned Fluid–Structure Interaction (FSI) simulations is systematically evaluated. In particular, a Non-Uniform Rational B-Spline (NURBS)-based isogeometric mortar method is elaborated which enables the transfer of fields defined on low-order and isogeometric representations of the interface along which the FSI constraints are defined. Moreover, the concept of the Exact Coupling Layer (ECL) using the proposed isogeometric mortar-based mapping method is presented. It allows for smoothing fields that are transferred between two standard low-order surface discretizations applying the exact interface description in terms of NURBS. This is especially important for highly turbulent flows, where the artificial roughness of the low-order faceted FSI interfaces results in spurious flow fields leading to inaccurate FSI solutions. The approach proposed is subsequently compared to the standard mortar-based mapping method for transferring fields between two low-order surface representations (finite volume method for the fluid and finite element method for the structure) and validated on a simple lid-driven cavity FSI benchmark. Then, the physically motivated 3D example of the turbulent flow around a membranous hemisphere (Wood et al., 2016) is considered. Its behavior is predicted by combining the large-eddy simulation technique with the isogeometric analysis to demonstrate the usefulness of the isogeometric mortar-based mapping method for real-world FSI applications. Additionally, the test case of a bluff body significantly deformed in an eigenmode shape of the aforementioned hemisphere is used. For this purpose, both "standard" low-order finite element discretizations and a smooth IGA-based description of the structural surface are considered. This deformation is transferred to the fluid FSI interface and the influence of the interface description on the fluid flow is analyzed. Finally, the computational costs related to the presented methodology are evaluated. The results suggest that the proposed methodology can effectively improve the overall FSI behavior with minimal effort by considering the exact geometry description based on the Computer-Aided Design (CAD) model of the FSI interface. [ABSTRACT FROM AUTHOR]

• Fast online implementation for covariance-driven stochastic subspace identification (COV-SSI) is proposed. • It does not require any additional parameters. • The error between the proposed method and the original COV-SSI method is negligible. • It makes near-real time COV-SSI possible. Fast online system identification is critical for the timeliness of downstream applications such anomaly detection, condition assessment and decision making. Although some very efficient algorithms, such as the fast Fourier transform (FFT), exist, they are unable to identify damping ratios or mode shapes. The stochastic subspace identification (SSI) method is currently a widely used system identification algorithm. However, its time-consuming operation prevents downstream applications from running online in (near) real time. This paper aims to develop a fast online implementation of covariance-driven SSI (COV-SSI). Time complexity is utilized to analyze the computational cost of COV-SSI. Two steps of COV-SSI are determined to be time-consuming and are accelerated. They are (i) the computation of the covariance matrix (accelerated through incremental aggregation in the dataflow model) and (ii) the singular value decomposition (SVD) of the Toeplitz matrix (accelerated through a fast truncated SVD approach). These strategies do not require any additional parameters; the error between the proposed method and the original COV-SSI method is negligible, but the method is significantly accelerated. Finally, some numerical examples are used to validate the speed and accuracy of the fast online strategies. [ABSTRACT FROM AUTHOR]

Recent advances of the numerical wind tunnel result in a flexible methodological framework, which enables the fully-coupled simulation of wind-structure interactions considering the arbitrary and modular inclusion of additional devices into the system – such as added mass dampers – for the mitigation of vibrations caused by the wind flow. This feature could be seen as an add-on to what can be done with experimental methods and enables further possibilities when the predictive character of such simulations is used for structural design or even for the development and optimization of the additional devices themselves. The procedure promotes an approach fully-solved in time domain using a multiply-partitioned concept to be able to deal with the respective components in an efficient way. It is crucial to adopt a coupled approach and analysis in time as the involved systems can heavily influence each other. The systematic preparation of each module accompanied by coupled simulations is presented, which aims to ensure and enhance the quality of the overall solution strategy. The effectiveness and industrial relevance of the concept is presented on a tall building undergoing dynamic excitation due to vortex shedding with an integrated vibration mitigation device. [ABSTRACT FROM AUTHOR]

We investigate the wind-excited torsional vibration of a parabolic trough solar collector using large eddy simulation. Past studies of wind loads on parabolic trough solar collectors have used rigid models, thereby neglecting possible feedback effects of structural motion on wind loads. Therefore, the primary aim of this study is to assess the extent to which feedback effects of the structural motion on aerodynamic loading can affect the response. To this end, we perform both one-way and two-way coupled fluid-structure interaction simulations corresponding to rigid and aeroelastic models, respectively. In both cases, the collector is exposed to realistic time-varying wind gusts in order to account for their effect on the flow instabilities around the structure. Comprehensive data obtained by numerical simulation is analyzed to provide new insight into the transient flow behavior around a collector and its interaction with the torsional vibration. We show that significant self-excited vibrations can occur for certain pitch angles. The onset of self-excited vibrations coincides with the synchronization of the vortex shedding process with the torsional vibration of the collector. [ABSTRACT FROM AUTHOR]

Reducing the intensity of wind excitation via aerodynamic shape modification is a major strategy to mitigate the reaction forces on supertall buildings, reduce construction and maintenance costs, and improve the comfort of future occupants. To this end, computational fluid dynamics (CFD) combined with state-of-the-art stochastic optimization algorithms is more promising than the trial and error approach adopted by the industry. The present study proposes and investigates a novel approach to risk-averse shape optimization of tall building structures that incorporates site-specific uncertainties in the wind velocity, terrain conditions, and wind flow direction. A body-fitted finite element approximation is used for the CFD with different wind directions incorporated by re-meshing the fluid domain. The bending moment at the base of the building is minimized, resulting in a building with reduced cost, material, and hence, a reduced carbon footprint. Both risk-neutral (mean value) and risk-averse optimization of the twist and tapering of a representative building are presented under uncertain inflow wind conditions that have been calibrated to fit freely-available site-specific data from Basel, Switzerland. The risk-averse strategy uses the conditional value-at-risk to optimize for the low-probability high-consequence events appearing in the worst 10% of loading conditions. Adaptive sampling is used to accelerate the gradient-based stochastic optimization pipeline. The adaptive method is easy to implement and particularly helpful for compute-intensive simulations because the number of gradient samples grows only as the optimal design algorithm converges. The performance of the final risk-averse building geometry is exceptionally favorable when compared to the risk-neutral optimized geometry, thus, demonstrating the effectiveness of the risk-averse design approach in computational wind engineering. [ABSTRACT FROM AUTHOR]

This publication presents a robust and efficient approach for fully CAD-integrated analyses of solids, which aims to reduce the current modeling effort for static and transient problems, including implicit and explicit dynamic simulations. Generating high-quality finite element meshes of solid structures is still a time- and labor-intensive process. Since embedded methods do not require sophisticated boundary-fitted meshes, they have gained popularity in recent years. However, most approaches tend to be computationally expensive due to numerous integration points, especially within trimmed elements. Moreover, their practical applicability in explicit dynamics is often limited because the classically used C 0 continuous discretization field combined with trimming leads to infeasible time steps. In the following, we present methodologies addressing both of these shortcomings. The basic idea is to embed a three-dimensional object into a uniform C p − 1 continuous B-Spline cuboid, where the solid boundary representation provided by CAD is used as trimming surfaces to distinguish between material and void domain. Our primary focus is on constructing highly efficient quadrature rules for both trimmed and full knot spans, which accelerates required matrix formations and, in particular, drastically reduces the simulation times of explicit transient analyses. To fully exploit the potential of the B-Spline bases employed, first- and second-order reduced integration schemes are investigated in addition to optimal quadrature constructions. Despite the appearance of arbitrarily shaped domains, trimmed knot spans are evaluated at most with the same number of integration points as required for full Gaussian quadrature while maintaining optimal convergence in the energy norm. For full knot spans, savings in the number of quadrature points beyond 90% with respect to full Gaussian quadrature are achieved without observing any degradation in accuracy. The proposed methodologies are critically assessed based on scientific benchmarks of increasing complexity and a detailed industrial example, completing the design-through-analysis workflow by performing postprocessing operations directly on the deformed solid CAD model. • CAD-integrated analysis workflow for solid B-Rep models based on trimmed B-Splines. • Highly efficient numerical integration schemes for trimmed and non-trimmed domains. • Modified point elimination algorithm based on non-negative moment fitting quadrature. • Extension of generalized Gaussian quadrature rules to non-tensor product domains. • Feasible critical explicit time steps despite arbitrarily trimmed knot spans. [ABSTRACT FROM AUTHOR]

Unsteady fluid-structure interaction (FSI) simulations are generally time-consuming. Gradient-based methods are preferred to minimise the computational cost of parameter identification studies (and more in general optimisation) with a high number of parameters. However, calculating the cost function's gradient using finite differences becomes prohibitively expensive for a high number of parameters. Therefore, the adjoint equations of the unsteady FSI problem are solved to obtain this gradient at a cost almost independent of the number of parameters. Here, both the forward and the adjoint problems are solved in a partitioned way, which means that the flow equations and the structural equations are solved separately. The application of interest is the identification of the arterial wall's stiffness by comparing the motion of the arterial wall with a reference, possibly obtained from non-invasive imaging. Due to the strong interaction between the fluid and the structure, quasi-Newton coupling iterations are applied to stabilise the partitioned solution of both the forward and the adjoint problem. [ABSTRACT FROM AUTHOR]

Abstract: Subject of this contribution is form finding of “optimal” structural shapes with regard to the load carrying behaviour of surface structures under certain load cases. In general, those optimal shapes prefer a membrane state of stress to transfer loading. Bending is omitted as much as possible. It will be focused on two different disciplines and related numerical approaches which deal with solutions of the mentioned task: form finding of prestressed membranes and general shape optimization. As design is an inverse problem both approaches share similar problematic properties as e.g. indeterminate in-plane location of surface discretization or necessary regularization and filtering of sensitivity and other data. As it will turn out, those remedies found for the very special methods of membrane design can be abstracted and transferred to general optimization procedures. That merges into elegant, numerical shape optimal design techniques which combine advantages of both approaches and allow for effective and efficient shape optimization of free formed surfaces, directly on the finite element mesh and for a large number of variables. Typical applications are, for example, membrane design, free form architecture and structural engineering, and metal sheet design. [Copyright &y& Elsevier]

Reinforced concrete is a widely used construction material in the building industry. With the increasing age of structures and higher loads there is an immense demand for structural health monitoring of built infrastructure. Coda wave interferometry is a possible candidate for damage detection in concrete whose applicability is demonstrated in this study. The technology is based on a correlation evaluation of two ultrasonic signals. In this study, two ways of processing the correlation data for damage detection are compared. The coda wave measurement data are obtained from a four-point bending test at a reinforced concrete specimen that is also instrumented with fibre optic strain measurements. The used ultrasonic signals have a central frequency of 60 kHz which is a significant difference to previous studies. The experiment shows that the coda wave interferometry has a high sensitivity for developing cracks and by solving an inverse problem even multiple cracks can be distinguished. A further specialty of this study is the use of finite elements for solving a diffusion problem which is needed to state the previously mentioned inverse problem for damage localization. [ABSTRACT FROM AUTHOR]

• Application of partitioned dynamic structural finite element analysis. • Stability and accuracy evaluation of non-conforming FEM-FEM coupling technologies. • Comparison of Gauss–Seidel and FETI-based FEM-FEM coupling approaches. • Effect of origin/destination designation on FETI-based coupling. • Partitioned coupling approaches computationally competitive with monolithic solution. Contrasting monolithically solved numerical analyses, partitioned coupled simulations confer the substantial advantages of software modularity, employing the best numerical techniques for each domain and readiness to extend the coupling to other physics and emerging numerical techniques. However, an additional interface problem is created and must be solved without significantly deteriorating solution accuracy. This application-oriented study examines non-conforming partitioned dynamic FEM-FEM simulations under approaches of varying complexity illustrating their ranges of applicability as well as key advantages and disadvantages. After presenting the pertinent aspects of popular coupling technologies, two numerical examples of different complexities are considered. Although the efficiently simple Gauss–Seidel approach is suitable for partitioned systems of dissimilar stiffnesses, such as structure and fluid/soil interaction, stability and accuracy deteriorate as domain compliances align rendering it unsuitable for partitioned structural analysis. Contrasting this, the more robust FETI-based method maintains stability over all stiffness ratios and demonstrates excellent agreement with equivalent monolithic structural responses in terms of displacements and stresses. [ABSTRACT FROM AUTHOR]

Due to their easy‐to‐shape and light‐weight characteristics, membranous structures are common in modern civil engineering. As all outside buildings they are exposed to wind loads and consequently undergo fluid‐structure interactions (FSI). In order to dimension these membranous constructions and to avoid the damage of the material by FSI, numerical and experimental investigations are necessary. The current paper presents the simulation part of complementary experimental and numerical investigations relying on an advanced numerical simulation methodology. [ABSTRACT FROM AUTHOR]